Z Axis Servo Choices for PM-833 - BIG or is small ok?

Hello all!

I just took the plunge and ordered the PM-833TV…

This is my first of what I am sure will be MANY questions…

Has anyone used the small (DMM86N, 750 watt) servos on your Z axis? Dr D suggests the (DMMA-15, 1.3 KW with brake). I get that at almost 200 pounds…

I contacted Dave at AZ CNC and asked him which conversion kit to order for the PM-833TV (He has two flavors) and told him that I wanted to attach a NEMA 42 on the Z axis. He repeatedly insisted that the 86N (NEMA 32 mount) motor was more than sufficient for the Z axis. I ended up ordering the 1/2" and he said he would include a 14 mm adapter.

I am happy to spend the extra for the more powerful motor. And I can fabricate the motor plate (I have a hobby machine shop with a full size PM Knee Mill and PM 1440 lathe). I just wanted to see what you guys think.

Thanks in advance for all the help I am going to bug you all for!

Weatehrford, TX

Hello Bruce,

I do not agree with the extra power needed for this mill. I converted this mill using a NEMA 34 1700-ounce stepper. I was afraid that the mill would drop when power was shut off, so I added two inexpensive gas struts to assist. I later removed one of the struts as I found it not necessary. At a later time, while doing some modifications, I removed the existing strut and it worked without it.

Today I use it with one strut just to partially off load the z axis. The 1700 oz. stepper works perfectly, and the z axis moves at 150 ipm without any issue. See my other post “Another PM833 conversion” for full details.

Best… Richard

This motor is definitely oversized for the Z-axis, but it shared the same face plate as my previous Nema 42 (2830 oz) Z-axis stepper, so it was a drop in replacement.

One reason to oversize a stepper motor is to get higher detent or no-current torque to prevent the Z-axis from dropping when the mill powers down. Of course, you can now buy stepper motors with electromagnetic brakes or as Richard and I have done you can use gas springs to remove some of the weight off the head.

My spindle head is particularly heavy with my upgraded Y551 motor and drawbar.

Hey Bruce,

I have been thinking about upgrading to a Y551 motor and was wondering if you had to change the OEM spindle bearings. If my memory serves me correctly, I think Dave Clements from Arizona CNC told me that the stock bearing are good to 6000+ rpm if you break them in correctly. DId you replace the bearings? What is your max RPM?

Best… Richard

Dr D.

I decided to go with all CLEARPATH Nema 34 Servos. The CPM-SDSK-3441S-RLN for the Z axis with the MPC034-24-003-T Nema 34 Brake. The tech said it would do the job with no issues (I asked about the counterweight gas spring and he just said “if you want to, but you don’t need it”…and CPM-SDSK-3432S-RLS for the X and Y axis. I also am not a fan of programming so I am doing the Acorn for control. I have the Mill and a lot of the electronics and the box. I am laying it all out on the bench for fitment in the electronic box. Pics soon. Debating on replacing the spindle bearings. I have them and the oil seal (and after watching you - I ordered an oil seal removal tool !! ).

Do you feel like the Y551 motor was a good choice? I plan to use a VFD and control it from the Acorn regardless, but I haven’t pulled the trigger on the motor yet. Just wondering how you like it.

Thanks for reaching out!
All the Best!

Hi Richard !

I am just starting the build. I have the Mill and most of the electronics and the Spindle Bearings and Oil seal. I have never run the Mill and don’t plan to until I work through the build. I have not pulled the trigger on the Y551 yet either, but I am leaning that way. I am curious how much Dr D likes his. As for the max RPM on the spindle, the bearing specs for one of them state 7,500 RPM and the other is 9,000 RPM - So,I would think that 7,500 is the upper limit with those bearings.

And thanks for the info about the gas struts…I decided to go with all CLEARPATH servos and the tech also said that they were indeed, not needed.

All the Best!

Thanks… Are those the specs for the OEM bearings?

Just an FYI. I purchased a new oils seal and never replaced it. It is very hard to remove. When I bought the mill, the spindle would get hot to the touch after running at high speed, but I just ran a very long job and it no longer gets hot. I think it just has to work it’s way in.

