Converting a PM-728VT or PM-833TV - Questions and Discussion

I am about to convert either a PM-728VT or PM-833VT mill. I read most of the threads on this forum and have found the information to be fantastic! I am hoping you can answer a few questions:

  1. With respect to upgrading the spindle motor to an inverter rated motor, I am unclear as to the reason why you need to modify the motor mount. I plan on using the same PDB from Greg Priest. Is this because a motor with an exact fit is very costly? My goal is to increase the low-end torque and increase the spindle speed to 5000 to 6000 rpms. I will not necessarily be doing rigid tapping. If I were to use a sensorless Vector VFD, do you know what max rpm is possible with the original motor?
  2. When adding the VFD to the Mill, did you have to add any RF filters or chokes to reduce electrical noise?
  3. The images of the Mill enclosures look great. One quick question. If you are planning on using mist lubrication (like a fog buster) and will not be using flood lubrication, do you need to slope the bottom of the enclosure. Would a flat bottom work? Is there something I am missing regarding the need to slope the bottom of the enclosure?
  4. With respect to tools slipping from TTS holders, here is an interesting video from Cliff at Threadexpress that gives some interesting insights: R8/TTS vs BT30 Spindle Tapers - Part 1 - YouTube

I hope you find it helpful.

Thanks again… Richard

OOPS… I for got to ask, can someone share the dimension of the enclosure for the PM-833?

  1. With respect to upgrading the spindle motor to an inverter rated motor, I am unclear as to the reason why you need to modify the motor mount. I plan on using the same PDB from Greg Priest. Is this because a motor with an exact fit is very costly? My goal is to increase the low-end torque and increase the spindle speed to 5000 to 6000 rpms. I will not necessarily be doing rigid tapping. If I were to use a sensorless Vector VFD, do you know what max rpm is possible with the original motor?
    I can’t remember what the 60hz rpm is of the stock motor but theoretically you should be able to triple it. The motor may have a lower safe max rpm rating however. The stock motor is a Metric frame. If you could find an inverter duty motor with a metric frame it is possible that it the mount may not need modify the mount. I had to move mine back to make room for the PDB too. But the motor is larger than the oem.
  2. When adding the VFD to the Mill, did you have to add any RF filters or chokes to reduce electrical noise?
    I did not. Keep the cords short and follow the VFD instructions to lower the noise. The images of the Mill enclosures look great. One quick question. If you are planning on using mist lubrication (like a fog buster) and will not be using flood lubrication, do you need to slope the bottom of the enclosure. Would a flat bottom work? Is there something I am missing regarding the need to slope the bottom of the enclosure?
    Flat would work just fine in this case.
  3. With respect to tools slipping from TTS holders, here is an interesting video from Cliff at Threadexpress that gives some interesting insights: R8/TTS vs BT30 Spindle Tapers - Part 1 - YouTube

Todd Candee
Todd@thecandees.com

Just adding on to Todd’s response.

I literally just released a video on swapping out the motor and VFD for rigid tapping: https://youtu.be/WwvvC96PKz8

I can only speak to the PM-833TV mill, but that motor is driving the spindle in an approximate 2:1 ratio, so if you just changed the driver pulley to match the spindle one you could get up to around 6k RPM. If you are the States, then it will be difficult to find an inverter duty motor with a metric frame. I found the 145TC Frame to be about as big as you can go without having to modify the drawbar or create a spacer between the spindle head and column.

Here is how close everything was:

I did not add any noise protection either.

If you are going to build an enclosure might as well add that bit of slope incase you decide to add flood coolant down the line. The fogbuster is great for slotting and clearing, but is a little inadequate when it comes to pockets, drilling, and tapping. I usually just stand by the machine and add a little extra coolant during these Ops.

The video is great. Very informative. Thanks for the shout out. I like the “mill” parameter setting in the VFD. Out of curiosity, what vector mode does that setting use? I set mine to vector speed.

I am able to tap larger and very small holes using a peck tap cycle. For my controller this required a post processor modification because camworks does not have peck tapping built in. It works great. The largest I have tried is 7/16 NC. I was able to tap about 1” in mild steel. I don’t think I could have done 1/2” however. The motor was really working hard.

For very small taps it is handy to prevent tap breakage. I also use cutting oil and not coolant for small and large taps as a precaution. I have only broken one tap since I converted to rigid and it was my fault. It was a #3 at about 5/8” deep and I neglected to turn on the peck cycle in camworks.

Todd Candee
Todd@thecandees.com

Yah man! Thanks for all the help with this upgrade.

You know I just realized that if you set the drive up with that machine tool profile it defaults to IMVF (Induction Motor V/F control). I had set my drive up manually and realized that was going to be hard to explain on camera, so I just plugged that profile. And technically scalar control works for tapping, but not as ideal. The drive can do field oriented vector control with and without space vector pulse width modulation. I am using field-oriented sensorless vector control. I will make a note of that on my website. Here is a list of all the parameters the machine tool profile sets:

I think any setting will work with rigid tapping as long as the demands meet the power and torque capabilities in the particular situation. It’s probably only important in larger taps.

