What's new
What's new

2x72 knife grinder motor and VFD choice?

Hydroracer

Plastic
Joined
Apr 21, 2020
I am building a 2x72 grinder for knife making. A topic of discussion is often how much HP, motor speed and VFD choice.

Since I'm taking the time to make it I want to choose the best motor (speed and HP) and VFD for optimum performance.

One argument says a 3600 RPM 3 HP motor. Another says to go 1800 RPM and 2HP.

The drive wheel will be 5" diameter and at 3600 RPM this produces around 4700 FPM belt surface speed. Target speed should be between 5000 and 6000 FPM for best results with modern low grit ceramic belts (this is for hogging off a lot of steel quickly). Sometimes, I would require slower speeds, but still have the power to apply a good load to it. The smaller the grit, typically the slower you run the speed.

So, would it be better to go with the 1800 RPM motor and achieve the high speeds via higher frequency from the VFD and have more torque at lower speeds, or just go 3600 RPM?

I have a basic comprehension of 3-phase motors and how a VFD works, but just want to be sure I'm not setting things up to where I end up with a failed or burned motor and ensure it can perform as desired.
 

gbent

Diamond
Joined
Mar 14, 2005
Location
Kansas
You have to overspeed a 3600 motor to get your desired belt speed. You want 3800 rpm minimum. With the 1800, thats over 200% rated speed. I don't think an off the shelf motor will take that much overspeed for long.
 

Steven-Canada

Hot Rolled
Joined
Jun 16, 2015
Location
Toronto Area, Ontario, Canada
I say, for the VFD use a KBAC-27D

It maxes out at a 2 HP supply

To get the 2 HP rating, you need to feed it 240vac single phase and 15 amps.
That's an additional cost to setting it up, to have the 220 outlet.

If you want the added flexibility, you can use a 3 phase 1.5 HP motor and plug it into ANY 130 vac single phase 15 amp outlet.
( stay away from GFID - ground fault protected circuits, it's code now to have GFI's in a new built garages

You have listed 3 horsepower above.
It's an extra cost in a larger more expensive vfd, extra cost in larger more expensive and less common motor, extra cost in larger wire size, dedicated 220 outlet, wire, breaker.

You don't need 3 hp.
Even among full time smiths, the use of 3 HP is unusual.
Most of those that have it are "bragging rights"
Some makers do use it, but they are full time smiths decades of experience, with a high presure high production grinding style.
Time is money for them and they are grinding off heavy forge scale.

That's not you. You're going to want lower speed lighter pressure, slower material removal while learning.

For the vast majority of full time smiths, they just use 2 hp.
Let the belts cut, full speed 2 hp with ceramic belts, start with thinner stock material - it's enough.

There's no built like over built ?
yes, but 3 HP is just not it.
Go 1.5hp plug it in anywhere, use the extra cash saved on belts and steel, a 3/4" small wheel and attachment , and learn to grind

Motor Speed
The KBAC-27 D has a jumper you can set for double speed.
You can even set it up to switch double throw back and forth.
1740 double speed vs 3400 rpm
No clear answer for you.
Some people like both.
When you get two grinders do one each way and test it out.
Buy two half price used motors, test them out each way.

VFD's lose torque at low speed.
I say higher torque at lowest speed is more important than high torque at high speed.
But compared to water pump, the load on a belt grinder is very low.

Bearings vs overspeed ?
I think that 1700 vs 3400 rpm motors will have the exact same bearings.
Heat in the windings would be the issue and belt grinder load is much lower than a water or hydraulic pump.


Same for belts

The same folks that tell you you need 3 HP, also say you need 36 grit ceramic belts.
I don't go coarser than 60 and that's for profiling and general fabrication.
36 grit scratches are huge, deep and by the time you get them out, you have nothing left.

Ed Caffrey mentioned this and I like this too.
Start your bevel grinding at 320 or 400 grit and half speed.
No time spent changing belts, no problem scratch you can't get out...
Great learning tool
 

Hydroracer

Plastic
Joined
Apr 21, 2020
Thanks, that makes a lot of sense. I realize that speed in an induction motor is rated due to balance. Some are balanced better.

