Way cool! Thanks for doing that, Barry. Most interesting.
-frank
Frank,
No problem! I enjoy that kind of stuff.
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.
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.
hman,
I see your logic in moving the friction force as far out from the centre of rotation - tool post centre - as possible. This creates a better lever arm to react the tool post torque (a result of cutting forces trying to rotate the tool post).
The thought of including the tool post on top of the compound had crossed my mind, but for analyzing the base designs ("Which is stiffer?") it makes no difference. Another way to look at it, I could have 'fixed' the t-slot (i.e. made it 100% rigid) and the overall deflections would have been less, but there would still be a difference between the two Bases. Maybe I'll give it a go, though...
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?
Tozguy,
Again, not a problem at all. Any other modelling (not that kind) or stress analysis stuff, I'd love to help!
An easy way to evaluate how parts interact is to follow the load path then look at the mating surfaces between the parts. In this case, it's easy to see the Base is being pulled 'up' out of the t-slot. There are two surfaces of the t-slot holding the Base where it should be. The contact stress on these two surfaces can be calculated knowing the applied Force and Area (Stress = Force/Area). So, to keep stress on the surface low, either lower the Force or increase the Area (assuming all other variables are held constant, like t-slot and Base dimensions). If the full area of those two surfaces is used, there's really nothing else to do to improve it. This assumes the base of the t-nut is as thick, long, and wide as it can be in the dovetail, if you know what I mean.
You posted a picture above with two base designs, one square and the other round. The round one would not use the full bearing area of the t-slot and is therefore inferior. That being said, we're not talking about highly stressed parts here, so I really doubt there are any issues whatsoever (similar to my comparison above, yes one is better, but by such a small amount).
And you're correct about cast iron vs plain carbon steel flexing differently, as this is related to the modulus of elasticity which is different between the two. Not only different, but steel has a linear elastic region of the stress-strain curve while cast iron is non-linear and calculating the modulus of elasticity is a bit more involved... to keep it simple, let's say the Tool Post Base is steel and the Compound is aluminum (to make it easier to visualize), you can now appreciate how the Tool Post sitting on top (clamped in place) helps prevent deflection of the less stiff Compound. In terms of optimizing t-nut (Base) shape considering the difference in modulus of elasticities, I would say apply the same logic as the previous paragraph. The stress levels are so low and the difference in stiffnesses is not worth losing sleep over.
I see. Thanks. Are you concerned about what cycling through that amount of deflection will do to the cast iron over time?
Pretty much what hman said. At these loads the amount of deflection and stress is so low.
I may have missed this in all the NASA type theories flying around here but make sure your threaded post can't screw down thru the T-nut and contact the compound. The threaded post can act like a screw jack and pick the T-nut up braking a lip off the compound.
it was mentioned above, good point though!
Barry
