# Lathe rear parting tool



## Pescadora (Feb 27, 2022)

Not mine, and no comment by someone unqualified to make one.


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## mikey (Mar 1, 2022)

Good job! Those of us who have this tool love it.


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## Pescadora (Mar 1, 2022)

deleted


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## Manual Mac (Mar 1, 2022)

I’ve been trying to come up with an idea for making one for my SB9.
Been thinking for years, & nothing’s happening.
But…I just remembered, I have this table I made years ago before I bought a mill. 
it’s 4”x 8”x 5/8”, with a spigot on the bottom so it will bolt to the cross slide with the compound off.  It’s as accurate as i could make it.
Thanks to you guys, I will now make one, roughly adapt those plans, bolt it to the table & Bob’s yer Uncle.
Cheers


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## Wheat.Millington (Mar 1, 2022)

Someone help me out here as a beginner - I can't get my head around why a rear mounted parting tool makes a difference??


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## Zertsman (Mar 1, 2022)

Every connection between parts from the Ways on the lathe bed up to the tip of the parting tool can have some sort of flexibility or movement.  On their own, they are small or possibly unmeasurable.  Add them all together and all sorts of strange things can happen.

Making a solid connection with no adjustment, (center height is guaranteed) removes all the other parts and their connections, lubrication and things from the equation.  many problems go away and you likely will have a larger operating envelope in some other areas for parting..   more materials, bigger/smaller diameters..   

on a machine tool, stiffness and rigidity is everything.


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## Pescadora (Mar 1, 2022)

deleted


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## mikey (Mar 1, 2022)

Wheat.Millington said:


> Someone help me out here as a beginner - I can't get my head around why a rear mounted parting tool makes a difference??



Have a look here. It might answer some of your questions.


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## SLK001 (Mar 1, 2022)

Pescadora said:


> It is clear to me now that all the problems with parting stem from the flexibility in the tower of moving parts that held the tool. Take most of them out of the equation and the difference is profound.



Yeah, but this method takes none of them out.  The cutting forces with this contraption is UP, away from the ways - and the apron isn't tightly held down against up-lift, so rigidity isn't what this is giving you.

Also, this design is likely to give you fatigue cracks at the flexure point over time, faster for aluminum, slower for steel.


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## Pescadora (Mar 1, 2022)

SLK001 said:


> Yeah, but this method takes none of them out. The cutting forces with this contraption is UP, away from the ways - and the apron isn't tightly held down against up-lift, so rigidity isn't what this is giving you.





SLK001 said:


> Also, this design is likely to give you fatigue cracks at the flexure point over time, faster for aluminum, slower for steel.


I yield to a superior opinion. I should not post on a forum where I know so little.


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## mikey (Mar 1, 2022)

SLK001 said:


> Yeah, but this method takes none of them out.  The cutting forces with this contraption is UP, away from the ways - and the apron isn't tightly held down against up-lift, so rigidity isn't what this is giving you.
> 
> Also, this design is likely to give you fatigue cracks at the flexure point over time, faster for aluminum, slower for steel.



I've used this tool for over 25 years and understand it well.

Are you basing your statements on experience or conjecture?


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## SLK001 (Mar 1, 2022)

mikey said:


> I've used this tool for over 25 years and understand it well.
> 
> Are you basing your statements on experience or conjecture?



What statements?  The forces ARE up and the apron isn't tightly held down.  And a design like this will crack over time.


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## mikey (Mar 1, 2022)

SLK001 said:


> Yeah, but this method takes none of them out.  The cutting forces with this contraption is UP, away from the ways - and the apron isn't tightly held down against up-lift, so rigidity isn't what this is giving you.
> 
> Also, this design is likely to give you fatigue cracks at the flexure point over time, faster for aluminum, slower for steel.



^^^ These statements.

Ordinarily, I would have let it go until the OP posted this:


Pescadora said:


> I yield to a superior opinion. I should not post on a forum where I know so little.



Clearly, he is intimidated by you and I admit that I do not like that.

I am the original designer of this tool and yes, it is a tool and not a contraption. It has taken *thousands* of chatter-free cuts in just about all machineable materials at 1-3 times normal turning speeds without a hint of chatter. It is extremely rigid and extremely accurate using very thin HSS parting tools that most hobby guys can easily afford. And it will part materials as thin as 0.010" thick and in diameters as large as the chuck will hold. I know it will do all of this because I have done all of this, personally and repeatedly. So, let's be clear. It is a highly effective tool. 

