Dumb idea of the day

Your idea is plausible. There's some math below.

There's (I think) a replacement Atlas 10"/12" lathe spindle on eBay right now for $110 plus reasonable shipping. It also comes with a set of replacement bearings and a MT3/MT2 adapter. Maybe pick it up "just in case" your project runs afoul? Having a back up means if you want to go through with it, you could go a little deeper knowing you'll be back up and running in a short time.

Contemplating my naval lint thinking about your project, what would fail first if you thinned out the spindle? The tube should be mostly under a torsional load; belt pulley turning the spindle against the load of cutting (plus a little lateral load from belt tension). I'd think you'd start slipping the belt before you'd snap the spindle in torsion.

Maybe do a little experiment with a torque wrench? Stick a socket on the torque wrench and chuck the socket in the 3-jaw. Clamp the motor pulley so it doesn't rotate and pull on the torque wrench. I'm guessing one of your belts will slip at some point and measure that torque.

Or consider this though I had my strength of materials class before Ronald Regan even considered running for office!

Torsional stress = tau = (T x L) / (J x G)

T = torque
L = length of the rod
J = Polar moment of inertia
G = Modulus of rigidity

If you bore the spindle larger, the only thing you are changing is the polar moment of inertia which for a tube is (I did pull this from the web):

J = PI x (D^4 - d^4) / 32

Your "as designed" J is: 3.1415 x (2.441 - 0.372) / 32 = 0.203 in^4

If you opened up the bore to 1", your J is: 3.1415 x (2.441 - 1.0) / 32 = 0.1415 in^4
If you opened up the bore to 7/8", your J is: 3.1415 x (2.441 - 0.5862) / 32 = 0.1821 in^4

When plugged into the torsional stress formula, going to a 7/8" bore would raise the torsional stress for a given torque by ~10%. Going to a 1" bore raises it by ~30%. Frankly, seems plausible, but it's your lathe and I've forgotten a lot of this stuff due to inactivity!


Bruce



eBay item# 165620016595

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Thanks for your input Bruce. This will most likely never happen. But now I have an understanding of the forces involved. Saved me the time to try to figure it out how to figure it out.

This thread can die a quiet death. I think that I got my answer.
 
mickri said:
Thanks for your input Bruce. This will most likely never happen. But now I have an understanding of the forces involved. Saved me the time to try to figure it out how to figure it out.

This thread can die a quiet death. I think that I got my answer.
Click to expand...
I kinda thought you would get there as you did - but your original link to Varmint Al's page intrigued me. It's a huge web page resource packed with lots of very good stuff, and must have take a lot of work to put together. I could not spot where the spindle mod was, but I was thinking that if anybody did this, then HF spindles must be somewhat "softer".

Maybe HF spindles are only "soft on the inside"! :)
 
Only speed read the thread, didn't see any mention of resulting loss of Morse taper length. Don't have to remove much from the bore, the M.t. will disappear rather quickly.
Apologies if it was and I missed it.
 
I figured that the MT3 might not survive if I increased the bore. But that may not be a problem because an adapter is used so a MT2 dead center can be used.
 
Varmint Al describes the spindle bore modification in the very first paragraph on his mini lathe page that I linked to. Interesting guy if you took the time to read his bio. He is getting up there in age. Last update on his website was in April 2021. Wonder if his family will keep his website online after he passes.
 
It's not a dumb idea, I've had to do it.
Bigger lathes, lots of measurin' an' other such fartin' about. Sometimes only part way, just to get the job done.
I cheated a little though, I had a university educated "almost engineer" machinist on hand to the trickier bits of soul destroying math.
Planning the same for my Atlas spindle. Drilling the bore left swirl marks like rifling. The cnc guy that did the finishing, didn't skim the bore.
Tooling to do it is ready to go, only looking to sneak up on next nearest size, fractional or metric, makes no difference to me.
So sooner or later I'll be fixing it or trashing it.
Want/need to do it, just not looking forward to it.
The thought of ruining it due to stress release is holding me back.

P.S. don't forget about the pin and IMG_20220127_094426.jpgkeyway's!!!!
 
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