- Joined
- Mar 26, 2018
- Messages
- 2,725
I have been doing some precision boring on the lathe lately, routinely hitting +/- .0002" with ease. I was feeling pretty good about myself (problem #1). I have a part that I needed to bore a bearing pocket for a 26mm angular contact bearing which was 3.75" deep into the part. It needed to be concentric to the other bearing pocket on the backside of the part within .0005" and on size -.0004/+.0002". I thought to myself, no problem!
Here is where the problems began. The part only had a .625" hole through, which forced me to go to a 1/2" boring bar (Problem #2). The boring bar stickout was 7xD so I would chatter at anything above 100rpm (Problem #3). I can be a little stingy at times and didn't shell out for a carbide boring bar (Problem #4). I needed an accurate depth to the bottom of the bore 3.708 +/- .002" so I had to use the compound slide on the lathe rather than the carriage (I don't have a DRO, Problem #5). This meant I didn't have power traverse feed to control the surface finish.
As I brought the bore into size I made the 5 final passes at .005" radial steps to get a handle on bar deflection. After each pass, I stopped to measure with a telescoping gage and a .00005" mic. Each pass was coming out right where I needed, within a tenth or two. I noticed that as I was getting close to final size, my surface finish wasn't very good (Problem #6). I was feeding too rapidly into the part (I really was trying to go slow). I decided that I wouldn't worry about it until the final pass where I would feed as slowly as I could to get a nice finish. My next to last pass mic'd in at .0048 from final size, so I dialed it in and cut the final pass going very slowly. The finish was awesome. I took the bearing and tried the fit, and uh oh, it dropped right in. So I mic'd the hole again and wtf?! it was .0076" oversize!!!
I sat there and pondered life and the universe. Then it came to me. Approaching final size in even steps is great, but if you don't use the same feedrate, the force on the boring bar is different. The higher forces with the faster feedrates tended to deflect my long slender boring bar away from the cut, giving a smaller hole. This isn't a problem as you compensate for the deflection when you measure after each pass until... I took the final pass at a fine feedrate. The lower forces allowed the bar to spring back into the cut, removing much more material than intended. The effect was greatly exaggerated by the length and diameter of the bar.
So now I have to decide if I want to rebore it larger and press in a liner, or allow the bearing to float a little in the pocket.
Lesson learned: In precision (manual) boring, approach your final size in even passes and identical feedrates. Make each cut identical to the last.
Here is where the problems began. The part only had a .625" hole through, which forced me to go to a 1/2" boring bar (Problem #2). The boring bar stickout was 7xD so I would chatter at anything above 100rpm (Problem #3). I can be a little stingy at times and didn't shell out for a carbide boring bar (Problem #4). I needed an accurate depth to the bottom of the bore 3.708 +/- .002" so I had to use the compound slide on the lathe rather than the carriage (I don't have a DRO, Problem #5). This meant I didn't have power traverse feed to control the surface finish.
As I brought the bore into size I made the 5 final passes at .005" radial steps to get a handle on bar deflection. After each pass, I stopped to measure with a telescoping gage and a .00005" mic. Each pass was coming out right where I needed, within a tenth or two. I noticed that as I was getting close to final size, my surface finish wasn't very good (Problem #6). I was feeding too rapidly into the part (I really was trying to go slow). I decided that I wouldn't worry about it until the final pass where I would feed as slowly as I could to get a nice finish. My next to last pass mic'd in at .0048 from final size, so I dialed it in and cut the final pass going very slowly. The finish was awesome. I took the bearing and tried the fit, and uh oh, it dropped right in. So I mic'd the hole again and wtf?! it was .0076" oversize!!!
I sat there and pondered life and the universe. Then it came to me. Approaching final size in even steps is great, but if you don't use the same feedrate, the force on the boring bar is different. The higher forces with the faster feedrates tended to deflect my long slender boring bar away from the cut, giving a smaller hole. This isn't a problem as you compensate for the deflection when you measure after each pass until... I took the final pass at a fine feedrate. The lower forces allowed the bar to spring back into the cut, removing much more material than intended. The effect was greatly exaggerated by the length and diameter of the bar.
So now I have to decide if I want to rebore it larger and press in a liner, or allow the bearing to float a little in the pocket.
Lesson learned: In precision (manual) boring, approach your final size in even passes and identical feedrates. Make each cut identical to the last.
