Tail Stock lock

[Susquatch]

after some futzing with eccentrics (it has been over 50 years since I have done this)

at 10 degrees off of top dead centre, my calculations end up at almost 2000 lbs clamping force... Tan (90 - 10) * 40 * 8 - still plenty of clamping force.

We have to stop meeting this way. Your post arrived at the same time as mine again! LOL!

You are gunna end up forcing me to do the math you know! Too funny!

Does that take bolt stretch into account?

 

[whydontu]

"Someone check my math please". What a cool and diplomatic way to put it! I'm impressed @RobinHood ! I don't think I've ever been challenged so nicely!

I think perhaps you read my note about the max tension in a 1/2 bolt as if I was suggesting that was what the tailstock was seeing. I didn't intend that. I think @Dabbler read my intent correctly. You and I are close.

After I retired, I stopped being as anal about doing the math as perfectly as I used to when I was younger. In my senior years, I've started to fly more by the seat of my pants. That's probably dangerous, and perhaps even lazy. But it is what it is.

That said your way of looking at it does cause me to wonder a bit about the way that overcenter cams work.

I only took a SWAG at the numbers on my first cut. I confess that I didn't do any math.

I readily admit that I could be wrong, but I don't think an overcenter cam works the same as a regular lever. In other words, I don't think that the 1/8" cam size to arm length is the right ratio through the full stroke. As the cam reaches the top of its stroke, it isn't anywhere near 1/8 inch anymore. It's much less than that. And therefore it ought to provide much more force than is readily apparent. Nonetheless, it isn't infinite because stretching of the bolt would limit the maximum available by undermining the lift. That's half the principle of over center - the other half being the natural holding force.

Unless I am wrong (which seems to happen in direct corelation to my swagging frequency), I think we have both underestimated the available clamping force.

Perhaps after I sleep on it, I might do some math. But I'm not gunna lose any sleep over it and I'm sure as heck not sure that my instincts on this are correct.

It’s got to be an age thing. I think we both predate calculators and ACad, so we learned to do things easy when possible, and precise when justified. A month or so ago I had a very green engineer send me a request to quote a level control valve for a wooden open-top water storage tank. The tank is 28 ft long, 9 ft wide, 3 ft deep. 756 cu ft. Junior P.Eng converted to metric, did his math, and sent me a tank volume value of 21,407.54 liters. I was going to be a snot and ask him if he had allowed for knots in the tank walls, but decided the sale was more important than my need to explain what significant digits mean.

 

[Susquatch]

It’s got to be an age thing. I think we both predate calculators and ACad, so we learned to do things easy when possible, and precise when justified. A month or so ago I had a very green engineer send me a request to quote a level control valve for a wooden open-top water storage tank. The tank is 28 ft long, 9 ft wide, 3 ft deep. 756 cu ft. Junior P.Eng converted to metric, did his math, and sent me a tank volume value of 21,407.54 liters. I was going to be a snot and ask him if he had allowed for knots in the tank walls, but decided the sale was more important than my need to explain what significant digits mean.

Yes, and yes. I predate calculators. I actually used a slide rule back in the day. I still have it. A slide rule was one way of making sure you never got out of line with significant digits.....

I went to bed with @Dabbler s math in my head. The end result is that I couldn't sleep.....

So back to @RobinHood's request fueled by @Dabbler 's example.

Fundamentally, the math in this example isn't as easy as it appears.

The leverage for an over center cam is not a simple 12" to 1/8" situation. The Eccentric is only 1/8" at the 90 degree point. As the Lever is rotated the eccentric leverage goes from 1/8" to 0.0". In other words, the leverage goes to infinity.

However, infinity isn't really possible. What actually happens is that the bolt starts stretching and the shoe starts bending as the force increases with rotation. Since the rate of cam lift per degree of rotation is going down as top center is approached, and the stretch/bending rate is linearly proportional to the lift, at some point, the bending and stretching will equal the lifting of the cam and any further leverage gets cancelled out. It is not possible to calculate this point because the plate bending is unknown. I suppose one could model the plate and bolt and establish a stress strain curve for the assembly but that math is not possible on paper let alone in my head. It would require a finite element analysis or at least a gross over simplification of the parts. However, it is certainly not infinite.

The other big unknown is the setup on this particular lathe. How much initial clearance is there between the shoe and the bottom of the ways? And where is the cam when that clearance is taken up? @Dabbler chose 10 degrees from top center. That's as good a choice as any and probably about right in practice.

But it isn't maximum.

To my great relief, I conclude that I'm simply not motivated enough to further assess the matter.

