Chucking Up Brass with no marks.

[jheal2]

Hey Guys.
I have a Brass Casting that needs to be turned on both faces and the OD. The ID is approximately 4" (has a 1 degree draft angle though).
I was going to just use an ID chuck (with steps so I can do the back face), but have been told that no chucking marks are allowed (which makes no sense since its a crappy cast finish anyways). I know that if I chuck this internally we will be getting marks, at least with any jaws that I can think of. The cast tolerance is also to large for any decent fitting mandrill.
I have a sample here and I cannot even figure out how it was made (no witness marks anywhere).
If anyone has any suggestions please let me know.
Thanks
Jordan

 

[petman]

How about soft jaws made of aluminum

 

[btrueblood]

Or plastic. Or wood.

 

[MikeHalloran]

How about an expanding aluminum pot chuck gripping the ID?
The bore is large enough to put a sturdy shank on it, so you can turn both ends and the OD in one chucking. ... unless there's so much detail on both ends that you absolutely must chuck it twice.




Mike Halloran
Pembroke Pines, FL, USA

 

[BillPSU]

Chuck a piece of material in the chuck (6" round) face it off and drill and tap a center hole. Make a taper plug with the draft angle. Drill and counterbore for a socket head cap screw. Pull the casting down against the chucked block with the tapered plug and capscrew. Face the end and turn the OD. Rechuck the part using soft jaws and face the other end. You will have to have a drive feature on the part to stop it from spinning with the tapered plug.

 

[jheal2]

Hey Everyone,
Thanks for the quick and thought out replies.
Mike I like your thinking about a pot chuck, something that I completely gapped on. I believe that something like that would work, except that we are worrying about the part slipping at all. There are 4 cast tabs on the ID near the front (what were making front)that we were thinking of making as sort of drive cogs. This is easy enough, just mill away at the pot chuck, but obviously with 4 tabs and 3 "jaws", when closing down, 3 of the 4 grooves made will need to be made bigger in order to close properly (will be closing at an angle to the tab).
Do you know of any good retailers that sell these, and further more do you know of any 4 "jaw" pot chucks? Google does not work well with anything manufacturing.
May be able to get the 3 "jaw" to work, definately get the 4 "jaw" pot chuck.
p.s. I am not the best with words, so just ask if you need any clarification in what I mean.
Thanks
Jordan

 

[MikeHalloran]

Okay, you have a 3 jaw chuck that can grip an ID.
Make up a ring that fits the ID of the part, with a taper, and has reliefs to clear the tabs, and has a cylindrical bore of its own maybe 1" smaller than the workpiece ID.
Sawcut the ring radially in one place so it can expand.
That's the simplest possible fixture you could use.
It may not be possible to machine both ends using it. You can turn it around on the 3-jaw, being careful to not move it relative to the workpiece, and have a chance of doing a decent workpiece in two chuckings.

I was thinking of something more complex, made from say 4-1/2" round x 6" long, where you would chuck the aluminum cylinder in your 3-jaw internally or externally, and machine your taper and maybe a shoulder at its distal end to hold your workpiece. You would also put a conical bore in that end, a blind tapped hole for a central stud, and six or eight radial slots. Insert a conical steel plug and draw it in with the stud to expand the fingers to grip the workpiece. You need to use a taper that's steeper than ~7 degrees per side so it won't be self-locking, or provide means for extraction.

Machine shops used to keep a stack of IBM cards around, even if they didn't have CNC machines, because IBM made the cards tough and hard, and uniformly thick. Modern card stock is probably not as hard or as uniform, but it should still do a decent job of preventing scars on the workpiece. I.e., wrap the fingers with a strip of cardstock before chucking the workpiece. The paper also improves the grip.



Mike Halloran
Pembroke Pines, FL, USA

 

[BillPSU]

There are 2-jaw chucks used for square and rectangular parts chucking. What is the volume for this part?

 

[jheal2]

Hey everyone. Thanks for the replies. I have done my research, passed it up to the higher ups to see what they decide. Not going to go with any homemade things, because production is quite high. About a couple thousand a couple times a year.
We will either go with the pot chuck idea, or end up just holding on one OD and doing a flip half way through.
Thanks
Jordan.