PM-940 DIY CNC or Turn Key PM-940M-CNC-VS

my end goal is definitely to make parts. With arizonavideo coming out with a full ball screw kit for the 940, it has opened the door to possibly building a machine- well rather assembling and wiring it. The only 'machine' work would be to ball mill the ways for the oiling. For me, I'd never want to sit and design and build the ballscrew parts.

Its going to come down to the spindle motor/vfd and if I want to go servo or not. If I do want to go servo I think I'll build one. If I am OK staying stepper I will probably buy a turn key and change the board. That's what things look like right now.

I'm going to make another thread about the servos in the CNC forum- see what I can learn.
 
Hi RA,

I just wanted to give you a couple of my observations. I purchased the PM 940M CNC VS turn key with the hardened ways and ALL other options (4th axis as well) that were available at the time... and after some initial issues it seems to run and operate fine. For my needs I am happy. I am still using the original poorly documented n-motion controller and Mach3 software and it all works just fine. Others have had difficulty, but mine essentially worked right out of the box. I admit documentation leaves a lot to be desired, but the original cost is minimal. As far as the e-Stop switch is concerned, it is a very easy fix and is probably the easiest thing to change about the mill. You just buy a new e-Stop which for a couple of dollars and then install and invert the setting in the Mach3. Phazertwo and I have both done this and it is written about. I looked at all of the documentation and circuits for quite a while and I am still puzzled as to why they hooked it up the way they did. Anyway, it is not a worry. Also, the ways are automatic oiling and the lead screw (split nut) bearings are balls. I have measured the back lash on all axes to the best I can do and they seem tobe within spec. (I got a spec sheet from the factory.)

I have read and looked into changing the n-motion controller and the Mach3 and even though I am a semi-retired electrical engineer/physicist and university professor I decided to try it for a while before adding the cost and extensive rewiring time. I am glad I did as I wanted to get parts made and it is all working. Because of the discussions I think people maybe just assuming that they need to change these things. I guess the question one needs to ask: Is your objective to work on the mill or is it a tool to make things? I think you could work on this mill, or many of the others, for a long time and still find things to add or improved. On the other hand, if you hobby is to rebuilt the mill then have a go. It will keep you busy for while. There are a lot of things you could change. (not the mill's fault).

I did do a lot of initial work in order to get it through my basement door.... I had to take it apart somewhat. However, this is my issue not the Mill's! I took all the steppers, the x-stage, as well as the big electrical box off. It came with the base/stand on a separate pallet. Before disassembling the machine and while it was till on the pallet, I made a dolly that would fit on the bottom of the mill as well as on the bottom of the stand so that I could just mount the mill (with the items mentioned removed) and wheeled it into my step-out basement. I only had to lift it about 2.5 feet to get it over the initial steps into the basement. Once inside and roughly located I removed the dolly from the mill and mounted it to the bottom of the stand, put the mill on the stand, then put the x-stage back on, then mounted the big electrical box to the side of the machine rather than to the back so that the doors open forward rather into the wall I push the mill upto (the mill is still on the Dolly). The steppers are a bit on the small side, but do the job fine. If I were going to change one motor it would be the vertical as it has to lift and hold a about 250#s (the vertical head with motor is heavy). On the other hand it I have thought about counter balancing this weight so that the vertical would move faster. Also, these steppers are about as big as the drivers can electrically handle so if you change the steppers plan on getting new drivers too and maybe a new power supply and make sure they still fit in the electrical box. I initially found the vertical axis to slip and when the power was shut off it could drop, but this was due to the nuts and the washers on the bottom of the screw having fallen off in shipping. I found these nuts and washers when I lifted it off the pallet. The lock washer in this assembly simply had not been put on correctly, but you do have to remove the electrical box on the back to gain axis to the end of the screw. Also, probably because I had run the mill before taking it off the pallet (to make the dolly) the vertical ways were loose. After I tightened these properly, the slippage completely went away and that small stepper holds just fine.

If you search upon my user name, B2, some of my early postings you can see my final mill, on wheels, with the electrical box on the side of the vertical post. https://www.hobby-machinist.com/threads/pm-940m-cnc-z-axis-bearings.59538/#post-608202 I do not have too many postings, except when I was trying to help a member get is Mach3 working, so it is not a lot of searching. This group that I was helping appeared to have gotten involved with a machine that did not come directly from PM or it had some previous issues. You can ask them, but I think their biggest problem was that the Mach3 initiation code was just messed up. Matt at PM, probably could have helped them out if they had ask him.

By the way, the x-axis table etc probably weights in at well over 150#s without the stepper mounted! You will have to take this, and the other stages, off if you want to change the oil path or the lead screw circulating balls.

Things I would like to have on my machine and do not:

1) 3-axis DRO without using the computer and Mach3.

2) Power threading (cannot do this reasonably with a VS 3phase motor, even if you provide spindle feedback. The VFD just is not designed for this). Since power threading does not take a lot of force I have thought about connecting my 4th axis stepper to the spindle spline and using it for this task. It turns out in my machine there is enough room between the high and low speed gear engagement that the spindle can be freed from the 3phase motor and then the stepper via a pully could be engaged to the spline of the spindle directly.

3) An easy way to quickly convert the operation between CNC and manual. Sometimes you have a really simple job, but writing code takes more time than making the part by hand! PM supplies crank handles but the stepper cogging makes it difficult. Also, this is a good reason not to remove the vertical handle. The crank gears for this motion makes considerable noise (also documented) so some have removed it.... but then you HAVE to have the CNC working to lift the head.

