I have a Makita straight electric grinder, the type with the 1/4" collet, which free speeds at 25,000 rpm. I'd like to be able to safely run 4" cut-off wheels rated at 19,000. Can I use some type of plug in rheostat to slow the tool without damaging the motor? I do have a tachometer to confirm the speed once this has been addressed. Thanks!
I put one on my old mini lathe about 6 months ago when the original control board burned out. It's working fine, almost always running it at low speed. http://www.harborfreight.com/router-speed-control-43060.html
Many new devices use PWM. Pulse-width-modulated, to supply power thru a controller. These fill in the pulse, as more current in needed, to do the job. I await replies from the electrical control gurus on this one, as a rheostat I believe would (affect) by lowering the voltage.
Buy a Variac to control the voltage. You only have to watch for overheating in your grinder. This works very well to control speed in my band saw so i can cut metal or without it I can cut wood at high speed.
Some AC motors don't lend them selves to reduced voltages well. Depending on the type of motor your grinder has, it may need to a control that varies frequency to slow it down, rather than reducing voltage. A little reduction through a rheostat could create a lot of heat, which can wipe out the windings quickly.
^^^There in lies my concern! I suppose I could first try it while paying close attention the tool body for excessive heat....
Buy a Harbor Freight 4" right angle grinder for $9.99. I bought one for the hell of it and it wont quit
Why dont you just buy a cheap HF angle grinder with the correct speed? Probably pick one up for the same price as a rheostat. Or just get the correct cut off wheels?....Just sayin.
This is the solution..... I used to have one of those Makita die grinders, and they're really not heavy-duty enough to swing a larger cut-off wheel. A 2" wheel should be about the max size you want to try on those.
I need to test the 4" cut-offs, ours versus the competition. I work for a large abrasive manufacturer.
I know right! I used my cheap HF angle grinder to cut the IFS out of my 34 and it wasn't it's first job either. I've had it for years. When it finally die's I'll go buy another one.
If there is no option to my original question, I will go buy an electric grinder specifically for these wheels. But it won't be from HF. Stuff from there may work for some, but not me.
What do you need to know? Many of us have tons of experience and have posted over the years. Which company do you work for (PM if you want to keep it quiet).
You could use something like that HF 'speed control', but it won't be good for the motor. It will cause more heat in the motor. You'll burn that Makita up sooner or later, with or without the speed control. They make dedicated angle-head 'grinders' that accept the cut-off wheels, I don't recall the exact brands but I have seen them.
Well I picked up my HF grinder years ago and some nicer B&D units on evilbay figuring the HF one wasn't going to last. The B&D backups are still sitting in my rafters waiting for it to die. True I don't work the living hell out of it every day but I do use it often. I've done frame work, built railings for my house, cut roofs off of cars, and cut up a bunch of scrap just to name a few things. I've done hundreds of jobs with it. For me; for the money you can't beat it. Now in an industrial situation; it may not last as long.
It's kind of a Mcguyver thing, but I built one for my belt sander. I used a dimmer switch designed for ceiling fans. I wired it into a box with an outlet coming out of it and have used it on my wood lathe and bandsaw at times now too. A regular dimmer drops voltage which is browning out the motor. A fan control or an electronic dimmer designed for led lights reduces the amperage. Look for a heavy duty one, and don't use it all the time or you could kill the dimmer, but I have had mine for a decade and it sure is handy for a few bucks.
We commonly perform what we refer to as a G-ratio test, G=grind. There is a formula applied after the test has been run to determine the cost savings our product will provide the customer. (I am an outside sales guy and don't always trust what our factory results show.) In the case of a cut-off wheel, I'll measure the O.D. of the wheel, make 10-20 timed cuts, and then measure the "used" O.D. of the wheels. The most common size that I run into is 3", but this time it is a customer using 4", hence the need to slow this grinder down...
You can't slow down an AC motor because its speed is timed to the AC frequency of 60 cycles per second. Bog one down too much and it will overheat. In other words, an AC motor is a constant speed motor. Best answer is to tell your cheapskate boss to buy you a new grinder. There are devices that will allow you to do what you want to do, but a plain rheostat isn't one of them. You could try the ceiling fan control (not a light dimmer) or the router control.
A shop in S.F called Carl's Electronics, sells a robust PWM power supply, w/an amp rating you choose to match the job. I tried building a PWM w/Radio shack pieces, but produced only 'smoke'! The supply from Carl's did the job just fine.