Todd Candee
Todd@thecandees.com

Thanks to both of you for getting back to me. Very much appreciated. I watched the latest video showing your motor upgrade. Very well done!!! This is undoubtedly a full fledged professional solution. It also gave me more to think about regarding selecting the appropriate mill for my conversion.

I can now see that a full fledged motor upgrade for this mill will cost around $1400.00. When you add this to the $900.00 cost difference between the two machines, you now have a delta of around $2400.00. There are still additional cost to upgrade the spindle bearings if you want to achieve higher spindle speeds. This is rather interesting because my initial goal of choosing the PM-833TV over the PM-728VT was for a heftier machine with greater structural rigidity. But here is the rub. For $2000.00 - $3000 I could consider adding an 18,000 rpm 3HP ATC to the PM-728. This would give the PM-728 an amazing ability to zip through aluminum like butter. Furthermore, as you increase spindle speeds to the higher end of the spectrum, you decrease the radial force on the cutting tool/spindle (assuming the feed rate is constant). This means that structural rigidity is not as critical. Again… this video was extremely helpful to me!

I wanted to share a thought. If others are looking at upgrading the PM-833TV for a DIY project and wanted to increase mill performance for less money, here is an option. Standard duty 3 phase motors can be run close to 10% of their stated RPM with the use of an inverter. If you keep the stock motor but swap the OEM inverter for a sensorless vector drive VFD, I think you would get a significant boost to the low end torque. Maybe not enough for rigid tapping, but certainly enough for mill work requiring low end torque. The key to success here is to make sure the VFD you choose has a programable output. You then program the VFD to close a contact when the VFD is running at lower frequencies. The closed contact is used to turn on an external fan mounted on the rear of the motor. This fan adds additional air flow when the motor is running at slow speeds. This whole conversion comes at just the cost of the VFD. You can also experiment with running the motor faster than the standard RPM, but you need to be cautious as standard motors don’t typically indicate their max safe rpm. I think James Clough did a similar upgrade to his lathe on his YouTube channel Clough42. One last note. If you use an analog input for the VFD, be sure ground the shielded cable only on the end of the cable that attaches to your control panel. Do not ground the shield on the VFD side of the cable. Grounding the cable on one side will allow the shield to work properly and ground EMI. If you connect the shield to the ground on both sides, there may be a slight voltage difference and you can have ground loop problems.

Thanks again for all your thoughts… Richard

You may not be able to do

I did want to

Thank you!

In the future, I am hoping to build a granite epoxy and linear rail machine around this spindle. Its my opinion that high speed spindles are better suited for gantry-style machines. With the work staying still and the spindle moving in all three dimensions, you can keep your feed rates up as you change directions and interpolate curves.

I wonder if they sell general purpose motors with a thermistor or some kind of temperature feedback. You could then just tie that external fan to that temperature. Or perhaps taping a temperature probe to the outside of the frame would be sufficient.

That is a great idea. If the VFD does not have a switchable output, many of the computer fans have a contact that can be wired to a thermistor.

I bought the 833VT as my first mill and I am not an engineer. I am adding a DRO and some motors to drive the mill, and considering your CNC conversion build. I bought this manual mill to make parts for a CNC I am building from scratch for woodworking (much larger table space and less ridged). However - being new to milling (but not new to machines and I have a bit of common sense) I have encountered a problem with the 833 VT one shot oiler that makes no sense. Since you deinstalled and reinstalled the oil system I thought I would ask. When I contacted Precision Matthews everyone was nice but zero answers or follow up. Here is the issue - when I pump the oiler once - then walk away for the night, way oil drains into the pan of the machine - pretty much drained the one shot oil tank into the stand, then out the drain onto my garage floor. I think the stand drain is a coolent drain but since I am not at the coolant stage no plumbing was installed. It also seems neither the x or y axis screws are oiled by the system. I watched your videos but did not detect if you modified the oiling system on that mill in any way. On my machine the whole system is not built well. All aluminum plumbing enters the castings press fit (finger press). And since the tubes are finger fit then bent over for clearance all of them crimp into the hole. All of the joints into castings leak - more oil than the ways get. Has anyone addressed this in any way or do I just have a bad precision matthews supplier build? Fixing it on the part of Precision Matthews would be too costly overall under warranty (you know - the five year one) because the entire machine will need torn down and every fitting done right - I think. I cannot imagine how much way oil would be wasted with an automatic oiler if this is normal.

I ended up removing the x axis and I am absolutely sure no oil gets to those nuts driving x or y, is that normal?

