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View Full Version : Mild steel or Chrome-moly for mill table leadscrew replacement



abn
04-11-2007, 02:32 AM
I want to do a custom x-axis leadscrew on my mill (10"x48" table vertical mill) and I'm too scared to go cutting on the original one in case I screw up and have to return to stock. I notice that the Keystone and Nook generic precision acme screws come in mild steel or chrome-moly alloy. Will I need the extra strength of alloy steel (1 1/4 5tpi size should give a pretty beefy minor diameter)...will there be any perceptible difference in wear against a bronze nut? What say you?

Swarf&Sparks
04-11-2007, 03:10 AM
Seems to me, the finish on the screw would be more important than the hardness.

SGW
04-11-2007, 08:44 AM
I'd use 1144, myself. It machines really well, and is strong.

I also wouldn't worry too much about wear. If you use it enough to wear it out in the next 40 years...make another one!

Oh -- re-reading your post, I guess you're thinking of modifying a commercial screw, not making one from scratch. I'd guess the mild steel version will be perfectly adequate.

gzig5
04-11-2007, 02:20 PM
It is my impression that the Nook leadscrews are held to higher finish and lead accuracy standards. I chose the Nook for a new cross-slide screw. If I had the option, I would go with the stronger/harder material. I think it is better to have the wear happen in the nut than in the screw. The nut can be designed to take up for wear, while you are stuck with uneven wear in the screw.
Greg

lazlo
04-11-2007, 03:00 PM
The Chromoly screw will be a lot more wear resistant than mild steel.
I wouldn't worry about the tensile strength of the chromoly screw (about 3 times that of the mild steel) -- if you're applying that much stress to the screw, something is really wrong :)

lane
04-11-2007, 09:39 PM
All said for home shop use it probley dont matter. I make them all the time for machinery I rebuild and use stressproof 1144 . Seems to hold up well and machines great.

lazlo
04-12-2007, 12:09 AM
I make them all the time for machinery I rebuild and use stressproof 1144.

Agree Lane, but if you read his post, he's talking about buying a Nook screw, and he's asking whether to get it in mild steel or chromoly.

I also used Stressproof to cut a crossfeed screw for a taper attachment I'm making for my lathe. Like S&S said -- the surface finish matters a lot more than the material if you're not thread grinding. By the way, I got the idea for using StressProof for the screw from Harry's article on rebuilding his Monarch -- that's such a great series!

I tried single-pointing an acme thread on a shaft of 4140 Pre Hard, but I couldn't get a smooth enough surface finish on it. Some day I may try to lap the threads, but like you say, the 1144 screw will likely outlast me...

Spin Doctor
04-12-2007, 05:05 PM
I think I'd go with 4140 TP&G myself. When threading the screw I'd make up a special tool so that the Acme threading tool has that much less work it has to do. But it sounds to me as if he wants to buy a blank screw and nodify it to his needs. I'd still go with 4140. Prehard 4140 is only 30 to 35 RC.