What sized ball for ball center?

Tozguy

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I am planning on making a ball center to hold a rifle barrel. The barrel bore is .243'' and will have a small chamfer cut with a regular 60 deg center drill. I would like the ball to contact the barrel chamfer approximately midway on the chamfer at the .250'' mark. What size ball bearing should I use?
 
To determine the contact diameter midway on the chamfer, we need to know the width of the chamfer. If it is indeed quite small, then a 1/4" (.250") ball will do the job, but will contact at a smaller diameter than .250", and quite close to the rifling. Going with a 5/16" (.312") ball will need a much bigger chamfer on a 60 degree double taper. A 7 mm ball (.284") might be a compromise. I am not a gunsmith, but it sounds like the reason for using the ball instead of a lathe center is to avoid damaging the rifling at the muzzle. Is the existing muzzle shape rounded? A 45 degree angle (90 degree cutter) would improve the geometry quite a bit, and simplify the math to trivial, but would need a 90 degree cutter.

At any rate, doing the trig will give you an answer before you cut metal. https://www.visualtrig.com/

Edit: 9/32" balls are out there, .281".
Second edit: And lots more! http://www.vxb.com/Loose-Steel-Cera...MIsYKWkL7Z1QIVQWx-Ch1KvgqrEAEYAiAAEgKVhvD_BwE
Third edit: 17/64" is .266", and that may be about ideal for you. There are some less expensive choices here: http://www.vxb.com/0-266-Inch-s/1154.htm
 
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Bob, thanks for your reply. The project is to taper a cylindrical barrel to meet IBS rules. The barrel o.d. is 1.235'' but the muzzle end has to be reduced to .9584'' or less in a straight taper over 18 inches. I don't have a taper attachment for the lathe so I will be offsetting the tail stock center. There will be an insert in the chamber end and the muzzle will be recut after the tapering job is done.

My calculations indicate that the tail stock center has to be offset by .1383''. Although it probably can be done with conventional centres, I would like to try ball centres because of the offset. Plus, I want to stick with a 60 deg. chamfer because it seems to balance axial and radial forces nicely.

My noggin got sore from trying to do the trig calculations so I consulted the tables in my Machinery handbook for segments of a circle and got a royal headache. But I stuck with it and calculated that a 9/32'' ball would have a ring of contact in the 60 deg chamfer of .244''. So 9/32'' looks like the smallest I could use. Since the chamfer can be cut any width, a 5/16th ball could work also but with a contact ring of .270''.

Thanks very much for the links and your comments, much appreciated, it got me back in the game. Mike
 
I suggest you get Fusion 360 for this sort of thing.
I have a math degree (and I actually use it), and I don't bother trying to do the trig on this sort of thing.
When you do a lot of math, making out the sketches and setting up the math is actually pretty trivial. Making sure you are actually getting the correct output is harder.
it is far easier to simply make a rough model of the barrel, and then make a ball to fit. You can either do it by eye, or by using constraints.

Fusion 360 is a huge boon, for getting good sizes for things with out mucking about with a bunch math.
 
Joshua, just watched a video on Fusion 360 for rank beginners. Looks like a great tool but I can only wish I was 50 years younger.
 
Ah! I thought it was an existing barrel that needed work as is...

Yes, .244" is very close to your bore size of .243". 17/64" is the smallest you can safely use. 5/16" is a heck of a lot more common than 17/64" or 9/32". If you are going to cut the barrel end off afterwards anyway, that is what I would go with. The larger bearing will give the barrel more support.
 
Yes bigger is better. There is also the sizes of round cutter available to cut the socket for the ball. It is starting to look like maybe a 3/8ths ball would be the most practical size plus it would work on a 30 cal. barrel if ever required.
 
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