Air Cooled 1.5kW VFD Spindle - Min RPM vs. Heat


I’m cutting some acrylic with my VFD-driven 1.5kW air cooled spindle. Generally I’m running much higher speeds, but my G-wizard calcs put me at about 7800 rpm for this test and I was getting a nice cut. For the first time, though, I could tell the spindle body was getting VERY hot to the touch. I decided to stop it and do some research.

I’m sure I had read 7200 RPM was the minimum for fan cooling on this spindle, but now I’m seeing 8000 RPM on the automatedtechnologies site where I bought it. Fine… I can up the RPMs to 8000 and reduce my cutting speed to retain quality (I’m in no hurry on the cut). I’m about to test 12,000 RPM at a much slower cutting speed to retain quality. I’m also using a single flute 1/4" Onsrud end mill made specifically for acrylic, so I’ve not seen any melt issues as of yet.

ANYWAY - my question is whether the 7800 RPM speed was doing damage? How hot is too hot? Perhaps it’s just going to be hot to the touch at lower RPMs but still within safe mechanical tolerance?

The reason I got concerned was I knew I was at the low end of what I thought was safe for proper air cooling and, at the higher speeds I’ve been using, the spindle has always been downright cool to the touch… it barely, if ever, seems to get above room temperature.


  • Aaron

Why are you increasing rpm and lowering feed rate? Typically you want to do the opposite of that. I have a 1.5kw spindle I’ve never run it even close to that slow but I know it can get pretty hot without issues. I would stay above 9000 no need to go any slower than that. I use the same onsrud mill to cut acrylic it works well at higher Rpms just INCREASE feed rate.

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Sorry - I should have been clearer. In G-Wizard, I’m reducing feed rate because I’ve opted to increase finish quality at the same time. Yes, I realize increasing RPM does increase the feed rate accordingly… but I’m taking great pains to avoid chatter and get the cleanest cut I can get. The resulting piece is going to require polishing (if all goes well), so I’m willing to expend more time on the cut to have less hassle in hoping the acrylic can be polished back to match the pre-cut clarity.

Might be a pipe dream… but it’s an experiment right now. Once I know what is possible with obnoxiously conservative settings, I’m willing to step it up on the cutting rate to find the best balance of speed and quality.

Does that make more sense?

My main concern in my previous message was knowing the low end of the RPMs to risk damage to the spindle. Published spec says 8000 (though I know I saw 7200 elsewhere). Your 9000 sounds safer, too.

Open to any suggestions, of course! Waiting for the spindle body to cool back down so I can judge again if it’s getting too hot on the next cut.

  • Aaron