If I remembering correctly, the OEM bearings were NTN, and I believe I purchased the same bearings from Misumi (4T-30206 and 4T-30207) once I realized that they would be fine at 5,400 RPM. Max RPM for these bearings is 6,300 with grease.

I have had absolutely no issue with the Y551 motor. I do a lot of rigid tapping with M3-M8 threads at 350-500 RPM. No issues with overheating - plenty of torque. I cut a small amount of steel, and the motor hasn’t been limiting factor rather the rigidity of the mill. With that said, I picked the Y551 up when it was $770. It is now $1250.

You can get a 2kW AC servo motor and drive for less than that. Servos have a higher power density for the same weight. The only issue would be if Acorn controller can interface with AC servo for spindle drive and you may still need an encoder on the spline shaft if you plan to rigid tap because there is a lot of backlash between the motor and the spindle.

Here is a screenshot from my video, showing the price ~ 2 years ago.

Dr. D,

I did see that the price had doubled. I am very familiar with running VFDs on my shop equipment (I have 3 - and now 1 Rotary phase converter as well) and have NO IDEA how to run an AC servo for a spindle motor, not that I am opposed to the idea. I even eyed the 3HP version of that motor, however you say that the limitation is the rigidity of the mill and not the motor power (and yeah, its too big as well :slight_smile: ).

So now I am just considering the Y551 and the VFD that you recommend…I do want to do Rigid Tapping… I may get the VFD first and run the stock motor on it until I can’t stand it, then switch…or I’ll order both and just eat the cost.

Dr. D,
Just to make sure I am understanding you correctly, the stock bearings are good for 6300 RPM with grease… Correct?

This conversation of motor selection is interesting to me. I currently have the stock motor and the same Durapulse VFD as Dr. D. I agree completely with David that rigidity is the limiting factor on this mill. I included an encoder on my build but have not tested the rigid tapping capability. Keep in mind that the stock motor has a base RPM of 1800 but actually turns just under 7500 RPM; the speed is reduced via pulleys to 3200 max RPM or about 2.3:1. If you were rigid tapping at 350-500 RPM, the motor would be turning 800-1200 rpm so I think you will have sufficient of torque.

I too primarily machine aluminum. For example, with a 1/2 inch deep cut and .0625 width of cut, you can see some evidence of vibration due to the rigidity of the mill. I have attached an image below that has been highly polished and you can see this in the finish.

I am currently contemplating either using a Black Max motor or 2KW servo motor as suggested above. The stock motor is rather loud and annoying on a long run. The advantage of the servo is that it weighs almost nothing and much less expensive. I found a few posts on the Centroid forum so I believe I can control it with Acorn. The positive of the Black Max motor is that it will be controlled exaclty as the stock motor so there nothing to modify electronically.

Some folks have suggested that the servo will not have the low-end torque of the Black Max motor. I am still investigating this.

To keep this conversion simple, I want to use the original pulley that is mounted to the spindle and put a gates pulley on the motor/servo of similar size. I’m not sure if these are available. I will also have to be creative with the motor mount to accommodate a pulley with such a large diameter.

I have limited knowledge on this topic so any suggestions are welcome.

Thanks… Richard


WOW ! That part looks FANTASTIC ! Well Done!!


Dude! A 500 thousands deep cut !? OK 62 thousands wide, but Still…Aluminum or not, THAT is a VERY HEAVY CUT ! I have a full size knee mill and I have never cut that deep with it! Sounds pretty rigid to me ! :slight_smile:

And I agree about control… I am leaning toward the Black Max…

All the Best,

Thanks for the kind words! This is a new hobby to me, so I have little to compare with how heavy I am cutting. I used the G-Wizzard calculator, and it had a feed of about 12 ipm at 3000 rpm with very little HP needed so I thought it was an easy cut. To be honest, I tried the same .5 depth and .03 width on a turner’s cube and go the same vibration.

One thought… my VFD has a breaking resistor to help with deceleration. I’m not sure how a servo would handle deceleration. I will research the servo a bit further and share what I find.

Best… Richard

It is very nice to just plug a motor into a VFD and have it work. Problems with servo motors are more difficult to troubleshoot.

I am 80% sure this is correct. Unfortunately, I can’t find any photos of the bearings that I pulled out. But when I was searching for bearings most of these “lower-end” thrust bearings are rated for at least 5500 RPM.

