How to fixture for cutting small bronze cranks

[nh_eng]

Hi all,

Trying to "slice" off 0.100" thk disks from a .500" diam bronze rod and then drill two off centered holes in each - this is for my first Stirling build (my first engine build of any kind). I am a real novice, so everying is quite challenging.

On my first pass, I clamped the .500 diam rod vertically on my milling machine vise and and "sliced" off some .100 thk discs with a slitting saw mounted in a chuck. The results were less than stellar - got nicks and dings at the end of the cut. Looking for a better way.

I also need to drill two off center .093" holes in each one, and they all need to come out the same. I was afraid to drill them before "slicing" them, because I didn't think I would be able to deeply drill such a small hole accurately enough.

Any help is much appreciated.

Paul

 

[arnoldb]

Paul, first of all, how many of these "disks" are required?

Well, to be honest, what Stirling are you trying to build, and for what parts are the "disks" - sounds like the crank webs.

From what you're describing, you have a job more suited to a lathe and a four-jaw chuck than a milling machine at the moment, but I could be wrong.
Some more information would be helpful in order to help you

Regards, Arnold

Edit: I Didn't read your post title - it is for cranks.

 

[kvom]

With such thin disks, my approach would be to face the bar in the lathe, then part off to a length of .125 or so. The parted side will not have a good finish, so I would then face that side on the lathe also. One way to do this is to take a piece of round aluminum and bore a .5" diameter pocket .1" deep, then mill three thin cross slits across the face. When you chuck the disc in this holder, the 3 chuck jaws will compress the pocket holding the disc securely. You can then face to .1" and get a good surface.

If your lathe chuck has 2-piece jaws, you can fabricate aluminum soft jaws, bore a pocket in them, and chuck the discs therein. The prior method is more likely to be easier.

There are several ways to drill the offset holes. Assuming your aluminum holder above is a size that fits a 5C collet, and you have a collet block, chucking the holder in the collet will compress the pocket, much as the lathe chuck does. You can then mount the collet block in the mill vise. Alternatively, you could remove the stock from the lathe each time, drill on the mill, and then return to the lathe for parting.

 

[Layne]

If you're only making 2 of them you should have no trouble drilling the .093 holes through both before slicing. That's the best way to ensure that the holes match.

Making a truly parallel disc is one of the most difficult things to do. One very handy tool is an "emergency collet", which is just a 5C collet with a blank face that you can bore to hold your disc. They cost $20-$30 and you can bore several steps in them or rebore it for different sizes as needed.

 

[shred]

I'm thinking you probably don't have a surface grinder handy, so either the 'pot chuck' kvom describes or the emergency collet would work, as would a superglue chuck-- put a chuck of something in the lathe, face it off, then glue the flat side of the crank web to it and face the other side carefully.

 

[nh_eng]

Thanks for all the great ideas. I have to make 4 of these. I am building Jan Ridders LTD Stirling "Nicole" http://heetgasmodelbouw.ridders.nu/Webpaginas/pagina_ltd_stirling_met_cds/cd_dstirling_frameset.htm. I think I will part them off, then go with KVOMs home made collet where I can finish them off.

You guys are awesome!

Thanks,

Paul