# Pm25-mv Conversion Planning



## sbx (Oct 17, 2015)

Hello, 

So, I am starting to document and get my plans together to convert my PM25-MV mill to CNC. I'll start a separate build thread in the Build log section, but wanted to post in the general section to help get some ideas together and solidified. Hopefully you all can help clear up some of the remaining confusion. 

Grand Vision: 4 Axis, Spindle control, flood coolant, full or mostly full enclosure, power drawbar and utilize TTS for toolholding (ER20 - Already in use).

Software: LinuxCNC (already familiar with Linux, hate windows) and want to incorporate a handheld game pad style pendant. 

Now, here are the couple main questions I have confusion on, that I hope for help. 

1. What tradeoffs are there for stepper motors to the ballscrews. I've seen builds using timing belts and others with direct connecting couplers and standoffs. But, I've never been able to pin down the benefits of one way over the other. Just not sure how I should start planning or where to look for more info. 

2. I've been looking into the Mesa 5i25/6i25 FPGA I/O cards as opposed to dealing with parallel ports and old PCs. Thinking of building up an mITX Atom (or equivalent) based PC with the Mesa card as the primary motion control interface. If I do this I am somewhat confused about which stepper diver(s) that would be optimal. I think if I understand what I've read, this would also eliminate or equal the performance of having a ethernet smoothstepper involved. Thoughts? 

3. Should I build the full enclosure from the start? Or build the mill, then finalize the dimensions and go forward building a full stand enlosure etc. The latter seems the most sensible, but also could be a pain disassembling a working set up. 

Or, just chime in with " I should have done this first or better." 

Thanks for any input you all can provide, 

sbx


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## T Bredehoft (Oct 17, 2015)

I enjoy my PM25, manual, spent 10 years punching in G Codes for CNC machining, my day  job. Having said that, I would start with a larger, heavier machine. I must assume you're planning on powering the head movement.  If so, build in an automatic column clamp. My PM25 moves .008 when I clamp it.  Just my opinion. 

Tom


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## sbx (Oct 17, 2015)

Tom - When you say Automatic Column clamp, are you meaning locking the milling head to the column? Based on the research I've done the stepper/ballscrew can hold its position well, hopefully eliminating the need to have a clamp. None of the BF20 type conversions I've read about incorporated anything like this that I can tell. I do agree that when I engage the column lock on the head that it can move and is critical to do whenever doing any motion in X and Y and wanting to maintain any sort of static Z position. 

Regarding a more rigid machine. If I had more space and/or my own house, I would have gone with an RF45 type. But I am currently renting a shop and have no idea what size space I will have in the future. So, the PM25 was chosen for being compact, 110v, and still somewhat movable.

Thanks for the input though. Will look into the z-clamp/ rigidity issue.


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## brav65 (Oct 17, 2015)

1.  My understanding is the advantage to using a motor and belt is the increase in speed that you can move the axis at.  I would thing that a direct connection would decrease the backlash in the syste, but you are limited to how fast you can move the axis.  In a hobby situation accuracy would seem to be more important than speed.

2. Not familiar with those options

3.  I would wait and keep your options open to modify your build as you go then build the enclosure.

Check out Hoss Machine he has plans for conversion of a G0704. You can mill all of the motor mounts, ball nut mounts etc. with your manual machine than make your conversion.  I ordered balls crews from Linear motion on eBay. The prices were very good and the parts appear to be of nice quality. I am working on a CNC router project that has stalled a bit due to work and family. I also ordered Wantai stepper motors, power supply, controllers and control board.

If all goes well with the router I may take a stab at doing my PM-25 as well. Good luck!


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## Rescue35 (Oct 17, 2015)

Sounds like a solid plan and good advice. For a controler, have you thought about running linux on a beaglebone black with a Probotix PBX-BB-BeagleBone-Breakout-Board? I am considering this for my upcoming lathe conversion using a g540 driver.

I like direct drive steppers for the simplicity and rigidity.


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## sbx (Oct 18, 2015)

Thanks for the suggestions so far!

Brooks - I am well aware of the Hoss site and plans. Plan to purchase them as additional reference. I am still unsure on the differences if any between the G0704 and the newer PM25-MV, but think I can figure it out. 

Rescue35 - I haven't looked into beaglebone too much, but did see some interesting arduino stuff related to the TinyG board. But, I think there are quite a few limitations to them with either LinuxCNC or it just being too new. The Mesa cards look slick and have a bunch of features all ready designed to integrate with LinxCNC like separate Spindle control, and no need for extra breakoutboards. So, that is why I went there. 

The more I think about the coupling concept, I think I'll just start with direct couple Oldham style connectors. That part can always be redone later without disassembling the entire mill.