I agree that the initial cutting forces at the TIP of the tool are up; that is self-evident. However, these forces are transferred to the body of the tool and into the body of the lathe via the bolt(s) that holds the body of the tool to the cross slide. That bolt acts as a fulcrum and the upward forces are transferred downward to the rear of the tool body to the cross slide, then into the saddle, then onto the ways. All slack or gaps in these structures are taken up and the result is an extremely rigid affair. This allows a light lathe to handle cuts that it normally cannot handle and it does so with no effort. I have studied this in more detail than it deserves in order to work out how these forces are transferred. If you have evidence to the contrary, please do share it.

To be clear, a parting tool used from the front increases these clearances in the bed and it is this arrangement that is less rigid, not the other way around. I've looked hard at this one, too.

As for cracking, maybe. My tool is well over 25 years old and while it is made of aluminum, it has shown zero signs of stress or cracking whatsoever but I admit it is early yet. Maybe in another 25 years it will show some sign but in the meantime I think it will just keep making trouble-free parting cuts for me and others who use this design. There are hundreds of us who do and I have yet to see a single negative report on this tool. I have seen negative comments on the design from those who have never used one, though.

So, @Pescadora, while your tool is new, it clearly works well for you and I suspect that it will continue to do so for quite some time. It doesn't matter why it works; just that it does. 

All of us are free to give an opinion or share our triumphs and I hope that you continue to feel comfortable and welcome here.


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## jwmay (Mar 1, 2022)

SLK001 said:


> And a design like this will crack over time.


I'm betting mine will last longer than I will. I'm willing to risk it. But fwiw, nobody said anybody has to use a rear mounted parting tool. We only say, "try it, and you will like it". There's not much reason to argue over theory. It works. Nobody who ever did it has argued. Afaik. Which isn't very far. 
Again, fwiw, do or do not. Makes no difference. I like mine. I'm not worried about the 1" thick cast iron cross slide holding the 2"x3"x4" cast iron block that holds the 2.3 mm wide parting tool. But I guess it might fail someday. 
I don't have the smarts of Mikey. But there's my little thought on it.


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## WobblyHand (Mar 1, 2022)

Not wishing to get into any disagreement, but why are there two slots?  Wouldn't one be enough?  Not hard to put one in, just wondering.


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## mikey (Mar 1, 2022)

The rear slot is an expansion slot to reduce the risk of cracking. It works.


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## Navy Chief (Mar 2, 2022)

@Pescadora 
Can you please repost your original post on this thread? This is something I am very interested in and was hoping to look at your photos and the plans again, and without the original post this thread loses a lot of context. Don't let the contrary opinion of one user dissuade you, if something works well for you (and a lot of other people) who cares what that one person thinks. 

Sent from my Pixel 5 using Tapatalk


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## jwmelvin (Mar 2, 2022)

mikey said:


> I agree that the initial cutting forces at the TIP of the tool are up; that is self-evident. However, these forces are transferred to the body of the tool and into the body of the lathe via the bolt(s) that holds the body of the tool to the cross slide. That bolt acts as a fulcrum and the upward forces are transferred downward to the rear of the tool body to the cross slide, then into the saddle, then onto the ways. All slack or gaps in these structures are taken up and the result is an extremely rigid affair.



I’d suggest that the tool mounting can impart a balanced pair of reaction forces to the cross slide but cannot change the net force applied to the cross slide. You don’t seem to agree and I’m interested in why?

To be clear, I have not used a rear-mount parting tool but believe the common report of improved performance. I do not think my lack of personal experience has any bearing on a discussion of forces or loading, and I think the discussion of both aspects is beneficial to the forum. 

I also lament that the OP removed such valuable content.


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## SLK001 (Mar 2, 2022)

Navy Chief said:


> @Pescadora
> Can you please repost your original post on this thread? This is something I am very interested in and was hoping to look at your photos and the plans again, and without the original post this thread loses a lot of context. Don't let the contrary opinion of one user dissuade you, if something works well for you (and a lot of other people) who cares what that one person thinks.
> 
> Sent from my Pixel 5 using Tapatalk



Yes, please repost your originals.  It was not my intention to "run you off" or to even intimidate you, but merely to have a lively discussion on parting.  The parting operation is one of the most difficult things to do on a lathe and not being able to master it has caused more hobbiests to abandon the hobby than anything else.  Your setup works well for you, so keep using it - it will greatly reduce the stress of parting.  Now exactly *why *it works is a mystery.  Perhaps it is precisely because the rear setup can momentarily lift out of a cut when a hard spot is encountered is why it works.  But this is just conjecture on my part.  I must admit that I do not like parting, because it gave me fits before I finally figured it out.  I use the exact same parting blade that you use and I have broken more than I care to admit during parting.  About every year or so, there is a thread on parting where the "front parters" square off against the "rear parters" about which is better and why.  Neither is better than the other if both work for the respective teams.  With discussion, one team can perhaps learn from the other and even help those new to the parting nightmare.



mikey said:


> The rear slot is an expansion slot to reduce the risk of cracking. It works.



