Another Mill Choice Dilemma

[ptrotter]

I have decided that it is time to finally get a mill . I have been very happy with my PM1340GT lathe but am getting to the point where I am limited in what I can do.

For no particular reason, I have always wanted a Bridgeport style knee mill but I don't have the space, and in reality don't really need, a full size mill. I seriously considered the PM935TS but decided that I want to do a CNC conversion and I think that is more difficult on a knee mill. Also, giving more thought to my use case, a decent bench type mill will be more than enough. So now comes the dilemma.

I think if I were to design my perfect mill, it would have a 2 speed belt drive, maybe 3:1 and 1:3 ratios, with a 3 phase 1800rpm
inverter duty motor. This would allow a good top speed and also pretty good power at lower RPMs. Of course that doesn't exist so I have to look at what is available and decide what compromises I can live with.

I have read pretty much every post discussing the pros and cons of the various mills out there and still can't make up my mind. I have a couple of required features and a few desired features:

Required:

1. Commercially available CNC conversion kit. I can handle all the electronics and steppers, but I don't want to deal with the ball screws and motor mounts.

Desired:

1. 3 phase. Almost a requirement, but could probably convert to 3 phase if not available initially.
2. Taiwan quality.
3. Decent low speed power
4. High speed maybe around 3000 to 5000 RPM. I'm not looking at a true high speed CNC ATC spindle.
5. Decent size work envelope
6. Decent weight and therefore rigidity.

I think this boils down to 3 possible choices:

PM940M
PM833T
PM833TV

PM940M Pros

-Heaviest
-Largest work envelope
-CNC conversion available

PM940 Cons

-Chinese quality - Replacing the lead screw with ball screws will offset some of this but still dealing with the casting quality.
-Gear drive - Limited high speed
-Only available in single phase. Should not be too difficult to change to a 3 phase motor. Would require either turning down the motor shaft or making an adapter to match the original shaft size and lifting the motor up to accommodate this. This would also allow for higher speeds within the capability of the bearings.

PM833T Pros

-CNC conversion available
-Taiwan quality

PM833T Cons

-Gear drive - Limited high speed.
-Only available in single phase. Should not be too difficult to change to a 3 phase motor. Would require either turning down the motor shaft or making an adapter to match the original shaft size and lifting the motor up to accommodate this. This would also allow for higher speeds within the capability of the bearings.
-Lighter weight than the PM940 but much of the difference may be in the 940's stand so the rigidity may not be much different.
-Slightly smaller work envelope than the 940.

PM833TV Pros

-Taiwan quality
-3 phase.
-Higher speed
-CNC conversion available

PM833TV Cons

-Lower power at low RPM. I'm not sure how much of an issue this is in a CNC machine.
-Built in VFD. I'm not fond of the built in VFD as I don't know what it is but this could be replaced to give more access to the VFD programming. This is useful if I want to control spindle RPM through the CNC programming.

As can be seen, all the machines have good points. I could go with a 940M or 833T and consider changing the motor and maybe eventually converting it to a belt drive. This would get closest to my ideal machine but requires a lot of work. I could go with the 833TV and get most of what I want on day one. My problem is that I over think things and also think I need more than I do. I am always scared that if I make any compromises, I will be mad about it in the future.

After typing all this and thinking about it, I am thinking my best option is to just go with the 833TV. It has almost everything I want and I will probably never run into its limitations. Not needing the axis powerfeeds and DRO pretty much offset the cost of the CNC conversion.

I am interested in everybody's thoughts on this. I need to do something soon before I drive myself crazy.

Paul

 

[markba633csi]

Seems like you are more than halfway there already, you've narrowed it down to one supplier!
-Mark

 

[parshal]

I have the 833TV. The low-end power hasn't caused me an issue yet. However, I haven't worked with much other than aluminum right now.

 

[7milesup]

I am assuming that you have seen --> this conversion done by Dr. D-flo. He does a nice job in my opinion, especially with the electronics end of it. I am very interested myself in doing this conversion on my 833T. My hesitation is spending that nearly $1000 on just motor mounts and ball screws. I that Heavy Metal gets their ball screws from VXB which I don't think are very high quality. I could be wrong on that though. If I had the dimensions of the motor mounts I would just make them myself.

 

[mksj]

Reviewing the information, it probably comes down to how much travel and drive system speed you need and less so about Chinese vs. Taiwan. Another factor would be available kits if you do not plan (or have the ability) to mill the parts to convert to CNC. I would probably go with the PM-940V-BASICw/HRDWYS, it is basically setup for conversion to CNC, it is a 2 speed Low 100-1640 RPM, High 350-5000 RPM variable speed head. Probably beyond 5000-6000 RPM the bearing would need to be switched out and change the pulley ratios. It has an option for hardened beds, you would want to add an automatic oiler. Seems to be a number of CNC kits available, this was also can include Clear Path or DMM servos. I had looked into DMM servos in the past and they have some very nice drives/controllers. If going with the PM-833, the TV seems to be a more logical choice for reliability and convert the head to a 2 speed pulley belt system. In both mills you are already starting out with a VFD rated motor and drive, the downside would be if there is any provision for interfacing its control with a CNC system. You could interface a control relay to turn on/off the drive. You also could check if any of the CNC retrofit vendors have something in the works for the PM-833TV. Both mills are close to the same price, but for a CNC conversion project I think the PM-940V has a bit more going for it.

