# What's the ideal RPM for you?



## blorp (Jul 19, 2020)

I see some people with fixed RPM rigs and others with variable speed. I think most adapt to what they can start out on, but I haven't read too much about ideal RPM in my frantic googelin'

Of course its up to preference as well as context, but I'd like to know why you like X RPM and what your use case is. Is there a range that suits you best assuming the motor is beefy enough to not stop you apply a feather's worth of pressure on the belt?


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## Bensbites (Jul 19, 2020)

I think it also depends on the application. Grinding steel at 36 grit vs 400 grit, I would want different speeds.


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## blorp (Jul 19, 2020)

Bensbites said:


> I think it also depends on the application. Grinding steel at 36 grit vs 400 grit, I would want different speeds.



This would be for 6" knives. Lets say 80crv2 either 120 or 80 grit. If you had to stick with one RPM (maybe two) what would it be?


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## Bensbites (Jul 19, 2020)

To be honest, my 1x30 I could go with the max speed and bog it down. The 2x72 variable speed I am building... the 1.5 HP motor was defective and I couldn’t get it to track. I figured it out this weekend. I will have more answers for you in a week or two.

by the way, rpm of the motor is not the right metric. You need to factor in the wheel size to figure out feet per sec.


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## blorp (Jul 19, 2020)

Bensbites said:


> To be honest, my 1x30 I could go with the max speed and bog it down. The 2x72 variable speed I am building... the 1.5 HP motor was defective and I couldn’t get it to track. I figured it out this weekend. I will have more answers for you in a week or two.
> 
> by the way, rpm of the motor is not the right metric. You need to factor in the wheel size to figure out feet per sec.



Good to know! I'd be looking at a 1x30 or 2x42

There are some high powered motors, but at much lower RPM (1500s) and I'm trying to see if those would be an issue.


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## Tim Rowland (Jul 19, 2020)

It really depends on the materials you are grinding.
For profiling out a blade with say a 36g or 60g crank it up to whatever your setup and belts can handle.
I personally profile with a 36g Norton blaze at 5,000 rpm
For pre-heat treat bevel grinding again a Norton blaze 36g or 60g (depending on stock thickness) at anywhere between 3500 to 5000rpm depending on the steel type.
Post heat treated and finishing the bevels start with FRESH belt (same grit as pre-HT) typically 500rpm slower than what I used pre-HT along with a lot of water dunking. 
As you move up the grits you will create more heat and need to slow it down and dunk more often.

Handles all depends on materials used, everything from the epoxy used to the oil content of the wood, is there an acrylic/plastic/resin type material as a spacer, is there a metal spacer, etc..................
The short answer is yes you can do all your grinding on one fixed speed............but you will hate yourself after using a variable speed setup and not just saving up before diving in with a fixed speed.


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## blorp (Jul 19, 2020)

Tim Rowland said:


> It really depends on the materials you are grinding.
> For profiling out a blade with say a 36g or 60g crank it up to whatever your setup and belts can handle.
> I personally profile with a 36g Norton blaze at 5,000 rpm
> For pre-heat treat bevel grinding again a Norton blaze 36g or 60g (depending on stock thickness) at anywhere between 3500 to 5000rpm depending on the steel type.
> ...


Thank you for the amazing answer, I guess my followup question is then: What's the minimum amount of money you'd spend on a VFD?


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## panda (Jul 19, 2020)

my car revs to 9k


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## Nemo (Jul 19, 2020)

When I started, my natural cadence was around 60/ min. It's up around 80-90 now although with high power intervals it'll often go up to 110-120.


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## Nemo (Jul 19, 2020)

panda said:


> my car revs to 9k


S2000?


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## Nemo (Jul 19, 2020)

Back on topic... from my experiemce (1 knife), +1 to what Tim said.

Variable speed is good because you can slow down as the knife gets narrower and narrower behind and at the edge and as you use finer grits. You don't want to heat the steel near the edge too much or you will spoil the temper.


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## Tim Rowland (Jul 20, 2020)

blorp said:


> Thank you for the amazing answer, I guess my followup question is then: What's the minimum amount of money you'd spend on a VFD?



There are quite a few great VFD out there.
In the knife making community the 2 most commonly used (at least in the states)are:

KBAC 24d or 27d depending on the size of your motor. They typically run around the $275 to $300 mark.
Lenze AC tech SM vector (my personal favorite) run about $475 to $500

On the high high end there is also the AC Tech MC 1000 and they run about $850


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## HSC /// Knives (Jul 20, 2020)

Many variables to this answer but in general on my hardcore 2 x 72 the motor is 1750 rpm, and I rarely run it more than 30% even on 50 grit rough grinding

most grinding is done at 20%

and I do it primarily by sound and feel


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## blorp (Jul 20, 2020)

HSC /// Knives said:


> Many variables to this answer but in general on my hardcore 2 x 72 the motor is 1750 rpm, and I rarely run it more than 30% even on 50 grit rough grinding
> 
> most grinding is done at 20%
> 
> and I do it primarily by sound and feel



Thanks for the feedback!


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## HSC /// Knives (Jul 20, 2020)

blorp said:


> Thanks for the feedback!


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## blorp (Jul 20, 2020)

What are your thoughts on buying used Baldors?


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## HSC /// Knives (Jul 20, 2020)

blorp said:


> What are your thoughts on buying used Baldors?


I would. If the motor fires up freely and doesn’t get hot or make noise I would totally buy used Baldor


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## blorp (Jul 20, 2020)

HSC /// Knives said:


> I would. If the motor fires up freely and doesn’t get hot or make noise I would totally buy used Baldor



Great, glad to hear that. There's a lot of them on ebay for decent prices even with the higher shipping prices


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