chucking a gear for center boring

[thomas harris]

I have a small gear that needs to be center bored so I can put and oversized bushing in the hole to compensate for the eggged-out hole. I was thnking of using the chuck itself for alignment, but that still leaves the problem of gripping the teeth. What could I do to grip the gear concentrically and securely. The material is quite soft so the hole should bore easily. (Change gear for Atlas lathe)

[SteveM]

Take a look at what I did for re-boring wheels:

http://www.chaski.org/homemachinist/vie ... hp?t=75448

The "shoes" fit over the chuck jaws, are machines in place to the proper diameter and will grip over a wide area, so as not to damage your gear teeth.

Alternately, you could bore a peice of tube/pipe the ID diameter of the OD of the gear, mark where the #1 jaw is on it (to be sure you put it back in the same place, then take it out and slit it front to back. When you put it back, it will be concentric and the slit will allow it to close enough to grip.

You would need to make sure that the gear is not wobbling in the tube. maybe machining a shoulder in the bore to back the gear up to would do that.

Steve

[thomas harris]

Take a look at what I did for re-boring wheels:

http://www.chaski.org/homemachinist/vie ... hp?t=75448

The "shoes" fit over the chuck jaws, are machines in place to the proper diameter and will grip over a wide area, so as not to damage your gear teeth.

Alternately, you could bore a peice of tube/pipe the ID diameter of the OD of the gear, mark where the #1 jaw is on it (to be sure you put it back in the same place, then take it out and slit it front to back. When you put it back, it will be concentric and the slit will allow it to close enough to grip.

You would need to make sure that the gear is not wobbling in the tube. maybe machining a shoulder in the bore to back the gear up to would do that.

Steve

I read all the posts, but the pics have apparently expired and are no longer viewable. What concerns me is chipping a toothe on the gear when chucking. I was thinkng of lining it with some kind of hard rubber, perhap an old reinforced printing blanket would work. The gears are made of pot metal, so it should machine very easily with low forces on the piece. This is partly a "practice project" as replacement gears are availible, both used and new. I want to make it somewhat simple, as this is likely to be a one time thing. I see how the "sleeve jig" would be a very good way to go, and still may take the time to machine one.

[SteveM]

I read all the posts, but the pics have apparently expired and are no longer viewable.

They are there, but you need to be logged in to see pictures.

The gears are made of pot metal, so it should machine very easily with low forces on the piece.

They are made of a material called ZAMAK, which is an alloy of Zinc, Aluminum, Magnesium and some other materials. Not exactly a hardened steel gear, and they get a bad rap, but many of them are still in service after more than 50 years.

I have some that are a bit loose on their shafts and sleeving them with bronze may be a good solution. I think you would want to make sure there is some form of keying to prevent the bushing from rotating int he gear, or maybe some Loctite product will work.

Steve

[magic9r]

Clamp it to a face plate & indicate to center. then bore it. That way you take no risks with the teeth as you can clamp inside the gear root diameter,
Regards,
Nick

[Rich_Carlstedt]

The classic way to bore finished gears, is to place a dowel pin in three equal places around the gear and chuck on them.
The important thing is for the pins to be as close to the pitchcenter of the gear as possible !
The pins may even be soft metal, as long as they are all identical in Diameter.
As a rule of thumb as long as the pin is slightly above the OD of the gear, it will work. the concept is that the spaces between the teeth are all identical and therefore concentric to the pitchcenter.
Zamac casting produce a challenge because of the brittle nature of the material.
I would chuck it up on the outside of the gear, and then place a pin temporarilybetween two teeth and use your dial indicator agisnt it (!), then recheck it in several places around the gear to confirm repeated indicator readings.
If so, then the gear is centered. if not, place carboard shilm between the teeth and jaws and recheck.

