PM-833 PM-935 Alternatives

[Xyzzz]

I’m looking at these 2 precision Mathew’s mills. Yesterday I was convinced about the 833. I am sort of convinced I want a 6” vise, looking at a GMT CNC, because it will work on its side, it has decent reviews, has an integrated fixed jaw, and is far less bucks than a Kurt, while seemingly nearly as good, maybe better in some ways. (Anyone have a Glacern Vise? your thoughts?). And 600 v 900.

so The 6” will —fit— on the 833, but is a bit oversized. I’m enamored with the 6” as there are a ton of off the shelf jaws, dovetails, sine jaws, fixtures, etc for the vise.

I mocked up the 935, and it will fit. A little tight, but as long as I don’t push the headstock way back (I don’t remember Ever doing this with my Bridgeport experience, but maybe my memory is poor, and maybe I’m missing a capability—but I can’t do this at all on the 833).

But 2500 bucks more. Thoughts? advise? has Anyone out there hands on both these machines?

Next, what are competitive alternatives? These machines seem like a LOT of value for the buck. Maybe the fit finish, precision, accuracy are lacking?

anyone have experience with the Turcite way treatment? I’ve read it is smooth as glass provides excellent stiffness, and maybe even a hint of damping?

Yes, I understand the Knee mill, at 2x the mass, is going to be far more rigid, but most things I’d be doing would not require insane rigidity. I’ve watched some vids of 1/2” end mills in Steel on the 833, and it looke just fine. knee mill nice? Sure. Will I notice? I don’t think so. I’m really looking for precision, accuracy, smoothness, and a 6” vise.

give up on the 6” vise, and save the space and coin?

also, the VFD on the 833 is key. I’m NOT a fan of changing belts, and the belt drive variable heads can be noisy. But then I could get a VFD on the 935 with a 3 ph pulley head.

anyone added a VFD to a PM pulley head?

Jeeze, probably no one will have read this far. If you did, I’m grateful.
what is your experience?

 

[7milesup]

Funny that you bring this up. I was just thinking about PM's smaller knee mill.
A few points...

• The variable speed belt drive head should be quieter than the gear drive head. My 833T is NOT quiet.
• The variable speed belt drive will allow you an easier conversion to CNC should that ever be a desire.
• Some folks have expressed disappointment with the Z-axis gib, myself included. Maybe they have corrected the issue but mine does not fit worth a crap.
• "Sneaking" down to a tolerance is much harder than bringing a knee up to a tolerance. Just yesterday I ruined a part because the head shifted and lowered itself by about 7 thou. I was not happy...

The only negative I have heard about a mill such as the 935 is the rigidity of the head. Since I have never run one (I have never run a Bridgeport either) I cannot attest to the validity of that claim. @davidpbest has done a rather in-depth look into the aforementioned statement.
There are a lot of folks out there that have the 833 mills and produce great work with them, but yesterday I was ready to put mine up for sale. Actually, if everything wasn't sold out or on backorder, I just might do that.

 

[Xyzzz]

Thank you. That is a great point, it IS really nice to nudge the knee up for accurate Z tweaks. I’ve also heard the step pulley head on the 935 is quite quiet, and with a VFD, belt changes are rare. I’m hoping someone can validate this.

 

[davidpbest]

Thank you. That is a great point, it IS really nice to nudge the knee up for accurate Z tweaks. I’ve also heard the step pulley head on the 935 is quite quiet, and with a VFD, belt changes are rare. I’m hoping someone can validate this.

I've sent you a DM with some analysis that might be helpful. My 935 J-head is the standard belt-drive unit controlled by a pretty sophisticated VFD setup, and I've not moved belt positions since those controls were installed. The head is VERY quiet in main gearing - a bit louder with the back-gear engaged, but nothing like the loudness of a geared head benchtop like Neil's 833T or my Rong Fu 45. My 935 build log is here if you're interested (six pages total). I posted an overview of the customization here on HM a while back.

One disadvantage of the 833 is that to for boring operations, you either have to manually lower the quill during the boring op, or you have to lower the head. The latter practice, even if done with power feed, is going to have a jerky feed rate as the head essentially "jumps" from position-to-position. You're not likely to get super precision boring ops accomplished without a power feed on the quill (something my RF-45 had, as does any J-head mill).