I have not seen an evidence of this. Similarly sized servo and induction motors should have comparable torque below the base speed of the induction motor. The AC servo will have a higher torque beyond the base speed of the AC servo motor, which will trade torque for RPM.

Very nice parts indeed!

Thanks David. I just started to research belts and pulleys. Something I know very little about. Do you have any specs on the stock belt. Mine is printed with JC Power 540-5M, which I couldn’t find any info on.

My initial goal was to leave the stock spindle pulley as is and fit a pulley to the AC servo. However, since the spindle pulley has about a 100mm diameter, then the pulley on the servo would have to be 200mm to double the servo speed. I don’t think this is feasible. Are there pulleys available that would allow me to replace the spindle pulley with a smaller option? I believe the spindle pulley has an ID of 28mm.

Any info you can share with respect to this topic would be appreciated. I noted that you used a somewhat expensive option on your build. Was this the only option you could find?

I reviewed your video on the BlackMax install. Would you be willing to share your motor mount Fusion file with me? This would eliminate the need for me to remove the motor to take dimensions. If you are not comfortable sharing… Not problem at all.

Also, for those interested in using these low cost AC servos with Centroid Acorn; here is a great link:
https://centroidcncforum.com/viewtopic. … rvo#p67067

Thanks… Richard

What specs are you looking for? I believe that is a High Torque Drive (HTD), 5mm pitch and either a 15 mm or 20mm width with a 540 mm circumference.

Unfortunately, if you want something smaller then you will likely need a custom pulley. I have a writeup on my guide about the TL pulleys that I used, but the versatility of this design requires large pulleys to be used, so may not be ideal for your application

If you go to the project page for my mill conversion (Manual Benchtop Mill to CNC Conversion) and scroll down the the Resources in the Downloadables you will find both the STEP and .F3D files for my motor mount. This will work for any motor with a 145TC face plate.

I pulled the trigger and ordered the VFD and motor yesterday. Motor is on back order. Oddly, I received the VFD and resister today. Damn fast !!
All the Best,

1 Like

Pulley Research… here we go:

  1. Madler North America offers Taper Lock pulleys that are similar to the McMaster Carr option but less expensive. For about $100.00 you can get a set of pulleys and bushings. They are HTD 8M pitch. McMaster Carr are Poly Chain design, also 8M pitch.

  2. Since I need about 1:1.80 ratio, the taper lock pulleys would be physically too big to fit the envelope of the PM833-TV. BeltingOnline.com offers plain bore pulleys that for literally a $3 or $4 dollar upcharge they will bore and cut keyways per spec. I will be ordering:
    -40-4M-25F for the spindle
    -72-5M-25 for the servo
    The total cost with both pulleys custom bored and keyed is about $130.00

This will give me a max spindle speed of 5400-6000 rpm.

One possible problem I am thinking about is that the spindle pulley has a hub, which makes it 38mm tall. This may preclude my ability of using the threaded flange that hold down the spindle pulley. Still thinking on this one. I may bore/tap four 8M holes for grub screws on the pulley.

David, one question. I downloaded the motor mount you used for the Black Max motor. I am new to Fusion 360 and trying to learn as much as I can. I noticed that as you created this bracket, you joined each successive body to the previous body as opposed to creating a new body. Is there some reason that you did this? How did you CAM each piece if it was all one body? Just curious.

Thanks… Richard


I am looking for the encoder belt specs (part number?). I got 2GT-280, 2mm pitch but…

And an .stl or preferably a .3mf file for the encoder pully that you made. And did you just let the belt run on the encoder spindle ? Oh and the encoder mount file if you don’t mind sharing as well.

All the Best!

Good morning Bruce,

I modified Dr. D’s encoder concept just a bit. If you look at my post (Another PM-833TV Conversion - CNC Mill Conversions and Builds - Dr. D-Flo (drdflo.com) you can see that I used the same 3D printed timing pulley but mounted the encoder differently. The post also explains how I modified the shape for the timing pulley for easy 3D printing. See link below for the encoder, timing belt, timing pulley and both Fusion 360 and Step files. I used a different encoder than Dr. D. It works well with Acorn. Let me know if I can help in any way.

Best… Richard

Thanks Richard,

I’ll check them out. And Merry Christmas!

All the B best!