LAmachinist
Plastic
- Joined
- Jun 19, 2020
- Location
- NM, USA
I would appreciate any guidance from the more experienced rebuilders here, having searched this question and not found an answer.
Background:
I am rebuilding and scraping a small vertical knee-type milling machine. The column, saddle, and table all use 60 degree dovetail ways with straight gibs. From initial examination, I had thought the condition was OK - just the usual wear and table droop expected for a mill that has seen some use since it was made in 1964. Not claiming this is was the tightest mill out there - but compared to a small shaper I recently re-built, the condition of the ways was in much better shape (wear, but no scoring). I figured I could get away with scraping only, without the need to re-machine much if anything.
Problem:
However, there is one big problem area - the knee-column fit. The column has a large male dovetail - the wide flat surface of the dovetail should not be bearing, the two smaller flats on either side and the angled surfaces of the column dovetail should be the bearing surfaces. Likewise on the back of the knee, the two small flats and the two angled surfaces are bearing surfaces (with the wide flat between the angled surfaces being fairly roughly machined, needing only to provide clearance so the other flats can mate.
The flats of the knee dovetail were unusual looking; they originally looked like someone had been at them with a file (draw filing them). Looks like there was also a casting defect someone filled with babbitt or solder. More concerning was the large (non-bearing) flat in between the angled parts of the dovetail: it had been rubbing against the column dovetail surface, as evidenced by the shiny wear pattern on the originally milled surface. Likewise, there was a bit of wear on the column surface that was clear during rough scraping. Obviously that was not correct, indicating that even before scraping there had been interference in the dovetail geometry.
After scraping the two flats on the back of the knee square, and roughing in the front of the column dovetail (to serve as a reference surface), the two flats of the knee still do not make contact with the two flats of the column (which still need to be scraped but seem within 0.001 inches so it shouldn't take much material removal on these surfaces).
The interference is, at worst, about 0.005 inches (estimated by feeler gauge with the knee resting on the column). So either the top of the column dovetail has to come straight down at least 0.005 inches, or the recessed flat on the back of the knee need to be milled deeper at least 0.005 inches. That would be a bit to scrape but doable with my Biax. Alternatively, I could always take the knee to a larger mill to mill down the flat surface between the ways.
Question:
What is the appropriate clearance for the non-bearing dovetial surface between the knee and the column? I started this project with those two surfaces interfering, scraping the actual bearing surfaces has of course made this inference worse. What clearance should I shoot for as I correct this? I imagine the gap should be big enough that future wear of the dovetail wont lead to these two surfaces contacting each other, but not so big that chips become lodged in it (there is no wiper or chip guard for that surface). I would guess the clearance should be a couple thousands- 0.004? That would mean I need to remove 0.009 since I already have 0.005 inches of interference.
Any guidance would be appreciated!
Background:
I am rebuilding and scraping a small vertical knee-type milling machine. The column, saddle, and table all use 60 degree dovetail ways with straight gibs. From initial examination, I had thought the condition was OK - just the usual wear and table droop expected for a mill that has seen some use since it was made in 1964. Not claiming this is was the tightest mill out there - but compared to a small shaper I recently re-built, the condition of the ways was in much better shape (wear, but no scoring). I figured I could get away with scraping only, without the need to re-machine much if anything.
Problem:
However, there is one big problem area - the knee-column fit. The column has a large male dovetail - the wide flat surface of the dovetail should not be bearing, the two smaller flats on either side and the angled surfaces of the column dovetail should be the bearing surfaces. Likewise on the back of the knee, the two small flats and the two angled surfaces are bearing surfaces (with the wide flat between the angled surfaces being fairly roughly machined, needing only to provide clearance so the other flats can mate.
The flats of the knee dovetail were unusual looking; they originally looked like someone had been at them with a file (draw filing them). Looks like there was also a casting defect someone filled with babbitt or solder. More concerning was the large (non-bearing) flat in between the angled parts of the dovetail: it had been rubbing against the column dovetail surface, as evidenced by the shiny wear pattern on the originally milled surface. Likewise, there was a bit of wear on the column surface that was clear during rough scraping. Obviously that was not correct, indicating that even before scraping there had been interference in the dovetail geometry.
After scraping the two flats on the back of the knee square, and roughing in the front of the column dovetail (to serve as a reference surface), the two flats of the knee still do not make contact with the two flats of the column (which still need to be scraped but seem within 0.001 inches so it shouldn't take much material removal on these surfaces).
The interference is, at worst, about 0.005 inches (estimated by feeler gauge with the knee resting on the column). So either the top of the column dovetail has to come straight down at least 0.005 inches, or the recessed flat on the back of the knee need to be milled deeper at least 0.005 inches. That would be a bit to scrape but doable with my Biax. Alternatively, I could always take the knee to a larger mill to mill down the flat surface between the ways.
Question:
What is the appropriate clearance for the non-bearing dovetial surface between the knee and the column? I started this project with those two surfaces interfering, scraping the actual bearing surfaces has of course made this inference worse. What clearance should I shoot for as I correct this? I imagine the gap should be big enough that future wear of the dovetail wont lead to these two surfaces contacting each other, but not so big that chips become lodged in it (there is no wiper or chip guard for that surface). I would guess the clearance should be a couple thousands- 0.004? That would mean I need to remove 0.009 since I already have 0.005 inches of interference.
Any guidance would be appreciated!