**RESOLVED** Help me decide

[kpa_jay]

I have a Bridgeport Series 2 Interact 2 I'm about to convert. I'm also going to use a VFD to run it off single phase. The VFD gives me the opportunity to use speed control, but since the Bridgeport already has the air-speed, I'm not sure which way to go, or use both?

Here's what I see. The motor max rpm is 2500. If I tie the VFD into the Centriod control, I can set the belt ratio so at the motor max rpm the spindle spins at the rated max of 3500rpm and leave the belt speed alone, using only the Centriod/VFD to control speed. Though I see this as not optimal for lower rpm's - perhaps just use the back gear. Alternatively leave the VFD fixed and adjust speed with the Air -Speed control, though with this approach I think I loose flexibility..

I wonder is there's some sort of mix of the two that works well?

Jay

[Maint_1]

VFD with a VFD rated motor that will allow you to run the range of speeds you need.

[CRM]

I know this is a late reply, but it might answer the question for others.
I have two Hitachi VFD's used precisely because it enabled me to run 3 phase motors on single phase power. The added benefit is that I now have variable spindle speed on my mill and drill press. There should be a parameter setting in the VFD that determines the maximum frequency output allowed. 60 Hz will spin the motor at the motor nameplate speed (in the U.S. anyway) BUT you can set the parameter for a higher value, in my case, 90 Hz. That will drive the motor at a maximum of 1.5 times the rated speed. You don't want to go nuts with this value because of balance and bearing life considerations.

I would keep some type of gear (or belt) reduction for spindle speeds below about 400 rpm because there isn't much grunt for bigger tools at that rpm.

[CNCguy]

TB008 (Rev1) - AC inverter spindle motors

Remove mechanical variable speed mechanism

AC inverters allow the speed of a standard AC induction motor to be varied electronically. The current generation of AC inverters offer extremely efficient operation, with full motor torque down to almost zero speed, at a very attractive cost. In addition, it is very easy to interface an AC inverter to a machine tool control in order to provide programmable spindle speeds capability. These factors make AC inverters a natural choice for machine tool spindle use.

Traditional variable speed spindles on machine tools usually employ spring tensioned sheaves and a mechanical hand crank to vary the spindle speed. Wide belts transfer power from the driven sheaves on the motor shaft to the driven sheaves on the spindle. The mechanical losses due to belt friction are considerable. A 3 HP motor may only deliver 2 HP to the spindle. Also, these systems are subject to accelerated wear, and the spindle speed selection is very imprecise.

While it is possible to drive such a mechanical system with an AC inverter driven motor and then vary the spindle speed by electronically changing the speed of the spindle motor, it can be extremely dangerous in some situations. If the mechanical variable speed mechanism isn't totally disabled, it may be possible to run the spindle at extreme speeds, causing the sheaves to fly apart, or to ruin the spindle bearings. It is by far a better idea to eliminate the mechanical variable speed mechanism all together. In this instance, the sheaves and crank mechanism are totally removed and replaced by a fixed ratio V-belt or multi-vee belt system with a 1:1 ratio between the motor shaft and the spindle. This allows the maximum amount of motor power to be transferred to the spindle, and also allows for more precise speed control.

[polaraligned]

Driving a BP spindle 1:1 is a big mistake.
While torque delivered from motor may be constant close to zero rpm's, the power at the spindle plunges as the rpm's decrease. Note that as the spindle rpm's decrease in the bp's, the torque is multiplied mechanically with the much steeper gear ratio to keep power delivered at spindle up.
Step pulley heads do great with a VFD, but the varidrive heads are not bad either.

[CRM]

Big mistake to drive the spindle at 1:1 ratio. "full motor torque down to almost zero RPM" is salesman talk. We personally made that mistake on a Monarch Lathe when we converted the old DC variable speed (vac tube technology) to an AC inverter and motor. we removed the 2 speed gearbox when installing the AC motor. Forget trying to drill CRS with a 1.000" twist drill. Tools take HP to cut. Large tools take HP at low RPM to cut. HP is a mathematical computation of Torque over Time (Torque x RPM).
Now, IMHO, the best solution in this situation is a Polygroove (serpentine) belt with about a 1.5:1 ratio while retaining the back gear assy for the large tools that need grunt below, say, 500 RPM. That addresses the mechanical inefficencies of the Vari-Drive. A micro switch could tell the control which gear range you are in and invert the spindle direction in low range so an M03 would always turn the spindle clockwise. I don't know firsthand, but I believe with a fixed pulley ratio, the software should be able to compute spindle RPM based on the frequency command to the VFD, and by looking at the micro switch state, could apply a different formula to compute spindle RPM while in low range.

[cncsnw]

I don't know firsthand, but I believe with a fixed pulley ratio, the software should be able to compute spindle RPM based on the frequency command to the VFD, and by looking at the micro switch state, could apply a different formula to compute spindle RPM while in low range.

That is exactly what it does.