The rear slot is not needed and it actually increases the risk of cracking, since it reduces the amount of metal in the post.  That said, it probably won't fail in any one's lifetime, since the flexure is small and infrequent.


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## jwmay (Mar 2, 2022)

I'd really like you to repost your content too.
And also, I part from either side of the spindle centerline. 
Why? Cause the rear mount post has a little parting blade that won't do more than an inch. The front mount is an old workhorse that changed my world when I first got it... and it'll do an inch and a half I think.


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## SLK001 (Mar 2, 2022)

jwmay said:


> I'd really like you to repost your content too.
> And also, I part from either side of the spindle centerline.
> Why? Cause the rear mount post has a little parting blade that won't do more than an inch. The front mount is an old workhorse that changed my world when I first got it... and it'll do an inch and a half I think.



Nice cross-slide.  Is it original to the lathe, or something that you made?


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## Pescadora (Mar 2, 2022)

State of mind is anything but intimidated. 

Sarcasm is how I respond to arrogance, but in truth, I should not post opinion in a space where I am not qualified.


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## Manual Mac (Mar 2, 2022)

Go a ahead and post.
Fresh ideas are always welcome.
I’ve been working as a machinist most of my life, and there’s lots on this site I’m not qualified to comment on.
but I still do.
and I like new ideas


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## jwmelvin (Mar 2, 2022)

Pescadora said:


> in truth, I should not post opinion in a space where I am not qualified.


You are qualified. This forum, like many, is based on individual experience. You sharing your experience, regardless of the original motivation or justification for the action, helps us.


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## Navy Chief (Mar 2, 2022)

Pescadora said:


> State of mind is anything but intimidated.
> 
> Sarcasm is how I respond to arrogance, but in truth, I should not post opinion in a space where I am not qualified.


What makes your experience any less valid than anybody else's experience?

Sent from my Pixel 5 using Tapatalk


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## SLK001 (Mar 2, 2022)

Pescadora said:


> State of mind is anything but intimidated.
> 
> Sarcasm is how I respond to arrogance, but in truth, I should not post opinion in a space where I am not qualified.



As a hobbiest, you're always qualified to post an opinion.


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## jwmay (Mar 2, 2022)

SLK001 said:


> Nice cross-slide. Is it original to the lathe, or something that you made?


Its a casting sold by http://www.mlatoolbox.com/S-4382.html


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## WobblyHand (Mar 2, 2022)

Pescadora said:


> State of mind is anything but intimidated.
> 
> Sarcasm is how I respond to arrogance, but in truth, I should not post opinion in a space where I am not qualified.


I read and saw your original posts.  Thought you did a great job making a tool that works for you.  Please update your posts with your original content.  We are a great big family at HM, and sometimes have dustups, but we all support each other in our endeavors.  Everyone, from professional machinists, to those who have never used machine tools before and anyone in between, are welcome here.  We all learn from each other, including good techniques, and lets just say, things we did, that we never should have done.  We even have a blunder thread.  Generally, this is a good place to hang out.  Please don't have the comments of one person drive your behavior towards the whole group.  Remember, if someone really, really gets out of line, report them.  HM very much wants this place to be welcoming.  They do enforce their rules.

Everyone is entitled to their opinion, period.  You did make something and it did work.  Therefore you are allowed and encouraged to speak.  You are more than qualified to discuss your project!  You are more qualified than me, who is a mere hack and beginner.  Come on back.

Been wanting to make one of these back mounted parting tools for both my mini-lathe and my G0752.  Neither of my lathes have tee-slots, so creating a firm cross-slide mount is slightly more complicated, but should be doable.  Would be nice if the drawings were re-posted.  Thanks.


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## mikey (Mar 2, 2022)

SLK001 said:


> The rear slot is not needed and it actually increases the risk of cracking, since it reduces the amount of metal in the post.  That said, it probably won't fail in any one's lifetime, since the flexure is small and infrequent.



Hmm, my first prototype did not include the rear expansion slot. After a few months some stress cracks began to show on the top shelf just where the front slot ended. This told me that I needed some flexure to reduce stress so I incorporated the rear expansion slot. Since then, no more stress cracks and there seemed to be better clamping force applied to the blade. It has now been over 25 years and not a single stress line has shown up so I think the rear slot is important.


