Lathe Tool Holder Rigidity Theory Question

[G-ManBart]

Edit to add: This isn't for me, it's just something that was discussed elsewhere.

Okay, this is pretty random, but it came up on another venue and made me think for a minute. The topic was getting carbide insert lathe tools on center when the tool is a bit large for the lathe. In this case, the tool needs to move lower to get on center and the lathe is fit with a standard Aloris-style QCTP. With the height adjuster nut raised all the way the tool holder hits the top of the compound before the tool is on center.

That leaves us with removing material from the bottom of the tool holder, allowing it to go lower or removing material from the bottom of the tool so it sits lower in the tool holder.

Since smaller lathes don't have an excess of rigidity, which solution is more rigid? Let's say we have to come down .125" or so.

My theory is that removing material from the bottom of holder makes it more likely to flex. A larger tool will be slightly more rigid than a smaller tool, but that would seem to be a smaller factor. Putting a rigid tool on a weaker shelf just doesn't seem to be the better option. I'd rather put a weaker tool on a more rigid shelf. Opinions? Guesses?

 

[tweinke]

In my opinion I would remove material from the tool.

 

[epanzella]

Take it off the tool. If the tool is too big to get on center in your lathe it will still be as rigid as the biggest tool you have that WILL fit your lathe. Why compromise a holder that you may use with a standard sized tool down the road.

 

[great white]

At the risk of being “captain obvious” here, a tool holder that won’t put the tool on center is the wrong tool holder for the lathe.

You don’t need to modify it, you need the proper tool holder.

 

[Jake M]

Since smaller lathes don't have an excess of rigidity, which solution is more rigid?

if one piece is sized correctly, and one is not sized correctly. There you go.....

My theory is that removing material from the bottom of holder makes it more likely to flex.

My theory is that neither option will add enough additional flex to exceed the margin of error when you measure it. Small lathes rely on low tool pressure to make good, accurate cuts.

 

[Jim F]

Use the correct tool for the job.

 

[RJSakowski]

Not all tool holders are created equal. I found that the thickness of the bottom shelf varies. Shars seemed to be the best in the low priced set. In addition, Shars sells an XL version of the 250-101 which is made for a 5/8" tool. It has a slightly thinner shelf (sbout .030") which may buy you enough clearance.

I expect the same holds true for the BXA tool holders.

 

[savarin]

I solved it by replacing the top slide with a solid plinth which gives me heaps more vertical adjustment and is more rigid overall.
I thread at 90', I only replace the top slide when I need to cut small tapers and parting is a total breeze on my flexi 9x20 lathe.

 

[G-ManBart]

At the risk of being “captain obvious” here, a tool holder that won’t put the tool on center is the wrong tool holder for the lathe.

You don’t need to modify it, you need the proper tool holder.

I should have pointed out this isn't for me. Someone else stated they have a lathe with an AXA QCTP and normally use 3/8" shank tools, but got some free 1/2" shank tools. They tried to use the 1/2" shank tools but found they wouldn't go quite low enough to be on center, hence the discussion.

I don't see the harm in taking a bit off a tool in a situation like this. After all, milling a bit off the bottom of a 1/2" shank tool and suddenly it's essentially a 3/8" shank tool, right? Sure, a bit wider and one size larger insert, but it should work the same as a 3/8" shank unless I'm really missing something.

 

[great white]

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…