Lathe Tool Holder Rigidity Theory Question

No, it won’t work just the same. It will perform just a bit less in terms of stiffness than the 1/2” shank it started out as. It has to — we removed a portion of its section so it has been compromised. To what degree we compromised it is really what we’re after.

I get that this is a theoretical scenario and it’s not a real situation, but I think in order to come up with any kind of realistic analysis you’d need to put some hard numbers in for the variables. There will be a compromise no matter what you do, but where is it going to be the least noticeable? Or is it a part of one and part of the other? Stiffness or rigidity is only one part of the performance equation. When you start dropping sizes in tool holders you also sacrifice some damping qualities as well, and that speaks to the quality of cut on the work. Anyone who uses a 1/4” tool bit and the switches to a 3/8” bit will be able to notice the difference — it’s subtle and they both still work, but you can feel the difference between the larger and smaller tool.

I think this scenario is really interesting to think about because it makes you look at thresholds and what limits there are before things start to slide and performance suffers. In this hypothetical case those thresholds may be quite close to being reached, I mean we’re not talking 1” tool holders here we’re talking 1/2” to 3/8” holders. Margins start to sneak up fast the smaller we get.

So, yeah, I get the question but I don’t think it’s as simple as asking which action will cause the performance to suffer. They both will but which one will you notice, and for that you need numbers on each to compare.
 
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great white said:
Doesn’t change the fact the tool is oversized for the lathe and its being modified to fit.

If you/they had already settled on an answer, why bother to ask? Just do it and see what you/ they get…
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You seem to be missing what I'm saying. It's not for me and I'm not doing anything.

The question was simply which method would be more rigid, that's all.

If you just want to debate what you're talking, i.e. what's the "right way" I'd say you're still missing the captain obvious answer. If you take a 1/2" tool and turn it into a 3/8" tool it's no longer "oversized" for the lathe. We're machinists who can take something one size and make it something another size when needed.
 
francist said:
No, it won’t work just the same. It will perform just a bit less in terms of stiffness than the 1/2” shank it started out as. It has to — we removed a portion of its section so it has been compromised. To what degree we compromised it is really what we’re after.

I get that this is a theoretical scenario and it’s not a real situation, but I think in order to come up with any kind of realistic analysis you’d need to put some hard numbers in for the variables. There will be a compromise no matter what you do, but where is it going to be the least noticeable? Or is it a part of one and part of the other? Stiffness or rigidity is only one part of the performance equation. When you start dropping sizes in tool holders you also sacrifice some damping qualities as well, and that speaks to the quality of cut on the work. Anyone who uses a 1/4” tool bit and the switches to a 3/8” bit will be able to notice the difference — it’s subtle and they both still work, but you can feel the difference between the larger and smaller tool.

I think this scenario is really interesting to think about because it makes you look at thresholds and what limits there are before things start to slide and performance suffers. In this hypothetical case those thresholds may be quite close to being reached, I mean we’re not talking 1” tool holders here we’re talking 1/2” to 3/8” holders. Margins start to sneak up fast when the smaller we get.

So, yeah, I get the question but I don’t think it’s as simple as asking which action will cause the performance to suffer. They both will but which one will you notice, and for that you need numbers on each to compare.
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I should have been more specific...I'll edit that. No, the 1/2" shank tool reduced to 3/8" height won't work the same as an unmodified 1/2" shank tool, but it will work the same as a 3/8" shank tool....actually better since it's still 1/2" wide rather than 3/8" wide.
 
I think most of you have missed the point here.

To me, it’s a fairly interesting question. Probably pretty tough to answer because . . . It depends. Each component’s compliance contributes, as in a series of springs. Determining how the thickness affects total compliance isn’t that simple. Both structures seem like cantilevered beams, where the tip deflection scales inversely with thickness cubed (and with the length cubed but that shouldn’t change here). So it’s pretty sensitive to the change relative to starting thickness.

While these aren’t simple beams, the tool holder supports the tool over some length of the holder below the tool. (I might guess around a third of the supported tool length, but since deflection isn’t constant over that length, maybe it’s effectively more?) The tool supports a bending load over the projecting portion, which wouldn’t have to reduce in thickness. The tool’s reduced thickness is inside the holder, and not subject to as much bending load because it’s clamped in the holder. The portion of tool with reduced thickness projecting from the holder seems, to me, short enough that it’s contribution to the total tip deflection probably isn’t too great.

Also, reduced holder thickness means that as the compliance exceeds the clamping screw preload, you further introduce additional compliance from the tool bending. That is a second-order effect that could be significant?

Based on the above hunches, I’d guess that reducing the tool thickness causes less increased deflection than reducing the holder bottom support. But it of course depends on the details. It’d be interesting to see an FEA simulation.
 
No matter how you twist and turn this, a tool milled to 3/8 x 1/2 will perform as good or better than the 3/8 x 3/8 tools the lathe was designed for. It might even perform a lot better if the carbide is thicker.
 
G-ManBart said:
You seem to be missing what I'm saying. It's not for me and I'm not doing anything.

The question was simply which method would be more rigid, that's all.

If you just want to debate what you're talking, i.e. what's the "right way" I'd say you're still missing the captain obvious answer. If you take a 1/2" tool and turn it into a 3/8" tool it's no longer "oversized" for the lathe. We're machinists who can take something one size and make it something another size when needed.
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Ok, I’m done with this round and round foolishness.

The answer has obviously been predetermined so any conversation on the matter is pointless.

ciao
 
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Rigidity issues and height problems can both be solved by getting a bigger lathe and appropriate sized tools and holders.
Simple eh?
 
I've ground tool holders for this reason, and I don't like it. It is surely the most common method, as every used tool holder seems to be ground to chamfer a small bore. I've got 50 lbs of HSS blanks that say I can probably do better than making a permanent alteration to another durable tool. It only takes a few seconds to change tools.

As to the weakening issue, a few mouse clicks in FEA will show you in full color how much strength is being compromised.
 
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