Question for KMG owners

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PT Doc

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Dec 24, 2011
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Ill be ordering a KMG soon and wanted to hear your input.

I'll be using a KB vfd. The motor is 1750 rpm and I am planning to use a 5" pulley on the motor shaft and a 2.5" pulley on the grinder shaft. This would spin the grinder shaft at 3500 rpm. With the 4" drive wheel this is basically 3500 sfpm. I have heard that running the 2x option on the kb is not a good ideas, so I was wondering if it would be helpful to go with a slightly smaller grinder shaft pulley of less than 2.5" to speed up the grinder. Another option would be to go with the 6" drive wheel and this would bump up the grinder speed by 50%. Keeping the 2:1 pulley ratio with the 6" drive wheel, this would be 5250sfpm. If going with the larger 6" wheel, I could change the motor to grinder pulley ratio to less than 2:1 since the 6" wheel would be boosting the rpm on the grinder.

A few questions.

What grinder speeds are you guys using and are you happy with them? Would you like the grinder to be faster?

My motor should easily be able to go down to 1/4 speed with vfd maybe even to 1/20 so I have room to slow it down but couldn't really speed it up.

I think the 6" drive wheel has better tracking capabilities than the 4" but that might be a minor plus. Any other advantage of 6" per 4" inch drive wheel?

If I know the target grinder rpm or sfpm that is ideal I can change the pulley ratios to make it work. I'd like to get this right the first time for cost reasons and just for time reasons also.

Thanks for the help.
 
PT Doc said:
Ill be ordering a KMG soon and wanted to hear your input.

I'll be using a KB vfd. The motor is 1750 rpm and I am planning to use a 5" pulley on the motor shaft and a 2.5" pulley on the grinder shaft. This would spin the grinder shaft at 3500 rpm. With the 4" drive wheel this is basically 3500 sfpm. I have heard that running the 2x option on the kb is not a good ideas, so I was wondering if it would be helpful to go with a slightly smaller grinder shaft pulley of less than 2.5" to speed up the grinder. Another option would be to go with the 6" drive wheel and this would bump up the grinder speed by 50%. Keeping the 2:1 pulley ratio with the 6" drive wheel, this would be 5250sfpm. If going with the larger 6" wheel, I could change the motor to grinder pulley ratio to less than 2:1 since the 6" wheel would be boosting the rpm on the grinder.

A few questions.

What grinder speeds are you guys using and are you happy with them? Would you like the grinder to be faster?

My motor should easily be able to go down to 1/4 speed with vfd maybe even to 1/20 so I have room to slow it down but couldn't really speed it up.

I think the 6" drive wheel has better tracking capabilities than the 4" but that might be a minor plus. Any other advantage of 6" per 4" inch drive wheel?

If I know the target grinder rpm or sfpm that is ideal I can change the pulley ratios to make it work. I'd like to get this right the first time for cost reasons and just for time reasons also.

Thanks for the help.
Click to expand...

The VFD is a very good idea.

I'm using the stock motor and pulley. The top end is fast enough for me, and the bottom end can go slower than I ever use. If I recall correctly, the stock motor is a 3400, so I'd think about maybe the 6" wheel.

Rob Frink at Beaumont Metal Works knows this stuff very well and a good guy to talk with - he helped me out with several purchase decisions and I have been happy with all of them. It's well worth giving him a call.
 
Can anyone else with a kmg chime in about preferred sfpm. Also advantages to 6" drive wheel. Thanks
 
I just converted mine from pulleys to a VFD. I went with a 5" pulley on the motor and left the 3 step on the drive wheel shaft running it in the smallest groove. Wide open runs it faster that I would probably ever want it to run. It screams! I tuned it to the settings Rob recommends and it runs excellent. And mostly I run it on about 70-75 on the higher grit belts. And slow down as I go up in grit.
 
One rarely has a problem needing a faster grinder. Slower is more often used. Go with the stock pulleys/wheels and don't worry about the top end. I would gladly trade a 8000SFPM grinder for one that would go at a dead slow crawl when desired.
 
What's wrong with the 2x option on a 1750 rpm motor? Rob's vfd setup page seems to recommend it.
 
javand said:
What's wrong with the 2x option on a 1750 rpm motor? Rob's vfd setup page seems to recommend it.
Click to expand...

Are you talking about the part where he suggests turning max all the way up? Wouldn't that be twice as fast with a 3400 as with a 1700 RPM motor?
 
Tait said:
Are you talking about the part where he suggests turning max all the way up?
Click to expand...

No
http://www.kbelectronics.com/manuals/kbac_manual.pdf

You can read the manual for more details, but the 2x option is a jumper that doubles the frequency and therefore rpm.
Page 8
Jumper J4

The max is an internal trim pot adjustment - basically sets an upper limit on the speed as far as I can tell
Useful in other applications where you want to limit speed to prevent damage.
 
1000-5000 SFM is a good and fairly typical range. my .02...
 
Maybe I imagined it.. I don't see it there either, but I could've sworn... *shrug*.. I recently switched both my vfds to 2x and I like it. Especially on the disc grinder, although I don't really need to run it full tilt.
 
What about the 4" vs. 6" drive wheel? Any 6"drive wheel users? Thanks for the input.
 
I'm using the 6" wheel, but I have a fixed speed motor (1425 RPM), so I guess that doesn't count. Anyway, for my purposes it's fine. As an aside, I only bought the 6" wheel because 4" was sold out at the time. A rare moment of impatience for me.
 
tryppyr said:
I'm using the 6" wheel, but I have a fixed speed motor (1425 RPM), so I guess that doesn't count. Anyway, for my purposes it's fine. As an aside, I only bought the 6" wheel because 4" was sold out at the time. A rare moment of impatience for me.
Click to expand...

Thanks for the input. After reading MANY old threads on this site and others, I'm left with the idea that the 6" wheel would probably help with the tracking a bit. Not that there have been many complaints about the KMG tracking.

I think that Ill end up with the 6" drive wheel and then if its too fast for me with the 1760 RPM and 2:1 motor:grinder pulley ratios I can always changes the grinder pulley size to make adjustments. The Baldor 3hp motor is supposedly good to a 20:1 turn down with inverter, so theoretically it should be able to be turned down to 88 rpm. So on the low end, 88 rpm with the 2:1 pulley=176rpm and with 6" drive wheel (1.57ft/rev)=276 sfpm. On the high end, it would be 5526 sfpm. I think that covers a pretty good range for now.
 
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