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## Pescadora (Mar 2, 2022)

Appreciate the support. I am usually a closed-mouth guy. Opinions are like you-know-what, and typically I like to keep mine covered. A while ago, I asked a question about parting. One of the nice fellows here answered me privately with some drawings and suggestions. Those are posted now in the first post. Took some measurements on my machine, which is a 1030PM, and then adapted the concept. 
Parting on the 1030 was always an iffy affair, with the tool holder tip held to a different height with every in or out adjustment, making height and squareness difficult to achieve, with the quick change post on top of the compound and that on top of the cross-slide. With chips piling up in the slot and the invariable dig-in and stall of the spindle. Every time. Took to dismounting the work and using a bandsaw. And then mounting in the mill's radius jaws and squaring off the end.

After a few weeks of spare time in development, had it done. Gave it a try on a chunk of 1.5 1018 round bar (hot roll). Blade tip was dull. Forgot to add lubricant at first. Did it matter? No. I sent my mentor thanks and a couple of photos. He suggested I post on here. Did that, but offered some unwarranted opinions also.  Sorry for that. Sorry also for the comment about arrogance. Came back on to delete that, but it's quoted already.

Can't recover the text in that post as it is gone. Here are the photos.


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## Pescadora (Mar 2, 2022)

mikey said:


> Hmm, my first prototype did not include the rear expansion slot. After a few months some stress cracks began to show on the top shelf just where the front slot ended. This told me that I needed some flexure to reduce stress so I incorporated the rear expansion slot. Since then, no more stress cracks and there seemed to be better clamping force applied to the blade. It has now been over 25 years and not a single stress line has shown up so I think the rear slot is important.


My top is thicker. Not by intent. My fulcrum is also wider. Not by intent either. Block was thicker. The slot for the blade is less than a thou of clearance. It will only go in if squared up, but slides fore and aft easily.

But you cannot clamp with a hex key. It takes a 3/16" Allan socket on a 3/8" drive ratchet to clamp the blade in place. 10-15 lb/ft of torque to close about .0005". I have no opinion about that.


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## macardoso (Mar 2, 2022)

Pescadora said:


> Appreciate the support. I am usually a closed-mouth guy. Opinions are like you-know-what, and typically I like to keep mine covered. A while ago, I asked a question about parting. One of the nice fellows here answered me privately with some drawings and suggestions. Those are posted now in the first post. Took some measurements on my machine, which is a 1030PM, and then adapted the concept.
> Parting on the 1030 was always an iffy affair, with the tool holder tip held to a different height with every in or out adjustment, making height and squareness difficult to achieve, with the quick change post on top of the compound and that on top of the cross-slide. With chips piling up in the slot and the invariable dig-in and stall of the spindle. Every time. Took to dismounting the work and using a bandsaw. And then mounting in the mill's radius jaws and squaring off the end.
> 
> After a few weeks of spare time in development, had it done. Gave it a try on a chunk of 1.5 1018 round bar (hot roll). Blade tip was dull. Forgot to add lubricant at first. Did it matter? No. I sent my mentor thanks and a couple of photos. He suggested I post on here. Did that, but offered some unwarranted opinions also.  Sorry for that. Sorry also for the comment about arrogance. Came back on to delete that, but it's quoted already.
> ...


I think it is a great idea honestly. I have a 12x36" lathe so parting is relatively easy for me, but I've read lots of posts of people with smaller lathes benefiting from this design. If I had T-slots on the cross slide, I would have built one already.

Glad you posted here and sorry for the rocky welcome. This is a great community and there are amazingly few kaffufles here. Hope you stay around and keep posting.

-Mike


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## macardoso (Mar 2, 2022)

There's also an old Colchester lathe video showing them running hydraulic tracers using the rear mounted upside down turning tools. I have a feeling they did that both for tool access and rigidity. The downward clearing chips is super helpful.






Right at 30 seconds. Wish my lathe ran that well


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## Manual Mac (Mar 2, 2022)

I just jury-rigged a set-up from loose parts I had laying around.
i Can Not believe how much better parting from the rear works for me!
I’m not *******rting you. What a break thru, and I can run it much faster with a deeper cut, & no digging in, or stopping the spindle.
This table failed for the intended purpose I built it for.
But I use it all the time in my mill vise bolting engine parts or fixtures to it either flat or vertical for machining.
i only wish I had built it 2 inches longer for parting from rear, but it works great the way it is.
in fact it works so well I probably will leave as is, (if it works, don’t fix it, eh?)
Even Harvey told me how pleased he is.


