Variable Speed KMG on a "shoestring" budget

Daniel Koster

Daniel Koster

www.kosterknives.com
Knifemaker / Craftsman / Service Provider
Joined
Oct 18, 2001
Messages
20,978
Here's what I've done, and hopefully a good description of how you can do the same with little electronics/motor experience and not much $$$.


grinderpic1.jpg

KMG grinder w/ reversable, variable speed (grinning boy not included...:D)


Parts List

Grinder parts

KMG-PL = $650

I swapped in a Serrated 2" Contact Wheel ~ $40 and also ordered a 2.5" Pulley and Belt = $19 (at the bottom).

Add to that a piece of Pyroceram from Steve Pryor ~$28



Motor parts

1.5 HP 120V DC Motor = $25

130 VDC Controller = $40

Pulley for Motor (2 5/8", 5/8" shaft) = $4

Single Pole, Double Throw Switch = $4

Kit Box = $6

10k Potentiometer = $3

Double Pole Double Throw On/Off/On Toggle Switch = $4

Control Knob = $2

The rest is misc. stuff that can be picked up at Radio Shack or Home Depot, etc.

18 ga wire - for Potentiometer
Power cable - (Green,White,Black) 16 ga
Plug end
16 ga wire - for DPDT switch
20 Amp fuse and fuseholder
Insulated female spade connectors
"Circle" male spade connectors (better than "tongue" type)


Tabletop:

Tool Stand = $20
3/4" Med. Dens. Fiberboard - 2'x4' = $6 (at Home Depot)
Misc. nuts/bolts = $4


Tools Needed:

Ratchet Drive/Sockets
Screwdriver, crescent wrench
Solder Iron, solder
Hand Drill, bits



Total cost = ~$875 (not including s/h)



Having a variable speed motor for your grinder doesn't have to be expensive, or feel like you're chasing the Holy Grail...:rolleyes:

I'll try to throw together some pics and a write-up. In the meantime, a big thanks to rlinger and Sando for helping me put this together.

Dan
 
grinderpic2.jpg




motorpic01.jpg




motorpic02.jpg

This motor is great because it has dual shafts. I left open the option of adding a wheel, etc. later.


controller.jpg

The reversing switch is on the top left, the potentiometer in the middle, and the on/off at the bottom left.
 
Here's a quick video showing the motor being turned on, the speed reversed twice and then turned off. (pardon the strange audio...:rolleyes: )

Grinder Video (165 Kb)
 
Dan the man!

Much cleaner setup than mine. You really did it up nice. However I don't see the shoestring?

Isn't that KMG a quiet, solid machine?

Where's the first knife?

Steve

Your next problem is going to be NOT buying more tools from Mr. Frink.
 
Pic of the circuit board with both switches added (including the power cord and fuse):

circuit-board.jpg



Pic after adding the potentiometer (switches pulled away):

board-overall.jpg
 
Really Dan that is nice.

BTW I see what looks like a shop elf in the first photo. Now I know how you did it.

Steve
 
The potentiometer to control the speed, where did you get that??? I need one for a extra speed control that I have.
I see I just click on it coool.
 
Dan, that is a slick outfit you put together there.

Looking at the circuit board and not knowing anymore about that controller (I didn't see a Minarik # on the web site) it looks as though it does not support reverse by design and I am guessing you are just switching polarity at the controller output, but that you did provide for removing voltage from the motor while switching. That should cause no problem so long as you allow the motor to come to stop before reapplying voltage.

Neat looking layout and I bet it runs great too.

IG, you can get a pot at any Radio Shack. His seems to require 10K ohm. Most I have seen so far use 5K ohm. Hopefully you have documentaion or can look up the vendor specs for the value you need.

RL
 
Dan, very clean workmanship throughout. Congrats on a job exceedingly well done! :) BTW, clear concise pictures too. Thank you for posting the info & pix for us.
 
Sando - The "shop elf" and his cohort are part of the reason it took so long to get this put together...:D


Mike - "me likey jerky..." :eek:


Roger - from the controller manual: Which was found here:

http://www.minarik.com/PDFs/250-0246.pdf
(this took forever to find, btw)

"Minarik drives supply motor voltage from A1 and A2 terminals. It is assumed throughout this manual that, when A1 is positive with respect to A2, the motor will rotate clockwise (CW) while looking at the output shaft protruding from the front of the motor. If this is opposite of the desired rotation, simply reverse the wiring of A2 and A2 with each other."


