Suggestions on Speed Ranges after VFD install

[Tomzo]

Greetings - I am finishing up my VFD control box and with any luck will have time this weekend to get it installed onto the mill (HF RF31 clone). My motor is three phase 2 HP 1725 RPM and I have the VFD set with a minimum frequency of 6 hz (1:10 from 60) and a max at 120. This gives me ~172RPM up to 3450 RPM at the motor. The existing motor is listed at 1720 RPM so it is similar from the outset but with the VFD if I want to I can push the speed to double.

My question is what those of you with more experience would find to be a good range. I want to have a lower speed to use a coaxial indicator, so I am leaning on the lower end, but wondered if speeds in excess of 3450 RPM would be desirable. I am generally going to be milling aluminum and some steel.

While I can always change the belt configurations to increase or decrease RPMs, the whole point is to do that rarely, unlike now. I am trying to figure out an "optimal" initial belt configuration.

Thanks

Tom

 

[matthewsx]

Just my thoughts on it.

Lower speed is only limited by the power you need for any particular job. Upper speed I would get an infra-red thermometer and watch the temps as you go up. Carbide tooling likes speed so it would be good to figure out how fast you can go before heating up the bearings too much.

How hot? Can't really tell you but with a thermometer you should be able to get a feel for what you think is safe. There should be some specs on the bearings somewhere, Rong Fu still makes the machine so you could ask them.

John

 

[mksj]

A usable speed range is more like 20-120Hz, although you could go lower say for indicating but would have very little torque because of the belt ratio. Most 4P (newish) motors mechanically are usable to 2X their base speed, new motors a bit higher, but performance wise you probably will get some fall off below 30Hz and above 90Hz. Since most high speed work doesn't require much torque, I would top out the motor at 120Hz. At some point if you decide to replace the motor you could go with a 2 or 3 Hp inverter/vector motors which have a much wider operating window maintain full power over a wider range. A 4P vector motor like on my mill runs from 20-200Hz, so is a direct drive single pulley and a back gear for the low speed range. W/O a back gear or low speed setting even a 3 Hp motor will bog down with low speed boring/heavy milling. Cooling tends not to be an issue within the speed ranges mentioned.

 

[Tomzo]

Thanks - yes, the slower speed is really just for when I use a coaxial indicator, which is not super often now as I am generally hesitant to use it at higher speeds and too lazy to get out the ladder and swap belts. I don't see myself doing much that requires any amount of power at such low speeds. The main question in my mind is whether I would find a use for speeds over ~3500 RPM.

 

[MikeInOr]

I usually run my mill between 30hz to 120hz. Outside of that range I will change the belts. I am not saying this is correct, just what I do.

 

[Tomzo]

Thanks Mike - Forgive my inexperience here but what is the max RPM that you would use on a mill like this?

 

[MikeInOr]

Thanks Mike - Forgive my inexperience here but what is the max RPM that you would use on a mill like this?

I will leave that to someone that is more knowledgeable about these mills than me.

 

[mikey]

The main question in my mind is whether I would find a use for speeds over ~3500 RPM.

The real question is whether or not your bearings will live anywhere near this range. You might want to contact the bearing maker to see what kind of rpm range they will tolerate. While you're at it, ask them what the normal operating temp of the bearings should be so you can monitor them.

 

[Beckerkumm]

It generally requires quite large bearings for 3500 rpm speed to become an issue, even if the bearings are under preload. Machines today tend toward smaller bearings. Load capability is sacrificed for cost and high speed. If there were gears or clutches involved, they could be a limiting factor but I would guess here it is more about how well balanced the spindle is. Dave

 

[mksj]

Over 3500 RPM, not really for a manual mill, in that range maybe if you had very small cutters, like engraving. Although the bearings should not be an issue, it depends on the bearings used, lubrication and preload. As Mikey mentioned, you would want to monitor the bearings if doing sustained milling at the higher RPM. I had a mill that was designed/spec'd for 50-3000 RPM, any prolonged use above 2500 RPM, the bearings area became too hot to touch. This was a new mill, but unimpressed with the overall quality and eventually sold it. If you have an RF-31 clone then I am assuming it is mainland China copy, and I would not expect the same level of bearings and overall finish as the Taiwanese RF-31. I see no reason to go above 3,000 RPM on the top end speed for the mill spindle.