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  #1  
Old 11-26-2013, 06:01 PM
Radar Radar is offline
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KMG grinder, speed question

I'm on the verge of ordering a KMG grinder. I can't afford a variable speed setup and have a 2 hp motor on hand. The problem is that it's a 3450 RPM motor, so I need to step it down. Using step pulleys available through McMaster, I can slow things down to a low end of 1275 SFPM and a high end of 3900 SFPM. In order to go slower than 1275, I'd need to put a really big pulley on the drive shaft of the grinder,,, which is doable, but then limits my range of speed options.

Given the collective knowledge here, what range of speeds should I target? Without further experience, my goal would be to finish as much on the grinder as I can, minimizing the hand sanding. What grit can I safely run at 1275 SFPM without risk of burning the blades or wearing out belts immediately? Would I be wise to buy a large diameter pulley for the sole purpose of running really slow for finishing in addition to the step pulleys?

Thanks

-Radar
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  #2  
Old 11-26-2013, 06:35 PM
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Ray Rogers Ray Rogers is offline
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That 3900 sfpm is about as high as most any of us would ever want to use. Even then, that kind of speed is only good for gross stock removal with very coarse belts. The Grizzly grinder runs at about 3500 sfpm and it is challenging to use finer belts on it without ruining the belt very quickly.

My grinder is variable speed and tops out at about 2000 sfpm and I rarely ever run it that fast (again, only for the coarsest work). Some guys would prefer something faster than that but most would find that a comfortable speed, I think. After the coarse work, I rarely run my grinder more than about half speed and often about 1/4 speed for the finest belts. I get very good use out of my belts.

So, if you have a 3450 motor you might want to go ahead with that really large pulley IMO ....


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Old 11-26-2013, 10:28 PM
Doug Lester Doug Lester is offline
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I have a similar problem with my Coote grinder. I run it with a 1 1/2" pulley on the motor shaft and 5" on the drive wheel. With a course belt it still takes metal off pretty fast and with the finer belts the work doesn't overheat as readily. My motor speed is the same as yours.

Doug


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Old 11-27-2013, 04:54 AM
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Crex Crex is offline
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Agree with Ray and Doug. You'll find that you spend most of your time in the lower speeds. The 2hp will give you the torque you need on the slower end. Just make sure you get your pulleys set tight with good drive belts. Junk drive belts can cause vibration and/or "hopping". Neither are good for controlled grinding.
I went to those "adjust a link" drive belts and they seem to run smoother for me.


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  #5  
Old 11-27-2013, 09:07 AM
Radar Radar is offline
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Thanks for the input guys, confirmed my suspicions. I think what I'll do is get the step pulleys and a separate large one (9" ish). The step pulleys will handle the mid-high speeds (1275 - 3900) and then I can swap in the large one to cover ~600-1200 range when I need slower speeds.

Great suggestion on the link belt, that's the direction I was planning to go. There aren't any seams and they tend to run a lot smoother in my experience.

-Radar
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  #6  
Old 11-27-2013, 01:44 PM
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R. Yates R. Yates is offline
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this is what I use http://www.tecowestinghouse.com/Prod...ives/fm50.html

http://www.tecowestinghouse.com/PDF/FM50%2808.04%29.pdf and here is the spec's for it .

Well worth the Money .

Sam


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Old 11-27-2013, 02:19 PM
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Ray Rogers Ray Rogers is offline
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Sam,

If I'm reading that correctly that device has a 3-phase output so he would need to find a different motor (assuming the 2 hp motor he mentioned is a single phase motor). If that's right I wanted it to be clear so he wouldn't run out and buy one thinking it would work with his motor. Still, at that price it could easily be worth the extra expense to get a 3 phase motor (which are generally cheap) ....


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Old 11-27-2013, 03:18 PM
Larry Peterson Larry Peterson is offline
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Friend RADAR,

I would like to share my opinion. I have a KMG and I use a 3 hp, motor that runs at 1740 RPM. I had to send for the larger drive wheel to get the speed I like. I use a 3 step pulley to get the slower speeds for finish work. I also use a BaderII with the small drive wheel for finishing work. I would use the faster motor and a 3 step pulley from the motor to the drive shaft to get the finishing and finger groove speeds.

It is my opinion that the KMG is much superior to the Bader II. I mounted the KMG on a rubber pad to dampen any harmonic vibration. I might also suggest purchasing your wheels and 1 1/2 square mounting arms from the company. I started by drilling my own but soon found that the factory arms were much more precision and tracked much better than the ones I made up. Aluminium is great for the wheel arms.

Good luck with your new knife grinder!, Larry Peterson
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  #9  
Old 11-27-2013, 05:25 PM
Radar Radar is offline
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Thanks all for the advice.

I pulled the trigger today on the grinder, it'll probably ship out sometime late next week, the painful waiting begins.

To clarify, Ray, you are correct, it's a single phase motor, so speed control is out of the question (until I get the uncontrollable urge to upgrade ).

$250'ish for the speed control isn't a bad price, but the motor I have cost me 25 bucks. Add the cost of pulleys from McMaster and I'm up to about 75,,, still far short of the upgrade to infinite speed control. I'm planning on going with 4-step pulleys, that will allow me the greatest flexibility in speed control but won't bring me down to the sub 1000 SFPM range. I can slow things down a little with a 6" that I have on hand but will hold off on investing in anything larger than that until I have a chance to assess how my setup works.

I purchased the basic platen machine with no additional accessories. I've heard that the 8 or 10" wheels are great, but I'm only doing flat grinding now and don't really have a need to do much profiling. I have a neighbor who works at a sheet metal shop and he's willing to laser-cut my blanks (at least wile the quantities are small, beyond that it may take some barley-bribing to keep things going). The next most useful accessory that I see is the small wheel attachment. I had contemplated making my own, but if the precision difference is significant, I may spring for the Beaumont parts. Thanks for the heads-up Larry. Also glad to hear that the Al arms work well. I was concerned that the set screws would mar the surface and mess up the fit of the bar in the socket.

So, if I didn't recognize it before, I'm all-in now.

-Radar
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  #10  
Old 11-28-2013, 12:07 PM
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R. Yates R. Yates is offline
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Yep you are very correct Ray had to go out and look at my motor here is the tag photo .

http://imgur.com/BSMnVIc,Untvt0L,9Bv...Ba9M,TRNNnj2#4

3 Ph it is although they do sell controllers for a single ph also .

Sam


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