Can someone tell me the merits or faults with three different materials for a machine spindle? I have available 1018, 1144 (stressproof) and 4130. I am making a spindle for a small horizontal milling machine, and can't decide which would be best. Thanks, Bobby
I hope I don't poop your party with this posting but making an accurate, reliable machine tool spindle is a hell of a project involving many machining steps and careful heat treatment and inspection.
A machine spindle is the heart and soul of the machine. It's the most accurately made part of the machine and it's made of a stable material that can be hardened and ground to close tolerances measured in tenths of a thousandth and cylindricities measured in millionths of an inch.
A machine tool spindle is not a routine piece of lathe work. Those who claim to be able to make an accurate spindle on a lathe simply don't know what they're talking about. The spindle of my Austrian built lathe turned a 2" diameter test specimin round within 35 millionths in 1972 when I first bought it. It still turns round (more like a pear shape) within 65 millionths in 1999. I can machine ball bearing fits on it with only ordinary care. I've made replacement machine tool spindles in the past where I've had all the resources of a complete machine shop. I could not make such a spindle using only my lathe.
If you have to make machine tool spindle, it's best to rough it out leaving 1/16" or a little more on it, stress relieving it, then finish machine it leaving grind stock on the precision fits and surfaces. The harden it, tempering it to Rc 55 or so. After heat treatment comes cylindrical grinding the bearing and gear fits between dead centers and the final grinding of the chuck registration features and the taper from the bearing fits.
Therefore, you don't make it of mild or free machining steel which can be heat treated only by carburizing. You need a through hardening steel. 1044 and 4130 are better bets but they have limited hardenability and both require a drastic quench to maximize the hardness raising possible problems with distortion and residual stress.
I reccommend you make your machine spindle of 4150 or 4350 or O-1 tool steel leaving grinding stock, harden it and draw it back to Rc 50, and grind it to size and finish.
I recall a spindle we replaced in a Giddings and Lewis horizontal boring mill. It was a 4" diameter bar about 6 ft long with a #5 Morse taper in one end, a bearing fit at the other, and two keys for about half the length. The 4" diameter was ground and lapped within 0.0001 of a perfect cylinder. It cost over $13,000 in 1982.
A machine tool spindle has to be 4 times as accurately made as the closest tolerence you expect to produce on it. You can make a lathe turned spindle of your 4130 and get adequate service from it but there's little likelyhood of it delivering the maker's accuracy such as turning a bearing fit round within 0.0002". The bearing fits simply have to be ground between dead centers and the chuck mounting detail and the center hole has to be ground in an internal grinder from the bearing fits.
The point of my rant is, you may be disappointed with a home made machine tool spindle and you may produce disappointing work with it unless you jump your spindle project through all the expensive hoops. If every bit of your work far into the future is all open tolerence where cylindrical diameters aren't particularly important, a home lathe turned spindle may do the job for you.
Forrest, I appreciate your honest answer. I understand this is a complex undertaking, requiring accurate precision work. The machine will be used for cutting slots in wood and plastics. It will only be used for this and needs to be accurate within .001-.002". I'm gonna try it. If it works, I'll be proud for completing it, and if it doesn't, then I can say I gave it a shot. Thanks again for replying, Bobby.
Yeah, well, you didn't exactly mention the service. I assumed a replacement spindle for a lathe or milling machine or something fancy. Oh, well, now you know how to make a fancy one.
You shouldn't have any problem if you used the materials you mentioned, but I'd steer for heat treated stuff. Durability doesn't hurt even for woodworking machinery, hell, especially for woodworking machinery.