Bridgeport knee wear

CJD · Apr 8, 2020

Yesterday I ran a big job on the CNC Bridgeport, where I had to move the knee a few times to accommodate the tool offsets. I messed up the job 6 times before I figured out I was running into issues with the knee wear. If I run the knee up and down the length of the quill , or about 4.5", it shifts the position of the work in the "Y" plain by .020". Tightening the gimball makes moving the knee harder, but does not affect the .02" slop. To the best of my checks, the table remains square with the quill, so only the "Y" moves.

My question is...is this normal wear, or is the knee always sloppy on bridgeports? Everything else on the machine is within .001" over the range of travel. It seems strange the knee, of all things, would be worn so much more than the rest of the mill.

I can account for it by planning tools with offsets within the quill range...just curious if moving the knee during a job is not recommended?

moonlight machine · Apr 8, 2020

CJD said:
Yesterday I ran a big job on the CNC Bridgeport, where I had to move the knee a few times to accommodate the tool offsets. I messed up the job 6 times before I figured out I was running into issues with the knee wear. If I run the knee up and down the length of the quill , or about 4.5", it shifts the position of the work in the "Y" plain by .020". Tightening the gimball makes moving the knee harder, but does not affect the .02" slop. To the best of my checks, the table remains square with the quill, so only the "Y" moves.

My question is...is this normal wear, or is the knee always sloppy on bridgeports? Everything else on the machine is within .001" over the range of travel. It seems strange the knee, of all things, would be worn so much more than the rest of the mill.

I can account for it by planning tools with offsets within the quill range...just curious if moving the knee during a job is not recommended?

No the knee is not sloppy on a Bridgeport. Is this a real BP or a china copy? If it always moves .020 the ways are not parallel with the quill. Have you trammed the head? .020 in 4.5 is huge. The ways could not be that far off even on a china pos, although many years ago I worked at a shop that had a Kondia that the knee ways were off in the X. with a 12" angle plate on the table over the full travel of the knee it would move about .015 or so. turn the plate and check in Y and it was 0. What is a gimball?

Newman109 · Apr 8, 2020

moonlight machine said:
No the knee is not sloppy on a Bridgeport. Is this a real BP or a china copy? If it always moves .020 the ways are not parallel with the quill. Have you trammed the head? .020 in 4.5 is huge. The ways could not be that far off even on a china pos, although many years ago I worked at a shop that had a Kondia that the knee ways were off in the X. with a 12" angle plate on the table over the full travel of the knee it would move about .015 or so. turn the plate and check in Y and it was 0. What is a gimball?

I think he means gibs, not gimball. Gimbal or gimbaling is .for example, what a rocket engine does under test conditions. where the thrust chamber is moved sequentially on an X axis then Y axis to test it's motion capability.

.020 is way too much for a 4.5" movement of the knee on any mill.

Gimbal - Wikipedia

rons · Apr 8, 2020

I remember a guy here was dog nuts on always moving the table up and down for plunge cuts. Never use the down-feed wheel. "And don't mention this again" was something
like the words in his rant. Well, how about that.

CJD · Apr 8, 2020

It is a real Boss Bridgeport. And yes, I meant gib...my spell check struck again.

Something came to light today. What was bothering me is I made the part the night before, and it was spot on. Then yesterday 6 straight parts were buggered. It was confusing me how it could have made the one good part if it was so far off. A bad knee would be consistent.

Well...here is the rest of the story!

I had an issue when I first put the Centroid brains in the machine that at the end of a job it would send the quill through my rotary table trying to go all the way back to the home position. I posted on the Centroid site for help, and wound up moving the rotary table to the right side of the mill table, so it was out of the way when the job finished. But then another poster started talking about “rest positions” vs “home position”. He got me fiddling with the coordinate systems, and yesterday on start up is when I did the fiddling. After messing with WCS, when I started the first job the quill slammed my centering drill so hard into my part that it shattered to dust! I rebooted, rehomed, re-tooled, and restarted my job next without thinking.

Today I did some easy facing, and the head was obviously nodded back. Then it dawned on me...when the tool slammed the work yesterday, it moved the head out of tram! The way I was measuring the “Y” axis change was by comparing the quill tip to the table. I had just trammed the thing last week, and never thought about the botched job screwing it up. Lesson learned. I was not measuring knee slop, but rather head tilt. Also, the power of the servos is impressive.

I re-trammed her and all is good now.

moonlight machine · Apr 8, 2020

CJD said:
It is a real Boss Bridgeport. And yes, I meant gib...my spell check struck again.

Something came to light today. What was bothering me is I made the part the night before, and it was spot on. Then yesterday 6 straight parts were buggered. It was confusing me how it could have made the one good part if it was so far off. A bad knee would be consistent.

Well...here is the rest of the story!

I had an issue when I first put the Centroid brains in the machine that at the end of a job it would send the quill through my rotary table trying to go all the way back to the home position. I posted on the Centroid site for help, and wound up moving the rotary table to the right side of the mill table, so it was out of the way when the job finished. But then another poster started talking about “rest positions” vs “home position”. He got me fiddling with the coordinate systems, and yesterday on start up is when I did the fiddling. After messing with WCS, when I started the first job the quill slammed my centering drill so hard into my part that it shattered to dust! I rebooted, rehomed, re-tooled, and restarted my job next without thinking.

Today I did some easy facing, and the head was obviously nodded back. Then it dawned on me...when the tool slammed the work yesterday, it moved the head out of tram! The way I was measuring the “Y” axis change was by comparing the quill tip to the table. I had just trammed the thing last week, and never thought about the botched job screwing it up. Lesson learned. I was not measuring knee slop, but rather head tilt. Also, the power of the servos is impressive.

I re-trammed her and all is good now.

ALWAYS tram any knee mill after a crash!!! I have a Centroid V2xT Bridgeport. Used to be a Boss. It has plenty of power to knock everything out of tram if you crash.

CJD · Apr 8, 2020

Yep, I know now. I even had to REALLY lean on the lock bolts to release the head, and it still had the power to move it almost a full degree out.

moonlight machine · Apr 9, 2020

CJD said:
Yep, I know now. I even had to REALLY lean on the lock bolts to release the head, and it still had the power to move it almost a full degree out.

You can over tighten the crap out of nod bolts and tilt studs and even a small crash will move the head. It is just the nature of the beast. A knee mill without this issue is the Tree Journeyman series. No tilt or nod. Same work envelope as a Bridgeport, weighs about twice as much. 40 taper as well. I have an extra one I would sell if you want it.

CJD · Apr 9, 2020

Thanks, but I just got this girl back in commission and need to spend a little time with her. This crash wouldn't have happened if I wasn't messing with something that didn't need messing.

gbent · Apr 9, 2020

CJD said:
This crash wouldn't have happened if I wasn't messing with something that didn't need messing.

The root cause of 90 percent of my crashes.

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