The definitive belt speed discussion?

D

DocsMachine

Joined
Apr 7, 2007
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I know the subject has been discussed before, but repeated searches keep coming up with widely varying results- or rather, opinions, advice and personal preferences. :D

Basically, what is an ideal belt speed or speeds, for a KMG/Bader/Wilton style 2x72" belt grinder?

There's single-speed machines, where the drive wheel is attached directly to the motor shaft. There's also multiple-speed machines that use a three or four-row cone pulley to give a range of three or four fixed speeds. And of course there's the variable speed machines, that use a DC or AC-3Ph motor for near-infinite speed selection.

But, there's also 1725 rpm motors and 3450 rpm motors, as well as 3", 4" 5" and 6" drive wheels.

In looking over prior posts here and on other sites, people have recommended anything from 1800 FPM to 5000, while still others- often in the same thread- say that 2000 is too slow and 4000 burns the blade.

And yes, I'm aware there's a great deal of personal preference involved- some old hands can really lean on a fast, coarse belt and shape a blank in a few minutes, while others need a light touch on a slower belt else they gouge and burn the knife.

But just for the sake of the argument. I'm asking for two reasons: One, just for the general interest in getting a sort of consensus all in once place, and two, because I'm building a KMG-style grinder, and I have quite a few options on things like motor speed (I have both 1725 and 3450) belt or direct drive, and drive wheel size. (I'm making my own, and I have material for both 4" and 6" wheels.)

So, what's a good belt speed, in feet-per-minute, for a single speed machine? What's a range for a multiple-speed machine? And what's a max speed/top speed for a variable machine?

And, if you have a multiple/variable machine, where do you use it most? High for roughing, low for detailing? What do you consider "low"? Half speed? 75% of top speed? 20%? 10? Do you run coarse belts faster or slower than fine belts?

Thanks.
Doc.
 
My single speed Square wheel is very fast, and great for profiling & hogging out steel.
If I went out to look at the KMG it is probably set somewhere between 50% - 70%. I don't pay much attention to what the dial says. I set the belt to what looks good, and feels right for what I'm doing at the time.
Bob
 
While I understand your question, the real answer is "Variable Speed". If you use a 3PH motor and a VFD, the grinder can run at whatever speed is needed. A second choice would be to use a DC motor and drive. This may add 10-15% in cost to the build, but will make the machine 200% more useful. I would not build a single speed unit unless it was a dedicated second or third machine in the shop, and only performed one function ( And I probably still would make it variable.).
 
There is no substitute for hp. My KMG is powered by a three hp ac motor with pulleys.
If you use ceramic belts in course grits they require high speeds combined with as much pressure as you can apply to reach their full potential.
If I only had a single belt grinder in my shop I would want it to be a full variable speed model that offered all the different speeds required by different materials being ground.

Fred
 
Variable speed is the absolute best,step pulley's next,and direct drive single speed last.
Stan
 
SBuzek said:
Variable speed is the absolute best,step pulley's next,and direct drive single speed last.
Stan
Click to expand...


What Stan said, pretty much sums it up. I have two grinders both set up with variable speed control, Wilton and Bader III.
I run from 70 to 80 percent of max to 10 or 15 percent daily, both machines.
 
I am not aware of any exact formula that tells what the optimum speed is for any given operation. You might want to drop Rob a line at Beaumont metal works, he may have a more definitive answer. For me I use high speed for hogging. I have plenty of HP so I can really lean in hard. After the initial shaping, scale removal and very rough bevels I move to my disc. This gives me a much flatter bevels and I find it easier to center and even them out. I run from 50% - 100% for my rough grinding on the disc. Then for finishing I drop down to 10%-30% max with finer paper. On the belt i primarily use the VS to do the handle shaping so I do not burn the material. VS is absolutely better for me. If any of my VS machines die they will be replaced with VS units for sure.
 
If there was an ideal speed for knifemaking there would not be the many dollars spent on setting up grinders with the various speed controls. Each maker, each process, each material require different speeds. Anything but a variable speed setup is a compromise.
The lack of control can be both dangerous and frustrating.

