New PM-728V-T Mill

Maplehead said:
Just curious, in general terms, how would you compare the 728 to the 727? I'm not looking for all the details but more on what is an overall better machine. (And will it be beefy enough for steel.)
Click to expand...

On the PM website there is a link to a “PM Bench Mills Comparison” chart. One place to find the link is in blue print on the PM-727V item description page.

However, the PM-728VT is so new that it is not included in the comparison. Hopefully PM will soon update the comparison chart to include this fine new mill.

I recently improved my ability to cut mild steel on my benchtop machine by sharpening the HSS cutting tool, adding even more cutting fluid and further maximizing my rigidity through improved workholding.

I also keep RPMs as low as possible when I cut steel.

On a few occasions (cutting steel), I was greedy with the depth of cut and it stalled the machine because the cutting tool was unable to advance. In these cases, I turned off the machine as soon as possible, hoping that I didn’t damage the machine. So far, the machine has always returned to its original operating condition.

When I took machining classes, the school had large, older domestic machine tools. The instructors told us to go no deeper than, say, .030” in a single pass. I never saw those machines stall due to too deep of a cut.
 
Last edited:
Maplehead said:
Just curious, in general terms, how would you compare the 728 to the 727? I'm not looking for all the details but more on what is an overall better machine. (And will it be beefy enough for steel.)
Click to expand...

Note that I don't have a 727, so I can only look at information from others.

With similar designs the consensus is that stiffness is related to weight. The 727 is about 27% heavier, but some of this is a heavier motor and gearbox vs the 728's BLDC and belt drive. Still the 727 is a slightly stiffer machine. Both machines are stiff enough to handle steel, as commonly demonstrated by even lighter machines such as the PM25 and G0704. The 728 is from Taiwan where the 727 is from China, and generally it is the case that the Taiwanese machines are a little better in some areas. The 727, being a lower priced machine, is an excellent value. The 728 is a little quieter, has much higher spindle RPM, a little larger workspace, is a little easier to move around and has the convenience of variable speeds vs gear shifting.

These mills are more similar than different. The cost, features or availability of accessories (like CNC kits) may drive the decision in one direction or the other.

Good luck in your decision.
 
Last edited:
Thanks Alan for the reply.
The write-up on the 728 is sort of like the mill is the mill to end all mills. God-like. Are the things like the triple bearings that much better than the 727?
One thing I do lean towards is a belt drive mill. Wish all these models were belt driven. But the 728 is $800 more, 100 pounds less and not DRO ready.
What you think?
 
To really answer this I think we'll have to see more actual results from the field. Theory and specs only go so far.
 
Last edited:
Looks like a very nice mill, look forward to hearing more of your review when it is up and running.

You cannot look at the weight as the sole determining factor as too stiffness, the type of materials, webbing, geometries, etc. all come into play. Weight does help to dampen vibrations. I think as stated the 727/728 machines are more similar than different, the choice is similar to if one wants a PM-1236 heavier lathe vs. PM-1236T which it lighter but has better fit/finish, less change gears, etc. As far as not DRO ready, not sure what is meant by that. You can always install a DRO on these types of mills, you typically need to fabricate your own brackets anyway. Lots of good DRO's these days in the $200-300 range.
 
One of the first tasks I have for this mill is to drill and tap some fiber reinforced plastic parts. I used to do this on the G0516's Mini Mill, but they changed the plastic and the Mini no longer has the low speed torque required. Since the tap is tapered it must be precisely stopped so using higher tapping speed doesn't work. I've designed a work holding jig optimized for this mill and here's the first 3D printed draft. It uses heat-set brass threaded inserts for the part hold-down screws, and the spacing of the mounting ear holes matches the T slots in the 728's bed. The pins and ring locate the part repeatably. It is very short which maximizes the Z space for the TTS compression-extension tapping tool. Once I get the right length hardware I'll try this out. The BLDC motor should have good low end torque though it is also dependent on the controller.

20200102_115642.jpg
 
That looks like a really nice jig.

How exactly do you put in the heat-set brass threaded inserts?
 
With soldering pencil and either a special tip or a regular tip, or with a hot air soldering pencil to heat the brass nut on the end of a long screw, then push it in.

In this particular case I sized the hole just slightly smaller than the nut, so I can pull them in with a screw from the top side. They stop at a shoulder in the hole, since I install the nut from the bottom they can't come through the smaller screw hole. This also allows me to recover the nuts if the plastic is discarded.

If the hole is too small then too much plastic must be displaced and the result can be unusable (it often fouls the threads), so a little experimenting may be needed to find the right tolerances.
 
Last edited:
Today I made some chips with the new mill. I drilled, milled and tapped one of the fiber reinforced plastic parts. The mill and the work holder performed nicely. The drilling and milling worked well, though the center finding was a pain in the neck. A DRO would help there, or a laser center finder.

In power tapping the hole the mill had enough torque to tap at fairly low speed with this large tap in tough plastic. But it is still a bit fast and not well enough controlled, it should be fine for CNC but but not as slow as I'd like for manual.
 
Last edited:
You must log in or register to reply here.
Back
Top