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Where to find upside down parting holder?

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TORQUIN

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#1
Anyone have a source for an upside down parting blade holder for BXA? Or maybe some plans to make one? I am not going to build a rear tool post on my Monarch, but upside down from the regular tool post would be an improvement.

Thanks,
Chris
 

TORQUIN

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#4
I have seen that one, but to use it on regular tool post, the way it's designed, I would have to rotate the tool post 90 degrees when I want to use that tool holder. I suppose I could just use it backwards, but the clamp would be at the back of the tool holder in that position.

Thanks,
Chris
 

Robert LaLonde

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#5
I have seen that one, but to use it on regular tool post, the way it's designed, I would have to rotate the tool post 90 degrees when I want to use that tool holder. I suppose I could just use it backwards, but the clamp would be at the back of the tool holder in that position.

Thanks,
Chris
If you used it backwards the rake would be wrong.
 

ddickey

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#6
You're talking about the Aloris?
 

ddickey

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#8
No. Actually downward.
 

Dan_S

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#9
No. Actually downward.

i think you're thinking about this wrong. To use a parting tool upside down in the normal QCTP mounting location would require turning the spindle clock wise, and that would definitely be trying to lift the carriage, just like threading with the tool upside down. If the lathe is big enough it might not be an issue, but I definitely wouldn't do it on a small lathe, or one with a threaded spindle.
 

Ken from ontario

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#10
This inverted parting tool is something that can easily be built in a home shop, It was one of my very first project for my mini lathe,many thanks to Mikey for talking me into milling it myself .
 

magicniner

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#12
A conventional lathe tool of any type in conventional orientation with the work rotating in the normal direction will place downward force on the carriage, a force diagram makes it clear -

LatheCuttingForces.jpg
A parting tool will not be subject to axial feed force but the rest is is relevant, pertinent and accurate.
 

woodchucker

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#13
I have always wondered why someone would want to part it upside down. The force indeed is to lift the carriage, rather than push into the bed.
 

Dan_S

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#14
I'm sorry dickey, but that's incorrect. I have background in physics and I'm 100% sure about this.

With the tool holder on the front of the cross-slide, the blade upside down, and the spindle rotating clock wise, the cutting action will try and lift the carriage in the front. This is why the first bullet point on the eccentric page is as follows.

https://www.eccentricengineering.com.au/index.php?option=com_content&view=article&id=31&Itemid=45
1: The inverted blade design significantly reduces the chance of a jam up and blade breakage.
When the cutting forces get to high, the carriage will lift and the tool eases out of the cut. Even if you have your carriage locked it will still lift, just not as much, as usually the lock is on the right side of the carriage.

When the tool is right side up, the cutting forces push the tool and the carriage down onto the bed. when the cutting forces are to high, or the set-up isn't rigid enough, the parting tool flexes down because that's the only way to relive the force. This usually leads to the tool breaking. It either breaks because the force is to high, or because the work rides up on the tool and breaks it. Excessive backlash in the cross-slide can exacerbate the issue.


The Aloris #7R style holders are designed to be mounted on the back of the cross slide with the spindle turning counter clockwise like it normally does. This to tries to lift the back of the carriage, but it can't as the back of the carriage have gibs that run against the underside of the bed and prevent the carriage from lifting. Even that wasn't exactly what they where designed for.

On a manual machine they would most commonly be used on a turret lathe like in this video.
 

magicniner

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#15
It is hard to visualize but running the blade upside down in reverse will direct forced downward. Eccentric makes one specifically for screw on chucks.
I think you have missed the point that screw on chucks will be unscrewed by a tool used as you describe above ;-)
 

ddickey

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#16
Seems to make sense and that is what I thought. If you're 100% certain I'll go with it. That video of the turret lathe has the blade mounted upside down so exactly the same forces.
My lathe parts very well with this set up.
 

ddickey

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#17
I think you have missed the point that screw on chucks will be unscrewed by a tool used as you describe above ;-)
you're correct. I guess you could only use a rear parting blade held upside down with a screw on chucks.
 

Dan_S

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#18
My lathe parts very well with this set up.
If you're using a post rigidly mounted to the top of the cross-slide, like you posted a while back, that is most likely why. Removing the compound from the equation greatly increases rigidity.
 

higgite

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#19
I'm sorry dickey, but that's incorrect. I have background in physics and I'm 100% sure about this.

With the tool holder on the front of the cross-slide, the blade upside down, and the spindle rotating clock wise, the cutting action will try and lift the carriage in the front. This is why the first bullet point on the eccentric page is as follows.
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
 

magicniner

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#20
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
You call the tool post mounting screw a fulcrum in a lever system which it isn't, the fulcrum is not part of the moving assembly, it is the point around which the moving assembly moves freely.
The rest goes generally downhill from there ;-)
 

Dan_S

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#21
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
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.
 

mikey

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#22
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.
 

magicniner

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#23
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 ;-)
 

Dan_S

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#24
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.
 

woodchucker

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#27
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.
 

magicniner

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#28
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.
 

TORQUIN

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#29
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
 

woodchucker

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#30
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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