VFD questions for a clausing 8520 mill

[skipd1]

I am contemplating adding a VFD to my 8520 and my question is how big a motor and controller and what speed do you leave the mill on after the VFD in installed??? My mill currently has a 3/4hp 1725rpm motor, but I would like to eliminate changing the belt positions all the time!!

Skipd1

[Forrest Addy]

You can't elimminate belt changes but you can reduce them.

An induction motor for all practical purposes is a constant torque source of mechanical power. Stating it differently: your three phase motor loaded to nameplate Amps delivers about the same torque from 60 Hz down to 5 Hz. If you reduce the RPM (Hz) setting you also reduce the motor HP proportionately: 3/4 HP at 60 Hz; 3/8 HP at 30 Hz; 3/16 HP 15 Hz; etc. That's plain motor physics.

Machining operations require more or less constant HP regardless of cutter dia for a given depth of cut.

You can finesse the need to change belts by increasing the motor HP. More HP = proportionately more torque. For example: if you have a 5 step pulley that changes spindle RPM over an 8 to 1 range and you increase the motor HP proportionately to say 7 1/2 HP, in theory you would no longer need the 8 to 1 range step pulley. If the driven pulley is mounted direct to the spindle you are home free. If there is a geared transmission maybe not. There may be limitations in the spindle transmission gearing even if you only draw 3/4 HP from the motor for any given spindle speed. You have to work ouit from observation and calculation whether the transmission gearing at highest gear settings will safely transmit 8x their designed torque. So there is no black or white answer to this proposal.

It boils down to this: you can turn the motor speed down for light cuts but as you take heavier cuts there comes a time you need the mechanical reduction.

[lakeside53]

I totally agree! You'll quickly find the belt/gear you use the most with the vfd. I was suprised how little I needed to "gear down" for many operations - some just don't need much power.

I typically limit the bottom end to about 15hz, and the top to 90 or 120 depending on the motor. With care - don't materially exceed the lathe's orginal top speed....

[RWO]

My BP runs at 1120 RPM @ 60 HZ, 185 RPM @ 10 Hz, and 2250 RPM @ 120 Hz. This set-up eliminates the need to change belt position for me. I have a JOG button that runs the spindle @ 10 Hz which is great for power tapping
and positioning a cutter tooth as needed.

RWO

[Forrest Addy]

RWO apparently you work on a small scale; that is, smal cutters and taps. I'm not nay-saying your preferred mode of operation because it obviously suits you. I work that way myself about 2/3 of the time: pick a belt range and dial the RPM I need.

I'm planting the concept in the minds of others considering running a machine tool from a VFD that their speed solution may not always be solved by dialing the RPM up and down. If they wish to run larger cutters at greater chiploads or tap larger holes most likely shifting the belt or even kicking in the back gear will be necessary to avoid stalling the spindle motor.

No harm will be done to a VFD equipped machine by a simple motor stall. The VFD will current limit and protect the motor. However stalling the tool in the cut is annoyiing and if power feed is in use you may break a cutter.

[nort]

I installed one on my clausing mill a while back. It is a teco model good for one hp. I used a lincoln 3/4 hp. three phase motor I got brand new on e-bay for 50 bucks. I mounted it remotely to the tool box beside the mill. I highly recomend teco vfds. they are worth every cent and more. It makes you able to do most jobs without belt changes. If I had to get another one of these to power a machine tool I would not hesitate. Only wish I had got one sooner. You have to control the vfd and it controls the motor. Dont fool around and drag your feet. This one simple modification makes your machine much more fun to run.

[skipd1]

Are you happy with the horsepower or would you rather have a little bit more horsepower now that you've been using it a while?
Skipd1

[nort]

I was stuck with the task of drilling 1/2 inch stainless steel plates out to 1 1/2 inches. Prior to having the vfd. I had to change belt speeds three times for each hole as I used a larger bit each pass. With the vfd installed I could start with a center drill and go to 1/4 inch drill and so on until I reached 1 1/2 inches. This is with the belt speed set at 600 rpm. I didnt run out of useable torque, it was more of a problem keeping the 1/2 inch shank bit from slipping in the collet. I did 12 plates 2ftx5ft. with two holes in each plate. Not bad for a 3/4 hp. mill. I suppose a one hp. motor would work ok, but 3/4 hp. is just fine for everything I do. I dont use a drill press anymore because of the versitility of the little clausing mill.