~Alden
 
Its funny, I started out with a single speed grinder, a very fast one, and it taught me how much control I needed to have. Now that I have learned that control, it comes in very handy. I now have a variable speed and I can grind full bore or slow it down to a crawl.
Other than Freds comments about the ceramic belts for hogging, no one has mentioned belts. What belt you are using can also be a determining factor in your speed selection. I started using the trizact CF(gator) belts a couple of years ago and found they work best with the grinder running full-on. As Fred mentioned some of the coarse grit ceramics are the same. The real difference for me is when it comes to the handle, that is where those slower speeds really come in handy.
For me a high speed grinder works great for all the metal work, it also teaches you when to change belts, good sharp belts run cool it when they dull that things really start to heat up.
Del
 
If I were stuck to one speed on my knife grinder, I'd probably choose 1500 SFM. Too slow for most stuff - yet too fast for fine grit finishing - but you could make it work. The life of your belts, the time to grind a blade and the quality of your work would suffer, but it would be workable.

I use a 2HP four pulley system that goes from 1000 to 5000 SFM. I use the second to fastest the most and the slowest the next to most. About a tie between fastest and next to slowest. But I use them all frequently.

I'd say an ideal setup would be VS from 200SFM to 8000SFM and 3HP. If I were a full time maker, this would be a priority to me.

In my opinion, a minimum setup for starting out as a hobby would be 1HP and 1000 and 3000 SFM. You'd outgrow that, but it would get you started.
 
I suppose I should clarify: I am building a variable-speed model. I have a 2HP 1725 3-phase and a 3HP 3450 3-phase, and several VFDs that can run either one. I;m a big fan of the VFDs on my machine tools, so that's a no-brainer. :D

The question is, how should I set the grinder up? If I put a 6" wheel on the 3450, that'd give me 5,400 FPM. Almost everyone says that's too fast. The 4" wheel would give 3,600, which sounds more reasonable.

However, in my case, that's an issue- the 3450 motor isn't sealed, while the 1725 2HP is. The 6" wheel on the 1725 will give me 2,700 FPM- which to me still sounds plenty fast, but some posts have said is too slow.

Yes, I could do some trial-and-error to mix-and-match parts to see what works best for me. (And keep in mind I'm making this as much for a general-shop grinder as I am a knife grinder- I'll occasionally be doing brass and aluminum on it as well.)

But I was hoping to get a consensus, a range for preferred speeds. I'd then build my setup to more or less match. What I didn't want to do is have a slow machine that I'm always using at top speed, or a fast machine that's always running at 20% (or whatever.)

Also, for the record, a sealed, fan-cooled motor should not be run for extended periods at low speeds. The fan is designed to move X amount of air to keep the motor within a reasonable heat range, and running slower than that reduces the airflow and thus the cooling.

No big deal for five or even ten or more minutes at a time- though much longer than that and I keep an eye on it with an IR temp gun. (Most motors have a temp range on the data plate.) But I would NOT want to run it all day at fractional speed. That's a good way to cook windings or break down the insulation.

Doc.
 
DocsMachine said:
The 6" wheel on the 1725 will give me 2,700 FPM- which to me still sounds plenty fast, but some posts have said is too slow.


Doc.
Click to expand...

Hey Doc,

That really isn't plenty fast for a top end. I think 4000-6000 would be a good top end range. I wish there were a way you could try it yourself first and see.

Some of the better belts in the lower grits can be used at very high SFM. 5000 really isn't overkill.

If you're planning to machine your blades and just finish on the grinder, a lower top end won't hurt you, but if you're wanting to do bulk material removal, I think you're going to want more speed than that.
 
I run the 3600 2hp 3 phase VS with the 6in drive wheel, and I am 100% happy with the top end. A 60 grit blaze makes steel vanish with the greatest of ease and light pressure. I love it.
 
While variable speed is best, I would say that if you had to go with 3 speed, I'd arrange it so that your ranges are around 3500 to 4000 ft/m on the high end and 700 to 800 ft/m on the low end, as those seem to be the range of speeds that are good for various bits of work.
 
i run my Diy KMG on a Omron Inverter ,
i own a 2Hp - 4pole and a 3 hp 4 pole motor , i have tested it at 50hz and up to 60 Hz,

at the moment i have vibrating problem . when i run it at 50hz against my motor 5" and drive pulley 3" , so i am trying way to kill the vibration , if work find and stable when i run it maybe 40hz . i do not know the speed , but it sound kinda scarey at 60hz ,

what i seen on youttube is that i see guys going up to 9000 - 12000 , which in my case i think i do not need it , cos it grind fine at 40-50hz at the moment , slightly slower but more control . cos at higher speed it also mean , higher chances that if you make a small mistake you hurt yourself or damage your knife .

sometime you do it slow in order to make it fast

Gabaski
 
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