Mill Choice PM-728VT or PM-833VT

[RJS100]

Hello,

I am 99% done building the electronics for a CNC mill conversion using the Acorn board and closed loop steppers. My goal for the mill is to make one-off prototypes mostly with aluminum and some other harder metals. Since I don't know exactly what I will be making, I could actually end up working with the harder metals rather than aluminum. The last time I worked a mill was in high school using a Bridgeport so I have absolutely no experience with smaller benchtop machines. I had originally planned on converting the PM-728VT, but have since seen lots of videos that stress the importance of rigidity. I am now considering the PM-833VT which is about twice as heavy. I have listed the key features of each below. While the PM-833TV has a bit greater travel than the PM-728VT, I most likely will not need the additional travel or slight larger table size.

It is now time for me to pick the mill to convert. I welcome any of your thoughts to help me understand what I would gain from the added heft of the PM-833TV or any other thoughts comparing the mills.

Thanks... Richard

PM-727VT
Weight: 375 lbs.
Motor: 1HP BLDC
Drive: 2 speed belt drive
Bearings: Angular contact
Lube: 1-shot
Max Spindle Speed: 4300
Made: Taiwan


PM-833TV
Weight: 750 lbs.
Motor: 2HP + VFD
Drive: 1 speed belt drive
Bearings: Tapered Roller
Lube: 1-shot
Max Spindle Speed: 3250
Made: Taiwan

[jpmsteadi]

Richard,

I own the 728VT but have not see the 833 in person. I have seen some videos of a conversion to that as well.

I would say that the 833 is the better choice if you think you might be doing anything but aluminum. The 728 does have a slightly faster max spindle speed, which is handy for small tools in aluminum, but in making parts I have found the BLDC motor of the 728 to not work particuarly well at any sort of material removal rates.

Having an additional HP of power and a proper VFD will make the 833 more useful I think.

That being said, I think you can do good things with the 728 as long as you stay within the parameters of it. Were I to purchase a machine again I don't think it would be the 728. ITs a great machine, but I outgrew it quickly.

[RJS100]

Thanks for your insight. That is very helpful. I will seriously consider the PM-833VT.
All the best... Richard

[theblinkingman]

I also have a PM-728VT and rigidity has most often been the limiting factor when trying to optimize toolpaths. I've done some small parts in tool steel with it but it's a struggle. There's a pretty small window of useable feeds and speeds between just rubbing and chatter. I've had a lot more luck with hsm paths with 1/4 or smaller tools than any kind of big cut, but maybe a better machinist could get more out of the mill.

It can chew thru 6061 pretty well. Enough so that I don't run it all out so that I don't get chips sprayed in my face. But even there, the doc of the hsm path is limited by column rigidity.

That being said, it has let me prototype what I need in a variety of metals, so for me it would be more about quality of life when choosing between the two of them.

[RJS100]

Thank you very much for you reply. It is a tough decision as the PM-833TV is about $900.00 more than the PM-728VT. On one hand that is a decent chunk of change. On the other hand, once you buy a lighter machine, you really cannot do much to improve the rigidity. Thanks again for your feedback... Richard

[RJS100]

OK... Thanks for your thoughts. I have decided to go with the PM-833VT. Thanks!