Parting Geometry

M

mickri

H-M Supporter - Diamond Member
H-M Lifetime Diamond Member
In making a drawing for a backside parting tool holder I was thinking about why does parting from the backside seem to have less problems than parting from the front side. My craftsman parting tool holder presents the blade at a slight upward angle to the work. On the front side if the blade catches the blade is forced down and rotates into the work. On the backside the blade is again presented at a slight upward angle. If the blade catches the blade is forced up and rotates away from the work.

My thought is that why backside parting works better is all about the angle of the blade to the work and not about backside or front side. Why can't the backside geometry be used on the front side? The blade would be presented to the work at a slight downward angle. If the blade catches the blade is forced down and instead of rotating into the work the blade would rotate away from the work.

Parting diagram.jpg

I can't be the first person to think of this. Has anybody tried this? Any reason why it wouldn't work?
 
Last edited:
Even with presenting a negative rake, which is what you've drawn, the lack of rigidity in the compound can still cause problems. The flex comes from the stack of components, not from the tip of the tool.
 
Not all parting blades are presented to the work at an angle. My holders place them horizontally, which makes it easy to re-install the blade when I resharpen one. In that situation, if the tip of the parting blade is at the exact center of the work the tip rotates away from the work so no dig-in can occur. This should be the case if the holder is moved to the back (and flipped upside down)

Not having a backside parting tool I can't say for sure if your drawing of it is correct or not.
 
I built mine to be horizontal as well. Mine is a backside inverted holder, a solid block held to the cross slide with t-nuts. I also generally run a plinth for the QCTP. Using the QCTP holder with the plinth normally helps compared to the compound.

I get better performance from the back side holder though. It's a little more ridgid, and the chips clear a lot easier. The cutting fluid also gets into the cut easier. I also don't have to adjust height every time I change the stick out.
 
Can't run my lathe in reverse because the chuck will unscrew. Not good.

Having the blade horizontal instead of at an angle works too. I am trying to use what I already have. If I make a tool holder dedicated to parting I would make it so that the blade would be horizontal. Something similar to this. https://www.hobby-machinist.com/threads/cutoff-tool-holder.79651/ adapted to my norman style QCTP
 
There was an excellent write up in Model Engineers Workshop a couple of years ago that went deeply into the maths around the various forces involved and long story short upside down to 'conventional' is better. If you can't run your lathe in reverse then you need a rear toolpost.
 
The reason the rear parting tool works so well is that if there is a tendency to jam the tool deflects away from the work. On a front mounted parting tool at the beginning of a jam the tool deflects into the work worsening the jam. Using a front mounted tool with the tool inverted and reverse rotation behaves like a rear mounted tool.
 
Too hot to be in my garage/shop this afternoon so I spent the time reading every thread about parting on the Model Engineers website. Did not find anything that came close to discussing the math and the forces involved in parting in any depth. The basic jest of what I found is that when parting from the front side as the blade starts to jam the backlash in both the compound and cross slide allows the blade to be sucked forward and then pushed down. With nowhere to go the blade gets wedged between the work and the compound, cross slide and saddle. Any upward angle of the blade makes this worse. When parting from the backside the blade gets pushed up but it is not constrained by the compound, cross slide and saddle so it moves up and away from the work instead of getting wedged between the work and the compound, cross slide and saddle.

That's where I am at right now.
 
Mickri, Just started Spring and about to start a job when a visitor arrives, they left then I saw your post so had to go look through old magazines. Found the original article which led to others getting into the maths.
Model Engineers Workshop issue 215, February 2015. Page 8.
Parting off in the metal lathe by Reg. Merryweather.
I'll try and scan it tonight and PM it to you as I am sure there will be copyright issues.
Sunday and first warm day in ages and I had a beer with the visitor. Going to be hard to get back to work now.
 
You must log in or register to reply here.
Back
Top