Grinder motor with fixed speed

W

wwillimon

Joined
Jul 16, 2024
Messages
17
I have 1725 rpm motor with a 5" drive. Is my rpm too low, or do I need 3450 rpm motor? It is a 2" 72" grinder.
 
It depends on what your plan to use the grinder for - if it's mostly used for profiling, then yes, it's too slow. If for finish grinding then it would "work" - sorta.

Your 1725 rpm motor with a 5" drive wheel gives around 2200 to 2300 SFPM belt speed. Rather than think about a new 1 ph motor, get a 3 ph motor and VFD. While if money is no object the expensive KBAC NEMA 4X VFD is a good choice, but that's getting close to $400 just for the VFD. For a hobby use (like myself) my suggestion would be this $60 VFD on Amazon B0BTCRP1LC..... ooops, I see then increased the price to $86. I ordered Nov 21 for $60 shipped. This one is a bit more expensive, but will take 120 vac input for a 220 VAC 3 ph output if getting 220 volt is an issue. Either put the VFD in an enclosure to protect from grinding dust, or as I did filters over the air inlets to help with metal grinding dust. I usually get 5 or 6 yrs from the NEMA 1 VFDs with inlet air filters, and never remember to blow out the inside with an air hose which I'm sure would help.

Here's an ebay item 375823148902 listing for a 56C 2hp, 3ph 3450 rpm motor for $171.35 shipping (plus taxes of course).

As other folks will tell you for general knife work, a variable speed grinder can't be beat. For 2 or 3 yrs I touted my 3 speed pulley setup as all that was needed. When I built the 2 grinder I went with a VFD, and boy there is no comparison. Try it, you'll never go back {grinning}

just checked on Amazon and it looks like the 2hp KBAC is not around $500!!! Way out of my league.
 
I rarely run wide open, or at the bottom end....but I imagine you need the 500-2800 range for most of the action my 2x72 sees. My 2 cents...VFD really a NEED to run a 2x72 well.
 
Ken H> said:
It depends on what your plan to use the grinder for - if it's mostly used for profiling, then yes, it's too slow. If for finish grinding then it would "work" - sorta.

Your 1725 rpm motor with a 5" drive wheel gives around 2200 to 2300 SFPM belt speed. Rather than think about a new 1 ph motor, get a 3 ph motor and VFD. While if money is no object the expensive KBAC NEMA 4X VFD is a good choice, but that's getting close to $400 just for the VFD. For a hobby use (like myself) my suggestion would be this $60 VFD on Amazon B0BTCRP1LC..... ooops, I see then increased the price to $86. I ordered Nov 21 for $60 shipped. This one is a bit more expensive, but will take 120 vac input for a 220 VAC 3 ph output if getting 220 volt is an issue. Either put the VFD in an enclosure to protect from grinding dust, or as I did filters over the air inlets to help with metal grinding dust. I usually get 5 or 6 yrs from the NEMA 1 VFDs with inlet air filters, and never remember to blow out the inside with an air hose which I'm sure would help.

Here's an ebay item 375823148902 listing for a 56C 2hp, 3ph 3450 rpm motor for $171.35 shipping (plus taxes of course).

As other folks will tell you for general knife work, a variable speed grinder can't be beat. For 2 or 3 yrs I touted my 3 speed pulley setup as all that was needed. When I built the 2 grinder I went with a VFD, and boy there is no comparison. Try it, you'll never go back {grinning}

just checked on Amazon and it looks like the 2hp KBAC is not around $500!!! Way out of my league.
Click to expand...
I do this as just a hobby. Don't have a fortune. I had been looking for a 3/4 hp, 3450 rpm motor. I guess I could set up a double shaft and speed it up. You loose a lot of hp by doing that though. Thanks for the advice.
 
Unless you already have the extra pulleys and shafts and all that stuff, it's not in really worth doing. By the time you get all the stuff your going to have spent almost as much as just buying a vfd and a 3 phase motor...
 
You need to be thinking of hp as well. You can have a 3400rpm motor but if its only 1/4 hp its not going to be as effective as a slower stronger motor. I started with a 1/4hp ac motor on step pulley and had to pay attention to belt pressure and had to put the motor on a hinge plate so I could lift it to start it, then slowly lower it to get the grinder going. I could stop it pretty easily.. stronger motors allow you use more pressure (and take your time). you can usually get a 1-1.5hp (tefc) motor for around 300 (at HF or PA) but eventually you will want to get a kbac for variable speed but you will pay as much if not more for one.
 
