2FL 1/8 End Mill Biting into 6061 and Corkscrewing

[Hyelander]

Hi all, first time poster. Everything I know has been self taught through trial and error and it's often been painful. . . now is another one of those times so any and all input is extremely appreciated! I'm using a

carbide 1/8" 2FL flat uncoated center-cutting end mill (largest my spindle allows NRR-2651) The end mill is 1.5" long overall.
25k RPM
Feed: 20.613 ipm
Plunge: 8.3 ipm
.01 inches stepdown
20% stepover

Whether I speed up the rpm and feed/speed and make the cut more shallow, or slow down for more strength, I seem to be getting this same result. While the end mill is cutting normally (not plunging), it begins to bite in and corkscrew down. Fortunately, I was by the mill and heard something going wrong and hit the stop button fast before I broke another tool. You can see how shallow the cuts are and this was rounding the corner during the second pass (it starts at the middle top of the block) that it decided to dig in and start cutting downward.



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[elfrench]

Are you using compressed air or liquid coolant to remove the chips?

[Hyelander]

The spindle draws air from my compressor and blows it at the block to clear chips, but I also use an eyedropper to shoot some Relton A-9 cutting fluid occasionally/manually to keep things lubricated.

This was very early into the second pass and the spindle was blowing the cutting fluid around the block as it was moving around so I let it be.

[elfrench]

Is there any aluminum stuck on the bit after the dig in? Do you clean your collets before installing a bit?

I usually use a DOC of .004" - .006" on my router when milling aluminum. A more rigid router should be able to handle more.

[Hyelander]

No aluminum stuck to the bit after the dig in, and I don't usually clean the collet after changing bits. I did a while back after a few failed cuts until I realized that the tool I was using had dulled and was the culprit. Previously, my settings were at .005 in stepdowns, with different feed/speeds but I ran into the same problem. They had worked in the past, which is why I'm left scratching my head now. Part of me is wondering if maybe I need a new collet (maybe one too many failed cuts has slightly damaged it), or if there's an issue with the aluminum... No idea. This is only happening with my 1/8 bit, everything smaller runs fine.

[grg12]

What collet type are you using? If its ER - are you sure its istalled correctly? ("click in" empty collet in nut then slide endmill in).
Maybe it's obvious but some people don't get it first time (for example - me ).
About cooling - I use airbrush spraying methylated alcohol (bbq fluid) at about 1mililiter/min rate.

Wys?ane z mojego SM-N950F przy u?yciu Tapatalka

[Hyelander]

I'm using CHB-3.175 from NSK for 1/8" end mills. . . it doesnt click into anything. I slide the collet in and tighten it with two wrenches that came with it.

[Jim Dawson]

Assuming that your Z axis is not moving, then the end mill must be pulling out of the collet. Is it possible that the endmill has a 3mm shank? You are really making a light cut, I would not expect the endmill to pull out. Maybe a defective collet or maybe not tightened enough?

[Hyelander]

Hi Jim,

That's correct, no z axis movement when this happened. I took the collet out and it looks great on the outside, I cleaned the grooves a bit and blew it out with compressed air. I took a couple pictures of the interior as best I could with a loupe and saw it had a bit of scratching. From your experiences, does this collet look like it's in bad shape?

I also measured the shank diameter and its hittin .1245 inches. . . could a .0005" make a difference ? Is that a normal tolerance?

[Jim Dawson]

I can't really tell anything from the pictures. 0.1245 should be fine. I guess I might tighten a lot tighter.

[Hyelander]

So I called NSK and the guy said it sounds like the interior of the collet is scored, which is what it looks like in the pics. Apparently these little collets are a one and done type deal, they don't really handle failure well. I suppose I'm extremely lucky to have used it successfully this long with all my learning failures. I guess I'll purchase a new one and see if I run into the same issue and report back . . . in the meantime, maybe I can try clearing with my 1/16" end mill. Less force on the collet?

[Jim Dawson]

I guess it can't hurt to try.