Another Mill Choice Dilemma

ptrotter said:
Maybe not optimal, but then how often do you have to change the speed enough on a single setup that you would need to change the speed range. Still better than a gear box. At least it should be reasonably quiet. Why would you have liked this on the 833TV but not on the 940 except that the 833 is Taiwanese?
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I do a lot of work in stainless and 4140, so I need low end torque for tapping and drilling. It would be a real PITA to have to change belt positions in the middle of machining a part that was otherwise fully automated via automatic tool changes. If your standing there changing tools with a drawbar hammer between operations, then of course a belt change is a minor irritant.

The spindle James is putting on his Grizzly Is 4 pole, or two-speed, so it can run at slower speeds without giving up so much torque using a sensor less vector VFD. Some specs are here.

Have a look at this video showing that spindle working in steel.

As for my comment on the 833, let me clarify. The 833TV most obvious drawback is lack of low end torque because at 100 RPM the VFD is putting out maybe 10Hz. So a two position belt would have been ideal on that machine for use as a manual mill. The fact that the 940V has a two position belt is great - provided its used as a manual mill. I’m glad PM didn’t try to span 50-3200 RPM without a belt change on the 940V, like they do on the 833TV. But both machines could just as easily been done with two-speed motors and no belt change to get similar benefit (but this would probably cost more). Not a big deal for manual machining to change belts, but pretty disruptive to change belts in a series of CNC operations that include an ATC.

I‘m not in disagreement with you about the utility of the 940V given your desire to do as little as possible for the conversion. It looks ideal, assuming the right things have been done with motor, VFD, spindle bearings for 5K RPM duty. Being from mainland China I am a bit suspicious. But as Mark points out, it does have 3 year warranty. Do think about a power drawbar - I spent the better part of a week on a Tormach without a PDB and it was excruciating.

In my case, I’m open to more substantial conversion efforts to attain perfection. Can the “Best & Jacobs full Custom CNC” be that far off? LOL
 
mksj said:
If one wants to be in the 8-12K spindle speed region, then going with a more purposely built CNC mill like the Tormach 1100MX will take you to 10K RPM. The pricing on the 3 HP ATC ISO spindles I saw listed was pushing 3K and then one needs to adapt it to your mill head
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1100mx is ~$20K now. Just saying.
 
I'm going to try to get more info from PM tomorrow on the 940V particularly regarding the motor and VFD. This could be a good choice for me. I don't think I am likely to need to do much outside of its capabilities. I'm going to spend some time doing research on servo/stepper motors and drives to see what makes sense from a cost vs performance/accuracy perspective. I have been playing around with some recently to get some basic knowledge.

The ArizonaCNC stuff looks pretty good. I sent him an email yesterday with a couple of questions and he responded fairly quickly. When I get a little closer, I might ask him for some recommendations as to which servo/stepper motors to use, he seems to offer several choices.

Is there any value to a PDF on a CNC machine? It is an option on the 940V and I am wondering if it is worth it if I am going go CNC. My milling knowledge is somewhat limited at this time. I suppose there might be times I just want to bore a single hole and don't want to bother programming it.

I need to stop analyzing this and just buy something.
 
davidpbest said:
I do a lot of work in stainless and 4140, so I need low end torque for tapping and drilling. It would be a real PITA to have to change belt positions in the middle of machining a part that was otherwise fully automated via automatic tool changes. If your standing there changing tools with a drawbar hammer between operations, then of course a belt change is a minor irritant.

The spindle James is putting on his Grizzly Is 4 pole, or two-speed, so it can run at slower speeds without giving up so much torque using a sensor less vector VFD. Some specs are here.

Have a look at this video showing that spindle working in steel.

As for my comment on the 833, let me clarify. The 833TV most obvious drawback is lack of low end torque because at 100 RPM the VFD is putting out maybe 10Hz. So a two position belt would have been ideal on that machine for use as a manual mill. The fact that the 940V has a two position belt is great - provided its used as a manual mill. I’m glad PM didn’t try to span 50-3200 RPM without a belt change on the 940V, like they do on the 833TV. But both machines could just as easily been done with two-speed motors and no belt change to get similar benefit (but this would probably cost more). Not a big deal for manual machining to change belts, but pretty disruptive to change belts in a series of CNC operations that include an ATC.

I‘m not in disagreement with you about the utility of the 940V given your desire to do as little as possible for the conversion. It looks ideal, assuming the right things have been done with motor, VFD, spindle bearings for 5K RPM duty. Being from mainland China I am a bit suspicious. But as Mark points out, it does have 3 year warranty. Do think about a power drawbar - I spent the better part of a week on a Tormach without a PDB and it was excruciating.