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## jbolt (Oct 19, 2015)

1. Timing belt drives allow for mounting the motors in a more compact profile is space is a concern. You also have the ability to gear the motor up or down. Belts and pulleys need to be fully protected from chips, coolant etc. 

Direct drive with the proper zero backlash couplers are easier to mount (less parts to make) and are more tolerant to chips and debris. Always protect from coolant. Direct drive will require your enclosure to be larger to accommodate the extra length of the motor/coupler/shields. Direct drive with dual shaft motors can be setup with hand wheels for manual use. 

2. Linux/Mesa is a great combo if you are familiar with Linux and don't mind doing command line entry or a little coding. We are running a Linux/Mesa combo on the CNC router we built for the High School. Ours is a servo system but it is very robust and much more configurable than a Mach3/windows system. I'm a Linux hater so I don't operate the machine. 

For a Windows based system the PMDX BOB's and Ethernet smooth stepper is an excellent combo with excellent support. PMDX also has some new offerings that may be even better. I'm running Chinese drivers and power supplies on my PM932 conversion and Gecko drives with a toriodal power supply on my CNC router. Of the two I prefer the Gecko drives. More robust and excellent support. I have had to replace two Chinese drives but I also push the limits of the machine.

3. Not necessary to build at the same time but it may dictate layout of other components such as one-shot oil systems, coolant pumps/supply lines/drain/holding tank, control box, etc. Plan ahead.

Some people like full enclosures. I kept mine simple to allow for running parts wider/longer than the enclosure will allow. 

Good luck with your project!

Jay


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## charlton (Oct 28, 2015)

sbx, do you have the newer PM25-MV that comes with the brushless motor and belt drive? I'm considering getting this mill for CNC conversion but I have read in other forums that it's not a Weiss machine and may have differences from the standard G0704/BF20. Are you planning to make the mounts etc. yourself or buy them from a kit? I'll be interested to know what you go with.

Cheers!


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## brav65 (Oct 28, 2015)

charlton said:


> sbx, do you have the newer PM25-MV that comes with the brushless motor and belt drive? I'm considering getting this mill for CNC conversion but I have read in other forums that it's not a Weiss machine and may have differences from the standard G0704/BF20. Are you planning to make the mounts etc. yourself or buy them from a kit? I'll be interested to know what you go with.
> 
> Cheers!



I have the PM-25 MV and it is in fact different in many small ways.  The most critical would be the end plates that secure the lead screws.  They are different than the G0-704. If you plan to make your own parts it would not be a problem.  If you buy a kit they would probably not work.  The other consideration is the spindle bearings are not designed for the higher RPM's used in CNC and would need to be replaced.


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## charlton (Oct 28, 2015)

I would prefer to buy a kit since I have no experience with doing any CNC conversions so I worry about messing things up colossally.  I understand that the bearings are probably not the tapered roller bearings but I wasn't sure why CNC would require higher RPMs from conventional milling. Are the spindle and feed speeds not typically dictated by the material and cutters?

Anyway, I don't want to hijack SBX's thread. I'll be watching attentively. 

Cheers,
Charlton


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## sbx (Oct 29, 2015)

Charlton, you're not hijacking at all. The more input the better, as this post was mostly to bat around ideas for the conversion of this mill.

Yes, it is the newer one, purchased this summer. The primary differences are the belt drive, brushless motor and a larger base to column mount (column bolts down to base vs G0704/Weiss bolting to the side). I still need to do some more research on exact differences, but do believe that Brav is right about different brackets being required.. I also thought that the spindle bearings were upgraded as well but not sure on this one. I'll worry about power after the main conversion is up and running.

I am thinking of going with a kit for ballscrews and mounts, mostly due to 2 factors.
1) My south bend 9A Lathe can't fit the ball screws through the headstock to machine the ends of the most commonly used size for the Z (20mm), and I don't own a stready rest, nor do I understand how I'd hold it and machine the ends.
2) Due to the lack of documentation on converting the newer PM25s.

There is a guy on another forum that was going to put a kit together, so I may go this route. I also may try to figure out the Z dimension and order it machined for me or pay a local guy and machine the rest myself using the Hoss plans as a rough guide... Just not sure yet. I'll pass on info for a kit if I can get more info on it.


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## charlton (Oct 29, 2015)

Would that person be ArizonaVideo (Dave)? I PMed him last night to get some feedback from him about the mill and to see if he had plans to put together a kit. I know there were some kits about the TM25's having holes that seemed to be drilled rather randomly (possibly with a hand-drill) though this also seemed to be the case with some of the Weiss machines as well...this would presumably make kits harder to make. Perhaps we can suggest a kit that's machined but that doesn't have the mounting holes drilled.

