I know this is an old thread but one of my readers referenced it in an e-mail on the topic. Here is my answer to him:
1. Usually, facing any metal in a vertical mill will create circular marks on the work. This can be caused by:
a. Chips being pulled through the workpiece by the cutter. This is pretty much normal and you cannot stop it. I have a small CNC mill that is perfectly trammed and a work surface after facing is so smooth I cannot feel anything with my fingernail when pulled across the surface. I use a vacuum to pull the chips off as they are formed. However, circular patterns are still cut into the workpiece but they are so shallow they cannot be felt.
b. The milling cutters are not perfectly ground and one or more tips are longer than the others and cutting deeper into the work surface.
c. Using milling cutters with too many flutes. My machine shop instructor told us that 3-flute cutters were the best compromise for allowing the chips to escape, that 4-flute (and more) cutters tend to drag the chips round and round, marking the workpiece.
d. Using square or flat milling cutters. I find that the Bull-Nose cutters (rounded corners) leave a much smoother surface than those with sharp corners. Boeing regrinds all the flat cutters into Bull Cutters before sending them into the machine shop. Probably one of the reasons tool suppliers are now starting to list off-the-shelf Bull Cutters.
e. The milling machine head is out of tram. If you can feel lines in the workpiece in the direction of table feed while you were facing, then the mill head needs to be aligned with the mill table. One cannot perfectly align the head but there are ways to minimize the misalignment.
i. One of the easiest mistakes to make after tramming (aligning) the head of your Bridgeport or similar mill is to tighten the 4 bolts that prevent the mill’s head from rotating without torquing them. Bridgeport recommends torquing the bolts on my mill to 25 ft-lbs and then to 50 ft-lbs. Otherwise, as they warn, “Improper tightening of these could cause a choppy quill movement.” What I found is without a torque wrench, I didn’t tightened the bolts enough and the head would then move on me under heavy cuts.
f. What I have found is a stream of air directed right on the junction of the workpiece and the cutter, where the chips are formed, would get rid of them as well as any other method and keep the workpiece and cutter cool at the same time. I had purchased a mist coolant system only to find that I usually could leave the fluid valve closed and just use the air.
g. I have yet to see any facing operation that did not leave marks in the workpiece. Everyone either uses secondary operations like lapping to remove the marks or if they will not be visible in the finished, work, ignores them.
Mike Rehmus
Editor, Model Engine Builder magazine
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