Pros and Cons of geared head and variable speed

[AllenV]

I have sold my RF-40 round-column ENCO clone and committed to purchase of a new square column bench-top. I am purely a hobbiest, doing this for fun. The enjoyment of precision work, planning order of operations and tinkering with set-ups are ends in themselves. I have no plans for CNC type machines. The mill will be paired with my Enco 13x40 lathe.

I am looking very closely at the PM-833T or PM-833TV as these offer a large working volume (large Y travel) and X and Z power feeds. Z power feed became a thing with me recently as I strained my shoulder cranking the head up on the Enco.

I just bought the steel to make a stand but have not cut any metal yet. I am delaying a mill purchase until August as we will be out of town for a while.

-> THE POINT OF THIS POST is to discuss geared-head versus variable speed and to let you folks draw my attention to some other machine.

I don’t want to belabor the point as several others, as well as PM’s website, have detailed the high speed, up to 3200 RPM, benefits of the “TV” coming at cost of low RPM power and torque whereas the “T” offers max power and torque throughout the 60-1500 RPM range. But, really, the belt drive “TV” is only twice the RPM of the geared head “T”. Frankly, the Enco topped out at 1970 RPM and I never felt limited in speed, we’ll not discuss rigidity or the round column of that machine. Am I missing something here? Talk me into a “TV” for duffer manual milling of whatever random project I dream up. Some project examples are making a solid-block tool post to replace the compound on the lathe and and converting a Mercedes distributor for modern electronic ignition control by toothed-wheel crankshaft position sensing. I’ll likely have to make the toothed wheel and other small parts for that as the best size is not sold as generic.

To help me visualize the performance envelopes of the 2 machines I found a table of spindle RPM for milling various materials at:
https://www.etantdonnes.com/MACHINE/TABLES/cuttingSpeeds.html

Then I overlayed two lines in red at roughly 1500 and 3000 RPM to show what the additional speed of the “TV” enables. The upper line is set on 1500 RPM and the lower line is 3000 RPM. It shows that only at cutter sizes below ½-inch, and then only for brass, bronze, and aluminum is the geared-head mill not able to hit the RPM in the table. Basically, for these materials and small cutters the geared head would just be run slower. Any issues here?

As I write this I am nudging myself off the fence and over to the geared-head machine.

 

[matthewsx]

I’ve heard of people converting geared head to belt drive, obviously not the other way around.

Also, gears are noisier.

John

 

[davidpbest]

The attached document should be helpful. It includes specific analysis of the geared versus VFD-driven PM833, including links to some videos. DM me if you have questions.

 

[Huub Buis]

The advantage of gear driven is that the lower the speed, the more torque you have. The disadvantage is (a lot) more noise.

The advantage of variable speed is very fast and flexible speed changes and (a lot) less noise. In general, depending of the type of motor, the torque will drop at low speeds.

If you run most of the time thick end mils, you can benefit from gears. If you run most of the time small end mils, you will benefit from the variable speed.

 

[7milesup]

David provides some very valuable information and his linked document is well worth the read.

I used to own a PM833T and I considered it a decent machine. I, and others, have had some issues with the gibs in the ways, in particular on the Z axis. Not a show stopper, but something to be aware of. The 833TV was not available when I bought mine, but had it been that would have been the route I would have taken.
The reasons are...1.) I wanted to do a CNC conversion later on, but converting a geared head 833T to CNC would have been a serious uphill battle. 2.) I felt that the 833T was loud. Even though I am half deaf (literally), it had this obnoxious whine at higher speeds, to the point of wanting to wear ear plugs.

If you have room for a full sized knee mill you might want to consider that. I sold my 833T and purchased a used full size Sharp knee mill that was not much more than a new 833.

 

[Janderso]

purchased a used full size Sharp knee mill

I recently viewed the Precision Mathews website looking at their bench mills. The price on these have really gone up.

I too bought a used FULL size Sharp mill. It came with tooling, a knee power drive and a X table drive, and a vise, it cost $4,000.
I love the mill but I don't like the size. It's huge!
10x50 table with a 3hp motor. But, I couldn't work with the small work window with the bench mills
By the time you have a vise and a drill chuck you are very limited on the bench mills. Great for model making though

 

[7milesup]

I recently viewed the Precision Mathews website looking at their bench mills. The price on these have really gone up.

I too bought a used FULL size Sharp mill. It came with tooling, a knee power drive and a X table drive, and a vise, it cost $4,000.
I love the mill but I don't like the size. It's huge!
10x50 table with a 3hp motor. But, I couldn't work with the small work window with the bench mills
By the time you have a vise and a drill chuck you are very limited on the bench mills. Great for model making though

You got a good deal on the Jeff. I bought mine for $6300, but mine has the smaller table, although it does have a 2-axis CNC setup. I love the rigidity of those big Sharps.

