Where to find upside down parting holder?

higgite said:
I'm not as sure of that as you are, Dan. Here is why. Please tell me if I'm off base.

The way I see it, the parting blade is a lever and tool post mounting screw is a fulcrum. If you try to lift the carriage/cross slide/compound/tool post assembly by lifting on the business end of the parting blade, the whole assembly will try to turn around the fulcrum, dipping in the front and rising in the back. Rotating the spindle/work piece clockwise against an upside down parting tool in a front tool post does just that. So, it seems to me that the upside down blade and CW spindle rotation combine to push the front of the carriage down, not lift it up. Am I wrong?

Tom
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I believe you're incorrect.

The tool, tool post, compound, cross slide, and carriage are all rigidly connected and will act as one unit. If they were all off floating in dead space, and you applied a force they would rotate around their combined center of mass. On the lathe they are going to rotate around a hinging point, and that's almost always going to be the rear of the bed as the gibs hold the carriage down to it.


a good example is a single kid on a seesaw. It doesn't matter if you apply an upward force between the child and the center pivot, or stand behind him, and apply it. The seesaw will always rotate around around the pivot because it's fixed. All you are doing is changing the effective length of the lever arm and the amount of force need to rotate the seesaw.

This is why its easier on big lathes, the kid (apron and carriage) is heavier, and the lever arm (distance to the back way) is longer requiring even more force to cause lift.
 
I sort of think you guys are missing the point, which is that a rear mounted parting tool with the lathe running in the normal direction, or an upside down front mounted tool with the lathe running in reverse, works. If you haven't used one then maybe you should try it. It works better than a standard front mounted tool on smaller, less rigid lathes and it even cuts more efficiently on larger lathes. Lots of advantages to a rear mounted tool.

You can go on for weeks arguing about how the forces are distributed. I have seen this discussion many, many times and it has never been fully resolved. Even Myford, who markets one, isn't sure how it works. While I have my own beliefs, I won't add them here (but I agree with @ddickey and @higgite). Suffice it to say that it works.
 
Myford do not recommend inverted parting tools from the front with the machine reversed, just for rear tool post.
I've never seen inverted parting tools from the front with the machine reversed suggested by any trusted source but it would be interesting to see a reference ;-)
 
it works better in the back for two main reasons.

Tool pressure has to be higher to cause the carriage to lift. This is because cutting on the back of the carriage has reduced the length of the lever arm the cutting force has to work on by the diameter of the work piece.

Removing the compound from the mix increases rigidity, and helps minimize chatter.
 
I don't have an issue parting right side up. So why would I want to put it under? If I had a rear one, I would agree. But I think that a front upside down might improve it, because it lifts out of the way if you go to deep while lifting. Maybe I'm lighting a fire here, but I think poor positioning of the parting tool (too high or too low (worse) ) and a poor feed rate are the reasons for not parting well. I don't mean to put anyone down, but I can't see a good reason for it.
 
I use a 3mm inserted tool the right way up in the rear QCTP on my Myford Super 7 with the lathe running in reverse.
Once I discovered that most parting issues were caused by tool alignment issues and incorrect feeds & speeds I stopped having those issues.
 
I won't discourage the discussion as it may be relevant to those with smaller lathes, but I doubt I need to worry about such forces as my small lathe's carriage probably weighs around 500 lbs and the big one, I'll bet the carriage weighs 1K or more. I gotta believe the parting blade will break before it lifts the carriage on either. The inverted method has been recommended to me by another machinist. I recently switched to inverted threading and am ecstatic about the results of that. I don't know the physics of it, but my threads come out much better with an inverted cutter. The only downside is I can't use my automatic stops when threading inverted.
I can make my own, but I was wondering about commercially made tool holders, and with so many knowledgeable people here, I figured if it did exist, someone here had seen it and used it.

Thanks,
Chris
 
I understand threading upside down to avoid crashing or an inside blind side stop. I would expect that to be easier, but not better threads. I have threaded upside down away from a stop for the reasons stated. I didn't notice they were better. But it's an easy thing to try again. I find upside down threading (while an advantages for a hard stop) to be a little awkward in that I have to trust because I can't see the cut or chip easily. Where as I can when it is upright. But I'll give it a try just to see, never say never. The upside down parting, that's a little different, as I don't have a holder, I could reverse my qctp, to the other side but that would be a lot of stick out, and defeat the test.
 
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