First time parting not too good

Regarding the rear tool post mount, it might be to do with some smaller lathes having no gib on the bottom of the frount of the sadle, this might alow it to hop up a bit if it grabs and springs ?

i see that most capstan lathes have a rear cut off tool, i always thought it was for speed, but the lever operation looks like you could feel the tool realy well during a parting off.


Stuart
 
Back when I built my rear mounted tool post I wondered why it worked so well. I'd read all the arguments about which way the forces were going and so on but it was really hard for me to see. To resolve this for myself, I put a preloaded dial indicator on the carriage, behind the tool holder, and tried parting from the front; I got a small positive deflection, meaning the carriage was lifted up. When parting from the rear there was no movement of the needle at all, suggesting to me that the forces were indeed pushing the carriage down onto the ways. Not very scientific, I agree, but it was good enough for me.

My reasoning is that the tool post and lock down bolt act as a fulcrum. A lifting force at the nose of the tool will attempt to lift the front edge of the tool holder but since the tool holder is firmly bolted to the carriage it pushes the rear of the tool holder down. This takes up any play in the works and increases the apparent rigidity of the lathe.

Perhaps just as important as which way the forces are running is the fact that the tool is lifted out of the cut when parting from the rear so dig ins are essentially eliminated. Couple that with a rigid tool holder and the girder-like construction of a T-type parting tool and you have a set up that works.

Again, if you have a lathe that is large and rigid enough to resist the cutting forces developed by a parting tool then this sort of work around is not necessary but on a small lathe it is a very big deal.

Stu, on a capstan lathe (turret lathe here in the States) ALL the tools are mounted on the same turret. I suspect these lathes part from the rear due to the design of the turret rather than to resist cutting forces. They are all CNC lathes as well so feel is a non-issue. They do work really well, though, don't they?

To the OP, I didn't mean for this to go off on a tangent. My apologies.

Mikey
 
Not sure what grade of cold rolled it was. It is just some practice metal I bought at Princess auto. I was turning about 300 rpm or maybe less. I tried some oil also.
 
A video in the thread that 'great white' linked to in post #2 explains it in terms of the relationship of the pivot point to the tool/work piece interface. I found it very enlightening. And a neat tool, also.
https://www.youtube.com/watch?v=KETVR9qtEmY

Tom
 
Garyg said:
Not sure what grade of cold rolled it was. It is just some practice metal I bought at Princess auto. I was turning about 300 rpm or maybe less. I tried some oil also.
Click to expand...

Gary, it might be better to practice on some Aluminum until you get the hang of this. Parting steel is not difficult with a good set up but you need to learn how to do it first. I would suggest 1" 6061 or something that won't deflect too much and part off close to the chuck. Use WD-40 as a lube.

Sharpen your tool. A 7 degree angle at the front is good. Hone any burrs off the top of the tool. Keep your parting tools sharp.

Be sure your tool height is as close to on center as you can get it or a few thou above center. You can check your tool height with a 6" rule. Trap the rule between the tip of the tool and the work and adjust the height of your tool until the rule is vertical.

Be sure your tool is precisely perpendicular to the work. The easiest way is to move the tool up to the chuck and align it with the face of the chuck.

Lock all gibs except your cross slide feed. You want everything as solid and rigid as you can make it.

Aluminum will part at higher speeds than steel but will also part at lower speeds and I would suggest about 300-400 rpm to start. Move the tool in until is just starts to cut and lube the little groove that you just made, then start to feed the tool into the work. You want to feed so that you can feel the tip cutting and watch for a continuous chip coming off the tool tip. Keep the cut lubed; you don't have to soak it but try not to let it run dry. You will hear the tool whine or squeal when it gets dry so lube it before it makes that noise. As you feed in about 2/3 of the way through you will need to slow down a little because the surface speed of your work is lower as you cut into the center. Just keep feeding so you can feel the tip in firm contact with the work until you cut through.

Give us some feedback on how things are going. Parting is not difficult and you will be doing this for as long as you do lathe work. It pays to find a set up that works for you and stick with it. Just come back here if things are not going right and we can try to trouble shoot along with you.
 
Is this the video you were referring to? I found it interesting as well.

[video=youtube;-RZRq0olsxM]https://www.youtube.com/watch?v=-RZRq0olsxM#t=12[/video]



Henrymac said:
I saw a video on youtube where the operator ran the lathe backwards when parting. Instead of pushing down and into the workpiece, the force was away from the spindle. If there were any binding or other undue forces on the workpiece, the tool would be pushed away from the workpiece instead of into it. I believe he ran the tool upside down also. Crashing was near impossible under these conditions. Does this sound like something that should be considered? Just my .02. :makingdecision:
Click to expand...
 
ICanBreakIt said:
Is this the video you were referring to? I found it interesting as well.

[video=youtube;-RZRq0olsxM]
Click to expand...

Yes, I posted it in an earlier post, but thanks. Would this technique work on a larger lathe? I would think so.
 
You must log in or register to reply here.
Back
Top