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Lapidary / Gemstone Community Forum
February 21, 2019, 05:16:42 pm
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SFPM ?

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Downwindtracker2
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« on: October 07, 2015, 08:41:47 pm »

 I have an arbor, a motor, 30,000 and 50,000 diamond wheels, and a switch, but no pulleys,yet. For polishing agate, what is the surface feet per minute , so I can size the pulleys ? The wife is the real rockhound, I'm just the millwright.
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« Reply #1 on: October 07, 2015, 10:28:02 pm »

For 8" dia wheels, 1750 rpm is a good place to start. Expando rubber wheels can't be run faster than that speed or they come apart. Not the same for diamond wheels, but that same rpm should be a good place to start.

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« Reply #2 on: October 08, 2015, 07:49:52 am »

I like that speed too for hard wheels.  For sanding belts I often slow the wheel down for a softer surface if the stone is less hard than agate.  Added drape will help to reduce flat spots.  I wish I had thought about that when I was learning.  I think I would have shortened the process.  Higher speed will grind faster for really hard stones like ruby and topaz.
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« Reply #3 on: October 08, 2015, 10:36:51 am »

I guess I should explain, in metal working our speeds are governed by the diameter of the work or the cutting bit, the material being cut, and the cutter bit material. So to simplify , the specs are given as surface feet per minute, a high speed steel bit on steel is about 100sfpm.

The wheels are Diamond Pacific's 7 5/8" Nova wheels, 14,000 grit and 50,000 grit ( I was wrong about the 30,000)

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« Reply #4 on: October 09, 2015, 08:56:36 am »

Ray
I am a newbe to machining so I know where you are coming from cabbing is not as complicated as machining metal although I suppose it could be but that would be a nightmare. That being said  to figure the sfpm from the rpm is pretty simple it' simply  figure out the circumference of the wheel  C=3.14 X 8 ( C= 3.14 X D) then divide by 12 to convert to feet, then your sfpm will be(C X rpm ) so the sfpm of an  8" wheel would be sfpm 3.14 X 8"=25.12 then
divide by 12 or 25.12/ 12=  2.09 ft   then for a 1:1 pulley ratio for a 1725 motor it would be 2.09 X1725 = 36 sfpm. A lot of machines use a three stage steep pully with the driving pulley  being the same size a the driven.
Usually the first pulley for the motor and the shaft is around 2 in, then 3  , then 4. You only need the steep pulley on the shaft.

Cutting rocks is more of a feel it thing with more pressure with harder rock and lighter with softer rocks. What the others said is all true it's kind what works best for you thing so I guess it may be more complicated since you don't have predetermined speed etc. to use. 
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« Reply #5 on: October 10, 2015, 02:03:24 pm »

After a couple of days, I thought " Why not ask Diamond Pacific?". I'm so used to forums that I don't always see the simplest way.

"Both our Genie with 6” wheels and our Titan with 7-5/8” wheels run at 1800 RPM, so you are looking at somewhere between 2800 and 3600 SFPM."

So your 1725, kennyg, is the answer. Odd that they run both at the same speed.

Thanks for all your help.

 

 
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