3/4hp is definitely underpowered but it it’s what you have to work with you could also get a larger drive wheel.
I swap between a 4” and 8” drive on a 2hp 1725 rpm motor.
 
Loveless didn't need variable speed. We used to walk 5 miles to school, uphill-both ways, in the snow.

You can make knives with a file. A 1 X 30 belt grinder is a big improvement. A fixed-speed 2 X 72 will make you think grinding is easy after the first two options.
A variable speed grinder is a wonderful thing to use. A new heat treat oven is great, too. And a fiber laser for marking. And, don't forget an array of different buffers-so you don't even have to change buffs. Maybe an extra 2 X 72 also. Then, you need a CNC controlled milling machine. Annnd--- a bigger shop.

How far do you want to go? How much do you want to spend? Hobby or for profit? How far down the rabbit hole?
 
Bill DeShivs said:
Loveless didn't need variable speed. We used to walk 5 miles to school, uphill-both ways, in the snow.

You can make knives with a file. A 1 X 30 belt grinder is a big improvement. A fixed-speed 2 X 72 will make you think grinding is easy after the first two options.
A variable speed grinder is a wonderful thing to use. A new heat treat oven is great, too. And a fiber laser for marking. And, don't forget an array of different buffers-so you don't even have to change buffs. Maybe an extra 2 X 72 also. Then, you need a CNC controlled milling machine. Annnd--- a bigger shop.

How far do you want to go? How much do you want to spend? Hobby or for profit? How far down the rabbit hole?
Click to expand...
Just a hobby for me. I am up in age, and just want to learn a little about knife making.
 
Just remember, the motorcycle, bass boat, golf clubs, etc are just for a hobby also..... well, the in case of a motorcycle it might be basic transportation replacing a car, but usually a bike is for fun weekend rides. Considering what can be spent on those hobbies the cost of a 2X72 grinder with all the attachments needed isn't all that bad. PLUS - tools tend to hold their value pretty good while that bike, boat, golf clubs in 3 yrs have dropped LOTS of value while the tools hold value nicely.

BTW, if you fill out your profile with location and age it will help us understand better what suggestions to make.

It sounds like you are pretty much my profile, enjoy learning new things and perhaps even building new tools?
 
wwillimon, you already own the 2x72, or you are considering and already have a motor? A lot of guys here will shame it but as a hobbyist myself, Grizzly's 2x42 works fine for me. It has variable speed already. I made a larger/better flat platen (the factory one is crap) & use 2x48 belts now. They work fine on my radius platens and contact wheels. You can order extra parts from Grizzly and make changing tooling quick & easy. Contender Machine Works makes some goodies for it. Good luck with whatever you do!
 
wwillimon - When I welcomed you to Shop Talk a couple months ago, I asked you to fill out your profile so we would know where you live. If you do, depending on where you live, you might get an offer for a free motor and maybe a can of quench oil.

In your hobby situation, I would use the grinder as-is. You can upgrade to VFD and/or another motor in the future if you think you need it. What brand grinder do you have?

I started over 60 years ago with files, sandpaper, and a sears 6" bench grinder. That was all I used for about 10 years. Years later I built a 3/4HP 1750RPM KMG clone. When I upgraded the motor to 1HP at 1750RPM I thought it was as good as it gets ... until I got a 1.5HP VFD Bader B# ... when I went to a 2HP Reeder VFD it was even better ... then water cooling .... then special platens ... then surface grinder attachments ... then specialty jigs ... it is a rabbit hole.
 
Last edited:
I think that grinding slow is a great way to start. Don't need fancy ceramic belts unless you want supersteels with all that alloy carbides, so you don't need the speed. That low RPM makes you take your time, and you'll screw up a lot less. Mistakes happen REALLY QUICKLY. Take your time and make nice grinds instead of lots of grinds that aren't so swift.
I'm older than you, have two variable speed KMGs, and rarely run them over 50% with a 4" drive wheel. I don't like this hyperspeed that everyone runs nowadays, though I get why they do it.
Learn how to grind on a single speed. Then try variable...
My 2¢.
 
Last edited:
One response I give for people who ask if 1750RPM is too slow is:
"It is 1750 times faster than hand filing"
 
This made me laugh. I built my first grinder in the late 70's with a 3600 RPM, 2 HP motor. The pully ratios were backwards and my contact wheel was probably turning at 5000 RPM. It was like throwing a twig in a wood chipper. Fast forward, a TW90 made a bit of a difference.
 
You must log in or register to reply here.
Back
Top