Surface Grinder motors choice.

[J Tiers]

.... I think single phase Baldor
buffer and grinder motors are cap run. Notice they start
real slow and don't seem to have a click from a starting
switch? I bet they have a potential relay to start them.
Anyhow, hunt those quiet single phase motors. They are
kinda rare.

--Doozer

Those may be "PSC" motors, OR have a solid state relay replacing the switch. If just one motor run type capacitor, probably PSC. Those have no switching at all, and can be very smooth, plus may have close to a 1.0 power factor at near full load.

[Doozer]

I used to (and still do I guess) collect single phase motors
that were exceptionally quiet. I found 1 out of 10 were
actually quiet enough not to vibrate around on a steel work
bench. Balance yes, but also extra potting on the windings
makes a difference. Interesting that Jerry notes that cap
start / cap run motors are pretty quiet. I will take notice
now that I hear him say that. I think single phase Baldor
buffer and grinder motors are cap run. Notice they start
real slow and don't seem to have a click from a starting
switch? I bet they have a potential relay to start them.
Anyhow, hunt those quiet single phase motors. They are
kinda rare.

--Doozer

[Mcgyver]

Mine has a v-belt pulley on the back of the spindle, but the pulley is loose and probably not the original one. I was thinking of using an automotive style belt on my machine as they seam to have very nice flexibility as well as a better balance job than vee belts. I figured I would turn a couple of pulleys for this grinder and see where that takes me.

buy a high quality belt.....Any variation in the belt can be a major source of vibration as it passes over the pulleys. Another vote for a well balanced 3P motor. The lighter the grinder, the more balance matters....SG grinding motors are suppose to be much better balanced than a run of the mill motor

[J Tiers]

A cap start motor used with a run capacitor, can be much smoother than an ordinary so-called "split-phase" motor having no capacitor. Relatively close to 3 phase performance can be obtained if the load is pretty constant. A grinder in the cut is relatively constant load in a given material and depth of cut, so it is at least possible.

A run cap can be added to a cap-start motor, even if it did not originally have one, although you do want to check the resistances of the windings to be sure you do not overheat the start winding with continuous current.

[Forrest Addy]

Dynamic balance of the armature is only a part of a single phase motor's vibration signature. They are notorious for torsional vibration and there is no cure for it outside of drastic de-coupling/isolation techniques. You can bet the factory motor was selected with vibration in mind. Off the shelf or used general purpose single phase motors are very likely a bad choice.

If the candidate motor motor hums when idling on the workbench, it's unsuited for a tiny surface grinder. The loudest sound you should hear in an otherwise quiet shop is the whir of the motor's cooling fan. Unless you can find a single phase motor that runs exceptionally smooth you should probably plan on using a 3 phase motor/VFD. That is not an expensive option, especially when perfectly good motors are junked every day and fractinoal HP VFDs's are less then $80.

[engineerd3d]

Mine has a v-belt pulley on the back of the spindle, but the pulley is loose and probably not the original one. I was thinking of using an automotive style belt on my machine as they seam to have very nice flexibility as well as a better balance job than vee belts. I figured I would turn a couple of pulleys for this grinder and see where that takes me.

[projectnut]

I don't know what brand of motors originally came on the MG 612 grinders. Mine has a Hoover (yes the same company that makes the vacuum cleaners). I believe it could be the original motor in that it was built at the Kingston Conley plant in New Jersey. This plant was acquired by Hoover in 1945 and sold to Rockwell in 1956. The plant primarily built grinders for Stanley, Walker-Turner, and Wissota. They also sold motors to other manufacturers for metal and woodworking equipment.

http://vintagemachinery.org/mfgindex...l.aspx?id=1993

Given that Hoover only made these motors from 1945 to 1956 and they were primarily built for grinders, and my machine dates to the mid 1950's I believe it is the original motor. My motor looks similar to this but in considerably better condition:

http://i.ebayimg.com/00/s/NjAwWDgwMA...WLAqq/$_35.JPG

This one is a 1 hp model.

