ACME steel types.

[Kingjamez]

In a CNC router application is there any difference between the steel types that ACME threaded rods come in? I've seen Alloy Steel, 1018, 4140, stainless, etc.

Any good reason to go with higher grade steel?

-Jim

[fatal-exception]

Well, as with anything, the different grades have different qualities.

1018 - mild steel, nothing really special, hardware store grade, rolled with low lead accuracy, 1/2 thou per inch typically
4140 - heat treatable alloy steel, if your not planning on hardening the screw, don't bother, if the screw is milled, then it will have a higher lead accuracy, possibly as low as 1/2 thou per foot
Stainless - more wear resistance than 1018, will not rust (obviously)

The type of ACME nut you plan on using may also dictate they type of material the screw should be made out of for long life.

Paul

[ger21]

I think that most 4140acme is already heat treated. It also has a black oxide finish, so may be less susceptible to rust. Roton only sells 1018, and here is what they say about 4140:

Alloy 4140 Heat Treated is only beneficial in very specific situations and is commonly "over specified". This is mostly because it is a popular material for bolts and fastener threads. Many users specify Alloy 4140 HT with the goal of increasing the wear life of the screw, but most screws are used with bronze or plastic nuts and a sufficient hardness differential exists between these materials and all steels such that performance of the nut materials is not affected. Alloy 4140 HT is also specified in the belief that it will enhance the stiffness of the screw. However, the stiffness of the screw is a function of the modulus of elasticity, which is the same for all steels, so elastic deflection under load and column performance will be the same. If strength is a concern as it may be on parts for aerospace or other weight sensitive applications then alloys may be desirable, however, most strength applications are better solved by using a larger size carbon steel screw in place of a smaller but more expensive alloy screw material. Also for high temperature environments (above 700° F) Alloy 4140 HT provides superior service.

[JohnG1931]

Dear Kingjamez
It seems to me that there is point that may be more important than the choice of material for your CNC screws. There is choice between ball screws and plain acme screws. Plain ACME screws have much more friction than ballscrews, and tend to need very expensive motors to drive them at reasonable speeds.
If you already have a lathe, you can make a ballscrew for youself, which avoids the cost ($500
to $800) to buy one. If you have a set of gage blocks (e.g. SKU 700-CL3 (10 millionths tol $69), or
SKU 532 50 millionths for $39). The 10 millionths blocks are preferable becaue tha calibration certficates allow a very precise determination of errors due to bearings at each end of the screw, and this will allow you to make a better leadscrew than you already have. The SKUs are for JTS Machinery
If you already have an air driven high speed spindle, or are prepared to make one driven by an electric motor you can make a very precise screw and nut indeed.

By all means write to me if you wish to know more. I am progressing rather slowly on equipment for myself, being 76 years old and not a energetic as I used to be.

If you don't mind waiting for a while, I may be able to sell you a set of leadscrews for a small lathe (7x14) or perhaps a bigger one (12x36), but it will be less costly to make your own.

Sincerely
john.gabriel@mindspring.com