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My PM-940 CNC Modifications

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cut2cut

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I've got some good news... ok, so it is a bit of both, good and a far bit less bad news. If you are like me, its good news, because I found what I believe to be the reason my mill has more backlash than it should and it is fixable. The bad news is, it probably will cost 6 angular contact bearings and milling some new bearing / stepper motor mounts to do this the "right" way however there may be a way to do it inexpensively too ( see below ). In explaining this I am going to focus just on the X axis but this should apply to all three axis. On my PM 940 CNC the X axis stepper motor is on the left side of the "table". A nut is used to tighten a set of bearings that fix the end of the ball screw directly to the "table". This nut, when tightened should apply enough force to remove backlash but the bearings should allow the ball screw to rotate without any friction unless the nut is overly tight. I had tightened mine up sufficiently to ensure I *should* have a minimum amount of backlash defined by the balls and the screw itself. The ball screws in the PM940 are supposed to be be of a good grade, so I was hoping for the best. Sadly, I'm seeing a minimum of .002 of backlash. I kind of put it out of my mind as other things needed attention but it always bothered me that my mill had a significant deficiency. HOWEVER, I remembered something I read about ball screw ends from china sometimes are designed poorly. Basically they use two bearings but stupidly the inner races are allowed to touch each other. Basically the two bearings are back to back, acting as one bearing. So even if they were two sets of angular contact bearings, if the inner races are allowed to tighten against each other they have no value other than allowing low friction when turning. The bearings side play is then what defines the amount of backlash. Check out my little video that shows the two bearings spinning together with just a small amount of force squeezing them together by hand.
Also, they don't appear to be angular contact...

One additional detail is that the right side of the table also has a nut assembly that holds the hand cranks. When I first got the mill I removed all the hand cranks and nuts to ensure that the ball nut assembly would not be bound up with opposing force.

There is one possible way to do this on the cheap and use the existing castings and possibly the existing bearings. Just find a sufficient washer and place it between the large outer races. I haven't tried to remove the existing bearings because they appear to have been pressed in with no way to press them out without damaging ( pressing via the inner races is not good, but maybe necessary ), I haven't tried. I'm going to order replacement bearings before I try to remove them. Then I'l at least have a way to machine something that willl replace the castings, etc, if needed.

sleepy... gnite

Jake
 
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phazertwo

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Great post!

Careful with those bearings you spec'ed... doesn't look like they are sealed.

PZ
 

cut2cut

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Great post!

Careful with those bearings you spec'ed... doesn't look like they are sealed.

PZ
Good point. I wasn't thinking about the exposed side ( toward the table ) !

btw, I was thinking that maybe you might need to remove the hand cranks as they might be causing some additional drag. If you have little backlash it might be that both sides are opposing each other, giving you very little backlash, but could wear out your ball screws quickly... if that is indeed what is happening. Hopefully, yours is just fine the way it is though...

Jake
 
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phazertwo

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I defiantly want to dig into mine. When I bought this I bought it as something that I hoped I could upgrade to be on par of very near a Tormach 1100... So motor/ball screw mounts are in my future for sure.

PZ
 

cut2cut

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Ok, some rather good news related to the backlash fix. As noted above I dug into my X table ball screw end bearings on the left side. Tonight I pressed out the existing bearings using a harbor freight hydraulic press. Added a McMaster 35mm ID 45mm OD spring steel shim in **between** the two bearings Large OD races of the existing angular contact bearings ( a side note... one had its shield removed so it was exposed so it was pretty dirty ). Then I pressed the old bearings back in sandwiching the single shim in between the two bearings and voila... I didn't do much of a test due to lack of time but my X axis is now below .001 for sure and could very easily be at or below .006 , whereas it was clearly .002 before. So I'm pretty happy now. New bearings on the way and I'l do the Y and Z when they get here.