Cheers,
Charlton


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## sbx (Oct 29, 2015)

Charlton, Yep. Same guy. There is a post on there showing an installation of one of them in the benchtop section.. I wanted to email him to get some more info. The brackets look real nice, and probably way better than I could do now without a DRO on my machine. I emailed him once and he confirmed he had a kit, but haven't yet followed back up. His Kit was exactly for the PM version per his words in the one email we've exchanged. I didn't specify because I wasn't sure of the details yet.


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## chevydyl (Oct 29, 2015)

My take on ball screw drive, I use a 3:1 drive ration since my servos spin 5000rpm max, don't need to spin them ball screws that fast. It also multiplies the torque by 3, it also increases the resolution per step into the millionths of an inch. I could have used MUCH less powerful servos to get the job done, mine are powerful enough that it will easily break stuff if asked to lol, my motor drive combo is good for 1hp, 3hp at the screw, and around 12lb-ft of torque at the screw. All that is nice, but belt drives are not backlash free, very close with the gt2 profile. I have yet to see backlash from my belt system. Also belts wear out, I put a good amount of tension on mine. If you go direct drive you need to make sure tell motors you choose are going to be more powerful than you think you need, you risk losing steps with steppers being underpowered.


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## joshua43214 (Nov 5, 2015)

I can shed a bit of light on some of the stuff on this thread.
I had a PM25-mv, it is a great little mill - a very impressive machine for it price point and size. Sadly, the column in mine was tapered making it impossible to raise the head when the gibs where properly adjusted at milling height. Matt ordered me a new column, then offered me to exchange or even upgrade it so I would not have to wait. I opted to buy the PM450G which has about the same size table, but is an entirely different class of machine. So the first major difference between Grizzly and QMT, is that as good as Grizzly customer service is, Matt will probably always do better and with less trouble.

On the subject of Weiss, there really is no such animal. I also thought some of these machine where simply Weiss machines that are rebranded, when I asked Matt about it last year, the response was surprising. It turns out that Weiss is just a handful of guys in an office. They just buy machines from Chinese factories and resell them just the same as Northern Tools does. Apparently there is nothing special about the brand beyond what ever custom options that ask for.

The PM25-mv has ABEC5 roller bearings in the spindle. They are pretty massive for the size of the machine and will handle pretty high RPM. The quill though is not of the same quality. You will lose a great deal of the accuracy of the bearings under the heavy pressure that CNC produces because the quill is not tight. I would work out locking the quill before I worried about the bearings.

The y-axis trunnion on the PM25 is very narrow. This means that the mill's work envelope is really effectively about 7" x 7". When you get to about 5" off center, the table sags and the either you lose z-axis accuracy if the gibs are loose, or the table becomes hard to move if they are tight. This is not really a big deal, since this is really a small mill and meant for small parts. You will want to consider counter weighting the table if you are concerned about table tilt on wider parts.

My understanding about Arduino is they lack the bandwidth required for good milling. Not only do they not have enough head room for closed loop operation, they are slow. Better to stick with one of the chips from Texas Instruments. TI has a really amazing line of chips that are designed solely to make motion control cheap and simple. I am talking stuff in the <$10.00 range for a chip. For a bit more you can get a full fledged motion control chip with a full library on board that will do far more than you can imagine.

My own opinion (that will probably get shot down), is don't waste your time and money on an open loop system. I do not for a moment buy the argument that a well designed and build open loop system does not loose steps. This is an engineers fantasy in my opinion. The cost of closed loop systems has come down so much in the last few years that there is simply no reason to not use closed loop.
Check out these folks, I know many people like them. They certainly have good prices and they do respond decently well to emails.
http://www.automationtechnologiesinc.com/


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## sbx (Nov 16, 2015)

Ok, so you've officially put a bug in my ear about the closed loop. I had it in my head already, but its worth thrashing the details around.

So, "closed" loop seems to be a bit of a gray area with many ways of going about it.. I know some systems are fully and redundantly closed. What I think this means is the servo systems have both encoders at the motor, and also glass or other scale systems employed for absolute position feedback. I don't think I need to go that crazy for this mill conversion, but am interested in adding some closed loop, or what I think may be towards that direction.

So, in a simple closed loop I think I would need a Servo or Stepper with an encoder at the motor, and a different driver.

Automation Technologies has a  couple options. What looks like a Servo with Encoder and also a stepper with an encoder. So, if I were interested in adding a closed loop capability, what else do I need to be thinking?


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## JimDawson (Nov 16, 2015)

IMHO, the only way to go is put the encoder on the load (table) rather than the motor/lead screw.  What you want to position accurately is the load, not the lead screw.  This automatically compensates for some backlash and any inaccuracy's in the lead screw.  Linear glass or magnetic scales work very well as the feedback device.  Personally, I prefer the Renishaw 1 micron magnetic scales, these give you a theoretical 0.000039 position accuracy.

