Manufacturing Tolerance on HR Bar Stock?

[tc7]

I am trying to figure out what size (thickness) bar stock to order for machining a 100" long rail whose final thickness will be 1.875"+/-.003; material will need to be 4140 normalized (i think). We have seen bar stock come in that is bowed and twisted but has not been a concern for us since we generally can cut these into shorter lengths for our typical small piece production. But these long rails are new to us and so I am not sure what bar stock thickness to order so that I can be assured that we can mill away the twist and camber. How will I know that I can get 1.875"+/-.003 of usable thickness over a 100" length if I start out with 2" thick stock? or should I buy 2.25" stock? or 2.5" stock? What are the rules?

Thanks for helping.

 

[TVP]

ASTM A 108 Standard Specification for Steel Bar, Carbon and Alloy, Cold-Finished, Table A1.4:

for diameter 5/8" over, and length < 15 feet, and C > 0.28% the straightness tolerance is 1/8" for any 10 foot portion of the bar.

That means theoretically a 2" bar would suffice if the diameter was exactly 1.875". Your cold-finished bar supplier should be able to tell you if they can meet tighter straightness requirements than those specified in ASTM A 108.

 

[ornerynorsk]

It might be in your best interest to have the bar straightened prior to machining.

 

[CoryPad]

TVP - the title refers to HR Bar Stock, which I presume to be hot-rolled steel. Your data is illuminating though, since it shows that standard cold-finished bar allows non-trivial deviations, so hot-rolled bar would be even worse.

Ornerynorsk provides good advice - a little straightening can replace a lot of machining.

 

[MikeHalloran]

Note also that the stock size has a tolerance.

What you receive will _always_ be on the low side of the tolerance range, even though you pay for nominal weight.



Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

There are several proprietary alloys that will replace 4140 that can be purchased finished or semi finished. Here is one that we used quite a bit, Kromite #3 from Associated Steel. This material carries a slight premium but is well worth it.

http://www.associatedsteel.com/pdf/07-08.pdf

Another approach would be to get the bar to where it could be finished ground to size. This approach minimizes the distortion caused by machining a HR bar.

http://www.americangrinding.com/divisions/grinding/