However, I can say with much better confidence that my initial swag was likely very low. With proper adjustment, I believe the clamping force can easily exceed several tons or more.

I don't recall ever seeing a description of how to properly adjust the cam. What I have done myself is to manually find the point at which the cam lift is balanced by the stretch and bending. That's the point where the cam starts to feel mushy and can be cammed over center without excessive force. Then I tighten the bolt (nut in @PeterT s case) until I can no longer reach the mushy point without forcing the Lever beyond reason. As @RobinHood calculated, that's a minimum of at least 3800 pounds. But who knows what the maximum is. It would be a good assignment for someone who has the time, the equipment, and the required motivation to do the modelling. That isn't me.

Hopefully I can sleep now. And I'd bet good money that everyone reading this has already drifted off.....

 

[Susquatch]

I haven't had mine knocked down, but just eyeballing the eccentric/cam, looks like it should have lots of mechanical advantage pulling tension. There are many lathes with a wrench/bolt style hold down, maybe wrench torque on threads can provide more pull? The cam lever is very convenient, I'm not about tp change it. Its not a horrible problem, but under occasional heavy drilling might back up a bit. Usually its hogging operation so not like I'm making a precision depth hole. More irritating than anything else. Not sure if the ways are more at risk or not by sliding. Boring is probably kinder to the machine. Actually might be my imagination but I never noticed it much until I went with this nice slippery way oil.

My tailstock has a square drive opposite the handle. It is supposed to be used with a torque wrench for those times when you want consistent clamping force or precision alignment. I've never used it.

However, I did measure it and found that it's more of a gimmic than real. The socket is too big for a half inch drive and too small for 5/8. It is roughly 0.5543210 (extra digits just added to pull @whydontu 's leg) . To use it, I'd have to make a custom driver. It's not on my priority list.

 

[Snocrusher]

Yes, and yes. I predate calculators. I actually used a slide rule back in the day. I still have it. A slide rule was one way of making sure you never got out of line with significant digits.....

I went to bed with @Dabbler s math in my head. The end result is that I couldn't sleep.....

So back to @RobinHood's request fueled by @Dabbler 's example.

Fundamentally, the math in this example isn't as easy as it appears.

The leverage for an over center cam is not a simple 12" to 1/8" situation. The Eccentric is only 1/8" at the 90 degree point. As the Lever is rotated the eccentric leverage goes from 1/8" to 0.0". In other words, the leverage goes to infinity.

However, infinity isn't really possible. What actually happens is that the bolt starts stretching and the shoe starts bending as the force increases with rotation. Since the rate of cam lift per degree of rotation is going down as top center is approached, and the stretch/bending rate is linearly proportional to the lift, at some point, the bending and stretching will equal the lifting of the cam and any further leverage gets cancelled out. It is not possible to calculate this point because the plate bending is unknown. I suppose one could model the plate and bolt and establish a stress strain curve for the assembly but that math is not possible on paper let alone in my head. It would require a finite element analysis or at least a gross over simplification of the parts. However, it is certainly not infinite.

The other big unknown is the setup on this particular lathe. How much initial clearance is there between the shoe and the bottom of the ways? And where is the cam when that clearance is taken up? @Dabbler chose 10 degrees from top center. That's as good a choice as any and probably about right in practice.

But it isn't maximum.

To my great relief, I conclude that I'm simply not motivated enough to further assess the matter.

However, I can say with much better confidence that my initial swag was likely very low. With proper adjustment, I believe the clamping force can easily exceed several tons or more.

I don't recall ever seeing a description of how to properly adjust the cam. What I have done myself is to manually find the point at which the cam lift is balanced by the stretch and bending. That's the point where the cam starts to feel mushy and can be cammed over center without excessive force. Then I tighten the bolt (nut in @PeterT s case) until I can no longer reach the mushy point without forcing the Lever beyond reason. As @RobinHood calculated, that's a minimum of at least 3800 pounds. But who knows what the maximum is. It would be a good assignment for someone who has the time, the equipment, and the required motivation to do the modelling. That isn't me.

Hopefully I can sleep now. And I'd bet good money that everyone reading this has already drifted off.....

Wow, when I designed and made my tail stock cam lock I just looked at some other peoples designs and adapted to my needs. The 1/8" eccentric figure I just pulled out of my hat.
If I would have known it could be so complicated and the math needed and the understanding of the forces involved to correctly design a working cam lock I would not even attempted such a project.
I can't believe that mine works at all, oh well even a blind chicken finds a kernel of corn once and a while.