4) Higher RPM, but I get by with the 3200.

PS1. I live in Pittsburgh and so visited PM. I think Matt is the owner and he has been responsive and helpful to me. The other folks who work there have also been responsive and helpful. Some folks have indicated that he is slow to respond to email, but I have the feeling this maybe due to his travels to the other side of the world to get these machines made.

PS2. Available, inexpensive CAD/CNC conversion software is limited. I have been using FreeCAD which is user group supported. For simple cutter motions some of the Mach3 Wizards are handy.

Good luck with your decisions.

Dave
 
I am hard on the n-Motion controller, but it did work for me to make a few parts before I swapped to Acorn. By far the worst part about the n Motion was the lag time between when the e-stop was depressed and the machine stopped moving (2-3 seconds). I hit my vice once AFTER the e stop had been hit, and that was the end of that. Admittedly, I expect too much out of Chinese hobby grade gear. After running some some gear from the big boys at work (Fadal, Haas, Mazak, Mori) the Acorn seems more like home to me, and gives me more confidence in the machine, which I tend to let run while I'm doing other things in the shop.

As Dave said, the mill is pretty good right out of the box, and pretty much ready to make parts, save for some things that may have come loose during shipping (all my limit switch wires came loose). If you want to start chasing speed and accuracy, there are lots of good upgrades to make to the mill, but none of them are necessary.

PZ
 
Wow PZ, your 2-3 second e-Stop response sounds abnormal. Just to be sure I had not overlooked a similar problem I just fired up my system (n-Motion plus Mach3) and ran an x-axis motion command. During this motion I hit my e-Stop. It stopped instantaneously. No observable delay.

I am going to guess that you had, or have, a computer or computer interface problem, not an n-Motion problem. The Mach3 software has to be poling the serial (USB) interface regularly to receive the e-Stop command from the n-Motion. Maybe yours was just slow or busy. I have run into this on other scientific instrument systems I have built, but never this long. If a computer has memory limitations, or bad memory, this can also cause a poling delay. Likewise, sometimes the OS or software is simply broken! Many people do not realize this the the OS, and software, does wear out and needs to be reinstalled! This ususally happens for folks who run the computer a lot. Data errors do happen and under some situations, the OS stores those errors! It is because the PCs are cheaply made. Years go the memory had a parity bit (9-bit bytes rather than 8) for error correction, but the manufacturers eliminated this and now rely on the software to fix the errors. I rebuilt by OS about every two years, but I work my machines a lot.

Before I took delivery of my 940M I was reading the specs and thought that it said I had to have a old serial printer port. So I looked around and found a used machine for sale for little of nothing... that had I have an old serial port, an old Dell that I am still running ( I would never recommend a Dell, but that was what I found. I usually build my own computers.) After I got the 940M I found that the n-Motion it had both a USB and a serial printer port interface so I just used the USB interface also the Mach3 setup software that came with the 940M was for the USB interface. The Dell had a couple of GBytes of memory installed and was a fast enough CMU (the requirements are pretty low). So, I completely rebuilt the old OS and software as a 32 bit Win 7 enterprise (an OS which I had). The only real software problems that I have are that the Mach3 software needs to boot up before I turn on the Mill. Otherwise sometimes they do not communicate at all, or it will cause the PC to lock up, or it causes the 940M stepper drivers circuit breaker to trip. I think this is a grounding problem somewhere, not really a computer or 940M problem. But I have never chased it down. When the Mill turns on there is a surge of current to the drivers and this may create a voltage spike on the interface. Anyway, it is not a safety issue, but is a nuisance, which I have figured out how to avoid.

Acorn maybe better, I just do not want to go to all of that work and expense. Maybe someday.

Dave
 
I do believe my issues had to do with polling, which I have also run into issues with testing equipment (did I mention I also hate LabView?). Anyway, I played with a setting called buffer time in Mach 3, with 3 different computers, and no matter what I had the delay issue. The computers I used were older... but a 2-3 second delay seemed extraordinarily high for a computer that has absolutely nothing else installed on it. I even went as far as finding on a old, old copy of Win2k pro that literally had NOTHING to it but the OS, and I still had the same issue, though it seemed slightly better. I do wonder, if I built a better computer (like I did for Acorn) if it would have run better.

At that point I was researching different controllers. Centroid acknowledges the polling issue and addresses it by requiring a computer that meets a minimum single thread speed, additionally, they have your turn off quite a few things in Win10 that can interrupt communication with the board itself. Last, they are running a Beagle Bone computer on the Acorn board, which handles all the processing itself (as I understand it) so if the e stop is hit, it doesn't need to get the signal all the way back to the PC, and then back to the board before it stops the machine. Cut2Cut and I had several posts about this very topic when we were trying to figure out which controller to switch too.

Acorn is a great solution, but it is far from the only solution. For someone at your level of understanding, Dave, I would suggest a good look at Linux CNC. If I ever build another machine, or upgrade the controls on this one again, I'm going to give Linux CNC a shot. Linux is by far a superior operating system for this type of work, and will be able to harvest the awesome speed of modern computers with out worthless interrupts that windows will always be plagued by. Also the ability to add glass scales for full positional feedback is simply irresistible to me. I know... I'm a nerd.

PZ
 
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