Today I was milling a 2" wide slot in a 3/4" thick 1018 workpiece using a 1 1/2", 4-flute HSS end mill with a 3/4" reduced shank running at 105 SFM at a very slow feed rate. I was taking full-width cuts at 0.050" DOC. Well into the 6th pass, at 0.300" deep, the end mill dug into the work piece. I shut it down and began the process of figuring out what went wrong.
The drawbar was not tight. It wasn't loose, but it was not "tight tight" like it should have been. I wondered if I had not tightened it up when I installed the end mill. Unlikely, but possible, I suppose. Upon examination, it appeared as if the end mill had encountered a very hard spot in the 1018. The end mill was ruined (for center cutting, anyway).
I decided to use a 3/4", TiN coated, 4-flute HSS end mill to finish the job. I inserted the R8 collet only to discover that it would not seat. It lacked a little less than 5/8" before seating. Now, I was concerned. With a mirror and a Maglite, I could see a ring up in the spindle. It looked like it might be located at about the place that the top of the collet was hitting something. After repeated attempts to slam the collet further into the spindle by hand, I switched to an old 3/4" R8 drill chuck that came with the mill. (I still use it now and then.) Slamming it into the spindle semi-seated it. Repeated extractions and insertions seated it further. Whatever was holding it back was not solid. (By now I highly suspected a spun collet had created the ring I saw in the spindle.)
Retracting the jaws of the chuck, I placed a 123 block on the table and brought the spindle down with the chuck hitting the 123 block with some force but nowhere near a killer amount. The chuck went further up in the spindle. Next, I "pecked" it in until it seated. Of course, it was difficult to remove but repeated drops of the drawbar unseated it.
Replacing the drill chuck with a collet, I tried to insert it by hand. No luck. I took a magic marker and "blued" the top inch or so of the collet and used the 123 block and the spindle down feed lever to seat it. Removing the collet (same technique as with the chuck), I saw the line created by the obstructing ring in the spindle, which you see below.
Deciding all was well and REALLY tightening up the drawbar, I took a 0.010" pass. The end mill was destroyed at the same spot. It must be a hard spot in the mild steel work piece is all I can figure out. (This is a good way to create side milling cutters, by the way.)
I replaced the end mill with a 3/4", 2-flute, TiN coated HSS end mill (I wanted to save my last 3/4", 4-flute, TiN coated HSS end mill) and restarted that 0.010" pass. All went well, but the sound and smoke told me that it had to work on that hard spot. After passing the spot, which seemed to extend about 1/2", all returned to normal. I took another 0.010" cut, again hitting the hard spot. The hard spot disappeared by the sixth cut and I brought the 2" wide slot down to within 0.0167" of total depth without problem. I will take it to depth tomorrow.
What do you think happened? I am almost positive that the drawbar was tight on the initial installation of the 1 1/2" end mill since it made five full length passes at 0.050" without problem. Let's assume it was tight for the sake of discussion.
Is a 1 1/2" end mill with a 3/4" reduced shank to much to turn with an R8 machine? Could the increased leverage of the big diameter have overcome the holding resistance of the 3/4" collet? Or, could it have been the hard spot in the 1018?
I have read about hard spots in mild steel. However, if such is even possible this would be my first experience with the phenomenon. If hard spots are myths and running a 1 1/2" end mill at 0.050" DOC is done every day in an R8 machine, then the only explanation that I see is that I failed to tighten the drawbar sufficiently when I originally inserted the 1 1/2" end mill.
What is the most likely explanation?
--Bill
Especially in steel.