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## SLK001 (Mar 2, 2022)

mikey said:


> Hmm, my first prototype did not include the rear expansion slot. After a few months some stress cracks began to show on the top shelf just where the front slot ended. This told me that I needed some flexure to reduce stress so I incorporated the rear expansion slot. Since then, no more stress cracks and there seemed to be better clamping force applied to the blade. It has now been over 25 years and not a single stress line has shown up so I think the rear slot is important.



If you drill a hole approximately 0.150 to 0.250" at the terminus of the slotting cut, you'll eliminate the two stress risers from the square ends of the cut.  In a sense, it's "drilling the crack".  You are deliberately introducing a crack into the metal.  The round hole (that you will eventually slot) will distribute the stresses around the perimeter of the hole and prevent cracking.  The slot with the hole will look like this: =======O.  Ideally, the mounting screw should be in the middle of the post, where tightening it down doesn't affect the slot, but mounting considerations have to be accounted for.

The fact that you are getting better clamping force means that the flexure is even higher with two slots.  With two slots, you now have four stress risers.


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## mikey (Mar 2, 2022)

Thanks. If it ever cracks then I will revise the design. So far so good.


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## SLK001 (Mar 2, 2022)

You guys that have the rear parting mount, please do this simple test:

Mount a dial indicator with the base clamped, or mag-mounted to the bed and the DTI to measure any lift of your carriage.  Then perform a normal (rear mount) parting operation.  Watch the dial for any deflection and note whether it is constant, or varying over the cut.  Also note how much varying (if any) that you see.  This information may lead to actually understanding why this method works so well.


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## Manual Mac (Mar 2, 2022)

I thought I’d better make a disclosure here.
My lathe is a SB9. =Small, but very accurate. But small.
And yeah, I was squealing like a birthday girl when I discovered how much better upside-down rear parting is.
But the Monsters I ran when i did this for a living would not simply dig in or stop the spindle if something went wrong.
Things get *Real Interesting* in a hurry with a large lathe. 
parting Is something I never really made peace with.
that said, I wish I knew about rear parting a long time.
So….enough said.


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## mikey (Mar 3, 2022)

Pescadora, I've been thinking about what happened in this thread. HM was created for guys exactly like you, beginner hobby guys who are entering the field. Nelson, the founder of HM, wanted to create a forum where new guys and more experienced ones could gather and share information in a safe and welcoming environment. He set up strict guidelines and the staff of HM enforces them with rigor. 

One of our members made you feel intimidated or defensive or unprepared to respond to him. I honestly don't think that was his intent but at your level of experience you were not prepared to handle that. This happens from time to time; it is difficult to discern intent from words on a screen but as you saw, the other guys jumped in to encourage you. There was no need for the mods to intervene; the members did that. This is exactly as it should be and Nelson would be proud of them and of you.

Nobody enters this hobby knowing everything. It takes time and experience to become a hobby machinist so be patient with yourself and try to also be patient with us, the forum. As you learn, things will get better ... and easier.

If I could give you some advice, engage as you are able, learn and share and do not allow others to intimidate or inhibit you. Share what you can, encourage others that follow you and do not tolerate aggression and inappropriate treatment of others. 

You are welcome here. Your trials, mistakes and victories are welcome here. And this goes for every one of the silent majority of new guys who may feel inhibited in some way. Put it out there and I assure you that the guys will be there to support you. 


Mikey


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## WobblyHand (Mar 3, 2022)

After a horrible parting session on my Grizzly this morning, I'm going to make one of these.  I don't know what happened, this lathe used to part ok. Something changed and now it is bad.  Never have seen that much movement before.  Something is loose or way off.  Bad enough that I gave up and used a hacksaw.  I haven't had to do that in a very long time. 

So I'm going to draw some version of this parting tool.  About how much height over the blade does one need to make this work ok?  1/2", 3/4"? About how wide does it need to be?  More is better, but is 2.5" ok, or does the blade need more support?  I will need to drill my cross-slide since my lathe has no tee-slots.  2 M6 or 1/4" bolts ok, fore and aft?  Or something larger?  I'm thinking of machining feet (really more like flanges) so I can bolt through it into the cross-slide.  I have some rough cut 1.625"x3.5" stock in 7075.  No 6061 this big.

Edit: I'm going to start my own thread on this.  Don't want to take away from this one.


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## mikey (Mar 3, 2022)

WobblyHand said:


> About how much height over the blade does one need to make this work ok?


1/4" thick top is fine.



WobblyHand said:


> About how wide does it need to be?  More is better, but is 2.5" ok, or does the blade need more support?


