Machining a precision rail - choosing material

[bigj2]

I have to machine and grind a soft precision rail that is approx. 2.000" x 2.000" x 100.00" long and has a series of drilled and tapped holes. The customer wants it to be straight and parallel after grinding within .001" over the entire length. What kind of steel should I use to achieve the required tolerances. He wants me to use 12L14 and I don't think that is the proper material.

 

[ornerynorsk]

Your customer is DREAMING !!! Not practical to hold .001 over 100 inches.

 

[IRstuff]

Not necessarily impossible, but you would probably need start with something like a single 100"x10"x10" block and then machine the rail into the block

TTFN

 

[Iskit4iam]

I've made ball screws that are longer and meet this requirement. We straightened them after machining.

It seems to me that this rail is only going to stay straight if it is supported properly in the installation. Is it feasible to "straighten" it during the installation process?

W

 

[limct]

The straightness tolerance should be achievable.

You should highlight your concern (over the specified material) to your customer.

12L14 free cutting steel is a rather low strength material, with only about 0.003~0.009% of carbon constitution.

Since straightness is so important, you and your customer need to consider how long can it be maintained under the actual working condition.

Regards,

 

[unclesyd]

You should be able to purchase the bar to size and if the holes are not too large or too many there should be no problem in drilling and tapping. That material may not be readily available but other free machining steels would be.

Here is company that I’ve used for ground stock and bases.

http://www.americangrinding.com/

 

[EdDanzer]

Any machining process will warp a 2” X 2” X 100” bar unless supported or straightened. Trying to hold .001” will require grinding with a way grinder after bolting in place.
The material choice will be driven by what is running against it. 12L14 will work if soft materials are running against it and a short life is ok. It may be less expensive to purchase hardened way material. Several companies make hardened steel guide way inserts for machine tools.

 

[grinderguy]

This bar must be fastened to suitable base to achieve this straightness in operation. Many times people use lasers now to align rails as they are assembled.
Inspecting an item like this is very difficult because the "free state",laying on a granite plate, may have a bow. As installed and aligned the bow is irrelevent.
This is very difficult to produce no matter how you do it. I would get my advice from people in the way grinding business.


Robert Setree

http://www.advanced-grinding.com

 

[bklauba]

One way to minimize the effects of residual stresses (probably the likely-est culprit threatening your project), whether contained in the "as-delivered" bar stock, or induced by rough machining operations, is to vibratory stress relieve the workpiece. Considering your tight tolerances, it should be done prior to rough machining, and again afterwards.

The rough machining should include not only the removal of the stock that approximates the required profile, but also the drilling of most of the stock associated with the bolt pattern. The stress relief might cause some distortion (usually minimal, over 100", a few 1/1000's" is possible), so, if the bolt pattern needs to be precisely in-line to that degree, leave several thousanths in the holes, which can then be removed with an end mill as part of the final machining.

Some prudence and patience during machining is also advised. Machining setup should include sufficient clamping, and careful monitoring (independent of the machine tool used, such as a laser, suggested by grinderguy) of the part during the clamping. The machinist should be willing to shim that final 1/1000 out, if need be to assure no preload, prior to machining. And watch out for that guy opening a door that brings in a cold draft!!!

By stress relieving the part when it is very close to net shape, and as much as possible of the stock removal accomplished, tolerances are easier to achieve. The longest part done using such methods that I have been involved with, was 714". It was straight within + or - 0.003" over the full length, a ratio a little tighter than your target, but similiar, after final machining and monitoring over a few weeks time during assembly. We have a case study on the 714" workpiece, that I can e-mail to you.

bklauba@airmatic.com