Torsional Beam Design Question

[durableoreo]

I would be concerned that you're going to great extents to build a rigid structure, but it appears that your torsional box beam sits on a wooden table top. Wood has the somewhat cantankerous tendency to expand and contract slightly with humidity with less than predictable uniformity. In most cases that really isn't an issue with the tolerances involved in typical manual hobby machining. But if you're going to the extent of building such a rigid structure under your lathe, I would want to consider how that expansion would effect your structure.

If the beam is stiff enough, there is no need to consider the table top. For example, the Hardinge design is a rigid body, with 3 mounting points. On much larger lathes, it is impractical to make the lathe bed stiff enough so there are 6, 8, or more feet, which then include the floor of the shop in the leveling process.

Whether it is stiff enough or not is a matter of calculation and measurement, not dead reckoning of the inexperienced. In any case, it won't be worse than simply bolting the lathe to a bench.

 

[StudioMachinist]

I am a little confused about this. How are you going to level the lathe? Shims? Why do you feel you need this "torsion bar"? In most situations the floor of the shop is the "torsion bar" and adjustable feet are spacers that get the lathe bed true.

To elaborate; your torsion bar is not ground flat or square. It will definitely warp the lathe bed when you bolt it down. Assuming it is rigid, there will be no easy way to adjust the bed level. Shims would work but that would be really painful and time consuming to do. Keep in mind that the lathe does not have to be level to the ground. The idea is to get the headstock and tailstock ends of the bed level to each other. Anything else will cut a taper. Sorry if I am telling you things you already know. I just don't want you to go through the effort and decide later to scrap it.

That table is really beautiful by the way!

As of now, the game plan is to shim the lathe level.

The reason I went with a torsional beam design is because, in my uneducated engineering mind, it's more like bolting the lathe directly to my slab.

I can probably explain better later and I'd like to do a write up on my experience with Quikrete's Precision Grout I poured yesterday anyways (I think I'm going to be very happy with it).

 

[rwm]

I suppose shims can be made to work for this. I wonder what the thinnest shim you may need would be? There has been some debate about how accurate of a level is needed.

 

[StudioMachinist]

I suppose shims can be made to work for this. I wonder what the thinnest shim you may need would be? There has been some debate about how accurate of a level is needed.

Going to start with the cheap feeler gauge I got somewhere for shim stock and order one of the budget levels branded shars/precision mathews/etc.

 

[StudioMachinist]

About the quikrete precisions grout I used for the beam...

The big concern going with grout/cement over epoxy is the possibility shrinkage. Quikrete's product says its non-shrink but didn't say if it was absolutely zero shrinkage or just much less than a normal bag of premixed grout. I rolled with it anyways.


Followed the directions for a flowable mix. After the first batch I realized I could have used even less water but I kept everything the same for the remainder of the pour. Pictures show the results 4hrs after the pour (not even close to hard at that point).

This morning everything looked good. The expansion didn't warp the tube.

Threw a freshly soaked rag onto top and bagged it again then had to leave town this morning for work. Might actually be 28 days before I'm back.

Ps: you can see where I added JB metal putty over the welds to make the best effort to keep it water tight (only actually welded the top and tacked the inside of the bottom).
- Also threw some glass fiber into the mix for the hell of it.
- The clamps were just precautionary.

 

[rwm]

So the grout expands?

 

[Dabbler]

An interesting design.

I've build 4 lathe stands over the years. I always make them torsionally flexible. The lathe is then aligned (leveled) using adjustable feet at the 4 corners. Hey, build it as strong as you like. I've used lathes with single piece cast iron bases weighing over 5000 lbs, and you can still twist them to get them perfectly aligned.

A modification of the above quote is: everything is made of rubber if you are measuring closely enough

 

[StudioMachinist]

So the grout expands?

In my case yes, while the grout was still soft. I don't remember reading about this property, so maybe it reacted to the putty I used to seal the welds? Hopefully it has stabilized and when I get home to check it there is more expansion or contraction.