Concerning belt grinder motor...what is optimal RPM?

dadarrow

dadarrow

Joined
Sep 10, 2007
Messages
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I am wanting to build a grinder. In looking for a motor I stumbled onto a 3 HP motor that runs at 3450 RPM. In reading some other posts I saw that this might be too fast for a grinder. Therefore I would like to know the optimal setting for a grinder. Any information is helpful as I am very new to the addiction! :eek:
 
dadarrow said:
I am wanting to build a grinder. In looking for a motor I stumbled onto a 3 HP motor that runs at 3450 RPM. In reading some other posts I saw that this might be too fast for a grinder. Therefore I would like to know the optimal setting for a grinder. Any information is helpful as I am very new to the addiction! :eek:
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I wish it was that easy, DaDarrow. but it really depends on your Surface Feet Per Minute. (SFPM) You can run step pulley's and reduce your RPM and increase your torque. However back to SFPM... I like to keep mine around 2000 - 2500 sfpm. It removes stock fairly fast but doesnt burn the stock very fast. For instance I run a home made 24" wheel at 355 RPM (using the formula SFPM = PI x D x RPM / 12) I get 2,230.5 SFPM. To get the 355 rpm I use a 5:1 reduction pulley. with a 3/4hp TEFC 1750 RPM motor. Your find the your SFPM will change with the diameter of the wheel your turning. Heres an online calculator to help you out.
http://www.discount-tools.com/sfm.htm

Jason
 
Depends, it's not just a diaper...


What grinder are you planning to put it on?
direct drive, step pulleys, vfd ?


You can make any RPM work with changes to pulleys and drive wheel sizes

I do have comments about your 3 hp though.
Some use a 3 HP, but for the most part they are using it on big blades to tang taper, or remove scale..and are FT makers who count lost seconds as lost dollars.

It depends on your situation, for the average user, even if the motor is free, the additional costs of running heavier wiring and breakers to handle that power are much more than going to a smaller motor and cheaper wiring for a 1.5 or 2 hp motor.

Especially when you are using pulleys to slow a grinder down, like the KMG or NWG you are setting up the ratio for lower speed and greater torque.

I started with a single speed 1 HP and I don't think I ever bogged it let alone stalled it.
With sharp 60 grit belts, steel just evaporates.
 
Most 220 circuits are 20A+ which will run all of the 3HP motors I've seen. As long as he's not having to put in a new dedicated run for it, the cost will be the same. Much easier to wire for a 220 grinder anyways. If it's free, that beats paying for it.

As far as being too fast - it won't be. Just choose your drive wheel and/or pulley setup to suit it.
 
How ever you set it up in the end its all about the speed the belt moves, in surface feet per minute.
The second need is to be able to change the belt speed. Pulleys or VSD.

I run a three horse power 1725 rpm, on a KMG which has a six inch drive wheel. It tops out at 5400 sfm which is where I grind blades.

Every one has a speed they are comfortable with. Quality ceramic belts can run at 7000 sfm.

Fred
 
Thanks Jason,
I will be ordering the grinder package from PolarForge...looks like our best option. I just got a real good deal on this motor and I am hoping I can make it work.
 
Thanks for the advice...wiring won't be a problem...shop already set up for it. I am blessed with a decent shop that I am converting for knife making. Just anxious to build a grinder. probably the GIB as we have looked at many options but feel the Polar Forge GIB will be a great project. Hope all this works out!
 
I have a feeling the GIB will work out well for you, and you will enjoy building it. I built one like it with a 3hp motor/VFD. The good thing is that it has the tooling arm design for versatility. You may want to get a smaller drive wheel, say 3." That would yield a belt speed of roughly 2708 sfpm with direct drive at 3450 rpm. Not too slow to remove material, not horribly fast for finishing. Variable speed sure is nice if you can swing it...
 
Salem Straub said:
I have a feeling the GIB will work out well for you, and you will enjoy building it. I built one like it with a 3hp motor/VFD. The good thing is that it has the tooling arm design for versatility. You may want to get a smaller drive wheel, say 3." That would yield a belt speed of roughly 2708 sfpm with direct drive at 3450 rpm. Not too slow to remove material, not horribly fast for finishing. Variable speed sure is nice if you can swing it...
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Not sure if you are still active but I have another question for you...I never build my GIB...I ordered it but life took a turn. However, I am trying to get it built now and don't have plans...I have the metal but not the plans. Do you know of anywhere I can track down plans and part list for the polar bear forge GIB setup? He's no longer available so I'm shooting in the dark.
dadarrow
 
Stacy E. Apelt - Bladesmith said:
I have a few sources for you:


www.lifesforge.com

Building a Grinder-In-A-Box | United States | Life's Forge

My experience building the 2x72 belt grinder kit from Polar Bear Forge.
www.lifesforge.com www.lifesforge.com



GIB grinder in a box plans - Search Videos

www.bing.com www.bing.com





GIB grinder in a box plans - Search Videos

www.bing.com www.bing.com
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Thanks! I have watched the 6 (actually 7) videos on the build which were very informative. I just watched the Ken Deckelman/BoltOnHooks LLC video. These are all great resources. I truly appreciate your information. The Life's Forge data was useful as well. I am assembling a parts list right now. I am also having my GIB parts loaded into a program where I can cut a few more units out. We (my sons and me together) can build 3 of these grinders. It has triggered my interest once again!
 
Ken H> said:
Dirk, you might take a look at this page where he built a GIB and gives pretty decent step by step info on the build:
https://www.lifesforge.com/building-a-grinder-in-a-box

I've not watched, but here is a YT series on building the GIB.
Click to expand...
Thanks Ken. This series was HUGELY helpful. He does a great job in listing materials used (hardware, wheels, motor, etc.). The cool thing is that while I was putting a spreadsheet together on a list of likely needed parts, I stumbled across a parts list I created back in 2012 in my drive files. I've been able to cross reference what I found in this video series to my list from '12 and now I have a better understanding of what I need.

I have access to a laser cutter and am going to try to assemble three of these grinders as a project with a few of my boys that have a desire to build knives. I may even catalog the process via video and see if it will help anyone in the future! I'm definitely a novice but I am intersted in the process and will be setting my shop up to accommodate these grinders! Wish me luck!
 
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