View Full Version : Boring Bars for the Lathe

02-13-2006, 11:01 AM
I bought a name brand (not Chinese) 4-bar set of small boring bars last year for use on my import 13x40 lathe. They're negative rake and use 21.51 CCMT inserts. I've tried various makes & grades of inserts and have yet to find the combination of speed/feed, tool height & insert that results in a decent finish, whether in aluminum, crs, or stainless. I'm aware of bar dia./overhang ratio for reducing chatter, but there just doesn't seem to be a happy medium with these bars.

The latest MSC sale flyer has a couple of pages of Circle C bars on sale, including S-series up to a 3/4" dia. that uses trigon WPMT-322GP inserts. I've got a project using 400-series stainless that requires boring a 2" hole in a 3.5" OD workpiece, and the heaviest of my CCMT bars (only 5/8") isn't up to it. Do you guys think that a 3/4" steel bar in the heavy duty boring bar holder of my Phase II BXA QC toolpost would do an acceptable job for this sort of application? Or do I need to start saving for a carbide bar?

02-13-2006, 11:39 AM
Tool height should be dead center. I've had the best results with a tapered shaft bar with hss cutter. For shallow holes up to 1.5" deep I use small bars with welded carbide tips or ground from hss blanks... again with a taper to the shaft. A little flex can help finish on a small machine.. I know it's sounds odd but that's been my experience. Have not bored any 400 SS but could be the property of the material giving the bad finish.. can you lap the bore to improve the finish?

02-13-2006, 12:03 PM
I have had pretty good results with Circle brand CARBIDE bars. I drill the largest hole I can within 10 thou of finished size with a drill/reamer in the tailstock. All of my CIRCLE bars except the 3/16" use triangle inserts. 1/4", 3/8", 1/2" 5/8" 3/4" all take triangle inserts. My method is shallow cuts, with high speed and low feed rates.
You didn't actually say why your holes are substandard.
I always check that the bar only extends as far as it needs to be for the depth of the hole, it is mounted very rigid, the set screw for the inserts is not wore our and seats the insert securely, ensuring there is not foreign material between the insert and the seat.

02-13-2006, 02:08 PM
Negative rake tooling doesn't work too good on small lathes, unless they are a high quality machine in good condition, and then, with light cuts, the SFM will need to be quite high. The benefit to negative rake tooling is longer life, less failure because of the strength of the insert, and you normally get double the useable edges on the insert. You'll have much better luck with positive rake tooling. I'm pretty happy with it on a 13" SBL that isn't exactly "tight". The SFM still needs to be up there though. .015R inserts with .003 to .005 feed work good with "normal" length x dia. ratios on the bar.

02-13-2006, 06:11 PM
The boring bar (Pafana SCLCR 4-2) I use on my 920 lathe uses these same inserts. A while ago one of the members on the Yahoo 920 group suggested using CCGT-AK inserts on lathes of this size. He says he uses this AK style exclusively on all his toolholders. They have much more backrake than the std ones.

I recently picked up 2 from KBC (#CCGT- 21.51) and was amazed how well they work. The finish is so smooth and you don't have to run the part at high SFM.

Try them and I think you will be a convert.