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The Newbies Arena Are you new to knife making? Here is all the help you will need.

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  #1  
Old 06-20-2012, 01:19 PM
Jim T Jim T is offline
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How slow should a grinder turn?

For the past few years I've been working on a single speed grinder with a 1 1/2 horsepower, single phase motor that turns at 3450 rpm.

I'd like to hook up a VFD (variable frequency drive) so I can alter the speed when necessary. How many rpms should the motor be able to be slowed down to for knife making purposes?

Jim T
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Old 06-20-2012, 01:39 PM
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Ray Rogers Ray Rogers is offline
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About 0ne rpm suits me pretty good. What I really mean is it should be able to slow down to almost stopped and everything in between up to full speed. Mostly, you might use about 1/4 speed or 1/3 speed but every now and then you'll find a reason to want the belt to creep by. It's not absolutely essential, but it can come in handy ....


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Old 06-20-2012, 01:47 PM
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Eli Jensen Eli Jensen is offline
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That was my question too. Should my new motor be 3450 or 1750?
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Old 06-20-2012, 01:56 PM
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Ed Caffrey Ed Caffrey is offline
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If your going with a VFD, you can use either speed motor and be good. If your grinder is a single speed, DO NOT use a 3450rpm motor! An example of why not is my very first "real" grinder....a Wilton Square Wheel. It came with a 1725rpm motor, but had a 10" drive wheel....which made the belt speed approx, 4600 sfpm (surface feet per minute). Make a "bobble" at that speed, and you can literally cut a blade in half!
The joy of variable speed is that you can run wide open for heavy grit belts (that's the speed the work the best, and last the longest), and you can dial down speed as you go finer and finer in belt grits.....this not only gives a better cut and finish, but also makes the belts last longer.


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Old 06-20-2012, 02:27 PM
Jim T Jim T is offline
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Thanks guys. Any recommendations on what type of VFD would work for my situation? Or barring that, a VFD supplier who could walk me through it and supply the proper piece of equipment? I understand that some motors might overheat if run at TOO SLOW a speed.

Jim T
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Old 06-20-2012, 02:32 PM
rscoffey rscoffey is offline
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Google Wayne Coe, Blacksmith, artist. He can fix you up.
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Old 06-20-2012, 03:32 PM
Doug Lester Doug Lester is offline
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For my Coote grinder with a 10" drive wheel the maker recommended a 1750 rpm motor if it was to be used with a three step pulley cone or it would be too much of a strain on the drive wheel bearings at high speed. With a single set of pulley wheels you can really slow a 3450 rpm motor down but you're stuck with a fixed speed. So depending on how you have the grinder set up either speed motor will do.

Doug


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Old 06-20-2012, 03:51 PM
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AUBE AUBE is offline
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Jim,

A month back I asked about VFD and Ed gave me a recommendation, he said "I would highly recommend a KBAC 24D or 27D..." After doing additional research I feel what he recommended is probably the best option. Just Google them. Keep in mind they come in AC or DC output so you need the proper one for whichever motor you are running.
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