As for a belt, the older models used a flat belt and pulley system. The later ones use a standard V belt. Over the years Sanford changed the flat belt and pulley design several times. At some point in time they stopped making and selling flat belts and pulleys. They made a kit to convert the older flat belt models to the V belt design.

The belt tension is somewhat different than on most modern machines. Here's a quote from the instruction manual.

"Belt tension should be such that the belt between pulleys can be compressed to within 1 inch of touching. This is rather loose, and should be so."

[Sparky_NY]

Well, it looks like I may have to make due with a GE industrial motor for now anyway. I heard that these grinder had very high grade balancing done on their motors in the past, I just don't know how well that relates to more modern motors. I guess I will find out.

Wouldn't hurt to check with a local motor shop or possibly a automotive machine shop to see what it would cost to balance the armature to a higher degree. A common industrial motor is not often used in a application that is so extremely sensitive to vibration as a surface grinder so I wouldn't expect the balancing to be as "precise"

[engineerd3d]

Bear in mind, although a single phase motor was offered originally, it may well have been (more than likely) balanced better than the average motor by the factory.

Well, it looks like I may have to make due with a GE industrial motor for now anyway. I heard that these grinder had very high grade balancing done on their motors in the past, I just don't know how well that relates to more modern motors. I guess I will find out.

[Sparky_NY]

Bear in mind, although a single phase motor was offered originally, it may well have been (more than likely) balanced better than the average motor by the factory.

[JoeLee]

According to the Sanford sales brochure the later MG 612 and 618 machines came from the factory with a 3/4 hp, 3450 rpm, 115/230 volt single phase 60 cycle motor. The earlier machines came with a 1/2 hp motor with the same speed and voltage specs. They offered as options 220 or 440 volt 3 phase motors, or a direct current motor.

My MG 612 came with the 1/2 hp 3450 rpm, 115/230 volt, single phase motor. It works fine and can easily grind a mirror image surface. I have old price quotes I found online for several of their machines. The base price in 1992 for the MG612 was $9097.00. If you add in all the options the price could easily climb to over $15,000.00. With the accessories mine had a list price of $11,546.00. I have no idea what the original owner paid for the machine.

Here are a couple pictures of mine:

My KO grinder is also three phase, I run it off a VFD but I've always wondered if running a grinder off a VFD can also cause finish problems??? I do like the ability to have a soft start and fast slow down or stop.

JL..................

[engineerd3d]

So it looks like the GE 1/2HP motor is going to be the one to put in. Is a DC motor advisable in a situation like this? Is it worth the investment?

Wow. @PROJECTNUT, that is exactly what my grinder looks like. Including the stand.

[projectnut]

According to the Sanford sales brochure the later MG 612 and 618 machines came from the factory with a 3/4 hp, 3450 rpm, 115/230 volt single phase 60 cycle motor. The earlier machines came with a 1/2 hp motor with the same speed and voltage specs. They offered as options 220 or 440 volt 3 phase motors, or a direct current motor.

My MG 612 came with the 1/2 hp 3450 rpm, 115/230 volt, single phase motor. It works fine and can easily grind a mirror image surface. I have old price quotes I found online for several of their machines. The base price in 1992 for the MG612 was $9097.00. If you add in all the options the price could easily climb to over $15,000.00. With the accessories mine had a list price of $11,546.00. I have no idea what the original owner paid for the machine.

Here are a couple pictures of mine:

[Sparky_NY]

As a benchmark, my Boyar Schultz 6x12 grinder had a 7 inch wheel and a 3/4hp 3450 rpm motor, 3 phase. Single phase motors cause finish problems on surface grinders. Not having 3 phase, I have a vfd on my grinder which also allows a slow start up and shut down which is nice.

[JoeLee]

I would go with at least the 1/2 HP especially if your going to run it at a 1:1 ratio to the spindle with a 7" wheel.
You could go with a higher RPM motor and reduce the spindle R's to the normal 34 - 3600 R's. That would give you a little more power.
I think most motors are going to be at least 3400 r's. That seems to be a standard for any motor.

Vibration is also going to be a huge issue. A regular motor is probably going to have too much vibration resulting a crappy finish.

JL.............