2" front to back is enough.



WobblyHand said:


> I will need to drill my cross-slide since my lathe has no tee-slots.  2 M6 or 1/4" bolts ok, fore and aft?  Or something larger?  I'm thinking of machining feet (really more like flanges) so I can bolt through it into the cross-slide.


I suggest two 10-32 SHCS to hold the blade and 10-32 or 1/4" to hold the body to the cross slide.



WobblyHand said:


> I have some rough cut 1.625"x3.5" stock in 7075.  No 6061 this big.


Go buy some 2" to 2-1/4" square 6061-T6. It isn't that expensive and I know for a fact that 6061 will not crack if you follow the general build I used. While 7075 is stronger, it is also more prone to crack with repeated stress/flexion. I would go with 6061. DO include both slots. The distance of solid stock between the slots only needs to be about 1/4" - 5/16" wide.

Be absolutely sure the bottom of the blade slot, and therefore the location of the tip of the tool, is on the exact centerline of your lathe. Find the centerline with cuts; DO NOT chuck up a pin in a collet and expect that to be accurate enough. 

DO include a ledge in your design. It should butt up against the side of the cross slide. This prevents the tool post from spinning under load and allows you to lock the tool solidly to the cross slide with a single bolt.


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## SLK001 (Mar 3, 2022)

WobblyHand said:


> After a horrible parting session on my Grizzly this morning, I'm going to make one of these.  I don't know what happened, this lathe used to part ok. Something changed and now it is bad.  Never have seen that much movement before.  Something is loose or way off.  Bad enough that I gave up and used a hacksaw.  I haven't had to do that in a very long time.



Before you begin on a different journey, investigate *why *you are having parting problems with your current setup.  If you don't, you could experience the same issues with a rear mounted tool.  It sounds like there will be significant mods to your cross-slide for you to mount the tool, so make sure those are mods you're willing to make.


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## WobblyHand (Mar 3, 2022)

mikey said:


> 1/4" thick top is fine.
> 
> 
> 2" front to back is enough.
> ...


Thanks for the tips.  I was going to make a ledge that bolted on, like @Pescadora's.  When I made a plinth for your straddle knurler design, I machined the ledge in, but it wasted a bunch of material.  As an aside, making that straddle knurler was such a great experience for me.  I learned a whole lot from your thread.  I love using it!    

I do have a piece of 2" x 2.5" x 6" 6061, and a piece of 1.5 x 3 steel stock. I was thinking the aluminum wasn't wide enough once I mill away 0.75 for the screw flanges, that only makes the holder 1.75" wide, which doesn't feel wide enough to me.  Centerline is approximately 79 mm above the cross-slide.  This tool is for a 10" lathe.  Ignore the single slot in this picture, (and the hole at the end of the slot) but look at the feet of the plinth.  In this model, the width of the base is 3" and the tool holding width is just under 2-1/4".  I'm playing around in CAD, and changing stuff for these simple drawings isn't difficult.  Only took me 15 minutes for the whole drawing.


What do you mean by find centerline with cuts?  If I made a center in the chuck, left it in the chuck, and then scribed it against the piece, would that be sufficient?


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## WobblyHand (Mar 3, 2022)

SLK001 said:


> Before you begin on a different journey, investigate *why *you are having parting problems with your current setup.  If you don't, you could experience the same issues with a rear mounted tool.  It sounds like there will be significant mods to your cross-slide for you to mount the tool, so make sure those are mods you're willing to make.


Will be doing that.  Something obviously loosened up, or changed.  I suspect a couple of things went south, but need to investigate further.  

As for the cross-slide, all I expect to do is to drill and tap two holes sufficiently far away from the dovetails.  There's 20 mm of space.  Lots of room for an M6 screw.  I've done something similar for my mini-lathe and attached a rear mount plinth for a knurler.  When not in use, the cross slide holes are capped with screws.


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## jwmay (Mar 3, 2022)

I did the measuring thing. I put two indicators on the carriage. One was at the top front of the parting tool block. The other was on the tailstock side of the cross slide. Both showed about .002" lift when parting. So there's that information for whomever may want to make suppositions, theories, and explanatory essays. Lol. I'll be machining.


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## woodchucker (Mar 3, 2022)

Zertsman said:


> Every connection between parts from the Ways on the lathe bed up to the tip of the parting tool can have some sort of flexibility or movement.  On their own, they are small or possibly unmeasurable.  Add them all together and all sorts of strange things can happen.
> 
> Making a solid connection with no adjustment, (center height is guaranteed) removes all the other parts and their connections, lubrication and things from the equation.  many problems go away and you likely will have a larger operating envelope in some other areas for parting..   more materials, bigger/smaller diameters..
> 
> on a machine tool, stiffness and rigidity is everything.