However, higher torque at low speed is better, suggests that the 1800 RPM motor is better suited for the task. Same horsepower at half the RPM means (roughly) double the torque since HP is a calculated figure that describes the torque and RPM relationship (torque x RPM divided by 5252). 1800 RPM on a 5" drive wheel is 2356 FPM. I don't know what a comfortable RPM one could over drive the 1800 RPM motor to, so just wagging it, say 2200 RPM. That's still only 2879 FPM. That seems a bit on the low side, even for 60 gritters and hogging...or is it?

I too thought 3HP was a bit high. Heck my Jet cabinet saw is "only" 3HP and that thing is a beast.

I think that 2HP and a KBAC 27D is probably the best combo. I have no issue with supplying it 240v as I have several 240v outlets I added to my shop. I can add more if need be.
 

michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
X =My bad. I was thinking about a grinder (you said grinder), not an abrasive belt. but still, if you can push hard enough to stall the sfpm you will not be happy.
A real belt machine like a Kalamazoo I think has a 3HP so that should give some idea of what may be needed for pushing steel to a belt.


The difference may be the cost of electricity and with a big blade you may be grinding a 3/4" blade width. That can be a lot so I would go with the 3 hp, plus you may be grinding carbide so it might be good to have a double pully for two RPMs. I was thinking that a strip on nylon tape might be set on your ways(if using roller ways), so to replace once a year for a few bucks.

 
Last edited:

metalmagpie

Titanium
Joined
May 22, 2006
Location
Seattle
Am building one now. I chose 2hp, 3600, 6" drive wheel. My goal is 600-6000 sfm. Yes, I bought the same KB drive as everyone else.
 

Cyclotronguy

Stainless
Joined
Sep 21, 2005
Location
Northern California
I'm not a knife maker, but I do get some of the special considerations knife guys consider for a 2 x 72. I have a 2x72 built on a Black Fox (tm) frame and love it. That said, I thnk it's impossible to have too much HP and too much SFM at your disposal within reason Some day you might want to "snag" a casting. Having enough HP to pull a 24 grit belt at 7000 SFM across a 14" contact wheel is a real joy. Im running a 3HP, 3600 rpm motor with a vector drive and there are times that I wish for more.
 

Hydroracer

Plastic
Joined
Apr 21, 2020
The grinder I am building is from the design Jer Schmidt came up with and posted on youtube. It's made from 3/8" stock and will take up to a 10" contact wheel. This is his revised gen 2 version. On the original, he used a 1.5HP motor and stated he wished he had used a bit more power on it. This thing is a beast.

I have all the steel stock on hand and the welder and plasma cutter ready to go, but just wanted decide on the motor and VFD before I start cutting and welding.

I'll use it for grinding on other things too, not just knives...like sharpening the blades on my 7-foot bush hog and figured 3HP, while it might be overkill for knives, might be useful for some of the other things it may end up grinding. I'd prefer to have a little more than I need in terms of power and surface speed and not need it very often than to wish I had it every time I use it.

The cost difference between a 2HP and 3HP motor and the larger VFD is of no concern, it's really not significant in the scheme of things when I consider the cost of an anvil, building a forge and heat treating oven and, at some point, a hydraulic press and all the various other tools and what not I'll need for making knives.

I dunno, kinda leaning back toward the 3HP set up.

 

Cyclotronguy

Stainless
Joined
Sep 21, 2005
Location
Northern California
Jer's design that lets you pivot between vert and horiz with no other adjustments is very cool.

That siad I frequently find myself setting a work rest on the "Black-Fox" belt grinder on a bias such that I have access to the full width of the belt when it's backed with a platen. This extends the life of the belt and probally the platen too.

Horiz seems to throw grinding swarf more efficiently than in the vertical mode. The versitility of the 2 x 72's is handy but that versitility make for less than convienient dust managent: plan accordingly!
 

Hydroracer

Plastic
Joined
Apr 21, 2020
Did some pricing on motors and VFD. Price difference to go from a 2HP motor and KBAC 27D to a 3HP motor and a KBAC 29 is about $85 to $100-ish dollars. Considering what I'm investing in everything else, that's a negligible increase.
 








 
Top