Very hot LMS 3990 motor

Hi guys,

I have a CNC converted LMS 3990. I built the control system myself, changed over to 300in-oz holding torque steppers and ball screws, running GRBL, using Fusion 360 for CAM, have been happy with the results thus far.

This weekend, I was running a .375" square EM 2 flute HSS at 2500 RPM, about 12-14ipm, about .030"-.040" depth of cut in a 1.5"x2.5"x7.5" billet of 6061-T6.

The 2D adaptive clearing would take a variety of cuts, many were light, there was only one pass per level that was even somewhat 'heavy' but I'd say there were 10 minuts or so between this pass. I was blowing chips out manually, and using WD40 intermittently during machining. Work piece was not getting hot during machining. Overall the part looks great thus far, and I'm happy with the results.

I noticed the 500W brushless spindle motor was too hot to touch, the head near the spindle was hot to the touch (I could still hold me hand on it), and the column was even getting warm.

My question is for other 3990 users, preferably those with CNC conversions, is this kind of heat normal? I was a bit worried to be honest, and fired off an email to LMS customer service to make sure there isn't something to be worried about. I don't feel I was pushing the machine too hard. The Chinese HSS end mill still looks great, literally brand new looking after the 2.5 hour operation so I don't think the speeds and feeds are a problem. I also don't think the depth of cut was too aggressive, so not sure why it's getting so darn hot.

Any help is appreciated. I found some other threads on this forum about the belts possibly rubbing in the head, or too much preload. It sounds like the poster was mainly discussing this with the 'high speed' IE: 5000 RPM belt conversion kit. I am using the standard setup with 0-2500 RPM spindle.

Just to update, I have contacted Little Machine Shop and also done some reading online.

Looks like the general information I have seen is the spindle motor should be in the 140-200F range running at full tilt (2500 RPM). 200F was identified by LMS tech support for multi hour operation.

I have a touchless IR gun I will use to monitor this temp next time I run the mill.

If it's still too hot, I will look at the bearing preload and also the belt tension. If it's not running too hot, I won't even bother messing with the belt drive or bearing preload nut.

Another update...

Looks like the brushless DC motor generally runs in the 140-150F range during a multi hour run. The head of the machine generally runs in the 110-130 F range. I am using touchless IR gun with emissivity set at .95 aiming at the stickers on the motor and on the head.

I think I will try loosening and snugging the spindle nut. I have seen some info on the net about bearing preload.

If you can comfortably hold the motor / spindle block - it ain't too hot!

That's the thing, I cannot touch the motor for more than a second or so without being burned after it's been running for a while. I think that's actually normal. I think some of the heat may be transferring from the motor into the head because it's all metal (there is no plastic gasket or anything to isolate it).

As far as the head, after running for hours is uncomfortably hot but not burning hot.

I loosened the spindle nut and retightened and just barely snugged it down. I also loosened the motor mounting plate screws that allows it to move back and forth in the slots and put the slightest tension on the belt.

I ran 2 OP's after doing this but they were coming back in with a smaller diameter tool to just clean up corners and such so the load wasn't that high. Seems like it may have helped but not that much.

Either way, I have emails to LMS so if something craps the bed I at least have documentation that I tried to ask them for help.