Note, I still believe it may be necessary to remove the "other side nut and hand cranks or at least loosen it to allow it to have lots of slop. Otherwise you could end up ruining the ball bearings or the ball screw itself.

and here is the link to the the 35mm ID x 45mm OD x 1mm shim I used :

https://www.mcmaster.com/#98055A346

Cheers,

Jake
 

phazertwo

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Well this is good news! Did you order the bearings from VXB you linked above?
 

cut2cut

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Well this is good news! Did you order the bearings from VXB you linked above?
Yes, hopeful they will get me by until I have time to make new bearing holders, then i’ll fork out the money for higher quality AC bearings with shields.
 

cut2cut

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TomS,
Not much to see really, its fairly simple. My first posts on this thread actually outline what I used to implement the pulley and belt system. The only "tricky" part was to create a key for the splines on the spindle. ( Whoa ! splines and spindle use the same letters !!! Spooky ! )

Here are a few pictures. The secondary pulley ( red 3d printed part with a small belt ) is for the encoder for rigid tapping. I'm still working on that... so stay tuned....

Nothing sexy yet... once I get to rigid tapping I'll be cranking more stuff out to make the mill look better.

Cheers,

Jake

IMG_0997.JPGIMG_1003.JPGIMG_0999 2.JPG

IMG_1001.JPG
 

TomS

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TomS,
Not much to see really, its fairly simple. My first posts on this thread actually outline what I used to implement the pulley and belt system. The only "tricky" part was to create a key for the splines on the spindle. ( Whoa ! splines and spindle use the same letters !!! Spooky ! )

Here are a few pictures. The secondary pulley ( red 3d printed part with a small belt ) is for the encoder for rigid tapping. I'm still working on that... so stay tuned....

Nothing sexy yet... once I get to rigid tapping I'll be cranking more stuff out to make the mill look better.

Cheers,

Jake

View attachment 257212View attachment 257214View attachment 257215
Thanks for the pictures. Looks like you have eliminated the spindle drive sleeve with your setup and "locked" it in position. I'm still using the drive sleeve because I need about an inch of spindle travel to actuate my tool changer. Guess I'll have to live with the spline noise.

Not my mill but the configuration is the same.
pulley3.jpg
 

cut2cut

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Thanks for the pictures. Looks like you have eliminated the spindle drive sleeve with your setup and "locked" it in position. I'm still using the drive sleeve because I need about an inch of spindle travel to actuate my tool changer. Guess I'll have to live with the spline noise.

Not my mill but the configuration is the same.
View attachment 257253
That looks nice. Yeah, if I wanted the quill action, I'd have used this method and not locked down the quill movement. Does it allow the use of a power draw bar ?
 

phazertwo

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Hey Jake, I don't suppose you could get the shaft dimensions off the old motor? I picture would be great too.

Thanks!
PZ
 

cut2cut

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Hey Jake, I don't suppose you could get the shaft dimensions off the old motor? I picture would be great too.

Thanks!
PZ
Shaft diameter is 24mm.
Shaft length is roughly 45mm
Roughly 166mm between holes ( very rough +- 3mm ) no time to check further right now
200 OD of motor plate
130mm inner OD of motor plate ( ridge )

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phazertwo

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Awesome, thanks!

I think I'm going to give it a shot with the factory motor!

PZ
 

TomS

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That looks nice. Yeah, if I wanted the quill action, I'd have used this method and not locked down the quill movement. Does it allow the use of a power draw bar ?
Yes, you can use a PDB with my setup. I have a Ultron Snap Change manual quick change tooling system on mine. Nearly as fast as a PDB.
 

cut2cut

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Doug, I totally forgot to post these pictures of the transmission cavity. Here they are. Sorry for the delay !

( the red 3d printed thing is to drive a belt to my spindle encoder )

IMG_1022.jpg

IMG_1024.jpg

IMG_1027.jpg

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IMG_0999 2.JPG

IMG_0997.JPG
 

pburgh

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Thanks Jake,
No problem, I was wrestling with my lathe all weekend, so I haven't done much with the mill. Is the collar with the phillips head screws below the spindle pulley where the spindle sleeve sits that has the small gear (33) attached? Also, do I understand correctly that TomS' pic about can use the quill feature?

