Milling rigidity question

[pminmo]

I have two 1.25" Thompson shafts 41" long and two Thompson double pillow block bearings. I'd like to use them for an X axis to build a machine in a conventional mill setup. My question is since these are unsupported shaft's, would they be strong enough resist tool chatter when milling mild steel. I know this is a pretty abstract question whithout getting into cutter size, speed, depth of cut.... Just looking for an opinion if it would be so limiting in cutter and cut sizes and speeds if I'm wasting my time.

[fkaCarel]

If I see this configuration lying on your table, I see the blocks opening up, under eccentric load. So with four blocks, which then don't have to be double, things get better. The stroke would be limited, which is again better. Put them as far apart as possible. And for the sake of the experiment, you can always add a dimension by using four rails.

Carel

[Geof]

You can draw something of an analogy to a boring bar on a lathe. You will have two bars each supported at the ends with the middle supporting a cutting tool. Typically boring bars can be expected to be stiff enough to not chatter if the length is not more than about maybe six times the diameter. In your case that is almost 8 inches but you have two supported at each end so maybe a 32 inch span is feasible if both ends of each bar are rigidly attached. In other words clamped down in the same manner as a boring bar would be.

[DareBee]

Also

With open bearing blocks like that, your shaft CAN (and should IMO) be fully supported.
Buy or build 2 mounting rails and drill the shafts to suit.

[dberndt]

I'm no engineer. But my beam calculations for these rods with only the load of a light table on top to be atleast .004" or something like that in the middle. With cutting forces this would surely lead to parts that were out of size in all sort of fun ways as the load moves with the table changing the point load on the beams. This might work for a wood router or something like that but I don't think you'd have much fun with this setup in mild steel.

Support the rail fully on in several places atleast.

[wjfiles]

The nature of your shaft would come into play.
Are they mild steel ?
I am using induction hardened and ground shaft for increased stiffness
Bfiles

[dberndt]

The type of steel you are using and its hardness has relatively little to do with how much deflection you will see over a span. The young's modulus (elastic bending) of all steels is within a fairly narrow band of ranges.

The difference in alloying and heat treating comes in wear properties and tensile strength. Better alloys will simply bend further before breaking. But the amount of bending pound per pound will be quite similar.

[drummond1]

A machines stress limitation is something all machinists learn to adapt to. Cutting forces and mechanical stress have been for the most part ignored by modern day machine tool builders. Thats why the life expectancy of a machine has so greatly diminished. Depth of Cut and surface speed will greatly reduce deflection stress. Your primary objective is to handle the heat between your cutter edge and material to be cut. Some great advances are out there in Cutter composition and Cutter geometry that reduce cutter pressures significantly. Either way.. the more rigid the setup the more snot you can put onto your tool. Its all relative.. Best of luck mate.

[pminmo]

These are Hardned Thompson shafts, 60C rock as I recall. I suspect to have a qualified shop drill and tap would be expensive.

I also have wondered since the cutter would always be directly above the linear bearings, if I were to incorporate sometype of bearing assembly if I could minimize the deflections becuase of the open shafting.

[dberndt]

Install a flat steel plate below the bearings and put uhmw or teflon slides on the bottom of the table. Make it .0005" clearance or something like that. Now you've got some way to fight deflection.

[tobyaxis]

Not a machine builder, but just my 2 cents,

The configuration is nice , but I would definately look into a way to make it more rigid than it is now. There are some great ideas here in this Thread that will help.

4 rails bolted down like linear ways is a good idea and mount your second set of pillow blocks with the opening facing in the opposite direction. example One set of rails bolted to the frame of the Y axis and the other to the Table. assembly will be a pain to line up. Over kill is better than not having enough.

Looks good though, good luck on your project.

tobyaxis

[DareBee]

Your "Thomson" rails are case hardened to a depth of about .06.
You can mark the hole locations and grind through the case hardenend surface. Now you can drill and tap with conventional tooling, carefully of course.

If you want to bring them to me I could always EDM tap them from hard ;-)