Parting Trick

[jschmidling]

There are numerous videos on tricks for using parting tools but I found one that seems too simple and obvious not to be used universally.

His graphic makes it seem very simple but I don't quite see how it is not dependent upon the position above or below center.

I tried it on a piece of PVC in my mini and it works as advertised. To me, the most obvious advantage is the way it pushes the chips out the bottom.

Doing it the normal way, I have to stop frequently to remove the tangled mess of chip, including using a scissors to cut it loose.

I am interested in some opinions of this....

js

 

[benmychree]

Cutting off has always been done like this on turret lathes, but on the backside revolving forward, It makes parting off a much more successful operation than the standard method, partly because chips have an easier exit and also coolant/lubricant has easier flow to the cut, and as is pointed out, the tendency to jam up as the model shows, is likely the most important factor of troubles in parting by the standard method. A great deal of parting problems go away when using heavier industrial rated machinery, and made worse with light hobby style machines.

 

[rgray]

Not a fan myself.
Rotation is trying to lift the tool, compound, and cross slide.
Sure if you have slack in those items the tool will be lifted up away from the cut, but then will be above center also.
Get rid of the slack in those places and you will be able to part off normal.

Plastics are always challenging as far as the cuttings. All strings and no chips.

 

[stioc]

I've never tried this method (though have seen it a few times) so may be I should before I say this but if you're reversing both the tool's orientation and the part's rotation then you're basically doing exactly the same thing just upside down. The graphics are an extreme case scenario. I don't think the tool can dig into work if it's held solid in a tool holder and chips while in this case fall straight down as opposed to spilling over but would that really make that much of a difference? May be a placebo effect?

 

[C-Bag]

Another consideration is you'd not want to do that with a 9x20 because you might unscrew the chuck. I've seen folks with 9x20's mount the cutter on the other side and upside down and then you're still in fwd rotation. They say that it works for parting. I'm ok with fwd parting now that I've got everything including the headstock bearings adjust correctly.

 

[stioc]

Another consideration is you'd not want to do that with a 9x20 because you might unscrew the chuck. I've seen folks with 9x20's mount the cutter on the other side and upside down and then you're still in fwd rotation. They say that it works for parting. I'm ok with fwd parting now that I've got everything including the headstock bearings adjust correctly.

Excellent point there, any heavy work in reverse can cause the chuck to unscrew itself from the spindle. I take it you have a 9x20 too? It's a fairly decent machine, way heavier and more rigid than my 7x14 I had years ago. But I think a 12x36 or a 13x40 will be in my future when I retire (and have a bigger shop). I use a carbide insert parting tool and it slices through aluminum like butter, that wasn't the case with my HSS parting tool. Haven't tried steel yet.

 

[mikey]

I don't think the tool can dig into work if it's held solid in a tool holder and chips while in this case fall straight down as opposed to spilling over but would that really make that much of a difference? May be a placebo effect?

A rear mounted parting tool or an upside down/reverse rotation parting tool works.

This is a P1-N rear-mounted parting tool (0.040" wide tool) cutting a groove in a 1-1/4" OD piece of 12L14 steel at 1200 rpm on a Sherline lathe. Note that the blade is purposely overextended to try to induce chatter. The cut is clean and chatter-free - look at the finish inside the cut. This part was later parted off an inch from the chuck without issues. A Sherline lathe weighs somewhere near 35# soaking wet so it isn't about size; its about how the cutting forces are transferred to the lathe.



This blade, mounted on this lathe, could not make this cut from the front without slowing the speed down really low ... maybe not even then. The rear mounted set up can cut just about any machinable material at high speeds from the rear with no issues whatsoever. I have not had a single issue when parting this way in nearly 20 years so no, not a placebo.

 

[stioc]

Good to know, I'll have to try the rear mounted version. I assume in the rear mounted version the blade isn't held upside down nor is the spindle rotated in reverse?

 

[mikey]

In a rear mounted parting tool holder, the blade is mounted upside down and the rotation of the lathe is normal, not reversed. The cutting forces lift the tool so a dig in is not possible. Moreover, these forces push down on the cross slide, thereby increasing rigidity. This transfer of forces has been controversial because it is difficult to visualize and I won't go into it here because it just leads to arguments. However, I am convinced that this is the reason why a little lathe like a Sherline can part from the rear more effectively than many much larger lathes can from the front.

 

[C-Bag]

Excellent point there, any heavy work in reverse can cause the chuck to unscrew itself from the spindle. I take it you have a 9x20 too? It's a fairly decent machine, way heavier and more rigid than my 7x14 I had years ago. But I think a 12x36 or a 13x40 will be in my future when I retire (and have a bigger shop). I use a carbide insert parting tool and it slices through aluminum like butter, that wasn't the case with my HSS parting tool. Haven't tried steel yet.

Yes, I've got the despised duo, 9x20&RF30 too. My 9x20 was not used much for anything more than trying to trick it out, but he had no clue about all the QC probs. Being a total noob when I got it I didn't know what was operator error/or mis adjustment. Over time and through the simple projects I found and solved the problems as they came up with the help of other 9x20 users like Savarin. What I've come to is with all the work I've done I understand its shortcomings and have been able to do better and better work. If I ever get to the point I need a larger lathe, I'm going to keep it as a backup as everytime a lathe goes down it is crystal clear I needed a lathe to fix it.