PM-932M or PM-835S on the fence

[M.T. Pockets]

Functionally the x and y are the same. The z is different depending on the machine. Can you give more details about what kind of work you're planning on doing? Theres a lot of pros and cons to each. The biggest con to bench mills IMO is head droop.

I wasn't suggesting the OP consider the 833, although that would be a good candidate as well, provided you get the geared head version. I am not convinced the variable speed belt-driven 833TV has enough low speed torque to do a good job machining harder metals like steel. The geared head unit certainly does.

While I won't argue the geared head has more torque, torque hasn't been an issue with my 833TV. I've drilled 3/4" holes in 1010 and some other mild steels and never stalled, but have had tools spin in the collet even with it torqued down. Of course it all depends on what you want to do but if drilling big holes in steel isn't going to happen often I think the TV option is the way to go.

 

[davidpbest]

Functionally the x and y are the same. The z is different depending on the machine. Can you give more details about what kind of work you're planning on doing? Theres a lot of pros and cons to each. The biggest con to bench mills IMO is head droop.



While I won't argue the geared head has more torque, torque hasn't been an issue with my 833TV. I've drilled 3/4" holes in 1010 and some other mild steels and never stalled, but have had tools spin in the collet even with it torqued down. Of course it all depends on what you want to do but if drilling big holes in steel isn't going to happen often I think the TV option is the way to go.

I'm anxious to see a real bake-off. Driving that 833TV motor at 10Hz to get at the published 50 RPM can't be very torqueé. Love to see it pipe tap 1/2" NPT: LINK.

 

[parshal]

I've stalled my 833TV tapping aluminum at the slowest speed. I've also broken 5 or 6 taps. Both of those results are surprising because, most often, the tap slips in the chuck first.

 

[M.T. Pockets]

David, that would probably not go well. However, I don't see myself needing to tap holes that size for my projects and if I do I can do it by hand for those rare events. I do however mill a lot of aluminum so the VT version definitely fits my needs better as I need the higher rippums a lot more than I need the torque.

Its all about what you need the machine to do and what fits best for your kind of work.

 

[davidpbest]

Totally agree. I see the 833TV as ideal for aluminum. And I don’t mean to dismiss it at all. Quite the contrary - it looks to be a very nice setup for nonferrous materials, and light duty work with ferrous materials. What I bristle at is this idea that a single belt-position mill with 3-phase motor on a VFD putting out 20hz and below can come close to it’s geared-head equivalent in low speed torque. Because the 833TV is advertised as 50-3000 RPM, without much qualification, this could certainly disappoint someone expecting to do the kind of tough steel milling the geared-head twin can do. That’s all I’m saying. Both machines have their place, but the buyer should be aware of the advantages and limitations of both versions of the 833, which was the original intent of my fully vetted and published white paper.

 

[Skowinski]

Functionally the x and y are the same. The z is different depending on the machine. Can you give more details about what kind of work you're planning on doing? Theres a lot of pros and cons to each. The biggest con to bench mills IMO is head droop.

While I won't argue the geared head has more torque, torque hasn't been an issue with my 833TV. I've drilled 3/4" holes in 1010 and some other mild steels and never stalled, but have had tools spin in the collet even with it torqued down. Of course it all depends on what you want to do but if drilling big holes in steel isn't going to happen often I think the TV option is the way to go.

Thanks for the comments and education everyone.

I'll probably never need to drill or tap big holes in steel. I think in general most all of my work will be on the lighter end of the spectrum, I won't need to hog off large amounts of material in a hurry, never will need to make multiple copies of a part. FYI almost everything I'll be doing will be motorcycle related, making small modifications to existing parts, and making some parts (steel and aluminum) for old classic bikes, stuff you can't really easily find and buy now.

The more I think about it the more the larger knee type mills are likely unnecessary for my work, and a bench mill will be just fine. So, at least I've gotten that far in the decision making process.

edit: what is head droop?

 

[wrmiller]

Head droop is gravity forcing the head on a bench/bed mill to tilt forward. When I started using larger bench mills, I counterweighted the heads (using actual iron weight plates) like you see on the large CNC bed mills. When properly done, this counters the weight of the head on the Z-axis screw and nut, and will help reduce head tilt/droop.

I also got in the habit of traming the spindle with the Z-axis locked. Then when cutting I always made sure I locked the head. (I had my Z-axis on the head, not the spindle). The only time I couldn't do this was plunge cutting/pocket milling, which was rare for me.

 

[mksj]

Travel (X, Y, Z) and Z axis height (table top to nose) becomes pretty important as you start to work on larger parts, in particular when you have the vice height, the part and then try to tap a part. Also the Y travel is often a limiting dimension on many bench top mills. The PM-833 and PM-840 have travel dimensions that are close to what you would have with a full size knee. QMT has a nice comparison sheet of their benchtop mills so one can compare the different models. I agree with David as too trying to squeeze such a wide operating RPM with a single speed belt drive, so more geared to higher RPM work. It should be OK for most work other than low speed (under ~200 RPM) with big drills and cutters. As far as material removal, one can use a rougher for rapid metal removal.

Per what Bill just posted, ead droop or nod, is where the head at the spindle end may lean slightly backwards due to wear and play in the gibs. The heads can be quite heavy, and over time you can get a bit uneven wear in the gibs, if you need accurate work I would lock the head and use the spindle. On benchtop mills vertical alignment of the column is not adjustable, other than shimming the column, on a J head knee mill the nod (front to back) of the head is adjustable.

 

[parshal]

As for tapping steel, I just tapped a 3/8-16 hole in an MT2 arbor at the slowest speed on my 833TV. The spindle stopped once but, mostly, the tap spun in the chuck. I can certainly see a tightly held tap would stop the spindle. I've found reversing speed to clear chips helps immensely but the chuck unwinds and out comes the tap.

I'm finding I power tap frequently and this tap stopping or slipping is quite annoying. But, the ability to change speeds with a dial vs moving a belt is much more convenient. It allows me to quickly find the sweet spot when fly cutting or using an end mill.

 

[davidpbest]

As for tapping steel, I just tapped a 3/8-16 hole in an MT2 arbor at the slowest speed on my 833TV. The spindle stopped once but, mostly, the tap spun in the chuck. I can certainly see a tightly held tap would stop the spindle. I've found reversing speed to clear chips helps immensely but the chuck unwinds and out comes the tap.

I'm finding I power tap frequently and this tap stopping or slipping is quite annoying. But, the ability to change speeds with a dial vs moving a belt is much more convenient. It allows me to quickly find the sweet spot when fly cutting or using an end mill.

I use a Keyed chuck for tapping. That way you can get more grip on the tap and it doesn't loosen when you run in reverse. The disadvantage is that the risk of breaking a tap goes way up, especially for 1/4 or M6 and below diameters. With the TV at low RPM, the motor is likely to stall out before breaking a 3/8" or M8 or larger tap.