Here's a different take on why it works, which I agree with.
I don't agree with that  sentiment that the cross slide is tighter in the rear, since it's more tight at the front of the cross slide (our side) vs the back..


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## mikey (Mar 3, 2022)

WobblyHand said:


> Thanks for the tips.  I was going to make a ledge that bolted on, like @Pescadora's.  When I made a plinth for your straddle knurler design, I machined the ledge in, but it wasted a bunch of material.  As an aside, making that straddle knurler was such a great experience for me.  I learned a whole lot from your thread.  I love using it!


Glad that tool worked out for you. It is a learning experience, that's for sure.



WobblyHand said:


> I do have a piece of 2" x 2.5" x 6" 6061, and a piece of 1.5 x 3 steel stock. I was thinking the aluminum wasn't wide enough once I mill away 0.75 for the screw flanges, that only makes the holder 1.75" wide, which doesn't feel wide enough to me.


Why bother with flanges? You only need the ledge plus a single bolt in the center of the tool body to hold it solidly in place.



WobblyHand said:


> What do you mean by find centerline with cuts?  If I made a center in the chuck, left it in the chuck, and then scribed it against the piece, would that be sufficient?


The centerline I'm referring to is the exact centerline of your specific lathe's spindle. The tip of the parting tool has to be on that centerline or very, very close to it. You can maybe be half a thou or less below it but you should not be above it. Here is a simple way to determine this critical dimension:

Chuck up a piece of stock, maybe 3/4-1" OD. Aluminum or mild steel is fine. Use a sharp HSS tool and take a skim cut of 0.010" deep to take off the skin, then another cut of 0.003" to get a clean surface, then a final cut of 0.001-0.002" deep with a fine finish. You can use a simple 3 jaw to hold the work.
Use a height gauge if you have one and measure from the surface of the cross slide to the top of that turned work piece.
Measure the OD of the work piece with a micrometer and divide that dimension in half.
Subtract 1/2 the OD from the height to the top of the work piece and that gives you your centerline or spindle height above the cross slide. Write this down someplace accessible.
You can approximate this dimension with a precision pin and collet but you will be off; cutting it is more precise. When you make your tool post, the bottom of the blade slot must be on this precise dimension or as close as you can get it. 

An add-on ledge is fine. You do not need to cut one.

Let me know if any of this is unclear.


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## jwmay (Mar 3, 2022)

it. Here is a simple way to determine this critical dimension:

All I did was set my parting tool holder on the carriage and slid it across a tailstock center. That left a layout line. Then I measured it. I don't know if it's a fool proof method, but it proved fine for this fool.


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## mikey (Mar 3, 2022)

woodchucker said:


> Here's a different take on why it works, which I agree with.
> I don't agree with that  sentiment that the cross slide is tighter in the rear, since it's more tight at the front of the cross slide (our side) vs the back..


The debate on how a rear mounted tool works has been going on for decades ... literally. Almost everyone involved has their guess, including Myford, and nobody has proven the mechanism behind it to universal satisfaction. So many guys have argued vigorously over this and the most vehement of them have been those who have never used a rear mounted tool post before. This is why I don't like to get involved in these discussions. There is no end to it.

I have taken the stance that it just works. Why it works is a matter of personal belief and I have mine but that is of no consequence. What matters is that it works and it works better than parting from the front. My Emco Super 11 parts just fine from the front and I have no issues doing so but I cannot part at the speeds that I can from the rear and cutting loads are higher when parting from the front.

I can tell you that I can part from the rear in almost any machineable material on a little Sherline lathe at very high speeds with zero chatter. I've parted 1.25" OD 304SS with almost no heat in the part and no work hardening at all. I've done 2" OD Delrin, mild steel, brass, 1144, 4140, wood and I forget what else with zero issues. After thousands of problem-free cuts I can say that it works ... it just does.


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## WobblyHand (Mar 4, 2022)

mikey said:


> Why bother with flanges? You only need the ledge plus a single bolt in the center of the tool body to hold it solidly in place.


I'd rather not do flanges, but the plinth height is 4".  That's a long hole.  Was thinking of using a long M8 SHCS.  An extended length P drill (which I don't have) will only drill to 3.64", a jobber length P drill drills to 2.82".  Guess, I could flip the piece to try to make the hole longer.  Haven't tried that before, but why not.  If I flip the piece for drilling, probably need to up the through hole size a smidge.  An M8 is as large as I dare drill into my cross-slide.  Want to have sufficient spacing from the edge of the dovetail, since there is a groove cut in that corner with a slitting saw.  


mikey said:


> Use a height gauge if you have one and measure from the surface of the cross slide to the top of that turned work piece.


