New collet Part 2

[phil_o]

I posted earlier that I was shopping for a new collet. I figured I needed a new 1/2" collet because I had a bit slip during a cut that ruined the workpiece. I was machining a bowl with a 1/2" ball nose end mill. The roughing cut was a raster that went fine. However when I did the finishing 3D cut I used an offset toolpath that started in the center and worked it's way to the perimeter, also it was a climb cut. The bowl was a 7" diameter and when the cutter got to about the 5" dia. point the bit began to slip, I didn't notice until about 6" and by then the bit had slipped enough to ruin the part.
My first thought was that the collet needed replacing. When I removed the bit the collet nut was very loose. I decided to redo the part and this time I did both the roughing and finishing cuts in a raster toolpath. I had no problems.
Now I'm thinking that if I had done the offset toolpath as a conventional cut the bit would not have loosened. Along the lines of tools that have a left handed nut to prevent loosening during rotation.
I'm posting to see if anyone can add to my thoughts and hopefully learn from my experience.

Phil

[Ajcoholic]

The nut tightens against the rotation of the bit regardless of the direction of cut. It should not loosen if you climb cut or cut conventionally

[Ger21]

Are you snapping the collet into the nut before putting the nut on the spindle?
If you are, then one of two things.

1) Make sure the bit is not bottomed out in the collet. If it is, it won't tighten correctly.
2) You're just not tightening the nut enough. Align the two wrenches so you can squeeze them in one hand, and squeeze very tightly.

[srwtlc]

Are you sure you had it tight to begin with and was the collet 'Snapped' into the nut before inserting the bit? Your finish toolpath shouldn't have been taking off much for material (0.040-0.050) and wouldn't have exerted much resistance force on the tool.

Edited...Gerry covered it.

[phil_o]

Thanks for all of the responses. They helped me rethink everything I did. However I still can't pinpoint what cause the slipping. The one thing I know for certain is that the collet nut was looser near the end of the cut than when I started the cut. Also the problem occurred after the rough cut which was much heavier than the finishing cut and it was doing fine until near the end of the finishing cut.

Phi

[curtiss]

I use a bit on medium strength lock tight on most bits which dries and takes up a few voids. Just a theory tho...

However sometimes you lose the electrical connection for the plate and have to use the clip.

[Burkhardt]

Based on the posts on this and other forum sites this just happens once a while as it happened to me (usually with longer 1/2" shank bits). My personal theory is one or a combination of the following:
- not tightened enough
- long bit leverage makes it worse
- wood dust in the collet lock grooves
- thread and nut/collet surface binding or not sufficiently lubricated
- collet/bit grip too much lubricated or debris in there
- long time rocking of the bit tip rattling the nut loose. I suspect a finish cut may be as bad or worse than an aggressive rough cut because the flute engagement is always at the same angle. This is just a theory but I had that, too after several hours finish cut.

I have now a set of ball bearing collet nuts which eliminate at least the nut-collet friction. Has not happened since but obviously this is no proof. I may just tighten the nut more.
I always make sure that the bit is not bottomed out so I guess I can eliminate this reason for me.

[phil_o]

I purchased new collets from MSCDirect.com. Very nice people to deal with. I made a mistake in placing my order and they were very helpful in taking care of my mistake, the return and proper reorder of the right item.
I will order from them again.

Phil

[chiloquinruss]

Make sure the little slots in the collet are clean. Also that the inside of the nut is clean. I use a brass brush to clean the inside of the nut and the air gun to blow out the slots in the collet EVERYTIME I change bits. A little dust in the slots will keep it from compressing on the taper of the shaft and therefore not tighten properly against the bit. Russ