[TiagoSantos]

I'm sure there have been a few other guys with this question..

I couldn't find much though, so here we go. I apologize if I'm missing something obvious, and if you do find other threads where this has been beaten to death, please link me to them!

I'm converting an RF-40 type machine to CNC. Got it all going with the stock leadscrews, then took it apart to install ballscrews. At the same time, I found an incredible deal on a brand new, but incomplete, RF-45. It's missing a motor and table, maybe a few other minor bits, but I have a complete column and head assembly that I can bolt onto my RF-40.

So the plan is to use my usable machine to convert the new head to belt drive, do whatever else I need to move the head up/down as my Z axis, rather than the quill on my current machine, then swap the column and head into my base/table.

Long story short, here's what I thinking for the belt drive. I have a brand new 2hp 3ph 3500rpm Toshiba motor and a VFD. The VFD manual tells me it'll run continuously with full torque at 6Hz. Seems a bit of a stretch, but if that is the case, theoretically I can run the motor from 7000rpm@120Hz, down to 360rpm@6Hz. Obviously those are pretty extreme limits, but I'm considering just going with a single 0.75 ratio on the belt drive, which would give me spindle speeds of 270rpm to 5250rpm.

For my purposes, I anticipate I'll run between 1000 and 3000rpm most of the time.

Does all of that sound reasonable? I know, ideally I'd be running within 20% of the motors rated speed the whole time, but.. Running a single speed belt drive rather than 2 different ratios would make the system much simpler.

[UUU]

If the motor is 2hp at 60Hz and you can get the same torque at 6Hz, then you only have 0.2hp at the lower speed. You could look up in a feed and speed program to see if this power is suitable for the cutters and materials you propose to run. If you want to put a shell mill through some steel it may not be enough.

That said, I often run my lathe at low motor Hz, to save the bother of changing the belts, and I'm surprised how well it performs.

[SCzEngrgGroup]

Any 3-phase motor and VFD will give nearly constant torque up to the motors base speed, and constant power above base speed. This means that below base speed, power drops linearly with decreasing motor RPM, and above base speed torque drops linearly with increasing motor speed. At low frequencies, the torque does fall off, though this fall-off is considerably less with a sensorless vector VFD.

Regards,
Ray L.

[TiagoSantos]

Hmm.. You're right, 0.2hp isn't much. Even with the 0.75 ratio, it ends up at just under 1/3hp. Granted I don't think I'd be running at those speeds very often, but..

Maybe a two speed setup with a 0.75 and a smaller ratio, with an adjustable tensioner or motor mount plate would be ideal. Definitely gets more complicated, but it's all doable..

[arizonavideo]

I'm running a single speed belt drive on a RF-45/IH mill with a 3 HP and belt drive at a .8 to 1 ratio.

If you want any low end torque you may want to go with a larger motor. Drilling with large bits in steel like a 3/4" is slow going with a 3HP, a 2 may not get it at all.

Milling with any end mill under a half inch is just fine and face milling with a 2.5" works fine and fast.

I put the pulleys on the inside. It looked like this.

[ame=http://www.youtube.com/watch?v=4dmfP5i2syQ]Single speed internal belt driive RF-45 LM-45 .MP4 - YouTube[/ame]

I also did a two speed with the more standard setup.

[TiagoSantos]

Yup I remember your setup, thanks for the inspiration

Are you running a 1700rpm motor? What's your max spindle speed, and do you miss having a higher speed?

I anticipate most of my work will be in aluminum..

[arizonavideo]

The single speed runs a 3600 3HP Larson. I also have a two speed drive made to use a 1750 motor. If I did it again I would get the 5HP for the low end torque.

My spindle was giving me some vibration over 4K right now so I stay under that most of the time.

I changed my spindle a few days ago so doing test now. I'm just starting to find the top speed.

The single speed gave a nice wide range of around 200 to 4K RPM. Compared to the stock 1800 RPM it came with that was smoking. Large drills and tapping with bigger than M10 taps was a problem on the low end the top end was fine.

You have to give up something with a single speed it is just the way it works. If you have two mills it is not to much of a problem but if you really want to tap and drill in one setup then you will need to give yourself some extra low end torque and perhaps loose some top end. It helps if you have a motor that you can run at 100Hz or so as you should not need much power for smaller bits so you just spin it faster. The fan get noisy at the higher speeds and I would guess they could be a problem if they came loose.

I'm trying the two speed now. I don't know if I will go back to the single speed or not. On the single speed I really liked the belt being enclosed and the motor was mounted on rubber so it was vary quiet. No way it would do a M12 tap in aluminum, I tried....

I have both, so time will tell what I think is better. Right now it is a toss up.

[TiagoSantos]

Thanks again for the details. I'm sure a two speed setup would be much more flexible, but I think for the time being, I'll go with a single speed setup. Maybe gear it down so my top speed will be around 4000 rpm at 100hz and learn to live with the limitations.. I don't expect to be drilling big holes in steel all that often, but who knows!

I do have the possibility of building a second machine.. At the moment, I have a complete RF-40 (that I originally converted to CNC) and I just took delivery this weekend of a new RF-45 that's missing a table. The plan is to use my RF-40 to convert the new head to belt drive, make the z-axis parts and brackets, then move the RF-45 column onto my old base, with X and Y already done.

I will have an almost complete machine in parts, and I've been thinking about machining the dovetails out of the old saddle and base, and mounting linear rails instead. Oh well, it'll keep me busy for a while, that's for sure.

[arizonavideo]

Check to see if the column will fit. They may be slightly different.

Good luck.

[TiagoSantos]

That was the first thing I did as soon I had the crate open! It looks like it'll work, but either way, the old saddle fits in the new base as well, so one way or another, I'll get it going..