Parting off question

I'm still with Cheeseking. The downward force on the tool can't cause an upward force on the carriage.

-Ron
 
Actually it can. On the back side. You have the tool pressure and the rotation pushing down on the front side of the lathe. For every action there is an equal an opposite reaction. The back side of the carriage can lift up. I was seeing it a lot on my little lathe before I did surgery on it. The front prism acts like a fulcrum. Push up high on the tool post and up comes the back side of the carriage.
 
I'm a "front tool carriage lift " skeptic. A diagram showing the how the load, effort & fulcrum combine to create the reported lift might change my mind, but I cannot visualise it yet.
Another reason given for improved performance of a rear mounted toolpost for parting, is that the rotating job tends to get pushed downwards, where the solid lump of lathe is. With a front TP, the job tends to get lifted up, and only the small amount of metal over the spindle bearings can resist the upward force.
I don't know if it's correct, but so many say that a rear mounted TP is better for parting, there must be something in it!

Jordan
 
Just think of the carriage and the tool post being like a piece of angle iron. Push the top to the side and the other end of the angle lifts.

That said, there are only some cases where it will make *that* much difference. Otherwise, all lathes would have rear parting tools.
 
Sorry guys, I'm still not seeing any "lift" on the carriage. I'm doing the force diagrams in my mind (going to put it to paper when I get the chance) and I just don't see where you're coming up with that. Pushing down on the tool tip will cause the rear of the qctp to have a "lifting" force on it as the leading edge of the qctp would be acting as the fulcrum but the "fulcrum" for the carriage would be the ways (at least the one at the front side) so the only way that an upward force would be seen at that point would be if the tip of the cutting tool passed over the center line of the lathe bed while still cutting. While parting from the front there is no way this would happen.

There may be other reasons parting from the back may be "better" but I can't see where this would be one.

Best regards,

-Ron
 
Sorry guys, battling a cold and haven't looked at the computer much at all.

First of all, don't let any of this bother you. If it doesn't make sense then just let it go. I've seen threads like this go on for months and it's never settled. We can talk about force vectors and gib clearances all day and not get anywhere - for my part, I will leave this here.

All I am saying is that if you are having parting issues then you might consider using one of these tools. In my experience, and those of many others, such a tool may just be what you are looking for. They have only been in use for over a hundred years so there must be some value to them, no?

And before anyone tells me to go buy a "real lathe", I'm going back to bed ...

Mikey
 
Hardly surprising, but I being a novice, I used to have real problems parting off with the lantern type tool post on the compound that came with my first Atlas, then I acquired a rear tool post to fit on a T slotted turret type cross slide I had, this was such an amazing improvement that I never had any more fear of parting off at all. I would even go to the trouble of changing cross slides just for parting.

Then I got a second much better Atlas, I spent a lot of time and effort rebuilding and solidly mounting it, it is very well adjusted now and has a new Acetal lead screw nut, zero backlash.

It now has a QCTP fitted on the compound on the standard cross slide and I can part off with equal confidence at the front now. The only difference is that the finish to the sides of the cut is not so good using the parallel sided tool that came with the QCTP, whilst the one I use in the rear tool post has relief angles at the sides and also tapered away from the cutting edge, and gets an almost perfect finish.

So my conclusion is that if your lathe is worn, flexible, or not too ridgedly mounted, parting is much improved using a rear tool post, which in the case of an Atlas, means using a much longer cross slide, which also helps.

I may be wrong, but I am of the opinion that the forces involved lift the cross slide and carriage at the rear just enough to really tighten the gibs, and also pull the spindle down tight, as Mikey says, the reasons don't matter but what I do know for sure is it just works!

The added advantage of having a parting tool permanently ready for use is nice when turning small diam parts too, but a QCTP and a well set up lathe can get you the same results and is so more versatile.

Just my thoughts

Bernard
 
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OK heres a different twist on this. I havent started parting last night, I must have done a million. This weeks
brainstorm is a .500 X 1 inch with a .60* sharp point (center) for a mill locater. Alright turn more than an
inch at exactly .500, turn the 60* sharp point. Part off at the one inch mark. Do the setup for parting, I use
a low belt setting not back gears, plenty of oil, about 3/16 into the cut that thing got sucked right in and
under bending the part, good thing its a flat belt stalling and a squealling. ??? I had to undo the tool post
unlock carriage just to get the thing out. Course the part is trash. No machine damage so far. Whats up
with that? Well every time me and the kid goes to HF I'll buy some HSS blanks and cut offs just to have in
the draw. So what failed, my freakin eyes. Upon examination somebody or some thing in a rice burning
shop ground the parting tool upsidedown. I just see some kid .50 cents a day working there I dont blame
the kid only me for not checking HF products. So parting is not difficult and the lifting thing is ?? like best I
can say is-- with nice crossilde gibs it comes down to the weight of crosslide plus apron holding the whole
thing down. In my (above) case the tool grabbed in and went under the work and this pressure may have
lifted the whole apron assembly up and jammed the tool, it had to, It bent the work. Example If I were
to do this on my GK It would have busted the tool or tool post. Im saying the apron & crosslide has to be
lifted with a chain fall which is no competition for a trival 1/2 parting on this lathe. So It comes down to
weigh and matter and ridgity and double checking what you do buy. shame on me, I just thought I bought
some parting tooling. HF love em or leave em........samuel
 
With regard to the carraige being lifted off the ways - it is possible if you are using an inverted (upside down) mounted blade. I think that is where the comfusion lies??
 
I don't see this leverage on the tool post being any more effective than pushing on the front of my truck to lift it's rear axle. It's pushing down. The rear may get a little lighter but the front is MUCH heavier.

At work we often part from the back (and thread) these are large machines and have no problem with the carriage lifting (powerturns and monarchs). If you have ever heard anyone cause carriage chatter on this kind of rig it is memorable, impressive and seldom repeated. these are pin mounted chucks so we can have tools mounted in the back and reverse the rotation to press on the ways.

My 12x36 in the garage might be equipped one day with a backside parting setup for the simple reason that in piece work, I can run the steps on the front tool post and then part from the back without changing that level of setup. ... Oh don't laugh, it'll always be square, and centered because it won't need to move (often). The rolled up parting swarf can fall away...but I know it will be lifting my 70# carriage and apron. I don't have the luxury of reversing (IMO) my screwed on chuck so it sits until I am well bored or see a future need.

Steve
 
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