 

[j-becker]

I was going through similar questions before I bought my 833TV and was especially worried about the potential lack of low-end torque. Talking to Matt he mentioned that they far outsell the TV for the T version (I remember something like an order of magnitude) and that he has yet to hear of somebody complaining in practice of the low end torque. My experience bears that out. I mainly use low rpm for tapping and the TV seems to be okay with the tap sizes I have used at around 100 - 120 RPM. And the lower noise of the mill itself is definetly a plus. On the other side I have rarely used more than 1500 rpm (I haven't done much aluminum). So you probably can't go wrong with either one. If space for example restricts you to a benchtop mill (I would love to have space for a Sharp or Acra knee mill or even better a Deckel) I would go for the 833TV again without hesitation.

 

[B2]

Hi @AllenV

I am looking very closely

A couple of additional comments:

I have a PM940M-VS-CNC. VFD with 2 speed geared head. 1.5KW 3 phase motor. Speeds from ~100 to 3200 rpm I also wanted the larger envelope and went this 940M route.

By far the motor's fan motor noise is louder than the gear noise. So compare motor cooling too. For my CNC operation I sometimes am working on the machine or the code without cutting far longer than I am actually cutting and this noise gets to be an issue. During cutting it is not uncommon to run a vacuum to collect the debris and this is loud. My hearing has already be badly damaged so, I always wear ear good plugs. ( Recommend: Etymotic Research ER20. I like these as they are inexpensive, last for years, can be easily cleaned, and are comfortable for hours. I also carry them with me when traveling on a plane. ) One can also add a switch to the motor fan, or a thermal switch to turn it on when the motor gets too warm.
A VFD can be added to most motors and mills, geared or belt driven. So variable speed it not really the issue ... if you are willing to add a VFD.

The geared head is heavier. This has both advantages and dis-advantages: The advantage is that it is heavier so has more mass while cutting, better rigidity. Dis-Advantage, for my CNC when the motors are powered down the z-axis stepper motor tends slip to allow the head to slowly drift downward.... unless the gibs are extra tight. Also, unless the gibs are extra tight, there is a small amount of what I call dynamic nod of the head when changing directions, up to down or down to up. This is due to the head tilting as the lead screw changes direction and the balls in the ball nut changing positions. With the gib loose, the spacing between the dovetail at the top of the saddle can vary with the weight/torque on the head. This is not all that much of an issue for a non-CNC machine. On my machine this represents about a 0.002" backlash contribution (in addition to any other backlash that is around). But this value is gib tightness dependent. If the gibs are tightened up this tends to go a way, but if too tight then the stepper might miss a step when going upward. For non-CNC tighten the gibs up a bit to eliminate the dynamic nod and install a DRO to read the vertical saddle position accurately.

By the way, when engraving one usually wants very high speed (>10,000 rpm) , but these bigger machines are not very good for this due to the backlash etc.

Good luck.

Dave L.

 

[AllenV]

Thank you all for these well-considered responses. A few thoughts jumbled together.

Geared head mills are noisier: I get that. But I have struggled to quantify the unpleasantness of the geared head noise level. I had my geared head Enco for more than 15 years before selling it. I never thought the noise was a problem. Of course, now I have tinnitus. But I blame loud music in my youth. Maybe PM mills are noisier? This prompted me to initiate a wide-ranging data collection project asking users to measure noise from their mills and post results. Here is the link.
https://www.hobby-machinist.com/thr...various-milling-machines.106851/#post-1049521
we’ll see what happens.


Why not a used full-size knee mill? Two factors really. I don’t want to dedicate so much space in my shop to a big machine which, once planted, won’t move ever again. And, New Mexico is a very small market for used machine tools. They are very hard to find within any distance I would want to travel to get one. Then there is the challenge of dealing with getting such a heavy machine into my back yard shop.

Bottom line is that I try to extract a “happiness value added per square foot” to everything taking residence in my shop. For me, I don’t see a big mill providing sufficiently more happiness quotient to offset the large footprint. And the large footprint means something else cannot come to live in my shop.

@davidpbest, thank you for you write-up on mills. This is valuable data and perspective.

@7milesup. Good to know you had a good experience with the 833T and thank you itemizing why you moved away from it.

@j-becker. A vote in favor of the TV. I noticed your 100-120 rpm tapping speed fits well with David’s note that power fall off below 120 rpm.

@B2. Thank you for offering your experience with noise. Also, I am trying to avoid buying a tool that immediately needs significant modification, such as adding a VFD, which might also need installation of a 3-phase motor, etc., etc.
Of course, in the future if I change course and get interested in CNC then many changes will be happily made. Part of the fun.
I do find the PM-940 interesting, but it may be that the 40-inch table is a bit much?

Keep the thoughts coming!