Bench Top Mill CNC Conversion (basic question)

[bretthl]

Should I purchase a gear head bench top mill with a spindle speed of `~1500 rpm or a DC variable drive with max speed of 3000 rpm. I like the PM833T gear head but am concerned about the low spindle speed. The PM30MV is variable DC at 3000 rpm. Am I making too much of this? Can the gear head be driven faster with a 3 phase motor and VFD?

 

[coherent]

In my opinion having the ability to set RPMs to any variable is the better option, especially considering the increased RPM. A gear head mill may offer more torque depending on the ratings of each. The spindle type and design itself (bearing types and specs) may be a factor if you want to change the motor and run at faster speeds, but I doubt the difference between 1500 and 3000 would be a factor. You can always use a different motor/vfd combo to meet your needs if the spindle can handle the new speeds. If not bearings are usually the limiting factor and can normally be replaced fairly easily. The least expensive option would be to simply get what you want to start with and not go through the cost and hassles of upgrading. Motors and VFDs can get a bit pricey. Another option to speed up a DC variable speed mill may be to install or modify a belt drive setup. Converting to a belt drive and choosing pulley sizes to run in the RPM ranges you want is a pretty common mod. Dual pulley diameters would even allow simple high or low RPM range changes. I would examine to your planned specific requirements. Depending on your mill/cutter size, number of cutting edges, type etc, most materials (like aluminum) should be ok under 3000 rpm for manual milling but for CNC rpms for best performance and quality could easily exceed that. Also remember with a CNC mill optimal RPM is directly influenced by feed/travel speeds which may depend on the machines capabilities.

 

[bretthl]

I like the PM833T because it appears to be a heavier (more rigid) mill than the PM30MV. I don't want to put time and effort converting a light weight mill. I want a mill capable of performing CNC operations in steel. I think I would be willing to trade faster feed rate for rigidity.

 

[cs900]

do you mean the PM30MV? I can't seem to find any specs on a 40MV model.

I would stick with your gut on this one. If you can afford the heaver machine I'd go with it, and just plan on upgrading the spindle motor (and maybe bearings) as part of your conversion. This is a pretty common practice on these mills and is fairly well documented.

 

[bretthl]

Yeah, PM30MV (if fixed it), thanks. From what I have seen on belt driven machines there is no transmission between the motor and the spindle. If you smoke the spindle bearings then change them out. But on a gear head machine would you be speed limited by the transmission?

 

[macardoso]

You will find the low speeds very limiting for tools under 3/8". I would suggest getting the biggest machine you can afford and fir, but if you will CNC it, you will probably want to look into modifying the spindle to reach 5000+ rpm

I would recommend a belt transmission for noise reduction.

 

[cs900]

Yeah, I'm not sure what the top limit of the geartrain is, but i'm very certain is under 5k. I stripped the entire geartrain out when I did my belt conversion.

 

[cjtoombs]

1500 RPM is going to be very limmiting on a CNC mill (or a manual one, for that matter). I have a Tormach PCNC 1100 and it's 5140 RPM max is fairly good, but for small carbide end mills in aluminum it's way too slow. I have the speeder for that, but it does eat up some vertical real estate. On the flip side, variable speed is great at the upper RPM, but at the lower RPM they loose power quickly, which makes using things like slitting saws a huge pain, especialy in steel. They require running the saws faster than they really should be and taking very light cuts. The ideal would be several speed ranges between 100 to about 5000 rpm that give good power across the RPM range. Not something you generaly find on resonably priced manual machinery.

 

[bretthl]

So based on spindle speed the PM30MV is the better choice because of the existing belt configuration and bearings can be upgraded and motor swapped out? Why do people do CNC conversions on gear heads? I think even QMT offered a CNC gear head bench mill.

 

[coherent]

It appears the PM30MV already has the variable speed and belt drive. You may be able to increase RPM by simply changing the pulley configuration/sizes. You shouldn't need to change the motor unless you just decide you want more HP. The existing bearings may be ok to 5000 rpm or more. You can prob get the specs on the stock bearings. If not it should be a fairly simple task to buy/install higher rated bearings that fit without any mods. If your spindle/bearings run really hot, you'll know. Your first project will likely be the CNC conversion... start there and once you know the machines capabilities, travel feeds and speeds after the conversion, then you can determine the RPM required to optimize performance and go from there.