The great SFPM Debate

[rides20]

..Alright everyone bear with me here... I'm trying to get into the "nitty gritty" of polishing, sure I can make stuff shiny BUT!....I'm confused about the whole SFPM thing....
Caswell's manual shows a formula of (1/4 dia. of wheel X Motor RPM) =SFPM, and for best results use a SFPM speed of 3600-7500.....Here's where the confusion comes in: If you use a 6" wheel on a 3000 RPM motor, you're turning 4000 SFPM...if you use an 8" wheel on the same speed motor, you're turning 6000 SFPM. How can you have a higher Surface speed, if having a larger diameter wheel theoretically is turning slower than a smaller diameter wheel running with the same SPINDLE speed

[fly]

Re: The great SFPM Debate

Your thinking of RRM's, it takes a 6" wheel longer to make a complete round per minute than it takes a 8". Look at the ratio, when you go to a 2" larger wheel you step up 2000 SFPM. Its not talking about RPMs its talking about SFPM..
Say the smaller wheel has to make two rounds to stay the same as the larger wheel making one round.

Does this make since (NEWBIE)
Just watch your rpms and you will be fine....

[fly]

Re: The great SFPM Debate

Hey Newbie lets try the flash chat....

[rides20]

Re: The great SFPM Debate

If you have 12" rims on a car, and you travel at X rpm...then you put on a set of 20"s (polished ofcourse!) at the same RPM, the 20s are going to GO farther in the same duration of time, but will actually make fewer rotations than the 12's...This sounds like the S.A.T's ! HAHAHAHAH..And ehhhhemmmmm....... I've been "upgraded" to Amateur. HAHAHAHAHAh

So all in all....the formula says Slower is better??

[fly]

Re: The great SFPM Debate

Yea just check your mph and you will see, ask the cop next time he stops you....Ha Ha Ha.....

[sswee]

Re: The great SFPM Debate

From years in the machine shop, let's see if I can clarify. The formula Caswell gives is a short cut for the calculations. The long way is to find the circumference by multiplying Pi times the diameter. Multiply the answer by the RPM running. This gives you the IPM (inches per minute). Divide this by 12" to get the SFPM (surface feet per minute).

On autos one tire size is approx. 3 mile difference on the speedo.

6" wheel at 3000 RPM's

3.14 * 6 = 18.8495" circumference
18.8495" * 3000 = 56548.6677" IPM
56548.6677" / 12 = 4712.3889' SFPM

6" / 4 = 1.5
1.5 " * 3000 = 4500

The Caswell formula is more closer to actual since when you bear into a wheel you are not running at the 6" diameter but at 4" to 5".

SS

[rides20]

Re: The great SFPM Debate

But isnt it still spinning slower by using a larger wheel?

[pickleboy]

Re: The great SFPM Debate

no bro you are still thinking in terms of rpm. the wheel is still spinning at the same speed. if you had a 4" wheel and turned it at 3000 rpm,it would be 3000 sfpm. and a 8" wheel at 1500 rpm would be 3000 sfpm. like your example of the wheels on a car. now spin them both at the same speed and you get 4"=3000 sfpm 8"= 6000sfpm. clear as mud right.

[sswee]

Re: The great SFPM Debate

No, a motor turning 3000 RPM will turn 3000 RPM no matter what size wheel is on it. The larger the wheel the higher the SFPM.

If you put a larger wheel on a motor and it turns slower than with a smaller wheel on the same motor.
1. the wheel is too large for the motor. (a Chevy 350 in a Kenworth)
2. the motor is weak and fixing to burn up.

[pickleboy]

Re: The great SFPM Debate

were you talking to me or him on this?

[sswee]

Re: The great SFPM Debate

I was talking to rides20. Try the flash chat.

[rides20]

Re: The great SFPM Debate

HAHAHAHAHAH....I think i'll just stick to polishing! I guess I dont need to know why I can make stuff shine!HAHAAHH..Actually sswee, that was a big help it is actually a tad clearer now. Thanks (I clicked your add reputation button) HAHAHAHAAHAHAH

[mpierich]

Re: The great SFPM Debate

..Alright everyone bear with me here...

Think of it this way...say a 6" wheel and an 8" wheel are traveling at the same RPM's. The circumference around the 8" is more inches than the circumference around the 6". That means that the surface of the 8" has to travel _more_inches _in_the_same_time_period_ than the surface of the 6", which equates to more feet per minute, or inches per second, or miles per hour even.

I figger when a piece grabs on the wheel and leaves my hand (which happens a lot less frequently nowadays), it's traveling at about 80 mph.

[fxstcguy98]

Re: The great SFPM Debate

My turn, think of tying a rock at the end of a 10' string, if you held the string at arms length and spun in a circle the rock will be hauling balls to keep up with your hand.

[mcaswell]

Re: The great SFPM Debate

Look at it like this

The CIRCUMFERENCE of a wheel is Diameter (D) x Pi (3.14)
Or roughly D x 3

So, a 4" wheel = 4 x 3 = 12.
One revolution of that wheel will travel 12" along the ground

A 6" wheel = 6 x 3 = 18
One revolution of that wheel will travel 18" along the ground

An 8" wheel = 8 x 3 = 24
One revolution of that wheel will travel 24" along the ground

If these wheels are mounted on your buffer, which does 1 revolution, a point on the circumference of will travel different distances, depending on the diameter of the wheel.

If the motor revolves the wheel 3500 RPM - then the distance of that point on each wheel will travel further as the wheels get bigger.

So, seeing as RPM is a time element, greater distances will be traveled in the given time of one minute.

Clear as mud eh??