Ball end mill slipping

[The Workshop Barbados]

During a roughing pass my 3/8" ball end mill starting slipped down out of the collet and caused some damage to the stock and I'm trying to figure out why.

I zeroed the bit correctly, tightened it (pretty tight), then after about 10 minutes of cutting, I noticed it was cutting too deep. It had slipped down by about 1/2". The first time it happened, I thought I must have made a mistake, so I re-zeroed, but the same thing happened again.

Settings were 120IPS, 40% stepover, 0.18" cut, mahogany. The bit is new as is the collet.

The cut was pretty noisy, but I find ball noses are always noisy, so I wasn't overly worried.

I switched out for an end mill and had no issues with this.

I'd be interested in hearing some thoughts on what could have caused this.

[srwtlc]

You must mean 120ipm?! Use a caliper or mic and measure the shank diameter compared to your end mill.

[The Workshop Barbados]

Yes, my bad, 120 IPM.

Are you thinking the shank could be undersized? It did tighten normally.

[don]

Yes, my bad, 120 IPM.
Are you thinking the shank could be undersized? It did tighten normally.
During my working years. I worked for a local company that manufactured new and reground and sharpened used tooling.

All tooling are made from carbide or Hss (High-speed steel) blank stock.
Most tooling are manufactured with a standard tolerance range from 0.0005" to 0.002".
So all new tools are not the same size as the shank.
The same goes for metric tooling (don't remember their tolerances)
Use imperial collet for imperial shank - metric collet for metric shank.

Manufactured Vbits have an angular tolerance from -2.0 to 2.0 degrees. So for everyone that is Vcarving measure the angle of your Vbits for best results!!!
All tooling can be resharpened to any specific diameter or angle you need.