Help with surface feet per minute

KnuckleDownKnives

KnuckleDownKnives

Time to make the doughnuts..
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Feb 12, 2015
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If anyone is good with this can you verify my calculations are correct?

Motor max RPM: 5600

Final pulley ratio is 1 to 1.625 (motor spins 1.625 times for every one of the belt drive pulley) Verified by balance drill mark on motor pullet at 12 o'clock and keyway on final shaft at 12 o'clock turned motor over till final drive was back at 12 o'clock and the motor pulley went around one full revolution and then to about the 7:30 position. So I'm calling that 1 5/8 turns or 1.625.

Final pulley max RPM: 3446.15 (5600 RPM / 1.625 pulley ratio)

4" Belt drive wheel circumference: 12 9/16" or 12.566" or 1.04'

Surface feet per minute: 3608.693 (3446.15 RPM x 12.566" / 12)

Edit: If correct how is this compared to other grinders?
 
Quite a few years ago I just went to Ebay and bought a laser tach. Cheap and very simple answer to all these questions.
 
1:1.625 = .615
.615 X 5600 = 3446 RPM (at drive wheel)
4 pi = 12.56 " (drive wheel circumference)
3446 PRM * 12.56 circumference = 43303
divide by 12 (inch to foot) = 3608.6 SFM


^ Yeah, I got what you got.

That's a good usable speed, but most professional knife grinders can reach 5,000, though not everyone uses it that fast. I seldom go over 3,000 SFM
 
Your calculations look correct to me. I think it's about right for speeds. (3-5K feet per minute sounds right, but I could not find a ref.)
 
pleasanthill said:
If anyone is good with this can you verify my calculations are correct?

Motor max RPM: 5600

Final pulley ratio is 1 to 1.625 (motor spins 1.625 times for every one of the belt drive pulley) Verified by balance drill mark on motor pullet at 12 o'clock and keyway on final shaft at 12 o'clock turned motor over till final drive was back at 12 o'clock and the motor pulley went around one full revolution and then to about the 7:30 position. So I'm calling that 1 5/8 turns or 1.625.

Final pulley max RPM: 3446.15 (5600 RPM / 1.625 pulley ratio)

4" Belt drive wheel circumference: 12 9/16" or 12.566" or 1.04'

Surface feet per minute: 3608.693 (3446.15 RPM x 12.566" / 12)

Edit: If correct how is this compared to other grinders?
Click to expand...

http://www.blocklayer.com/Pulley-Belt.aspx
 
Nathan the Machinist said:
1:1.625 = .615


^ Yeah, I got what you got.

That's a good usable speed, but most professional knife grinders can reach 5,000, though not everyone uses it that fast. I seldom go over 3,000 SFM
Click to expand...

Thanks for the reply, yeah I was thinking they could go a little faster, I wanted to figure the math as now knowing what my speed will be I may up it to a 5" drive wheel to get a little more speed if ever needed. If I get a 5" drive wheel it should get me up to somewhere in the 4500 sfpm range.

I watched one of the videos in the stickeys and the old man was running at 7500 sfpm. Man that thing is smokin... That's be like dragging you knuckles on the pavement at 90mph....:eek:
 
For a 4" drive wheel it is easy - just use the drive wheel RPM as the approx. SFM. That is pretty close to the real number.

As Karl mentioned, a cheap tach is a good thing to have around for all sorts of things. Many read in SFM as well as RPM.
 
I'm really considering going with a 6" or 7" drive wheel so I can obtain a higher sfpm in the 5400 - 6000 range and be able to do some serious quicker stock removal if needed. Having a 2.5hp motor and the pulley arrangement I have (image in above comment) do ya'll think I'll have enough torque out of the 2.5hp motor to handle it? It is a treadmill motor..
 
A treadmill motor rated at 2.5HP may deliver a real HP of less than 1 HP. To figure out the real HP, look at the amps drawn on the plug into the wall. That will tell you the actual HP. A good rule of thumb is 7 amps draw per HP. If you look at a lot of higher RPM motors ( like routers, vacuums, and treadmills) they often state high HP but only draw a few amps. You see 5HP shop vacs, and 3HP routers that draw only 5-6 amps. These are using the misleading "rated HP".

In electric motors, 1HP equals 746 watts. This is the theoretical calculation. Most motors only deliver about 85% of the rated power. 746 watts divided by 120 volts = 6.2 amps.

The other way to know that a HP rating is not realistic is that any electric motor that plugs into a standard 120 volt wall socket won't be more than 1.5 HP. At 2HP, and above most all motors are connected to 240 volts

I would stick with the 4" drive wheel for now.
 
Stacy E. Apelt - Bladesmith said:
A treadmill motor rated at 2.5HP may deliver a real HP of less than 1 HP. To figure out the real HP, look at the amps drawn on the plug into the wall. That will tell you the actual HP. A good rule of thumb is 7 amps draw per HP. If you look at a lot of higher RPM motors ( like routers, vacuums, and treadmills) they often state high HP but only draw a few amps. You see 5HP shop vacs, and 3HP routers that draw only 5-6 amps. These are using the misleading "rated HP".

In electric motors, 1HP equals 746 watts. This is the theoretical calculation. Most motors only deliver about 85% of the rated power. 746 watts divided by 120 volts = 6.2 amps.

The other way to know that a HP rating is not realistic is that any electric motor that plugs into a standard 120 volt wall socket won't be more than 1.5 HP. At 2HP, and above most all motors are connected to 240 volts

I would stick with the 4" drive wheel for now.
Click to expand...

Yeah, I was kinda thinking that myself, but wasn't sure considering that it has all of that DC conversion circuitry.
 
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