Grinder motor question

R

Ram4x4

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Jan 1, 2017
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I am currently building the Jer Schmidt Gen 2 knife grinder he designed. For those with experience and you had to do it again, would you go with a 2HP or 3HP motor? 1800 or 3600 RPM? Drive wheel diameter? I will purchase the appropriate VFD for speed control (KBAC 27d or 29 depending on motor).

I'd like to just get this right the first time and not have to do it over or refit later.
 
I just watched a video of the Jer Schmidt Gen 2 build. A very good grinder - I didn't determine what thickness steel he's using - 3/8"? OR is that stuff 1/2" thick? Lots of work. If the parts were waterjet cut out would save a LOTS of work.

With a grinder like that, there's no reason not to use a 3hp motor. The extra hp will never be a problem, you'll only use the amount of hp needed. I use 2hp and have never bogged the motor down, but a couple other full time knifemakers insist a 3 hp is best.

1800 vs 3600 rpm? I've got both. In theory perhaps the 1800 rpm might be best, but in real world you'll never really know the difference. At 50% speed the 1800 rpm motor will still be giving 100% of the 3hp, where the 3600 rpm motor at 50% speed will have less hp. That really doesn't matter cause at 50% speed you're not going to be hogging and not using all the hp anyway. With the grinder you're building I'd go with the 1800 rpm motor. The extra weight of the 1800 rpm motor won't be an issue with that solid steel grinder you're building.

Diameter of drive wheel? I use a 5" so my 3600 rpm motor running at 3800 rpm gives 5,000 SFPM belt speed which is plenty for me. Some folks say you need 6,000 SFPM which would need a 6" drive wheel. It's usually best not to over speed the motor in excess of 4,000 RPM, with 5k being the absolute max. With a 5" drive wheel you'll have 5200 SFPM at 4K rpm, and just as important to me is the slow speed which is used for precise shaping of scales, sharpening blades, etc the 5" drive wheel is 350 SFPM at 266 rpm.

Seems like I remember it's usually about 10% of rated rpm as the min speed to run a motor due to cooling, and perhaps other factors? So the 1800 RPM motor at 180 rpm would give 280 SFPM belt speed with a 6" drive wheel which is good while giving 6200 SFPM at 4,000 rpm.

You mention using the KBAC VFD, I "think" they only have a double speed which would give you a max of 3600 rpm so I think the 6" drive wheel for sure. 1800 rpm motor with the double jumper set would give a range from 280 SFPM to 5600 SFPM. The 5" drive wheel would be a range of 236 SFPM to 4700 SFPM which isn't bad at all. With the KBAC drive I think I'd opt for the 6" drive wheel.... I think anyway :)
 
I use a 2 HP and that's more than plenty for my needs.

The KBAC can be set to 2x, but there is also a fine adjustment on top of that with a potentiometer (I think it is between 0.7-1.1 or so, but I'd have to check the data sheet).
 
The Jer Schmidt grinder is all made from 3/8". I already have the steel. Cutting and shaping will be no issue, I have a portaband, several angle grinders, a plasma cutter, and a small cheapo 1" belt grinder.
Ok, so 3HP it will be. I was only concerned between the 1800 and 3600 RPM because I thought about torque at lower speed, but as you say, if I'm slowing it down, it's going to be for finer work, so won't need excess torque. I think I'll just go with the 3600 RPM and a 5" drive wheel. Changing to a 6" drive wheel later if I think I need more belt speed would be easy. Cost difference between a 2HP and a KBAC 27 vs a 3HP and a KBAC 29 is only about $90-ish dollars. Negligible in the overall scheme of things.

Thanks all! Off to order parts!
 
That does look like an exceptionally well designed grinder. By using his plans to print the patterns, gluing patterns to metal for cutting out should work really good. He has some really good ideas on the design. Good luck and please do keep us posted WITH PHOTOS! of your build.
 
Ken H> said:
That does look like an exceptionally well designed grinder. By using his plans to print the patterns, gluing patterns to metal for cutting out should work really good. He has some really good ideas on the design. Good luck and please do keep us posted WITH PHOTOS! of your build.
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Too much welding , too much fabricating parts .A real challenge to make it even for experienced craftsmen . And I don't see many options for fine adjustment when you finish with welding after you assemble the grinder . What could be better, more precise than drilling a few holes to make a grinder ?
 
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