Ideas? Need 1/4" diameter expanding stub mandrel for a blind hole.

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R.G.

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OK, so I got myself talked into doing a "simple" favor. For reasons too numerous - and probably hilarious to people who are not me :)) - I will be making some turned metal decorative knobs for some electronic equipment. No big deal, chuck in some 1" diameter steel, turn the profile, finish the top, and part it off.

Uh... it needs a 1/4" hole for the shaft in it. Oh.

OK. Face the end, center drill, drill the blind hole for the shaft, turn the profile, part it off... oh. Need to re-chuck to face the top to length and polish.

Gotta knurl a band on it too, so plumber's wax, Gorilla glue, beeswax, etc. will not hold the second clamping enough.

So I decided - no problem. Make a 1/4" stub mandrel that - um - fits in the the hole and expands a bit to hold the mostly-finished knob for doing the top.

Best I've come up with it so far is a 1" diameter round drilled through about 1/8", and the last 13/16" or so turned down to 2.49999999" and threaded inside, then split, and expanded by a... ah... closer/expander rod through the headstock

About here I decide there has to be a better way to do this. Ideas?

There's about 24 of them that need done.
 
Maybe hard to describe. A piece with 0.249 diameter shoulder long enough to fit the depth of the hole on the knob. Drill and tap it 8-32 from the back. Use a taper tap if you have one. Counterbore as necessary to set it up so that when you saw a slot the length of the 0.249 shaft, a socket set screw will run into the bottom of the thread and swell the faux shaft out. This piece could be roughly 1" diameter and 1" long, plus the 0.249 section. It will need to be chucked/unchucked for each part unless you have a really long allen wrench, which of course you could make.

There are other ways, but that came to mind first. Of course, you couldn't do any heavy work on it, like the knurl. Are these getting a set screw (grub screw) to lock onto the shaft on the radio?
 
Am I missing something here? From the description of the knob, I think I'd knurl the OD, finish the end, and part off all 24. Then, I'd turn them around, stick 'em in a collet, face 'em and drill the hole. Failing the collet, a simple split fixture would work fine with a chuck.

If I'd already parted off 24 pieces, I'd toss 'em in the drawer and start over before I'd try to rig up an expanding collet. . .
 
Thanks for the suggestions!

My tool kit is relatively primitive; I don't have collects that would do the work. I was also worried about chucking on the knurled portion ruining the knurl, and the knurled portion is the only good section available for a re-chuck from the top.

That's why I came up with the idea to do the work from the bottom first, doing the center hole first, profiling, and knurling on the first chucking, then parting, re-mounting using the center hole, and doing the top surface. This wouldn't be a problem if I had expanding collets in 1/4" diameter which were suitable for use in a blind hole. That's the thought that led me to thinking "hmmm... make what amounts to an expanding mandrel where I can expand it from the back end."

@Tony: Good description. I like that.
@MM: yeah, but I don't have 5C anything yet.
@Frank: The right think might be to set all the blanks in a careful row on the work bench and then wash out my mind with some scotch.
 
Thought from a woodworker...

Depending on how demanding your second set of operations is, you could make a block from a hard wood (think oak or maple) that would grab the knurled area. Thought through carefully, you could set it in a 4 jaw independent, and use the 4th jaw to clamp/unclamp the block without loosing center on the block. This would also work with an aluminum block...

Basically just a square with the OD of your knob bored in the center. Then cut open along one side with a bandsaw to give it enough room to compress and hold the piece.

Good luck!
-Tim
 
If I understand your dilemma correctly, all that is left to do is the knurling. If so, you can simply use a piece of shaft with a slightly smaller diameter than the OD of the knob, chuck it up and spot the center, drill to accept a 1/4" rod that protrudes just enough for the knob to fit/center onto, place a piece of gasket or rubber material, to provide friction, between this shaft and the knob, then sandwich the knob between the chucked shaft using a live center in the tailstock against a piece of aluminum, wood or any other suitable material that will not mar the knob and knurl it. You may have to go slowlyly and use quite a bit of force from the tailstock, but as long as you do not get too carried away it will work. I have had to use this exact set up before and it worked for 6 pieces I made that sound very similar.

Of course I may have totally misunderstood your issue and am way off course.

By the way, I noticed you are in Austin. Coincidentally I was born there and raised just slightly east near the huge spot in the road called Cele!

Good Luck!!
 
Use a relatively good fitting rod and superglue it into the bore.
Let it cure overnight.
turn away and then soak it in acetone.

Holding the shaft in a vise (soft jaws) give the part a rap or two with a PLASTIC hammer and the bond will break.
 
Normally I would just do the knob profile and knurl in the same operation as the drilling of the hole and then just part it off. Then if needed make a stub arbor to mount it on after you drill/tap the set screw and clean up the cutoff end.

DSCF1439.jpg

DSCF1439.jpg
 
franks idea sure sounds good to me. . . . . it sure would be faster than trying to cobble up an idea that most likely woud be a failure. . . . . if you don't want to chuck up on the knurls on your final operation, make the wood collet as suggested as your final operation. . . even a round adapter with a slit in it should work.

jmho

davidh
 
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