Regards,
Doug
 

TomS

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Thanks Jake,
No problem, I was wrestling with my lathe all weekend, so I haven't done much with the mill. Is the collar with the phillips head screws below the spindle pulley where the spindle sleeve sits that has the small gear (33) attached? Also, do I understand correctly that TomS' pic about can use the quill feature?

Regards,
Doug
Yes, with my belt drive conversion setup I have about 1" to 1-1/2" of quill travel.
 

phazertwo

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Hey Jake, anyway you could snap a picture of the name plate off your motor... The way mine is mounted it's hard to read.

Also, do you know what the gear ratio was for high or low?

Thanks
PZ
 

cut2cut

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Hey Jake, anyway you could snap a picture of the name plate off your motor... The way mine is mounted it's hard to read.

Also, do you know what the gear ratio was for high or low?

Thanks
PZ
C66420E3-C032-4F34-ABB2-5BC952188529.jpeg
 

cut2cut

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Yeah, I was wondering if the site was causing huge issues , or you were having huge issues with the belt conversion. Actually, that part is significantly easier and less stressful than replacing the bearings in the spindle, imho.
 

phazertwo

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Thanks for the pic!

Yeah, I was wondering if the site was causing huge issues , or you were having huge issues with the belt conversion. Actually, that part is significantly easier and less stressful than replacing the bearings in the spindle, imho.
Think I'm going to do the bearings at the same time... I'm thinking AC bearings will be better for the PDB.

PZ
 

cut2cut

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Ok, long time, no see ! Well, here we go, POWER DRAW BAR TIME ! I've collected a bunch of the necessities and now need to mill the parts. I am in need of someone that can make a steel "top hat" for the belleville washers since I don't have a capable lathe and not much experience with it either ! So anyone out there, please let me know if its in your wheel house and can do it for me !? ( see attached drawing ( in inches ))

I'll be posting details later today but here is the render of the model. Its adding some weight to the head, so I'll be using struts on the head to take some of the load away. Details to come !

Cheers,

Jake

Screen Shot 2018-10-31 at 12.34.45 PM.jpg

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phazertwo

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popcorn.gif

I'd reach out to Shooter123456. He's pretty handy on the lathe, and seems willing to help!

PZ
 

cut2cut

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Haha, love the popcorn gif. i’ll see if mrshooter can help. Much appreciated.

Cheers,
Jake
 

cut2cut

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Welp, this Power Draw bar project has been kicking my ass. I think I have an issue with my Z axis stepper which I'm in the middle of trouble shooting. So its putting the possibility of me upgrading to DMM servos to the forefront. ( Phazer, calm down.... Don't eat that popcorn too fast, I wouldn't want to be the reason for you untimely demise ! :)

So, my question is, is it worth the extra money to go for DYN4 drivers, or just go with the cheaper, and less powerful DYN2 drivers ? Also wondering if the 750Watt will be adequate for the Z or will I regret it. Likely I'll add a counterweight as the gas struts I just added don't seem to be doing the trick so that would help a lot. It is still possible I could be facing a g-code gremlin. I will say, my Z axis has always been noisy and the Z ball screw has a whip to it so there are many things that could be adding up to my Z issues. I'm tempted to also buy a Z axis double nut kit from ArizonaVideo but really wouldn't look forward to installing it. That PM940 column and head is heavy and tough to deal with. Alas, servos are the way to go so I figure I may as well go for it now and enjoy the benefits regardless of what the real issue is with the Z.

Jake
 

phazertwo

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Zn0ny8E.gif

Well, I'm equally excited to watch some DMM servos get hooked up to a 940!!

I think the Leashine 86HS85 stepper on our Z axis actually only produces about 2-3 nm of torque at the speeds and resolution we run it at... So I would want something better than that on my Z. I've been thinking a 1.0 KW or even the 1.3 KW and don't worry about a counter weight. or gas struts. Though I also want my Z to be able to move at least 100 ipm, preferably even faster. I would also do 0.75 KW drives on my X and Y... I'd rather have too much than end up wishing I had bigger motors later.

