Accuratly bore the ID of a 6 inch OD aluminum gear from 3 1/2 to 4 5/8?

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dansawyer

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The project is to replace a telescope drive gear assembly. The current main gear has a 4 5/8 bore to attach to the scope. The replacement gear currently has a 3 1/2 bore. I have a 9 inch South Bend lathe.
My first thought is to bore a 'jig' the OD of the new gear to hold the new gear without munging up the teeth. If the jig is not removed from the lathe between boring the hole to hold the new gear and boring out the new gear center there there should be no alignmnet issues with the new center hole. The only accurcy issue will be the actual boring of the center hole.
Is this correcth thinking?
 
It seems like what you propose would work. If you have a face plate, just mounting the gear on there and dialing it in might be another option.

If you have a 4 jaw chuck, then wrapping the gear with some thin sheet metal to protect the teeth would work also.
 
Is it a thick, flat gear? How about mounting it on a face plate on spacers? Lots of tap-tap-tapping to center it initially but then you just bore it.
 
I often do as you are proposing.
Put your jig block in the 3 jaw and bore it to a nice snug fit on the gear.
Then use a hack saw to cut thru one side of the jig with it still mounted in the 3 jaw.
Now put in the gear and tighten the 3 jaw, the saw cut will allow it to compress like a collet to grip the gear and maintain center to the spindle.
Bore the gear.

One thing to check before you start, Often the gear blank is turned on a different machine than the one that will be doing the hobbing, sometimes the od is not concentric to the bore or to the pitch dia of the gear Not all gears are top hobbed. What is ultimately important is the bore to the pitch dia of the gear. You can get close enough in checking by using a pin that will touch the tooth face at about the middle of the tooth depth and measure the distance from the pin to the bore surface. It should be the same dimension no matter where on the gear you measure. You can also go around and measure from the top of the tooth to the bore to check for that concentric.
 
The gear is about 1/2 inch thick. It is flat.
Is it accurate to use a micrometer to measure the bore to teeth at different points?
My thought was the jig method would essentially limit the final centering to the accuracy of the lathe. The final bore diameter would be limited to my ability to not overshoot the boring process. (that is an area where I often have trouble. :( )
 
Flyinfool said:
I often do as you are proposing.
Put your jig block in the 3 jaw and bore it to a nice snug fit on the gear.
Then use a hack saw to cut thru one side of the jig with it still mounted in the 3 jaw.
Now put in the gear and tighten the 3 jaw, the saw cut will allow it to compress like a collet to grip the gear and maintain center to the spindle.
Bore the gear.

One thing to check before you start, Often the gear blank is turned on a different machine than the one that will be doing the hobbing, sometimes the od is not concentric to the bore or to the pitch dia of the gear Not all gears are top hobbed. What is ultimately important is the bore to the pitch dia of the gear. You can get close enough in checking by using a pin that will touch the tooth face at about the middle of the tooth depth and measure the distance from the pin to the bore surface. It should be the same dimension no matter where on the gear you measure. You can also go around and measure from the top of the tooth to the bore to check for that concentric.
Click to expand...
I was going to drill and tap 4 clamp downs in the jig. That way the center would be clear to bore. I was going to start with a 1/2 inch square aluminum plate and bore out a 1/4 deep snug fit. That and the clamp should protect the teeth and ensure the gear center and the final bore are concentric.
Am I missing something?
 
dansawyer said:
The gear is about 1/2 inch thick. It is flat.
Is it accurate to use a micrometer to measure the bore to teeth at different points?
Click to expand...

In my opinion, no

My thought was the jig method would essentially limit the final centering to the accuracy of the lathe. The final bore diameter would be limited to my ability to not overshoot the boring process. (that is an area where I often have trouble. :( )
Click to expand...

Make a sacrificial arbor and leave it freshly machined in the 3-jaw that barely fits (less than 0.000,5") the ID of the gear.
Now take a mechanics ruler and place one end on the ways and the other side on the gear and use a dial indicator to read the runout of the gear.
This will tell you if the outer teeth are cncentric to the bore.

I suspect that the gear has a few holes for being mounted to a shaft. You can use these to hold the gear to standoffs on the mounting plate or to another arbor.
Mount the gear and tap-tap it around until the teeth are concentric (as good as you can get it; 0.000,1 if you can!)

Now you are in a position to bore to size.
 
If you go the route of making a jig to hold the OD of the gear one way to keep everything concentric is to make a mark with a Sharpie on the jig. Then make a mark on one of the jaws and use these marks to align the jig when boring and when holding the gear. This will get you very close even if your chuck doesn't run perfect.
 
The final bore diameter would be limited to my ability to not overshoot the boring process.
Click to expand...
Wait a minute now... Isn't that why a lathe has reverse? :)
 
Do you have the proper tools for measuring the ID? An inside mike would be ideal. Next best is a telescoping gauge and an OD mike. Dial and digital calipers may not be accurate enough.
 
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