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brianc

Yet another knife grinder build

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My grinder in a box (GIB) should be waiting for me when I get home tonight. It’s one of the 2 x 72” kits. I’m hoping to get an idea from some of the experienced knife folks out there as to what motor may be appropriate in terms of hp and RPM. I had initially settled on 2 hp and 3600 rpm with a VFD speed control, but in reading the various forums some comments made me re consider this decision. Some folks pointed out that with the VFD, if you are running at 50% speed your torque is substantially reduced as well. This is something I am currently researching. They also pointed out that running at less than full RPM; the fan won’t be cooling as efficiently, leading to potential motor overheating issues. I’m new to knife making but not too mechanics so I realize there are choices you can make in terms of drive pulley diameter to change belt speeds as well, but I haven’t found any sort of chart that spells out appropriate surface footages etc. for the different grit belts so I can better bracket in my desired speeds.

Any comments or suggestions from the more experienced folks would be much appreciated so I can get the appropriated drive parts on the way. I've found little bits and pieces on my research, but nothing approaching any sort of consensus (not that I expect too with this crowd)

 

Thanks

Brian

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You'll need a three phase motor if you intend to use a VFD on a motor larger than about half horse.  The torque curve is absolutely going to be impacted by lower synchronous speeds but that isn't necessarily a big deal because lower speeds are generally reserved for higher grits where you're not hogging material off.  So lower torque at lower speed isn't such a big deal.  There are VFD's that input single phase power and output three phase.  Be sure to enclose the VFD in something dust proof and don't forget to give it some kind of cooling fan.

 

As for cooling - a Totally Enclosed Fan Cooled (TEFC) is recommended for harsh environments like a grinder.  Most folks put a gooseneck lamp on their belt grinder- why not add a cooling fan? 

 

I found this via googling:http://mcfinishing.com/resources/abrasivetech1.pdf

 

Hope that helps.

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How many of the name brand grinders are sold with variable speed motors and do they have the issues you mentioned?

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KMG was where I had initially come up with a TEFC 2 hp 3600 rpm motor.

 

I'm not big on re-inventing the wheel if I don't have too. I also only want to build one grinder (for now), so there isn't that much difference between 2 hp and 3 hp motors for example and enough folks weighed in and said they wished they had a little more "oomph" I might veer that direction.

 

The above referenced chart was interesting, but only gave one speed for steel with no regard to grit size. This may be one of those acquired things where most folks go by feel as far as how fast they run their various belts? Is there a maximum surface footage one should not exceed? That would help me calculate input rpm and drive pulley diameter.

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the speed max limits would be more because of the bearings of the wheels/rollers than the grit.  Having said that I prefer to run the higher grits like 600 and above at 2500 or a lot slower, and I use 40 and 120 grits at about 5k to 6k

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I have been selling motors and VFDs for about 5 years.  I have a 1.5 hp motor on mine and can not bog it down even at 25%.  I suggest an 1800 rpm motor.  1800 rpm motors have more torque than 3600 rpm motors and are better motors than 3600 rpm motors and they cool better.   I sell a cast aluminum motor that cools better than the rolled steel motors.  I believe that your fears are unfounded.  I am still using my original motor and VFD.

 

E-mail me or call if you have questions or if I can help you.

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