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Brian Madigan

3450RPM motor, SFM

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I was digging around the shop last night and found a 3450RPM 1HP motor from a grizzly dust extractor unit I scrapped a while ago.

Currently I am running a 1725 RPM 1/2HP at 3665 SFM (2:1 pulley ratio).

If I run the 3450RPM motor at 1:1 with the 4" drive wheel, I get 4"*0.2618*3450RPM=3612.84SFM

So 3612 surface feet per minute is a little less than the stepped-up 1/2HP motor.

BUT, from what the machinists say, 3600SFM is too slow for serious grinding, and 5500 with water cooling is where you want to be at.

 

Anyone running a 2x72 or similar at 5000+ SFM? Wet?? I'm guessing older AO belts won't like that kind of speed. Good because I don't buy them anymore.

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im running about that fast and i see no real need to go faster but some times i would like to slow it down a bit i did notice that when i went to the 1.5 hp at that speed that i could burn the back of the belt which i couldn't do before

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Burn it on the platen or on a contact wheel? I should shoot for a 3 speed setup again, since I do use the platen about half the time.

I have trouble finding really good step pulleys that are round enough. The cast zinc ones made here in Chicago have all been prone to going off center.

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Who is telling that 3600 RPM is too slow? SFM? Are we having a unit of measure miss-match? My HC grinder runs about 2500 rpm at it's fastest and that is often too fast.

 

I don't know anyone that uses a water cooled grinder, everything tends to rust, and you need to wear a rain coat while grinding.

 

I'm confused, but I want to help if I can.

 

Geoff

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I run a 3450 rpm motor on my Coote but I have a 1 1/2" pulley on the motor and a 5" pulley on the 10" drive wheel.

 

Doug

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I have a grinder that runs 5000 sfpm, but it is not wet. Standard 2 X 72 more or less a Bader copy made with the parts I had at the time. The idler is actually a Kalamazoo, I think. I need at least two horsepower to be happy with the platen, and three is much better. :)

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I used a 1hp Burrking knife maker for many years. It has a Baldor 1hp 1750 rpm. It stepped up with pulleys to double out put speed. So 3500 rpm at the drive shaft then a 6"to 8" drive wheel depending on setup. (The contact wheel is the drive wheel on that design) so roughly 5395 to 7329 SFPM on one HP. It worked very well for me for a long time. I am jumping to 3hp variable speed direct drive on the next one and I am starting to think with all that power I should up the drive wheel from 4" to 6" which would bump me up into the 5400 SFPM range at full speed. At first I wanted to be able to reach further into slow range with the smaller wheel, but now that I have seen how slow it can go I think I would be fine with the larger drive wheel and getting the top end back.

Patrick

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I got it all set up to run at around 7200SFM. I can see that these belts really like to be run fast. I'm running 4":2" pulley, 4" drive to an 8" contact wheel. It will bog down the motor if I'm hogging out a profile on the platen or flat grind. But hollow grinding on the contact wheel is really smooth, no bogging at all.

First 'test' grinds:

IMAG0160.jpg

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SFM = Surface Feet/Minute. To calculate, see the formulas here:

 

http://en.wikipedia.org/wiki/Surface_feet_per_minute

 

These are for lathe operations, but substitute drive wheel diameter for stock diameter:

 

drive wheel diameter (in) X 0.2618 X RPM

 

(RPM of the drive wheel, so you'll need to take any drive ratio into account, multiply or divide that by the motor RPM)

 

the magic number in that equation is PI * 1/12, but I don't know where 1/12 comes from.

calc "3.1417*(1/12)"

0.261808

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The 1/12 is just feet to inches. You want feet/min, but the drive wheel diameter starts off in inches.

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