Crazy Idea For A Small 4-jaw Chuck On A Milling Machine

TIGL

TIGL

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I've got a wacky idea I want to run by all of you before I turn a nice chunk of 7075 into chips. As mentioned in this thread, I bought a 7/8"-20 tap so that I could make different kinds of attachments for my boring head arbor. The fly cutter I made was wildly successful so I was thinking about trying something really out there: a 4 jaw chuck with an R8 arbor. The goal of this would be to be able to use my mill to turn larger delrin and polyprop rods than I can currently with my 1" collet without having to thread each one to fit this arbor.

My plan is this:

1) bore bore out this 2.5" concentric blank I made out to a 2" ID, as deep as I can without hitting the arbor.
2) make small flats on 4 sides of the blank using my vice so that my jaws are exactly 90 degrees apart
3) drill and tap holes into the flats of the blank so that I can use 4 bolts as the jaws.

I have 4 questions:

1) what bolt size should I use? I'm waffling between 3/8-16 for holding force and 1/4-28 for the ability to make fine adjustments
2) should I use 1 or 2 screws per side? 1 makes adjustment easier, 2 (probably) makes it easier to keep the workpiece rotating parallel with the chuck (1/4-28 would make fitting two much easier)
3) should I put the screws perpendicular to the axis of rotation or angled slightly towards the interior of the cavity? Angling them might help draw the workpiece against the face of the chuck which would help keep things square.
4) is this a stupid idea? am I going to spend a lot of energy and frustration on something that works marginally well at best?

Thanks!
 
Congratulations, you have just reinvented the 4-jaw chuck, all humor aside this arrangement will work fine with POM, (delrin) and polypropylene, these materials normally are produced in round form at considerably oversized raw diameters for finishing so make the collett chuck considerably larger then the nominal stock size that you intend to use. In larger sizes these materials are often not terribly round at all, I'm talking above 3-4 inch diameters.
If you want to do lathe OPP's with a mill invest in some lathe tooling that will be held in the vice. Aluminum specific insert tools work well on these materials, a dab of dish washing soap in water applied with a spray bottle works wonders with the finish on polypropylene and CPVC, this may be anathema to hobbyists due to the possibility of rust and more importantly discoloration.
Good Luck
 
They are out there .Been around for a while now.
 
TIGL said:
I've got a wacky idea I want to run by all of you before I turn a nice chunk of 7075 into chips. As mentioned in this thread, I bought a 7/8"-20 tap so that I could make different kinds of attachments for my boring head arbor. The fly cutter I made was wildly successful so I was thinking about trying something really out there: a 4 jaw chuck with an R8 arbor. The goal of this would be to be able to use my mill to turn larger delrin and polyprop rods than I can currently with my 1" collet without having to thread each one to fit this arbor.

My plan is this:

1) bore bore out this 2.5" concentric blank I made out to a 2" ID, as deep as I can without hitting the arbor.
2) make small flats on 4 sides of the blank using my vice so that my jaws are exactly 90 degrees apart
3) drill and tap holes into the flats of the blank so that I can use 4 bolts as the jaws.

I have 4 questions:

1) what bolt size should I use? I'm waffling between 3/8-16 for holding force and 1/4-28 for the ability to make fine adjustments
2) should I use 1 or 2 screws per side? 1 makes adjustment easier, 2 (probably) makes it easier to keep the workpiece rotating parallel with the chuck (1/4-28 would make fitting two much easier)
3) should I put the screws perpendicular to the axis of rotation or angled slightly towards the interior of the cavity? Angling them might help draw the workpiece against the face of the chuck which would help keep things square.
4) is this a stupid idea? am I going to spend a lot of energy and frustration on something that works marginally well at best?

Thanks!
Click to expand...

Your idea will work fine, I made a 4 jaw chuck 20 years ago for a wood lathe that I still use once in awhile.
My lathe spindle was 1 x 8 threads. I had that size tap and a piece of 2-1/2" black iron pipe that was threaded on one end. I bought a cap for it, cut the pipe to 2" long. The hard part was drilling a 15/16" hole for the tap. I drilled with a 3/4" hole saw after flattening the top of the cap with a file. The holes saw drilled a 7/8" size hole. I had no machine tools except a drill press at the time.
Tried to tap the hole after boring with the hole saw, the hole was a bit to small for tapping. I spent at least an hour carefully filing the hole larger til the tap would work. I used 1/4-20 hex head bolts for jaws, that's all I had at the time and figured I could always re-tap larger if needed. The bolts worked fine and never did need to re-tap them.
I think I spent at least 8 hours making this chuck with limited tools. Today I could make the same thing in 2 hours or less.
mike
 
MANY moons ago, I bought on fleabay a chuck with R8 mounting. Can’t recall if its 3 or 4 jaw. What do you mean, can’t recall, you ask? That’s because, at the time, I thought it would be nice to add to my Bridgeport box of toys. To this day, I have not needed it at all. I don’t even know where I put it? I wasted money on it and it’s taking up space in my shop somewhere. Personally, I would get a more meaningful toy for the shop…Dave
 
Just noticed the date of the original post. Deleted my reply.
 
Please, never ever worry about how old a thread is. A single post at any time could turn the thread into a whole new ballgame. Everyone , feel free to bring to life any thread you have interest in.

"Billy G"
 
I agree with Bill G, as your contribution could help someone in the future.

Sent from my SM-S320VL using Tapatalk
 
TIGL said:
I have 4 questions:
1) what bolt size should I use? I'm waffling between 3/8-16 for holding force and 1/4-28 for the ability to make fine adjustments
2) should I use 1 or 2 screws per side? 1 makes adjustment easier, 2 (probably) makes it easier to keep the workpiece rotating parallel with the chuck (1/4-28 would make fitting two much easier)
3) should I put the screws perpendicular to the axis of rotation or angled slightly towards the interior of the cavity? Angling them might help draw the workpiece against the face of the chuck which would help keep things square.
4) is this a stupid idea? am I going to spend a lot of energy and frustration on something that works marginally well at best?
Thanks!
Click to expand...
OK ... here's what I would do if I were making such a chuck:
1) Probably use a 5/16-24 or ⅜-24. Larger diameter than ¼-28, but still a relatively fine thread. And be sure to cut the end of the bolt off flat and square. Then chamfer the edge to about the same depth as the threads. "Store bought" bolts tend to have concave ends, which can gouge the workpiece (especially plastics, as mentioned in the original post).
2) One per side. Locate reasonably far "out."
3) Perpendicular. Remember, centering a workpiece involves loosening as well as tightening the screws. Though tightening may drive the workpiece inward a bit, loosening might cause it to lose contact with the "bottom." Also, placing the screws at an angle would mean that only the edge of the screw would make contact, and could lead to gouging. If necessary, sandwich a spacer (plus maybe a piece of foam or low durometer rubber) between the end of the workpiece and the mill table, to help hold it in place while adjusting.
4) As others have pointed out, it should be do-able and practical.
 
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