I also examined the threads on the x axis and found the screws had been damaged most likely when they sat in a box with 20 others screws, and people pulled screws out of the box dragging the end on the screw down the length of the screws still in the box - leaving grooves in those screws - which look a lot like tap threads - but threads that are not right for the brass nut used for the screw. I contacted precision matthews and was basically told not to worry about that. I touched up the screw removing all the places the gouges in the threads had sharp metal bent into the screw valleys so that metal damage stopped chewing up the brass nut. This is another reason I am looking at your CNC build because this machine has way too much slop (lash). Since you switched the screws and nuts for ball screws maybe this was not noticed on your machine or not present. Anyway - thoughts on the one shot oiler problem? I am not finding much on the web.

There is definitely a problem with your oiler. Pumping the lever once requires a bit of force and only a small amount of oil will be dispensed on the ways. Check for leaks at the base of the reservoir.

The oiler is mostly meant for the ways. I rub a little EP2 Grease on the ball screws every now and then. This sticks around way longer than oil but you could add some fittings/tubings so you can supply those X and Y nuts with oil.

Yes some of the oil will leak out, but if you have the Precision Matthew stand this oil (which isn’t much in my case) will collect in the corners of the pan.

These imported mill/drills are all unique in that they use very low cost components with high variability. This comes with the wallet friendly price tag. These mills are a project in and of itself. I purchased the 833TV because of the belt drive and nice quality of the ways. I have pretty much replaced all the other components in my CNC conversion video.

Just by reading your post, it doesn’t seem like you are not looking for a second project with having to build your large CNC router. Perhaps you can return the mill and use the proceeds towards having your parts manufactured by xometry or a similar service?

Thank you for the feedback. I do have the stand. The oil floods the pan on the stand then drains out the coolant drain.

I agree I was not looking for what amounts to a total mill rebuild.

It sounds like your system does not leak as mine does.

Have a great day!

Lenny Smith
LenSmith701@gmail.com
(C)407-925-4397

If you moved the position of the oil pump to above the ways the oil can sometimes siphon out. In Thai case I believe there is still something wrong with the pump.

Todd Candee
Todd@thecandees.com

Hello Lenny,
I was just getting ready to pull the trigger and order this mill, but it seems that you are a bit disappointed with it. If you were going to do it all over again, would you buy another machine? Other than the issues you mentioned, are you happy with the mill as a whole?

Thanks… Richard

Here is a video on that you may find helpful for enhancing the one shot oil system:

PM-25MV CNC Conversion Part 2: One Shot Oiler - YouTube

Good luck… Richard

When I add up lost time that needs to be invested, hidden damage that has to be corrected etc. my math shows it would be better to select a better quality mill. If you have time on your hands and as Dr Flow says- your really buying just the casting and intend to do a complete rebuild and overhaul then this mill is for you. Precision and well built as it is advertised it is not. In the quill there is more slop than in my Jet drill press and there is no adjustment.

However - if you have time and this is a hobby - it runs out of the box and it’s heavy. There is something to be said for that. Quality is really not there, but you can elbow grease your way past that.

I find so many simple things (like no lifting eyes) that would have improved this machine for little cost.

For example - why buy this thing with a “one shot oiler” if that infrastructure needs to be totally removed, piping replaced, holes tapped, anti oil back flow valves purchased and everything reinstalled?

However if you have less capital going in and you need a project - this is a way to get started.

The road to a quality machine costs the same - either get it up front for higher initial investment or get it over time by continued additional investment until the quality you want is achieved.

The 833 TV is not a precision machine. And you cannot re-crate and just send it back.

The damaged threads on the x axis is an example. That damage was eating at the nut, which would have materially increased lash in a matter of a few hours use. That screw was clearly laying in a pile because there were multiple drag marks down the length of the screw. It took hours of careful file and emery work to get the threads fixed to the point of no longer tearing up the brass nut. The precision matthews team basically offered nothing and thought it was good that I found it and cleaned it up. They should have instantly shipped a new screw and replacement nut.

I have not found that customer service to be the same as many posts make it appear to be - which is a secondary letdown.

Good luck.

Lenny Smith
LenSmith701@gmail.com
(C)407-925-4397

I think there are two main problems - one is no backflow valves exist and second all the insertion points in the casting leak heavily, and appear to be the main cause for line drainage. Because of the install method apparently used - push in and bend- the aluminum plumbing dimensions could not be designed for a tight fit and it is clear the piping does not have great tolerance control. I need to tap all the oil entry points and install proper fittings, back flow valves and see where I end up.

Hi Lensmith, I have been looking at the PM833 for a while now and have already made a list of stuff I am planning on changing (thanks to Dr. D & Tcandy). If you were to get a different mill what would you pick? I have looked at several and the PM seems to be about the best, but it’s hard to find things on the internet.

For what is worth, I have been doing the same research as you. I placed an order for the PM-833TV a month ago. After looking at everything on the market I could not find another option unless I went with the PM-940V which for me requires too large a foot print. I agree with Len and others that in this price range, you need to plan on doing a bit of re-engineering. Otherwise you are looking at higher end equipment like Tormach. For a hobby grade mill, I think the Precision Matthews actually gives you lots of quality for the price. Just my two cents.

Richard