I need to make up some long-term tooling jigs, and I don't want to use mild steel as I find it very hard to get really good finished on mild steel parts. These will have precise ID's and OD's on some locating spigots, will have threaded holes in the 5/16" to 7/16" range for clamping, and will have mostly aluminium workpieces mounted on them, for turning in the lathe and drilling on a rotary table. No welding on these jigs, and they are about 5" diameter and 1" thick.
What material can I use to get good surface finish from turning on the lathe to size, yet be fairly tough?
12L14 is the easiest to machine steel there is. Nice finish always. But poor on the toughness test.
4140 machines almost as well as CR or HR, not too bad anyway. Much tougher and can be hardened if need be. Works to just flame harden in spots where it might be needed. VERY hard to weld without high skills.
--Leadalloy rusts like crazy. If you're going to use a steel with a high lead content I'd suggest keeping them well oiled when not in use. Get something like Cosmoline or LPS-3 to coat them when not in use.
Are you making drilling "jigs" or "fixtures" for holding parts? Most drilling jigs use hardened bushing, button locators and supports inserted into the body. You can make the body out of aluminium, assuming the thermal expansion won't be a big issue, if you want.
As for milling fixtures, why is the surface finish that important? I'm being taught the German tool and die apprentice curriculum and they go gaga over appearance. Perhaps it looks nice, but it's just as functional as something like mild steel (in most circumstances) with that so-so finish I normally get.
Anyway, You may consider using inserts in some kind of desirable condition like hardened, or finely finished... You could make supports and locators out of O1 drill rod, gage stock or just HR tool steel stock. Just screw and dowel (as required) them into your fixture after hardening (drill and ream before hardening...). If you don't have a grinder, you can hard-turn/mill parts once tempered to the ~50-55Rc (or less) level for a good finish (use carbide tooling of course, roughly 1/4 normally SFPM).
If *tough* is good enough, you can use 4140 for your inserts and file (or use sand paper and a plate of glass) them to a nice finish.
However, it's hard to tell what you really need with your description. You said long term. Does this mean a thousand parts, a hundred thousand, a million? What kind of positional or concentricity tolerances are we talking about? In some applications, annealed free-machining steel will deform excessively with use and cause your parts to be located incorrectly. In some instances, aluminium locating surfaces are sufficient.
"Brian" wrote in news:01Kwb.110997$ snipped-for-privacy@nnrp1.uunet.ca:
I suggest 4140 PH (pre-hardened) will come in about 32-38 HRC. Provides a decent finish..and is tough, can be ground if needed without additional heat treatment.
Hi robin. I'm using an aluminium fixture now, and these are fixtures by your definition. Specifically, these are plates mounted on either a rotary table or a lathe faceplate, onto which are mounted aluminium automotive flywheel blanks for machining. What I do is machine a 1" pilot hole in the blank and use it as the reference for all turning operations, for concentricity. I don't make more than a half-dozen a year. HR Mild steel would suffice, but I just happen to get a crappy finish turing it. I need to make the pilot diameters to .001", and they need to retain that over say
50 mounting cycles.
I appreciate all of the feedback. 4140 is certainly available locally in small quantities as bar cut-offs, and I will try to find some stressproof. I also take your point about not really needing a great finish for a fixture.
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