Most modern gears are concentric to the OD from the start, but do not " Assume" this !
Rich

[gzig5]

Thomas,
I recently had to rebore a 24 tooth gear for reversing lever on my Craftsman 12x36. I took a hunk of aluminum about an inch long and 1/4" larger in diameter than the gear, chucked it in the three jaw and bored it to the OD of the gear. I left a shoulder for the gear to rest on inside so it would be square. Before removing from the chuck, I marked the position and number of each jaw on the OD of the bushing. This lets me remove and re-install with a pretty high confidence of everything running true. After removal from the chuck I cut a slit down the length of the bushing which allows it to clamp on the gear. Put gear in bushing, align in chuck and then bore the gear ID. Takes longer to describe than to do. Resulting gear works perfectly.

Soft chuck jaws would acomplish the same thing, but I didn't feel like messing with the square holes. If you had a lot of parts to do, that would definitely be the way to go. Hope this helps.
Greg

EDIT: I just realized that depending on the diameter of the gear you are working with, the bushing may not be the best way becaus of the large diameter. But, I have done this using steel pipe and PVC pipe for the bushing material.

[thomas harris]

The classic way to bore finished gears, is to place a dowel pin in three equal places around the gear and chuck on them.
The important thing is for the pins to be as close to the pitchcenter of the gear as possible !
The pins may even be soft metal, as long as they are all identical in Diameter.
As a rule of thumb as long as the pin is slightly above the OD of the gear, it will work. the concept is that the spaces between the teeth are all identical and therefore concentric to the pitchcenter.
Zamac casting produce a challenge because of the brittle nature of the material.
I would chuck it up on the outside of the gear, and then place a pin temporarilybetween two teeth and use your dial indicator agisnt it (!), then recheck it in several places around the gear to confirm repeated indicator readings.
If so, then the gear is centered. if not, place carboard shilm between the teeth and jaws and recheck.

Most modern gears are concentric to the OD from the start, but do not " Assume" this !
Rich

I like this tip! So... if the gears are in a multiple of the number of chuck jaws(three or four) then you can grip between two gears with each jaw. Correct? Should I be striving to bottom the dowel onto the space between the teeth so no sideways pressure is put on them?

[MikeC]

Grabbing a dowel wedged between two teeth on a Zamak gear sounds like a sure fire way to break at least one tooth. Three jaw is absolutely out on this job if you want it true. I'd grab it in a four jaw with maybe some aluminum pads to protect the ends of the teeth. Leave a bit sticking out and inidcate off the outside of the teeth. In this case, I'd be far less worried about having enough grip to hold the gear than breaking it due to excessive clamping pressure. I like the faceplate idea, too...if you have one. That's probably the best way.

[GaryHart]

If you got some aluminum tube with bigger od then gear and a smallel id then gear. Cut a hunk to fit in 3 jaw chuck. and chuck up not to tightly.
Bore for nice fit of gear to fit inside tubing.
Slide gear in so it bottoms out on shoulder.
Tighten chuck a little more to grab gear.
Check with some pins to see if od is close to pitch circle.
If lucky and they are your good to bore center.
If not use some pins close to pitch circle contact as mentioned earlier.
Bore tubing out bigger for aluminum to grab the pins and retighten chuck.

[10 Wheeler Rob]

I would make an aluminum ring with a counter bore out of someing like 6061 T6 aluninum, about 1/8 to 3/16 inch thick radially beyound the teeth and a counter bore steep inside so the gear will bottom out to be square with the ring. I then would saw a spit in the ring and chuck it in a 4 jaw with the gear held in the counger bore. Center to the bore on the gear and rebore to the new size.

Dose the same job as set of soft jaws but can be made in minutes. Works great for smaller items.

On larger peices I have jutt used some aluminm shims undedr the hard chuck jaws to hold items I don't want to marr or damage as well. Again in a 4 jow to allow good centering. Specially true if there is an od number teeth on the item. Just make shure to use thick enouth shim material, to be suficantly rigid and storng enough not to deflect and come lose.

Rob