grinder speed?

[Ratking20]

i have a 1750 rpm 1.5 hp motor on my homemade 2x72 grinder with pulleys

im new to this, i believe i would get my motor at half speed, full speed, and double speed

when would i use each speed?

 

[john april]

the higher in grit you go, the more friction there is and the blade will heat up faster. i have a kmg with a 3 step pulley and 1.5 hp motor, i use the medium speed and when i get to 400 grit, i use the slow speed from there on up. for shaping wood handles i use the slow speed because you are less likely to burn the wood.

 

[BelnapCustomKnives]

I do it a bit different. I like to grind very fast and hard. For initial grinds of bevel and profile I run at 7200 surface feet per minute on the belt with a 36 grit. After heat treat I slow up to 3600 sfpm to begin details. Then finish at 1800 sfpm. The cubitron grit belts really like the hard pressure and high speed for the heavy removals.

 

[Howard1]

How do you figure SFPM. I have a 1hp motor at 1750 rpm at 60 cycles running a 2x72 grinder. Is that 10,500 SFPM? What is the advantage to more horsepower and/or 3500 rpm?

 

[Rhinoknives1]

Even one finds their own sweet spot in speed for different grits & belt brands & the pressure you grind at....Generally, the finer the belt the slower speed I grit at. ......

 

[kuraki]

How do you figure SFPM. I have a 1hp motor at 1750 rpm at 60 cycles running a 2x72 grinder. Is that 10,500 SFPM? What is the advantage to more horsepower and/or 3500 rpm?

(Motor rpm * (π*drive wheel diameter))/12 = surface feet per minute

 

[BelnapCustomKnives]

How do you figure SFPM. I have a 1hp motor at 1750 rpm at 60 cycles running a 2x72 grinder. Is that 10,500 SFPM? What is the advantage to more horsepower and/or 3500 rpm?

Belt surface feet per minute is the speed the belt is moving in feet per minute. It is a function of the Belt drive wheel rpm x belt drive wheel circumference in inches then divide the product by 12 to get feet per minute. Example 1750 RPM x 4" dia drive wheel (4" x 3.1416) 12.57" = 1833 SFPM.

 

[Howard1]

Belt surface feet per minute is the speed the belt is moving in feet per minute. It is a function of the Belt drive wheel rpm x belt drive wheel circumference in inches then divide the product by 12 to get feet per minute. Example 1750 RPM x 4" dia drive wheel (4" x 3.1416) 12.57" = 1833 SFPM.

So that sounds crazy slow. Thanks for the help. Now I know why everything takes so long to do.

 

[Kevin McGovern]

Yep. When I went from a fixed speed 1750 rpm motor to a vfd, double the rpm and a larger drive wheel, the sky's openen up and angels sang. ( Did you know angels sound like steel flying off in a shower of Sparks?)

 

[BelnapCustomKnives]

So that sounds crazy slow. Thanks for the help. Now I know why everything takes so long to do.

That is why I run a 3450 rpm motor with a 3 step pulley. At top speed I get 7200 sfpm for heavy removals. Then mid is 3600 sfpm, and low is 1800 sfpm. If you are trying to make a knife at 1800 sfpm it will be very slow and painful. Modern ceramic grit belts like 3M cubitron do not perform well at those low speeds. They like 6000 sfpm and up with heavy pressure to fracture the grit and keep it sharp and cutting. I get very long life in excess of 50 blanks per belt at 7200 sfpm with heavy pressure. When I got ready to build my first sander I saw this video.

. I decided that I wanted my machine to have that speed capability. Very glad I did. I feel that many are using underpowered and too slow of machines. Just my opinion. I love to make knives but I do not love standing forever in front of the sander to make one blade.

 

[Howard1]

That is why I run a 3450 rpm motor with a 3 step pulley. At top speed I get 7200 sfpm for heavy removals. Then mid is 3600 sfpm, and low is 1800 sfpm. If you are trying to make a knife at 1800 sfpm it will be very slow and painful. Modern ceramic grit belts like 3M cubitron do not perform well at those low speeds. They like 6000 sfpm and up with heavy pressure to fracture the grit and keep it sharp and cutting. I get very long life in excess of 50 blanks per belt at 7200 sfpm with heavy pressure. When I got ready to build my first sander I saw this video.

. I decided that I wanted my machine to have that speed capability. Very glad I did. I feel that many are using underpowered and too slow of machines. Just my opinion. I love to make knives but I do not love standing forever in front of the sander to make one blade.

How does HP figure into this?

 

[Pittknife]

It depends a lot on your belt and what type of grinding you are doing.

Run a 36 grit belt at full speed in a sharp platen will result in most the grit flying at your face.

Using small wheels you can't go pass slow.

Big wheels anything over 120 grit with ao you need to go slow, you'll burn through the belt.

That is why I'm a big fan of variable speed, there are certain finishes you need to be able to adjust speed in between that you can't get without variable speed. Plus it's a time saver

 

[BelnapCustomKnives]

How does HP figure into this?

I don't have the book answer for that but seat of pantsometer seems to indicate 3 HP to operate happily at those speeds and heavy belt loads. My current sander has a 5 HP which is on there because I already it sitting a shelf. I am working on a new sander that will use a 3 HP with VFD. Will see how it handles the loads. I really like the tons of power the 5 HP puts out at high speeds. Hoping the 3HP will perform well.

 

[Howard1]

I don't have the book answer for that but seat of pantsometer seems to indicate 3 HP to operate happily at those speeds and heavy belt loads. My current sander has a 5 HP which is on there because I already it sitting a shelf. I am working on a new sander that will use a 3 HP with VFD. Will see how it handles the loads. I really like the tons of power the 5 HP puts out at high speeds. Hoping the 3HP will perform well.

Thanks everyone for the help. I was grinding today and although it may not be perfect, my rig does carve metal. But now i know where I am headed. All the best.