# Endmill shattered in collet

**URL:** <https://forum.sienci.com/t/endmill-shattered-in-collet/25303>\
**Category:** 📄 General\
**Created:** [December 28, 2025, 5:59pm UTC](https://forum.sienci.com/t/endmill-shattered-in-collet/25303 "2025-12-28T17:59:32Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![iuliancalin](https://forum.sienci.com/user_avatar/forum.sienci.com/iuliancalin/32/6213_2.png) [@iuliancalin](https://forum.sienci.com/u/iuliancalin)\
**Post date:** [December 28, 2025, 5:59pm UTC](https://forum.sienci.com/t/endmill-shattered-in-collet/25303/1 "2025-12-28T17:59:32Z")

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so i was cutting some wood with a 4 mm 2 flute endmill at 1200mm feed 24000 rpm 2 mm deep, all went well up to a point, the endmill just shattered in the collet  
first i had a 3mm endmill 3.5mm deep 1000mm feed 24000 rpm , then i switched to a 4mm endmill 2 mm deep, that was weird  
i never seen that one happening in wood, any ideas?

the type i was cutting was compact HPL board

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**Author:** ![gwilki](https://forum.sienci.com/user_avatar/forum.sienci.com/gwilki/32/366_2.png) [@gwilki](https://forum.sienci.com/u/gwilki)\
**Post date:** [December 28, 2025, 6:39pm UTC](https://forum.sienci.com/t/endmill-shattered-in-collet/25303/2 "2025-12-28T18:39:17Z")

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@iuliancalin I would bet that the mill overheated. I just read up on HPL. It seems to have a lot of glue and resin. Both of those dull bits very quickly.  
Add to that your very low feed rate and a very high speed, and you end up with a dull bit getting very hot.  
I’ve never cut HPL, so I am not really in any position to provide you specific parameters. I cut acrylic and polycarbonate at 10000rpm and 100ipm=2500mm/min. A higher rpm or slower feed produces a hot mill and melting. Extreme melting snaps the end mill.  
With luck, others on here have milled HPL and can provide more valuable advice.

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**Author:** ![mick\_s](https://forum.sienci.com/user_avatar/forum.sienci.com/mick_s/32/9655_2.png) [@mick\_s](https://forum.sienci.com/u/mick_s)\
**Post date:** [June 28, 2026, 2:32pm UTC](https://forum.sienci.com/t/endmill-shattered-in-collet/25303/3 "2026-06-28T14:32:39Z")

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> [@iuliancalin](#):
>
> compact HPL board

First impressions - compact HPL is a resin/paper product similar to phenolic. It’s not wood. Phenolic is hard stuff. Industrial CNC router manufacturers use it for their vacuum grid tables.

The correct chipload for phenolic for a bit diameter that size is .004" to .006". That works out to about 12,000 rpm and 100 ipm feedrate at a depth equal to diameter. Your chipload was closer to .001" - way too low. As @gwilki said, that results in the bit overheating really quickly, dulling almost immediately and unable to cut.

Hope this helps.