Eagle surface grinder motor and pulley selection

I have an Eagle surface grinder (not sure of the exact model - the bed measures 21 x 8" and the serial number may indicate this is model "4" ) that is currently fitted with a 1.25HP 380/440V 3-phase motor. This is a 2-pole 2850 RPM motor.

My intention is to remove the motor and see if I can dig out the star point and rewire as delta. Alternatively, I have numerous other 3-phase motors to hand which have a nice compact form-factor and will fit the existing space. Newer motors (e.g. 80 frame) will likely foul the lead screw.

Off the top of my head, I have a choice of 0.75 HP, 1 HP and 1.5 HP all of which are 4-pole motors therefore running at 1425 RPM @50 Hz. To maintain the grinding wheel speed I would need to increase the motor pulley size. Under no-load conditions this would obviously work fine but would it perform poorly when grinding? I suspect the higher speed motor would be the better option but as I have some "free" motors the only cost will be my time boring / bushing a pulley from my junk box and trying this out.

Your comments, please.

Edited By AJAX on 16/04/2022 08:00:50

Are you really sure this motor is two pole? I somehow doubt it.

Yes my Eagle motor was 2 pole I fitted a 4-pole and had to make a new pulley to suit. I Went all-in and made poly-vee pulleys.

If you're going to re-motor it watch the overall length of the motor/pulley combination or you'll find that the z-axis screw will foul the back of the motor.

Hi Pete,

Mine most definitely isn’t. Motor pulley is larger than the spindle pulley on mine, which is a series 4.

Well, it's 2850 RPM on the data plate so I would assume so.

Pete, why did you opt for poly vee pulleys? Was slippage an issue? I've only tried the machine once, and that was at the seller's premises. All seemed to be good.

I'm aware of the z-axis screw issue, and that's why I would prefer to use one of my existing motors which I believe will fit.

Mine is probably a series 4 based on what I've seen on the Lathes website. However the table size didn't exactly match the descriptions. It's 21 x 8" with a single t slot.

I had to make a bigger pulley and had a small pulley plus the belts were easy to get at the motor factors who have a very small selection of vee belts now.

After looking at the spare motors I had (several would fit, but all were 1420 RPM) and considering the need to change the pulley and belt, I ended up splitting the hard wired Star point on the original motor. This actually turned out to be very straightforward. It took about 1 1/2 hours to do.

Have I missed something or WHY are you digging out star point ? Are you intending to use a VFD ? If so pay special note to the belt/pulley ratio and the resultant wheel speed if using an increased frequence ie above 50Hz ! Noel.

Must admit, mine took longer than the 1 1/2hours claimed. But I did do mine piecemeal, over about three days. Easy job, really.

Yes, I'm using a VFD. Frequency will be limited to 50hz. I would have been happy with single phase for this application but I had this motor and it fits the space.

Variable speed can be a boon on a grinder. If you find your wheel is too hard you can slow it down so that it behaves 'softer'.

Pete, I had a look at your album photos and noticed you have added a spindle lock for grinding wheel changes. I was wondering about doing that myself when I inspected the spindle and saw the hole. Do you find it a worthwhile modification?

Another question, I was wondering about adding a slide lock (not yet considered "how" in any detail) to be used when dressing the wheel. Or don't I need one? Last time I used a surface grinder was 35 years ago...

Oh yeah the spindle lock is very worthwhile. There's no other handy way of holding the spindle to loosen the nut.

I too had a thought about a gib lock but because of the design of the gib it's not so practical. Either way I haven't found it necessary for dressing, the slide stays put.