correct order to machine a cylinder on a lathe

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dansawyer

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The majority of the parts I am trying to make involve making some form of a cylinder. The work typically involves a center hole, square faced ends to the hole, and turned sides parallel to the hole. One such part is a gear blank. I am having difficulty centering the hole and or holding the blank firmly to turn the gear edge if I drill the hole first. The second is a cylindrical spacer. With this part f I turn the sides, face one end, cut off the cylinder, face the second end and then try to drill the hole the hole is out of alignment.
The gears blanks are 3 to 6 inches in diameter and 1/4 inch thick. The spacers are 1 to 1 1/2 inch in diameter and about an inch long.
My lathe is a South Bend 9 in reasonable shape. It has a three jaw chuck. I am using hss cutting tooling.
My questions are: What order should this work be done? Which is machined first, the hole, facing the ends, or machining the sides?
 
If you're not able to bore the hole and machine the OD at the same set up, you're better off mounting the part on a arbor to machine the OD after machining the bore.
 
Yes you usually would have to make an arbor with either a nut or bolt on the end to secure the blank for turning
Best to hold the arbor in a collet then the arbor can be removed and re-mounted easily without losing accuracy
Mark
 
Tapered arbor to turn your part between centers to get final O.D. of your gear blank.
To cut the gear it should also be done between centers for accuracy.
If your chuck is not easily removable you can make a center out of any scrape stock chucked in the lathe and cut to a 60 deg angle.
It will be accurate as long as you leave it chucked up. If removed just cut it again after reinstalling it to true it again.

If you don't have tapered mandrels or the hole in the gear blank doesn't fit any of your mandrels you can make a tapered mandel out of scrap also. Machine it between centers also.

3-6" gear blanks are flimsy if they are aluminum and would need support on a mandrel if they are 1/4" at the center. That would be unusual as usually the center would be wide and have the set screws to anchor it to a shaft and a keyway.

Tell us more and we can help better. Steel? aluminum? gear blanks have hub centers?

Your cylindrical spacer if done in a 3 jaw should have inner and outer dimensions turned all in one set up.
Use stock longer than needed so your entire part your making is outside the chuck jaws. Turn outer then drill inner then usually bore to final size and then part off.
If the parting tool does not leave a clean/straight enough end it could then be rechucked to face that end but inside and outside should not be touched as the 3 jaw will not be concentric enough.
 
OD to ID concentricity is typically not super critical on gears unless they are running at high speeds where balance becomes a concern.

What is important is that the pitch circle be concentric to the ID when cutting the teeth.
 
Update: Creating an aluminium gear blank for a 180 tooth 10 teeth per inch gear.
The calculated diameter is 5.7296 inches. The center hole is 1/2 inch. The arbor is made from a 6 inch piece of acme rod.
The turning of the blank the correct diameter went smoothly. The final diameter measures to within +/- 1.5 thousandths., However the hole is not in the center. I placed the blank back in the arbor and measured run out with a dial indicator.
I was hoping for +/- 1 thousands. I was disappointed to measure +/- 3 thousandths.
My first thought is this is an arbor issue.
What is a reasonable diameter for an arbor?
What are the best ways to create the original center hole? I am thinking a centering drill and then a final finishing with a boring tool. Is this the correct approach?
Dan
 
I would do the center hole to size first. Then machine the O.D. to size on a mandrel turned between centers.
You say "arbor" , I say mandrel, I don't think we're talking about the same thing.
I don't think you'll get within .001 unless you use a tapered mandrel between centers.
1/2" center hole in a gear that large seems very small.
 
Thank you for pointing out the correct vocabulary. I was using the term arbor to refer to an adapter that attached to the chuck and held the part to be machined in place. I will switch to mandrel.
The gear is part of a telescope drive. Typical fast revolutions are on the order of one rpm. The gear attaches to a 1 inch diameter shaft. The forces are not great, the telescope is balanced. This gear will be part of the declination drive, the drive mechanism has a spring to maintain contact between the drive and the gear in question.
I plan to cut the teeth this weekend.
 
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