Facing off gears

[Nelson]

How to chuck small parts in lathe

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How would you set up some plastic gears 1" diameter, 24 teeth, 0.360" thick in a lathe to face them off to turn them down to 0.270" thickness?

Can it be done with a 3-jaw chuck? If so, how?.

Would this be better done in a mill? If so, how?


Nelson

 

[Hawkeye]

Nelson,

Since you are facing, rather than turning diameter, the three-jaw should work fine. The gear is 24-tooth, so that is divisible by 3, meaning that the jaws can be set on gear crests. I would suggest slipping a piece of pop-can aluminum between the gear and the jaws. Actually, the aluminum may not be needed for plastic gears, but I'll leave that reference in for those doing a similar operation on metal gears.

The trick is going to be in getting the gear to sit parallel to the face of the chuck. What I would try is to set the gear up with a bit more than the amount you want to remove sticking out past the jaws. Tighten it only enough to hold the gear and aluminum in place. Fasten a stick of hardwood in your toolholder and run it gently up to the face of the gear. with the lathe turning slowly, advance the carriage just enough to push any high spots on the gear into alignment with the low spots. When the stick is rubbing lightly all the way around the face of the gear, back it off and shut down. If the gear is still sticking out far enough to take off what you need to, tighten up the chuck to hold it firmly. If it got pushed in too far, move it out and try again.

 

[geotek]

I would use an expanding arobor. Or cut a matching diameter (match the ID of the gear) on a piece of scrap, press the gear on, and face it. A little blue locktite helps.

 

[Tony Wells]

I use 3 pot magnets stuck to the face of the jaws and pull the part gently into contact with them to get parallel with the chuck, then snug it down. This works if your chuck jaws are ground true and co-planar. Since you have that much to face off, you probably can't do the entire job that way, but you could face off the hub area to finish thickness, then make a close fitting spud that has a tapped hole in the center. Finish turn the spud diameter to a close sliding fit to the ID of the gear, with the spud length to the shoulder a little less than the now finished hub thickness. Using a suitable washer, clamp the gear onto the spud and finish facing the remaider of the gear.

 

[HSS]

You could turn a hollow jig closed on one end with an inside shoulder located .270 from the end to support the gear, pour the jig full of fixturing alloy, press the gear into the jig and alloy, down to the shoulder. When the alloy cools and solidifies, chuck it up in the 3 jaw and face it off down to the jig.

Pat

 

[mikew67]

Since you're working with plastic, all the suggestions to fasten to the hub and/or the face of the gear are appropriate. Also, making a completely enclosing soft collet. It's just too easy to deform the teeth if you try to apply point pressure on the teeth. Use high speed, very sharp tool, and very light cut. (Rake angle will vary with type of plastic. Acrylic likes zero, some other plastics like high positive rake. I've had success with carbide inserts made for aluminum machining.)

And why use a lathe? Put the gear on a bolt, fasten with a nut and chuck in a drill. Spin up and apply the face to some coarse abrasive paper. Trim the hub, where it's covered by the bolt head, with an X-acto knife. You can even do it with the hub spinning. Or lay some abrasive paper on a flat surface and rub the gear on it by hand

Mike

 

[martik777]

I use an adjustable spider or some parallels to space the piece away from the chuck face.

http://www.homemodelenginemachinist.com/index.php?topic=1235.msg8925#msg8925

3 set screws in place, or in addition to, the round spacers works too.

 

[Nelson]

Do you have any photos of how you made one of these:

(as depicted in http://www.homemodelenginemachinist.com/index.php?topic=1235.msg8925#msg8925)

 

[martik777]

Here is one of my small ones, just turn a 2-3" disc, make cut outs for the jaws (I drilled some holes and hacksawed, filed the rest) and tap in 3 screws for adjustment. There are NO holes drilled in the chuck! If I need accuracy, I put a DI on the compound and adjust the screws - only takes a minute or 2. You could also make some spacers in addition to, or in place of the screws. In the 2nd pic I am reducing the washer's thickness by .003" Before I had the spider I just used a spacer (drill bit, tool bit, metal bar etc) between the chuck face and the stock (3rd pic). Good I idea to remove the spacer before turning on the lathe! Last pic is one I made from 12mm laminate flooring - not really rigid enough

 

[martik777]

martik777, very cool idea but what keeps the spider tight against the chuck? Just the work piece? :huh:

Jerry. :tiphat:

When I dial in the spider I apply hand pressure or push the compound against it, then I hold the work tight against the spider while tightening the chuck. The spider will sometimes flop around in the jaws but won't go anywhere.