Warped steel?

Did you check the flatness of the ground stock before you drilled it? You can always grind it flat again.

Les

No I didn't ... but it came all nicely wrapped :-) I can just finish drilling all of the holes and bind it down but I didn't want to introduce more uncertainty. I will be using this technique in the future and just wanted to find what I did wrong. I guess I'll just have to try again and check the stock first. Thanks

You are worried about .003" warp over two feet? Someone said you can "grind it out"? HOW???? As soon as you put that on a mag chuck it will lay down nice as nice.

I say just bolt it down and be done with it. Or glue it down. It takes what, a few pounds pressure to make it lie flat?

Grant

Kev> No I didn't ... but it came all nicely wrapped :-) I can just finish

Well, I was concerned with any movement. I didn't think I was going to have to measure the full length of the stock to determine the general precision. If I can't depend on the metal to hold the precision I paid for then I could just as well bought cheaper steel and milled it. If the metal can loose it's tolerance with such a small amount of stress (2 bolt holes drilled and fastened) will it move while performing it's task (mounting a precision slide). I was hoping not to have to go to a harder metal just to hold the tolerance I wanted. Yes I can get this to work but I wanted to know if buying 2 feet of steel for $135.00 is worth it. If when I drill two holes in it it then becomes worth $10.

Kevin, Unfortunately, this is not unusual. It is caused by internal stresses in the material before you received it. It can be much worse and the amount is unpredictable. It is usually caused by poor or no annealing before the final grind. Of course, the final grinding itself can induce similar stresses if done too quickly. Steve

Wouldn't help. All steel has the same elastic modulus. The only difference hardening makes is to move the point where the steel yields. So to get more stiffness, you'd have to change the geometry of the part, ie make it thicker, or an I beam shape, etc.

The better solution to your problem would be to use fully annealed steel. Since it has all (or nearly all) residual stress removed, machining it won't cause it to warp. It'll be just as stiff as a piece of hardened steel. It will wear faster, though, if it will be sliding against another part.

Gary

The screws are probably causing it to bend. If the surface you are mounting this to is fairly straight you could add a few more screws specificly close to the ends to help straighten it. The 1/4-20 are producing easyly enough force to bend a 1/2 thick bar. To ensure straightness it is better to use smaller screws and more of them. You want just a little force but in a lot of places for straightness. Good Luck

You shim the part before turning on the chuck. You can use cellophane (.001" thick) as shim stock, or anything that will fit under the part before activating the chuck. You can get amazing flatness when you get on to the process. Taking light cuts and constant flipping of the part is also a part of success. Removing stock equally from each side is also important.

Harold

Thanks to all. I learned a lot. I'll probably redesign.

Ted

Harder won't help. All steels have essentially the same modulus of elasticity so hard steel is stronger but not stiffer.

BeamOut(0 12 24)(30E6,I_rects 2 0 .5)Beam 0 24(0 24,w÷24) pos'n pt. distrib shear bend. slope def'l force force stress stress 0 -5.208 0.434 -7.812 0 0.0004 0 12 0 0.434 0 -375 0 0.003 24 -5.208 0.434 0 0 -0.0004 0

Note the stress of 375 inch pounds in a steel that could take tens of thousands. Only 10.4 pounds of distributed load or about 6.5 pounds point load at the center would cause your 3 thou. How was it supported for your test? The weight of the bar itself (no extra load) supported at the ends would give about 2 thou deflection at the center.

Ted

Which isn't much more than the piece weighs (about 7#).

Ned Simmons