In my case, I’m open to more substantial conversion efforts to attain perfection. Can the “Best & Jacobs full Custom CNC” be that far off? LOL
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I see your point on the speed change but I need to draw the line somewhere or I will never get anything. Even the 940V or 833TV is much more than I probably need. I am looking at them more for the work size, particularly the X axis, than anything else. If changing belts becomes and issue, I guess I could always change the motor to a 2-speed motor and leave the belt in 1 position.

That ATC spindle is quite impressive. If Matt could get a Taiwanese casting without a head that might be an interesting way to go. Of course you need a mill to make the parts to do that.

I do expect to see a full blown CNC version from you at some point, probably with automatic tool changing as well. I once started a project to rebuild an old Southbend lathe. I took it all apart and started cleaning and painting the pieces. That was about 8 years ago and it is still in pieces in the corner of my shop. I now know better than try that again. I'm better at making small incremental improvements that don't interrupt the use of the tool for too long like the one shot oiler on my lathe. I also have an airplane in my shop that I started building about 15 years ago that is 90% done that I need to finish. I don't need another major project but I will watch yours with great interest.
 
ptrotter said:
Is there any value to a PDF on a CNC machine? It is an option on the 940V and I am wondering if it is worth it if I am going go CNC. My milling knowledge is somewhat limited at this time. I suppose there might be times I just want to bore a single hole and don't want to bother programming it.
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To me, a mill without a PDB is like a lathe without a QCTP - even worse. Watch this video. Then watch this typical milling sequence, noticing the tool changes, and imagine dealing with the drawbar every tool change.
 
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No reason to have a PDF on a CNC machine. I think David is referring to an ATC or some form of quick change system. I might be missing something in CNC of stainless, you are still using an RPM in the 1-2K range for 1/4-1/2" end mills. Tapping requires very little power for smaller sizes and would require some form of positional control (encoder) for the motor. I do not see a motor/spindle designed to spin in the 10-12K+ range having enough low speed power for tapping unless it was significantly oversized. I also do not see the advantage of a 2/4 pole motor when a 4 pole motor can spin to 6K RPM or more, which is much higher than a 2P 60Hz motor. I was interested in the DMM drives in the past because they were AC servo and there was no issue with generation if using a mill in a manual mode (although this is probably not a factor in a full CNC conversion).
 
So I spoke with Matt at PM at length about the PM-940V. It sounds like they had it built for those who want to do a CNC conversion. It has the same casting as the PM-940M, just a different head. The head has a 2 speed belt drive and a new spindle and 3 phase motor. Matt says he has run it at high speed for 24 hours and the bearings only get a little warm. The VFD is a Delta VFD so it should be possible to get the programming information if one wants to change anything or add CNC spindle speed control. It looks like a good choice for my needs. It comes in at the same price as the 833TV w/stand but is heavier, has a larger work envelop and a higher top speed. It also looks like the CNC conversion is easier on the 940 than the on the 833. They have already sold a couple so I guess I need to get my order in soon. I'll probably drive down to their warehouse and pick it up.
 
I agree - the motor is a reasonable choice. I'd extrapolate it is running from about 15 Hz to about 225 Hz at high speed. Maybe the upper limit is capped by the bearings, in which case the maximum rpm would be at a lower frequency. Not sure what the torque curve is like, but it seems reasonable that you could keep to one of the two belt speed options when milling a part (the speed choice governed by the material being milled).

As for making it ATC-capable, the R8 might not be optimal. But you can leverage manual mill tooling. So, also a reasonable choice for non-production. (Don't think I want to venture into ATC quite yet.)

The question of Chinese vs Taiwanese remains. But if you have to swap in the ballscrews, then you are disassembling and reassembling every part of the machine. There is still risk in the finish of the surfaces that matter, and the precision in holes.

Finally, I think that an oiler system would be either a requirement or very highly desirable in CNC. I don't see that the 940 comes with it.
 
ptrotter said:
David, I was questioning the need for a Power Down Feed for CNC, I definitely see the need for a Power Draw Bar.

Paul
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Sorry I misread your question. No need for PDF on a CNC machine. If you plan to use the machine as a manual mill for some time prior to cnc conversion, and you intend to work with a boring head, you'd want PDF.

Thanks for the update about the 940V.

I spent many hours yesterday digging into CNC control software - Mach3, Mach4, LinuxCNC. These bring a bewildering complexity that I do not want in my life. EVER! So I will be exploring alternatives that actually are well supported, or I will give up on the idea of doing a CNC conversion and go with Tormach.
 
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