It is possible to close the loop with a stepper system, but a DC or AC servo system is the best.


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## countryguy (Nov 17, 2015)

I 2nd Jims idea.  I would like to do this on my Mill at some point at least on X & Y to start.    If you use the Renishaw (or Equiv per a post I did here) you will still need a controller that can accept and use the feedback for positional control.    If you find a Mach3/Stepper setup that can do this I'd be very interested.      Here are two I investigated as full kits.  .... http://machmotion.com/ http://www.ajaxcnc.com/      Hope you keep us up to date on what you find / buy!


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## sbx (Nov 17, 2015)

Countryguy - Check out Mesa Cards. Its not Mach/Windows but rather Linux CNC. Which I am planning on going with. They have what seems to be a large number of different FPGA IO cards along with Daughter cards that can take servo encoder inputs. At least from what I have looked into so far. I think I can go either stepper or Servo with encoder feedback using these card combos.

I'm still digesting all of this so, will probably just post here and there as more questions come up. I am intrigued by adding the full closed loop. Just don't want to break the bank. Its is just a PM25 after all. Trying to blend the cost/performance ratios and see where it all comes out. At some point accuracy is going to be maxes out by the mill's capabilities itself. One day I would love love to have a true VMC to retrofit. But for now, I just want to dip my toes into it with this little mill.


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## SEK_22Hornet (Nov 22, 2015)

I have an Ajax DM45nc mill which is about 10 years old. It was factory built by Ajax using the Centroid CNC10 controller and software. It is a brush type DC Servo system based on Linux CNC. One problem they had with machines was vibration causing read errors on the hard drive when the PC was mounted to the column of the mill, so they used a Compact Flash card as the hard drive.  Just mention that in regards to how you package the electronics. I love the DC servo system. The servos are belt drive to the ball screws, about a 2 to one reduction (just guessing). 2000 line encoders are what they recommend for the feed - and the encoder is on the motor shaft. I actually have a G0704 mill I'd like to convert as well, so I'm thinking about the same things. I'm a PC guy so the Linux CNC is a bit intimidating to me, but I do like the way it operates in the Ajax form. I just looked at the Automation Technology website and they have some very nice looking prices for DC servo motors. The Gecko G320 drive lets you use it just like a stepper motor. I need to look into this to see how it tells the controller the drive didn't move properly, but it kinda looks like there is an error output that may take care of that. Looks like the encoder feeds back to the gecko drive.  More research needed on my part. It looks like a Nema 23 drive might be big enough for the x and y axis with one of the nema 34 for the Z axis. Prices look like about $200 per axis and up. Not sure how a hybrid stepper works, but with an ecoder on a stepper, it would seem possible to get some odd feedback unless the encoder was closely matched to step size, so I'd vote for Servo. Just my thoughts on this.


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## chevydyl (Nov 22, 2015)

I think the error output works in a away that the commanded steps from the controller are then commanded by the servo drive, then the drive counts the steps from the encoder, if they don't match or it doesn't do it


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## jbolt (Nov 23, 2015)

Just a comment on the Automation Technologies servo systems. We are using their 600 oz servo on our CNC router which works great though we are using a high end driver by another company. We initially used the encoder they sell. Their encoder is a single ended unit and we had a number of issues with noise causing position errors. We switched to a US Digital differential encoder which is much superior. I think the AT encoder is more suited for applications like vending machines vs precision CNC.


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## justlie (Nov 25, 2015)

SRX,

Did you order a kit from arizonavideo for the mounts and ballscrews? 

General question, why can't we use the factory screws? Are they too fine of a thread for the stepper motors? Would it be and option to gear them and not drive them with the stepper directly? 

I am planning on getting a PM25MV here shortly so I am just trying to gather some information.


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## SEK_22Hornet (Nov 25, 2015)

There are a couple of very good reasons to use ball screws in place of the original acme thread lead screws. One is the turning effort on the ball screw is much less, reducing the power lost in moving the table. Another is reduced backlash. Ball screws are by design low backlash devices, which improves overall accuracy. It is possible to do CNC without using the ball screws, but on a mill the size of the PM-25, the Chinese ball screws are well worth the cost, in my opinion. Automation Technologies sells the kit for $189 for the G0704.


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## chevydyl (Nov 25, 2015)

it has nothing to do with the pitch of the screw, in most cases, you will be ordering ball screws of the same pitch, as said above it has to do with friction, ball screws are something like 90% efficient, acme screws (factory) require a lot of torque to turn, and would wear out quickly from the repeated higher turning speeds of CNC systems, and then you also have backlash, now, acme screws can be had that are close to backlash free, but they are both expensive, and require an even higher amount of torque to turn, because both the nut and the screw have much tighter, closer fitting tolerance. even then, they still wont hold up like ball screws to the higher speeds of CNC.


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