Tapered Seat Finish

[kwhizz]

We are turning some parts that have a 60 Degree Internal seat in them....It starts at about 7/16 and tapers down to about 3/32 where it meets the center thru hole.....we are putting the taper in with a small Boring Bar because of the small thru hole........we are having trouble with consistancy of the surface finish.....material is 4340 ....28-32 rc......Is there any tricks or special Tooling for getting rid of the circular boring finish grooves.......need to be a 1Rms or better on Profilimeter readings.....

Thanks
Ken

[307startup]

It sounds like a combination of a VERY small radius insert & much smaller feed rate is going to be necessary to accomplish that finish...

[ctate2000]

Use a center lap to finish the operation. Buy it from MSC. Put the lap in drill press and turn slow if you can't see the flutes as the tool turns you are going to fast. Hold part against tool with hand pressure and lap to achieve finish. You may want to leave a thou or three for finish as the lap will cut slightly. The lap is not usually agressive enough to move the hole but I would verifly the dimensional correctness on the first few parts. Try to get the best finish possible from the machine tool before lapping. Oil the lap good each time and blow the flutes clean before each part. You can use the machine tool to do this if you are trying avoid hand work I would feed to depth and dwell for 1 sec or so with lots of coolant.

[cdlenterprises]

Your not going to hold a 1 RMS finish using any conventional machining method that I know of. Lapping as mentioned before is probably your best bet. This is a huge shot in the dark but you may be able to burnish it some how, but the small size may be a problem. I would give the guys at Cogsdill a call and see if they can help....