PM25MV faster than 2500 RPM spindle but with rigid tapping?

[hackish]

I've read that going faster than about 2500 on the PM25 or G0704 results in a lot of vibration. I assume this is imbalance or poor quality bearings. I'll be doing a lot of AL material on my PM25 and manually calculating feeds and speeds it appears that most times I will need significantly more than 2500 RPM.

I was wondering if anyone had insight into what upgrades would be necessary to be able to support a higher spindle speed like this? With all of that, I'd like to do rigid tapping.

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[CL_MotoTech]

Belt drive is a big help in that department which I think the PM25 has from the factory I'm not sure what people are doing to get more RPM with the PM25, but I would guess spindle motors and pulleys are the main areas of adjustmet. With my G0705 I just simplified everything as much as I could and it vibrates very little, even at 8k. Good bearings and oil lubrication are a must. The machine at times certainly vibrated, especially when I did my first upgrade to a 6k spindle, but with adjusting bearings and lubrication I was able to get everything under control. At this stage there is almost no detectable vibration.

You'll probably want to add a rotary encoder if you are serious about tapping. Depending on your control this may or may not be viable.

[hackish]

I haven't decided on the drive yet. The PM25 comes with belt drive, encoder and a brushless dc drive. I was thinking of using a servo, but they are usually limited to 3000 rpm and I think sub-degree accuracy is overkill while going that fast.

[CL_MotoTech]

My BLDC encodes at 6 times per revolution but for small tapping that really wasn't quite enough, as a result I added a proper rotary encoder with better resolution. It was inexpensive. My BLDC is good for 6k (though they make and 8k model), I just overdrive my spindle via the pulley ratio. I don't do any large tapping (m8 at the biggest). 8k really helps one 3d contours, I can up my feeds and drastically reduce run times. For most cuts though, especially with roughing, I try to keep the BLDC closer to the meat of it's torque.

[skrubol]

Most of the low-inertia AC servos I've looked at have a 3000 RPM 'rated' speed and 5000 RPM max speed. Full torque is available up to the rated speed, and full power is usually available from rated to max, though sometimes it drops off a bit at the top end.
Either way, you will can always play with pulley ratios. You'll probably want at least an index pulse on the spindle unless you're running 1:1 with a timing belt (which I wouldn't if you're trying to get rid of vibration.)

[AMays]

I know it has been several months since your post, but I wanted to both respond and see if you had changed out the motor. I can tell you that if you plan on exceeding the stock 2500 rpm that you need to consider upgrading the bearings. They are standard, cheap tapered roller bearings that reputable companies list as maxing out at about 6000 rpm. Since you don't want to run them at max all of the time and Grizzly doesn't specify them as NSK, NTN, etc., I would think that the rating for them is significantly lower. A BLDC or 3-phase AC motor driving the spindle with a belt should help with the vibration, too, so long as the shafts/bearings are adequately supported and balanced. I mill aluminum almost exclusively, and the 2000-2500 rpm limitations on these low-cost machines should be considered. Overall, I don't have major problems; I just have to recognize that I have a speed limitation. It's a hobby, not a profession, so I'm not losing money babysitting a slow mill.
There are dozens of websites, YouTube videos, and forum threads of people who have made these upgrades to these machines. Hossmachine is a good one. His website has changed, and not everything has migrated, so you may have to do some digging. He documents everything very thoroughly, though. Keep us up-to-date on what works for you!

[CL_MotoTech]

Hands down the biggest improvement I made to my spindle as far as vibration is concerned (outside of maybe the belt drive) was to get rid of the quill assembly bearings and all and then mount the upper pulley directly to the spindle. At 6k there is effectively no detectable vibration, at 8k there is some but still less than it was at 4k before. I spent $15 shipping included on the bearings, they are KML (if I recall correctly) brand. They were eBay specials. After more than a year on them they are getting noisy when I drill, I may replace them with another $15 set. I use an oil drip with Marvel Mystery Oil for lubrication, no grease. I went back to tapered bearings, no AC's. For prolonged 8k the spindle definitely gets hot, but I'm sure it's still less than maybe 160 F, so no where near the operating limit for the bearings.

[shooter123456]

My first spindle speed upgrade, I made new pulleys and got rid of the quill and 2 large bearings. I don't remember the exact pulley sizes, but it got me up to 5500. I replaced the tapered rollers with some relatively inexpensive angular contact bearings.

Next, I replaced the motor with a 2 HP AC servo with a rated speed of 6000 and a max of 7000. I haven't made new pulleys for it yet, so right now the top speed is 15,400. It can't handle nearly that speed, I keep it limited to 7500 until I can work on the spindle more. The problem I have is that the threads on the spindle aren't square, so the part I have screwed on for my PDB to grab on to wobbles. It vibrates more and more as speed increases.

For rigid tapping, you will probably want an encoder on the spindle, even if the motor has one. It will just make things simpler in the end.

[hackish]

Shooter, which AC servo and controller did you go with? At the moment I'm looking to convert to CnC before addressing the spindle.

[coherent]

I added a belt drive and upgraded the spindle bearings in mine and have had no vibration or heat issues since doing so. There are mods which allow the quill to be used manually, but I eliminated the quill and with a cnc'd machine, I don't see it as a loss. The max rpm obviously depends on the pulley sizes you choose. My goal was around 4k to offer more flexibility with aluminum. With my current pulley setup I max at about 5000. The original bearings ran hot even at lower speeds after any continuous use (even after proper adjustment and lube).

[arizonavideo]

Most of the spindles I have seen have too much preload. The stock China TRB bearing and not great but should be as good as most other bearing from China. The most common cause of failure is crap in the bearing from all the grunge in the system with too much preload.

If you wash out the old grease and make sure the preload is correct and re fill the grease with something like Mobil SCH460 most people can get 4 to 5 K out of the spindle with little problem for years. You can do this with out removing the bearings.

I have seen some spindle that were just not balanced so like a lot of the China stuff YMMV.

If you really have to have over 5K for most of your milling then just plan on going to AC bearing. They run cooler and can go 6K all day long.

The performance difference at 4K is a lot more then you might think. With a 3/8" cutter the surface finish is much better and the cutter will throw the chips out much better. Its a huge improvement.

If you plan on doing a bunch of work with a 1/4" and 1/8" cutters and lots of 3D work then you will want higher RPM.

[shooter123456]

It is a Lenze synchronous servo. Model is MCS09F60LC40B0. The drive is a Lenze PositionServo E94P drive. They are a bit overkill, but I got a good deal on mine.