PM 940M CNC conversion Kits

 

[chipping]

Ptrotter - excellent summary of the options.

I am also considering a path to CNC. What I would love to see is Precision Matthews offering a stripped-down mill, as referenced by davidpbest in the other thread: https://www.hobby-machinist.com/threads/pm1340gt-mods.86651/post-776456

What I envision is the frame (base and column) and table with precision ballscrews and mounts for NEMA motors. But no motors, head/spindle, controllers, or software. That way the end-user can customize based on cost, quality, features, and materials to be milled.

The Dr. Flo video referenced by 7milesup is excellent, but shows the complexity of the conversion. (And PM needs to stop gluing the column to the base with paint.) Converting to CNC is more complex, time-consuming and expensive than necessary because of the extra work that went into making it a manual mill.

Bonus points if the 833 is offered with an iron cast base - perhaps the PM-940 one, although it might need new mounting holes.

[And if PM ever wanted to offer a CNC-only machine, then the design choices open up. But I acknowledge that is a lot more design/engineering/manufacturing/customer support, so I am opting for what seems to me the simplest path.]

 

[Skowinski]

After 3 months of wandering back and forth through the entire PM lineup from the 727's to the 935, I ended up with an 833TV. There's always some tradeoff it seems. I was a bit concerned about low RPM power with this mill. But, decided that the better high end RPM was worth the tradeoff, and anyway I'm not likely to go much down below 200-300 RPM for my intended uses.

For awhile I was about to buy an 833T and put a VFD and 3-phase motor on it, but the expense and possibility of exploding the gearbox if I pushed the RPMs too high worried me.

I figured if the proprietary VFD that comes on the 833TV fizzles out I can just replace it with a mainstream Hitachi or Teco VFD. At least I have the 3-phase motor already.

I have wondered if a 2 step pulley could be adapted to the 833TV?

 

[davidpbest]

Ptrotter - excellent summary of the options.

I am also considering a path to CNC. What I would love to see is Precision Matthews offering a stripped-down mill, as referenced by davidpbest in the other thread: https://www.hobby-machinist.com/threads/pm1340gt-mods.86651/post-776456

What I envision is the frame (base and column) and table with precision ballscrews and mounts for NEMA motors. But no motors, head/spindle, controllers, or software. That way the end-user can customize based on cost, quality, features, and materials to be milled.

If you’d be interested in a stripped down 940 for CNC conversion, I would be as well. If we could get a handful of people interested, maybe we could talk Matt into sourcing such a configuration.

My interest in doing a CNC conversion is increasing rapidly - I need a new project to sink teeth into. The 940 with hardened ways looks like the most desirable path so far in my research. Assuming the ways are hardened, the China versus Taiwan quandary strikes me as nearly irrelevant if in the conversion process the positioning guts are all replaced, the spindle is upgraded in some manner to achieve 5k RPM. I’m still intrigued with the idea of using some kind of replacement spindle more suitable for CNC work rather than upgrading the OEM head, but haven’t done much digging on the replacement spindle options. I’d love to hear more on this aspect from others who have deeper experience than I do. Mark suggested the 940V with a hint toward retaining the OEM spindle/motor, and while that sounds like an expedient path, for me at least it causes the ”China v Taiwan” question to pop up again.

I’m also not sure that 350 pound cast base is ideal if the intent is to ultimately have an enclosure. I think I’d rather fabricate my own stand with better facilities for chip evacuation below. My experience building my own platform for the 1340 lathe was both gratifying, and the end result superior to the option offered by PM.

 

[Watchwatch]

I’d be interested in a stripped down PM Taiwan bench top mill for CNC conversion. No motor, ball screws installed, no motor, axis and motor mount options.


Check out Clough42 spindle upgrade for his Grizzly. Very impressive what a $3000 12k spindle can do on a benchtop mill. That video series, actually the test cuts, really made me want a CNC.


Sent from my iPhone using Tapatalk

 

[mksj]

OEM spindle/motor, and while that sounds like an expedient path, for me at least it causes the ”China v Taiwan” question to pop up again.

You get a 3 year warranty, it either holds up or fails. If you want a higher speed spindle or worried about longevity there are better bearings. If the motor eventually fails, then replace it with vector motor, and if you want power tapping add an encoder. I worked with a number of individuals that went with the PM-932 in the past (before a 940 was available) and they added a Marathon Black Max 3 Hp motor. More recently there have been some AB (same as Baldor) vector motors that one person went with a 3 Hp in his mill and another a 5 Hp 130lb brute in his lathe.