Shop Doc – Holding Tight

Today’s Machining World Archive: June 2006, Vol.2, Issue 06

Dear Shop Doc,

I have a customer who needs me to hold a .0005 tolerance ID on a part made with C12L14. I’ve been having trouble getting the tolerance close enough on my 6 spindle Acme bar machine. I’ve been trying for days to ream it up to specs, but I’ve never had to go this close before and I’m starting to run out of patience. Are there any tricks to getting the tolerance right.

Holding Tight

There is a technique, which may help you hold your tolerance on the part, known as ballizing. It can actually give you tolerances as close as .0001-.0002. Ballizing isn’t anything new, but usually it is utilized after machining, when you have a ball that you press through a through hole to size the ID.

What you can do is this; First, ream your part the best that you can, and then afterward, silver solder a carbide ball to the end of a shank and put it in one of your open spindles–probably your fourth position spindle on a six spindle Acme. At the same time, for this to function properly as you’re sealing everything off, you will need a little hole that will go through at an angle that doesn’t interfere with the OD of the part, to let air behind there escape as you’re ballizing the part. Then run your operation. It’s just in and out, so it doesn’t limit your cycle time of making the part. You can now complete the part on the screw machine and hold a very close tolerance, usually getting better than a 10 – 6 micro inch finish on the hole.

Things to remember

The balls are made of carbide, so you will have to experiment a little bit with size because what you’re really doing is displacing material, and it will spring back. So if you would have to hold, for example, closer than .0005 tolerance, then you will need to experiment with two or three different balls to find the result you are looking for. Also, like anything, the balls are only as good as how close you ream. In other words, the reamed hole would need to be held close to keep consistency on the ball that you’re working with. They both work a little bit together. So if you had a hole that was varying .002 before you ballized it, I still think you could ballize it and hold it within .0005. If you wanted it any closer, you would probably have to ream within .001. Then, I think you would be able to hold within .0002 or better. Also, you need to make sure that you are using, the same gauging that your customer is using.

Good luck!

Henry Bradlock
St. Joe Tool Co., Bridgman, Mich

Share this post