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Old 03-15-2016, 03:29 PM
ss caustic ss caustic is offline
Join Date: Jan 2015
Location: Canada
Posts: 17
direct drive 2x72 question

hey guys I've been googling /searching quite a bit on this but have found mixed info.

I'm sure it's been asked or spoken of before at nauseum but I can find any definitive info.


I'm a little confused as to how the sfpm is determined ,and what is really required.

I'd be using ceramic/coated belts most often , but will also be using small contact wheels and polishing belts.


basically I'm trying to determine what rpm (1725 or 3600 ) of 3ph motor to use on a direct drive 2x72.

now this will be a vfd driven motor looking at 2 hp or 3hp.

so far I've read that:

1) some guys are using a 1725 rpm with 6" drive wheel and using the vfd at 120hz to get higher speeds.

(but this would seem to me to end up burning out the motor faster)

any one have pros /cons to say about this?

2) some are using 3600 rpm with 4" drive wheel,( some say you loose torque)

is this really an issue with 2hp or 3hp motor?

any thought on this?

any and all help appreciated.

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Old 03-15-2016, 03:50 PM
damon damon is offline
Join Date: Jul 2013
Location: NE Tennessee
Posts: 410
my 2hp doesn't bog down easily at my guess would be youd not have to worry about loss of torque with either 2 or 3hp. so my guess would be either set up would get the job done quite well.

the extra speed should cut well without getting heavy fisted on grinding.... let the speed do the work, where as at lower speeds..... the torque would be more beneficial.

just my thoughts. though I'm not a pro on the subject of motors. just using my experience as a jeweler with lower speed/high torque flexshaft motors vs 300,000rpm/not much torque micro motors, and which one works better at which tasks.

hope this helps
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Old 03-15-2016, 03:55 PM
ss caustic ss caustic is offline
Join Date: Jan 2015
Location: Canada
Posts: 17
is your 2hp on a direct drive? or pulley system?
if direct drive what rpm is the motor?

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Old 03-15-2016, 04:55 PM
damon damon is offline
Join Date: Jul 2013
Location: NE Tennessee
Posts: 410
ive got this one....

3" pulleys...
4" drive wheel...
3450 rpm
SFM..... whatever I tell it to be with the VFD
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Old 03-24-2016, 11:23 AM
Ron Neilson Ron Neilson is offline
Join Date: Jul 2012
Posts: 2

I have built 4 direct drive grinders. All four used the KBAC 29 (1P) Variable speed controllers.
Two were 208V 3HP 1740 RPM Leeson motors, which I use in my shop. Two were the same controllers using 208V 2HP 1740 RPM Leeson motors. With the controllers set up for 120V; With the KBAC set up for 120V, the max motor HP is 2. Set up for 220V the max motor HP is 3. I am using 6" drive wheels. The 3 HP units have more power than I have ever really needed. Running these grinders with 60 grit belts at medium speed, I have tried to bog them down grinding every kind of steel I could find, pushing as hard as I could, they are simply put amazing. I have used many single speed AC, and a couple variable speed DC grinders over the past twenty something years and nothing I have ever used has come close to the power of these setups. The 120V 2HP units I built for people who did not have ready access to 220V. They too are very powerful and plenty adequate for anything most people are wanting to do. The KBAC controllers are amazing, I would highly recommend them to anyone wanting to build a variable speed grinder. The AC variable speed grinders are head and shoulders better than any VS DC units I have ever seen. The DC units vary the speed by raising and lowering the voltage, which raises and lowers the power accordingly. These AC controllers raise and lower the speed by raising and lowering the frequency. Simply put there is little noticeable loss of power at low speeds. As a note, The controllers have a bunch of jumpers, dip switches and rheostats. All of them must be set to the proper configuration or the units will not perform as desired.
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Old 03-24-2016, 02:29 PM
ss caustic ss caustic is offline
Join Date: Jan 2015
Location: Canada
Posts: 17
thanks for the replies,

at the end of the day I deiced to go with a 5 " drive wheel , leeson 2hp, 3 ph 1800 rpm, 220v and leeson vfd nema 4 ,220v ,1ph input power.

hope I'm not disappointed , but It will likely be just fine.
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Old 03-26-2016, 12:06 PM
C Craft's Avatar
C Craft C Craft is offline
Steel Addict
Join Date: Jul 2008
Location: N.W. Florida
Posts: 280
Personally I don't like the direct drive approach unless you are going to do a variable speed. You will find once you use such a grinder there are times when you need to slow down the cut speed.

You were talking about KMG, they actually offer four different motors! Two they consider single speed use, a 1 1/2 hp. motor and a 2 hp motor. They also offer two with their variable speed setup.

The specs on KMG's 2hp motors they offer, (single speed) are as follows.

The KMG was designed around this motor using the step pulley system. Will run on 110 or 220V.

Heavy Duty Single Phase Hi-Torque Motor are designed specifically for severe "Farm Duty" applications. The motors have double sealed ball bearings.

1725 RPM
7/8" shaft
220VAC single phase 60hz.
Gasketed Capacitor Housing and Conduit Boxes
Copper Windings and Varnish
Manual Overload Protection
Totally Enclosed Fan Cooled
Double Shielded Ball Bearings
Rubber Boot Over Manual Reset for Weather Protection
Capacitor Start
Service Factor 1.15
High Starting Torque
Mounting is standard NEMA 56HZ frame, bolts directly to our SUBP plate

Beaumont offers their motor their 2hp at $390.00 plus shipping.

The one they show in the pic when the site opens shows two starting capacitors but, when you go to the page with the actual motor for single speed 2 hp it has only one starting capacitor on it! Not sure what that is all about!!

This is that same motor on ebay for $224.99, plus shipping!!

I bought one brand new from an eBay seller his price included shipped to my door. I had to watch for about a month and even had to do a little poor mouthing by email to the seller but finally got it delivered to me for $200.00 Whaaaaaat I am not above doing a little poor mouthing for a better price!!

So if you can get is shipping and all for $200.00 your doing pretty good. I run mine off of 220V and I have never bogged it. I use the 4-3-2" Step pulley, that I also got of eBay by watching the prices!

You also have other options. There is a way to use a 3-phase motor without having 3-phase power and often these are cheaper because 3-phase is often commercial and the market is a little softer because the average Joe doesn't have 3-ohase power!

By the way if you still are having problems figuring the formula for grinding, take a look at this page their is also a calculator available on this site!

Here is a good thread from another forum on the subject as well!

C Craft Customs
With every custom knife I build I try to accomplish three things. I want that knife to look so good you just have to pick it up, feel so good in your hand you can't wait to try it, and once you use it, you never want to put it down !
If I capture those three factors in each knife I build, I am assured the knife will become a piece that is used and treasured by its owner!

C Craft

Last edited by C Craft; 03-26-2016 at 12:13 PM.
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Old 03-26-2016, 12:19 PM
ss caustic ss caustic is offline
Join Date: Jan 2015
Location: Canada
Posts: 17
thanks fro the reply,

ya I'm planning on using a leeson vfd ( variable frequency drive) for speed control.

thanks for the link on the belt rpm
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