Lathe rear parting tool

[mikey]

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?

 

[SLK001]

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.

 

[mikey]

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:

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.

 

[jwmay]

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.

 

[WobblyHand]

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.

 

[mikey]

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

 

[Navy Chief]

@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

 

[jwmelvin]

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.

 

[SLK001]

@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.

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.

 

[jwmay]

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.