As for DYN 2 vs 4... i'd go with 4. They can be programmed to turn an output on if they loose position, which would be very nice. Also, they run on 240 VAC, so there is no need for a big DC power supply, which nice for a few reasons. Not only does that mean less wiring, but it means more room freed up in the cabinet. Only thing that might mess that up is the 24 VDC holding break... not sure if needs an external 24 VDC. I'll have to do some more reading on it. Please keep in mind that I have priced NONE of this out, I have only read spec sheets and am dreaming big.

PZ
 

cut2cut

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View attachment 280498

Well, I'm equally excited to watch some DMM servos get hooked up to a 940!!

I think the Leashine 86HS85 stepper on our Z axis actually only produces about 2-3 nm of torque at the speeds and resolution we run it at... So I would want something better than that on my Z. I've been thinking a 1.0 KW or even the 1.3 KW and don't worry about a counter weight. or gas struts. Though I also want my Z to be able to move at least 100 ipm, preferably even faster. I would also do 0.75 KW drives on my X and Y... I'd rather have too much than end up wishing I had bigger motors later.

As for DYN 2 vs 4... i'd go with 4. They can be programmed to turn an output on if they loose position, which would be very nice. Also, they run on 240 VAC, so there is no need for a big DC power supply, which nice for a few reasons. Not only does that mean less wiring, but it means more room freed up in the cabinet. Only thing that might mess that up is the 24 VDC holding break... not sure if needs an external 24 VDC. I'll have to do some more reading on it. Please keep in mind that I have priced NONE of this out, I have only read spec sheets and am dreaming big.

PZ
The sales guy at DMM has been very helpful. I asked about DYN2 vs DYN4. Basically the DYN2 drive powered by our existing 48VDC power supply with their 86M-DHT .75kW servo would run at 1200rpm. With our 5 turn per inch ball screws that equals 240 ipm. ( I could be wrong and our big DC power supply provides 60VDC ? which would give us 300 ipm ) Basically speed is directly related to the voltage. The DYN4 drives apparently double the speed by being 220/240 V AC but they have no other performance advantage over the DYN2 other than top speed. They both have the same holding and "cutting" power. I perceive this as being that the motor is the limitation, not the speed control, however voltage dictates rpm, so the DYN4 has the advantage of 220v AC but if you don't intend to go faster than 240 ipm, I am told there is no advantage. I, however, have opted for the DYN4 due to exactly what you said Phazer, being able to remove the existing big DC power supply and basically a cleaner wiring and if ever, being able to use the equipment being more versatile if used on another setup. The DYN4 drives are fairly large, so its almost a necessity to fit them in the existing cabinet. I do plan to add a counter weight, but regardless, will also pay the hefty cost for 24v brake option as added insurance if the counter weight ever should fail it could really result in a nasty crash. The added benefit of adding a Z counter weight, is that I will be able to get the .75kW ( 750watt ) motor for all three axis. Also, they have servo motors with 14mm shafts, so it should basically drop right into our pm940cnc with little to zero fuss as the 4 mounting holes are with 2mm ( on the diagonal ) of being correct, I don't see this as being a "show stopper" however could require some small modification to the servo motors to make it fit.
However, if you go with the "N" version ~ 86N-DHT the bolt pattern is exactly the same so it drops right in with only one issue, the shaft is 1/2 inch, which requires different shaft couplers / "lovejoy" couplers. I think VXB bearings has the shaft couplers needed for $50 each. ( Correct me if I'm wrong, but I measured 14mm on the motor side and 16mm on the Ball screw, at least on the X axis. ) If they are 14x14 then they can be found much much cheaper on ebay but expect them to take longer to arrive from China. All in all, the DYN4 DMM Servo setup should cost around $1500 to $2000. A few hundred less if you go with DYN2.

attached is what I milled last night, one of two for the PowerDraw bar project.

ok, enough for now, gotta join the thanksgiving festivities...


Cheers,
Jake

IMG_0633.jpg
 

phazertwo

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One other note, at least on my machine, the power supplys are just transformers. The Stepper drives run off 60VAC, which means I would need to get a big DC power supply if I wanted to run DYN2 drives. The smaller of the two transformers just provides 24 VAC for all the Chinese control circuitry.

PZ
 
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