QCTP Base Question

I'd be willing to agree (with no evidence other than my own visceral perception [gut feel]) that the rectangular (square) style of T-nut is indeed a bit more rigid than the round. But come on, guys ... we're not talking "farmer tight" here. I've never had to tighten the toolpost nut beyond "good and snug." I seriously doubt that the tool post nut will be the part that flexes when the lathe tool is under load ... especially for the types of lathes many of us have. It's an interesting discussion, but I don't think the OP has anything to worry about.
 
q20v

Thanks for your positive response. Just to be clear I am not saying that there is a problem with either style of T nut.
However, when I faced the same situation of mounting a new QCTP without the capability of milling the blank, I opted to use the existing round T nut and save the new square blank for posterity. I did not see the point of having two round T nuts.
It did require that a new bolt of the proper dimensions be threaded to fit the old round T nut and that was a fun exercise in itself.

It turned out to be relatively easy to do a bit of milling on the lathe. So I have both style nuts now.

Looking forward to seeing the results of your analysis.
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I don't have either style. My setup uses a rectangular piece of steel about 1/2" x 2-1/2" x 3-1/2" with a drilled and tapped hole in the middle for the stud that fits my compound. Been using it for years and the tool post has never moved.

Just saying.

Tom S.
 
A 6" vise is overkill for a RF45 style mill. A 4" is more appropiate for that size mill & IMO 5" max. I have a 5" GMT vise on my PM45 & it's slightly too big. Not enough Y axis travel to make use of the 5" full capacity. Better to save your money rather than getting something too big & most importantly the weight. I take my vise of the table quite often, a 6" is still light enough for me to be carried by hand but I'm glad I have a 5". I also have a 4" vise as well. I prefer the 5" though.

But those GMT 6" Premium vises are pretty nice. I'd love to have one but don't need one on my current mill. But if you plan on upgrading to a full size knee mill in the future than the 6" will be perfect.


Here's what the 5" looks like on my mill.
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I couldn't even complete this cut without my bellows & DRO scale getting in the way. Not enough Y travel & the 5" vise is not even maxed out.
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Here's what a 6" vise looks like on another PM45 (gt40's)
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Barry -

That's indeed slick! And it's a great representation of what I do: I relieve the inner part of the QCTP mount just a bit, so that it only makes contact on the periphery, increasing friction as far out as possible, thereby resisting rotation on the post.
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Wonder if/how the analysis would differ if the QCTP had a perfectly flat bottom and pressed down against the top of the compound, keeping it from bowing upward. My guess would be that most owners of decent lathes have not needed to relieve the bottoms of their QCTP mounts.
 
Barry, thank you for going to all that trouble for us. That is very enlightening. Much appreciated. If you need more excuses to run stuff through that software I am sure we can help :)

Would you have an opinion or data on which style of T nut would be 'friendlier' to the dovetail on the compound? Compounds are often made of cast iron which flexes differently than steel. For a given torque value on the bolt, which style of T nut would spread the stress on the dovetail more evenly?

hman, do I understand correctly that the recess you cut in the bottom of the tool post leaves part of the lips of the dovetail free to deflect upwards?
 
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It does. The recess I cut was something like .010" or .015" deep, but that's plenty to allow the deflection that Barry's analysis indicated (.0006" for the round option, .0004" for the square). It's an "unintended consequences" type of trade-off to making the QCTP less subject to rotation. But as I'd mentioned previously, I don't make the retaining nut up "farmer tight". I use a wrench that's cut down to about 6" length.
 
I see. Thanks. Are you concerned about what cycling through that amount of deflection will do to the cast iron over time?

Your idea kinda has me imagining a cupped or arched T nut that would reduce the concentration of deflection in the area surrounding the bolt.
 
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