convert rf45 type to belt variable speed and some cnc info?

[dcprecision]

Hi TPPJR,

I really enjoyed the photos. Attached is my solution which took 1/2 hour to build and install. I used 90lb. gas springs which give an effective zero head weight at the angle shown.

[Ron111]

Guys,
Any ideas what the spindle bearing are rated at, as far as rpm, for the rf45 clones and the IH?

Ron

[TPPJR]

Originally Posted by dcprecision I agree that you can't have both worlds. The best power solution I can think of is to install a 3 phase 2 speed motor that is inverter rated and run with a 1:1 or lower belt ratio. This gives a spindle rpm of lets say 300 to 2500 low speed and 600 to 5000 high speed. So far as torque is concerned. By reducing the ratio to 1 : .75 you can still get around 4000 rpm high with improved torque. Even when using a 3/4" end mill you still want at least 1500 in aluminum and 750 in steel for optimum material removal. The only application I can think of requiring high torque / very low speed is when drilling very large diameter holes ( 5/8" + ) with a drill bit. If its CNC, you can mill them. My RF-45 came with a 2 speed motor from the factory so I am set to experiment when I get around to it.

Looks like we have the same machine. I don't think I would go over 4000 rpm, probably stay about 3k. I think that is around the limit for the type of bearings the RF has. Wish it had "real" spindle bearings. I would want to do the conversion mainly to get away from the head noise and heat.

Thomas

[TPPJR]

Originally Posted by dcprecision Hi TPPJR, I really enjoyed the photos. Attached is my solution which took 1/2 hour to build and install. I used 90lb. gas springs which give an effective zero head weight at the angle shown.

Looks good. I am still waiting for my 150lb spring to come in to replace the 200lb. I will be done with the counter balance then. Did you loose any travel?

Thomas

[dcprecision]

I would run it faster, at least 4500 rpm. If the bearings give up the ghost, so what, you can always replace them with quality units. I am more concerned with the motor itself. A motor designed for inverter use has an armature that is resin impregnated and bonded and is criyically balanced. This keeps it from flying apart under the high "G" force loads encountered at high rpm. One other point!!!! I estimate that the RF type gear drive system is probably loosing at least 40% of its power and possibly more at high speeds due to heat production. A belt is going to be 95% + efficient. Just think of all the power you gain vs. gears. This is power where it counts on a cnc and far offsets the torque lost at very low spindle speeds.

[TPPJR]

Originally Posted by dcprecision I would run it faster, at least 4500 rpm. If the bearings give up the ghost, so what, you can always replace them with quality units. I am more concerned with the motor itself. A motor designed for inverter use has an armature that is resin impregnated and bonded and is criyically balanced. This keeps it from flying apart under the high "G" force loads encountered at high rpm. One other point!!!! I estimate that the RF type gear drive system is probably loosing at least 40% of its power and possibly more at high speeds due to heat production. A belt is going to be 95% + efficient. Just think of all the power you gain vs. gears. This is power where it counts on a cnc and far offsets the torque lost at very low spindle speeds.

Remember I am not shooting this down. I like the idea. Actually the bearings are quality from the factory at least mine were. When I did my rebuild I replaced them anyway with good ones again. The problem is that they are the wrong kind of bearing for that kind of speed. The best will not hold up. If you go belt drive why do you have to over speed the motor to get 4500? Just adjust the pullies that you use to give you the rpm you want without seriously over speeding the motor.

Another thing is that the gear box lube is what helps carry away the heat from the upper bearing. Once that is gone the bearing will run hotter still.

Thomas

[dcprecision]

Thomas,

I understand what you are saying. I think that the vast majority of heat is being produced by lubricating fluid displacement by the gears themselves not spindle bearing friction. Just like a car engine, the vast majority of heat is not from combustion or bearing friction but by oil friction between the cylinder walls and pistons. I think that the bearing temps would be substantially lower without the gear system. I have not disassembled the RF head so I can not say definitively what type of bearings are employed. There is no bearing cooling needed on drill presses or knee milling machines, so why on a RF spindle.

I was under the impression that a belt drive conversion is going to fit internally within the gear box using a timing belt. No access for belt changes. If a ratio of .8 : 1 were used for improved low speed torque, a motor rpm of 5625 is needed for 4500 spindle rpm. I think that 1:1 ratio is more practicle with a 2 speed motor. This gives a good power range of 1000 to 5000 rpm considering gains in transmission efficiency.

[TPPJR]

The bearings are conical bearings, about the worst you could use. The preload changes with changes in spindle temp etc, etc. The bearings produce a ton of heat. Try this, run your spindle (not the head) to the lowest positon then run the mill full speed for 20 minutes. Then feel where the lower bearing is, (where your quil stop is attached) it will be quite hot, you may not even be able to keep your hand on it. Then feel up the spindle from there, the temps will drop but you are getting closer to the gearbox. The gearbox does produce a bunch of heat, most of the heat you feel when you touch the gearbox is from gear friction but the upper bearing runs hotter than that and the oil is on the outside of the casting the spindle slides up and down so the heat is getting transfered to the oil from the bearing too. You would just loosse this heat transfer if you go to belt drive.

What benifit would there be to having a two speed motor if you went belt drive? I have a two speed motor now but have the VFD wired to the high side only. Two speed is not needed.

The worst thing I feel in this whole process is not having the mill to make the modifications that I would need to make. I would have to take the mill down to do the work then would not have a mill to do it with. I would love to do some tinkering around with this without any permanent mods if that is possible.

Thomas

[dcprecision]

Hi Thomas,

I see what you are talking about with the bearings. When you say conical, do you mean Tapered or Timkin roller bearings? Can they be replaced with an angular contact at the bottom for thrust, and a radial at the top??

The reason I was thinking about using both the high and low speed options on the motor is that I believe that torque characteristics may be better at low spindle rpm using the 1725 rpm windings at higher frequency.. I don't know for sure about this, it is just a logical guess...Can anyone advise on this.?? Inverters change the frequency of input to change speed and usually have a range of something like 0 to 120hz. (cycles per second).... This gives up to 2x stated RPM obtained on standard 60hz. power. Using the low speed setting, you would be driving the motor at about 35 hz. at 1000 rpm rather than at about 18 hz. on the high speed setting. This, I think, may yield higher torque and smoother operation at low speed because you get more power pulses per revolution.

Dennis