If you look at the switch in the first circuit board photo you'll see 8 wires tied to 6 pegs. Here's the idea:

dpdt.gif


In position "1" positive power comes from A1, negative from A2. When matched with the pos/neg of the motor, results in a CW rotation.

By tying the leads in position 2 to position 1 (and by switching them) the pos/neg reverses resulting in a CCW rotation at the motor, and there is no danger of shorting since position 1 is technically disconnected.

This only works with a DPDT - Double Pole Double Throw switch where the Center position is "Off".

The only danger is if you switch the direction without waiting for the motor to wind down. Fortunately, it slows down real quick, and I don't really have a need for switching it back and forth relentlessly.

That's also why I "braced" the motor with a heavily built mount. Testing the motor without a brace on my table resulted in a few unintended mexican jumping bean contests... :eek: It seems that even if you let it sit for a few minutes at rest, it will still "buck" when you apply current in the opposite direction.

Again, thanks for all the help guys.

Hopefully, I'll get this all written up and posted here soon. It should make it easier for the "next guy" down the road...
 
Yes Dan and that is what I ment to say. Because the motor is a generator it can easily cause an output SCR to short if voltage is disrupted and reaplyed while the motor itself is generating a voltage and also because load increases dramatically since the drive is struggling to redirect the motor. In fact with DC drives not specifically designed to be reversed during operation it may even be a good idea to turn power off to the drive itself before reversing direction.

You really have a great talent for diagramming things. You make it look easy.

RL
 
Originally posted by rlinger
In fact with DC drives not specifically designed to be reversed during operation it may even be a good idea to turn power off to the drive itself before reversing direction.
RL
Click to expand...


Yep Roger, it is a good idea...just ask me how I know! :) I've got 4 Rob Frink Flat horizontal disc sanders (2 flat, 2 beveled) and I'm running basically the same motor/control setup as Dan has shown in the above diagrams...I get to working and get busy and have blown two controllers so far by reversing them before they stop...the flat disc's don't have as much friction in the system to slow them down like the KMG does...anyway, replaced the output MOSFET and put them back in service...unfortuantly, it's a pita when they're down and you're waiting on parts! Anyway, long way to say, I now turn power off and wait for the system to come to a complete stop before reversing! :) Hey Dan, those dual shaft treadmill motors with the DC drive make great variable speed buffers, I set up two of them...very nice! :) Also, very nice step by step tutorial, that was way cool of you to take the time to put that together for everyone!

-Darren
 
If they don't watch out, Steve and Roger are going to become the defacto variable speed gurus on here! If it weren't for those two I'd never have gotten my disk grinder running. Roger even fixed the stupid controller I paid way too much for on eBay.

Good job Dan. I'll use this (or your write up at your website, hint) when I build my horizonal one.

Darren, good idea about the var speed buffer!

Dave
 
Great info Dan! Thanks for taking so much time to put it all together.

In regards to reverse/braking, I wonder if there is a way to build a simple circuit using a power resistor to dissipate the inertia and brake the motor before reversing the polarity. When playing around with DC motors, I'm amazed with the amount of rotational resistance when you simply short the input wires. For a braking circuit, you would disconnect the input power, then short the armature over a power resistor to kill the inertia, then induce the reverse polarity.

It is fun to think about anyway.....

Thanks again,
Rob
 
Rob, that is the way I theorized it in my thoughts but have never tried it (braking circuit). Maybe someone here that has done that can give us the results of their experience. I saw here once upon a time where one of us used a stove top heating element as the breaking resistor. Too small a resistance will be hard on the motor; for example, in practice, simply shorting the amature would not be good.

RL
 
Hey Dan, nice job on the setup you did. I noticed the motor is open though. Be careful of any grinding dust. It'll toast an open motor quick. Maybe you could build a shroud over it to keep of the grinding dust??
 
Originally posted by rfrink
Great info Dan! Thanks for taking so much time to put it all together.

When playing around with DC motors, I'm amazed with the amount of rotational resistance when you simply short the input wires. For a braking circuit, you would disconnect the input power, then short the armature over a power resistor to kill the inertia, then induce the reverse polarity.

Rob the RC car boys do that all the time, this type of circuit is built into the Electronic Speed Control (ESC) You buy the ESCs with or without brakes and they will lock the wheels up but I have never seen a schematic on one. My lathe has a variable speed control and a forward/reverse switch, the switch is as stated above DPDT with center off . With the lathe running you cant go directly to reverse, without going through the off position. When this happens you then have to turn the speed control till it clicks off then switch to reverse and turn the speed control back on before it will start agian. I know this is not a brake but it prevents you from going straight to reverse while the lathe is running.
Click to expand...
 
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