Lathe rear parting tool

[Pescadora]

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.

 

[macardoso]

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.

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

 

[macardoso]

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

 

[Manual Mac]

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.

 

[SLK001]

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.

 

[mikey]

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

 

[SLK001]

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.

 

[Manual Mac]

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.

 

[mikey]

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

 

[WobblyHand]

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.