No height gauge.  Did buy one, but returned it, since it did not match the photos.  Pics showed a carbide scriber, I got a steel one.  Sort of a bait and switch.  Got a refund, but haven't found a replacement.  Some day...

Could measure the height from the bottom up, with some 123 blocks, ground stock and shims on the cross-slide.  Not sure how accurate that would be with tolerance stack up, but it should allow a measurement.

I'll redraw the tool holder, assuming a 2.5" x 2" x 4" piece of 6061.  (Have a chunk on my desk.)  My drawing is parameterized so if I need to change a dimension, I just type it into the spreadsheet and whamo, the drawing is updated.


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## mikey (Mar 4, 2022)

WobblyHand said:


> I'd rather not do flanges, but the plinth height is 4".  That's a long hole.  Was thinking of using a long M8 SHCS.


You could always countersink the hole for the head, right? Drill from the bottom then drill from the other side and countersink it so a shorter screw will work.



WobblyHand said:


> No height gauge.  Did buy one, but returned it, since it did not match the photos.  Pics showed a carbide scriber, I got a steel one.  Sort of a bait and switch.  Got a refund, but haven't found a replacement.  Some day...
> 
> Could measure the height from the bottom up, with some 123 blocks, ground stock and shims on the cross-slide.  Not sure how accurate that would be with tolerance stack up, but it should allow a measurement.


However you measure the height, get it dead on. I would just go buy a digital height gauge and be done with it. You will need it again someday.



WobblyHand said:


> I'll redraw the tool holder, assuming a 2.5" x 2" x 4" piece of 6061.  (Have a chunk on my desk.)  My drawing is parameterized so if I need to change a dimension, I just type it into the spreadsheet and whamo, the drawing is updated.


I use paper and pencil - rub, rub, draw - whamo, updated!   

This tool is really simple. All good tools are really simple. The tool does not take much stress so it doesn't have to be huge; it just has to be solid. The ledge and a single locking bolt is sufficient to resist all cutting forces. The two slots work; they prevent cracking and allow the top ledge to lock down solidly, accurately and only requires snugging lightly with a hex wrench to lock the blade in place. The top ledge only moves a few thousandths of an inch so cracking is very unlikely. If it hasn't cracked in over 25 years then I would say it has proven itself. 

Try not to overthink it. I've already taken out anything in the design that doesn't absolutely have to be there.


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## WobblyHand (Mar 4, 2022)

mikey said:


> You could always countersink the hole for the head, right? Drill from the bottom then drill from the other side and countersink it so a shorter screw will work.
> 
> 
> However you measure the height, get it dead on. I would just go buy a digital height gauge and be done with it. You will need it again someday.
> ...


I have moved my new design into my thread in Projects.  Not nice to continue to clutter up someone else's thread.  Sorry @Pescadora don't want to take away from you accomplishments, or fill your thread with my dumb questions.  Time for me to discuss (my issues) in my own project.  https://www.hobby-machinist.com/thr...-parting-tool-for-a-g0752z-g0602-lathe.98539/


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## Manual Mac (Mar 6, 2022)

Manual Mac said:


> I just jury-rigged a set-up from loose parts I had laying around.
> i Can Not believe how much better parting from the rear works for me!
> I’m not *******rting you. What a break thru, and I can run it much faster with a deeper cut, & no digging in, or stopping the spindle.
> This table failed for the intended purpose I built it for.
> ...


As an update I went ahead & made some tooling for the rear parting tool, & added 2” to the table with the tooling.
i am using an old Williams 2020 5/8” cutoff holder turned upside down.
once again I can’t believe how good this set-up works
no more digging in, stopping the spindle, & I can use much higher RPM. 
and the pucker factor is (almost) gone.
And (once again) this is a SB9, a larger lathe would have similar results I suspect, but proceed with caution.
Cheers


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## mikey (Mar 6, 2022)

Very cool, Mac!

Try rear mounting your knurling tool back there. The increase in rigidity has a significant impact with that tool as well.

As your confidence in your parting tool grows, parting becomes a routine non-event. Grooving, thread reliefs become simple tasks and your efficiency improves ... all because you moved your tool back there. Who woulda thunk it?


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## Manual Mac (Mar 6, 2022)

It even turns fine.
Sorry, I’ll stop, I’m veering way off topic.


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