You should have no problem with 6" vises on the 833. I ran two Kurt 6" vises on my Rong Fu 45 with no problem.

 

[Xyzzz]

Thanks! I suppose if you can run two 6” vises, one will be OK on an 833. As far as boring, yes a Bridgeport can do that nicely, but I could probably add a servo or closed loop stepper (on the quill handle) with an Arduino and get infinitely variable power quill. I’d have to figure out some way to engage/disengage.

I’m A big fan of VFDs, so I’d get the variable speed version. That makes me a bit wary of low speed torque for power tapping. is anyone running a Tormach superfl-style fly cutter in stainless on this class machine?

X

 

[Xyzzz]

7miles,

in reference to:

• "Sneaking" down to a tolerance is much harder than bringing a knee up to a tolerance. Just yesterday I ruined a part because the head shifted and lowered itself by about 7 thou. I was not happy...

id suggest (and I’m as much asking as suggesting)—what about adding a couple gas struts that provide more loft than the weight of the head? Then the backlash would be biased in the direction you want, so sneaking “up” would be a no brainer?

I can’t believe I’m the first one to think of this, and it isn’t a standard feature, so maybe other people have some thoughts?

X

 

[mksj]

Heads on bench mills often will incorporate a gas strut or counterweight system in a CNC application. In non-CNC, the gibs need to be locked otherwise there is head nod/movement. On a knee mill the knee movement is not typically used for fine adjustments in height, movement is way too course with the hand crank. A fly cutter uses minimal power, and you would not be running it a very low RPM, face mills you would run at much higher RPM so cutting power should not be an issue. A servo/close loop to the quill handle will not work because their is too much backlash, you would need to operate it directly on the Z-axis leadscrew, a complex setup which you may only rarely need. I have used my PDF once on my knee mill after having it for many years. There is the 940V if you need a wider operating speed with more low speed torque and it is available in a PDF version, albeit is a rougher build quality.

It all comes down to your space and budget, the PM-935 after adding a VFD is almost twice the cost of the 833TV, when you get into that price range there are quite a few full size knee mills and used ones in the 3-4K range that should be in good condition. If you have the space and the ability to move a 1+ ton mill, I would go with a full size knee at the end of the day.

 

[matthewsx]

You guys are practically neighbors, if I had the chance to pick @mksj 's head in person I would definitely take him out for a nice steak dinner before making my decision....

John

 

[davidpbest]

7miles,

in reference to:

• "Sneaking" down to a tolerance is much harder than bringing a knee up to a tolerance. Just yesterday I ruined a part because the head shifted and lowered itself by about 7 thou. I was not happy...

id suggest (and I’m as much asking as suggesting)—what about adding a couple gas struts that provide more loft than the weight of the head? Then the backlash would be biased in the direction you want, so sneaking “up” would be a no brainer?

I can’t believe I’m the first one to think of this, and it isn’t a standard feature, so maybe other people have some thoughts?

X

IMO, best practice on a benchtop mill is to use the quill in conjunction with the quill DRO to set tool heights. Moving the head should be reserved for gross positioning moves to increase/decrease Z-height for differing tool heights (like switching from an end mill to a drill chuck with long jobbers twist drill). And the head-to-column locks should be engaged after repositioning the head to improve rigidity and tolerances.

The need for power feed on the quill is really a function of the types of projects you do and your workflow preferences. Unlike Mark, I use the power downfeed on my 935 (and RF-45) all the time. I can’t imagine trying to hit tight bore tolerances (under 0.001”) with a boring head without power downfeed. But again, it has a lot to do with the type of projects you tackle.

As for fly cutters, I’ve used the Tormach Superfly, but much prefer a dedicated face mill. This has been thoroughly discussed in other threads including this one.

For the past 9 months I have been using a 4” Haas facemill and liked it so much that I just recently bought a 2” version of the same face mill and love it. The insert geometry provides 8 cutting edges per insert, and outstanding performance on surface finish and low power requirements. These are shown in the following photo, with more information on them here.

 

[Aukai]

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