SFPM, Cant decide.

JTknives

JTknives

Joined
Jun 11, 2006
Messages
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I cant decide on what SFPM i want on my belt grinder. i am setting it up with a VFD so i will have full range from high to low but i don't know what i want my high to be. i was planning on doing like a 1740 rpm motor with a 6" drive pully to a 2" shaft pulley and a 6" drive wheel which would give me 1740 x 6" / 2" x (6 x 3.14) / 12 = 8195 SFPM :D. but i onley have material to make a 5" drive pulley and drive wheel which would give me 5691 SFPM which i a lot more then i am ruining right now which is 3642 SFPM. i run Ceramic blaze belts and i here that thy cut tones better and faster speeds. I just dont want to have to get more material as i have 5" stock but will5691 SFPM be fast enough.
 
7000SFM is the normal max,so 5691 should be fine.

Did you ever get those DVD's boxed up to send back?
Stacy
 
I am extremely happy with my speed of 5700SFPM, I cannot complain. The steel comes off real fast with a fresh belt.
 
Good Lord! thats over a mile of SFPM! Gotta question though, running an overdrive set up like that, doesnt it make it REALLY easy to stop the wheel with just a little bit of pressure?
 
I am running a 2hp motor, and I have not stalled out my rig. Then again too, I am afraid to really lean on the piece when I am spinning that fast. I dunno who it was, but last year I think it was Deker who posted pics of what happens to yer knuckles when you slip against a 36 grit belt spinning real fast. If I need to push hard, it must be time to change the belt...
 
You can also over-speed the motor with the VFD's max speed setting. For example, you can bump the belt speed by another 10% if you set the max speed freq to 66hz instead of 60hz. That will take your 5700 sfpm to about 6300.

I'm a "round off" guy when it comes to shop math. 5691 = 5700.

I've been running a VFD on a VS band saw at 120hz for about 8 years. The 3450rpm motor has been running at 6900 rpms so far, no problems. This is extreme and the motor doesn't have much torque...but I rarely need much so it works great. Anyway, it demonstrates some of the versatility of VFDs.

-Rob
 
As mentioned above VFDs can increase your speed as well as decrease it. I have installed one on my 2hp mill so I don't have to change belts as often. I have run mine at 160hz no problem and the motor doesn't get hot. Of course at that speed I am using small carbide cutters so there isn't much load. I would think that at something like 80hz you would get an additional 33% of speed with no ill effects on the motor. You want the reduction like you will have so that it won't easily stall at say 10hz. Constant torque isn't true constant torque. At low hz the motor is getting less and will stall easier than at 60hz. I would set it up like you plan or even a bit slower. You want that low end to work well to. I have a lot of reduction in my 2 hp VFD disk grinder/ buffer and I can have it zinging or at a mere 5hz it still has enough power to work.
 
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