You will find that your grinder has a "universal" brush type motor. The ideal speed control for these is the plug-in type that they used to plug electric drills into before they got all high tech and variable speed. I made one at high school about 30 years ago, but I reckon you could probably still buy a kit to make one up. They were known as a "triac" speed controller. All the other methods (variac, light dimmer), will result in an overheated/lower torque motor. Later hand drills incorporated the triac controler into the trigger. If you find a dead hand drill which has the trigger mounted speed control, you can swipe this unit and modify it for use on your grinder, as long as the wattage of both tools is about the same.
I have a HF electric die grinder. I use those flap wheels (that spin like a drill, not like a grinder disk) and they are not rated for the speed those electric die grinders spin at. I put on one of those HF speed controlers to slow it down. Its been over 8 years of almost an hour a day of use, and not burned anything up. The only issue Ive had is replacing brushes, but since it gets used as much as it does, thats to be expected.
I have 4 of those HF angle grinders, $10 a piece on sale. One for the sandpaper flaps, one for grinding wheels, one for cutoff wheels and one is a spare in the drawer. This way I don't have to swap wheels all the time. Beat the crap out of 'em and the spare is still in the drawer. I take the guards off immediately for clearance, and I have the scars on my hands to prove it.
Incorrect. You absolutely CAN slow down an AC motor, and I've done hundreds of them in both single phase 120/240 and three phase. It just requires a frequency controller. With a frequency controller I can get them so slow you can hardly tell they're turning, and they don't overheat.
Thanks for the info (interesting stuff, it's like catching up with Popular Mechanics from before my time), and I was reading this and that because I picked up one of those grinders for half-off (yippee), but I'd already collected some bits rated for somewhat slower speeds, initially to use in a drill (then I looked for 25000 rpm types, and a lot of them don't even give speed specs, at least from online stores or manufacturers, hmm well it seems as though accurate to specific info is not trending). Likewise I checked around for additional tool specs, and the grinder is at least 400 watts (at 115 volts), except some sites were listing it as 250 watts, so a 300 watt plug-in dimmer switch (by Leviton) was something I tested to see how it responded (then I double checked the wattage on the tool, which isn't stated directly, except in a Makita-India catalog, and that's amps x volts). This test worked out better than the router speed control from HF, which I tried first (maybe I got a dud though, and didn't want to mess with it at that point—compare funny video review about a similar one). Then I tried a 5 amp multi-fan speed switch (by Legrand, which I found at the local hardware store, on clearance again), wiring that into an all weather extension cord by Ace (which is 14 gauge and good for 15 amps up to 50 ft, yet is more flexible than usual, so it won't flip around), and these go together with an aluminum outdoor electrical box (which makes for a better heatsink than other boxes I suppose, and would keep dust out—besides me omitting the water resistant foam gasket from its lid, because what looks to be polyethylene could melt into the switch, I guess). That works like the dimmer in effect, and is designed for fan motors at least (higher capacity, no doubt). Still on a roll, I later found a good deal on a 15 amp multi-fan switch (by Raffel Systems, which looks to be the same as those branded as Dayton, though some are 10 amp, with a blue vs white wire under the hood, or all black for the Legrand, and the extra red wire is auxiliary for something like a power indicator light, but my cord has lighted plugs). I hooked it up the same way (with less stepwise instructions there, comparing the schematic to a multi meter), and put it in a wider box, with a box extension (for 3-inch deep aluminum, as they recommended 4 for a standard steel box, and aluminum is something like 5-times better at dissipating heat, or as far as codes go in walls, there was plenty of room in there for the conductors). Whatever else this larger setup could come in handy for, these are supposedly industrial strength switches, with a double gang sized heat sink too. I might have also tried the variac and whatnot, but didn't have major enough plans to compare everything, just for kicks. I'm not too concerned about torque either, as it would be low at such high speeds to begin with. I think the rationale for electric drills having a speed lock button, which only works from the highest of their variable speeds, is that they work basically the same way as the fan switch. You cannot lock them into a lower speed with the button, because they would respond no differently, and get just as warm that way. Yet the fact that they have variable speeds, which go pretty slow, means they can be run variably, as such, for a while without problems too, and Makita also makes a variable speed version of their electric grinder (which I've seen re-branded as well, or maybe it's the other way around). Pretty compact either way, well it still weighs as much or more than a 3/8" drill (or variable speed contraption, and all of that for a knob)!
This is not accurate. There is no control out there that limits amperage, that is false. All of these dimmers/controllers work by reducing the voltage, the difference between a light dimmer and a fan speed control is the fan speed control is designed with components to handle the inductive load of a motor while a light dimmer does not need that for the resistive type of load from incandescent lights (these do not work on fluorescent lights). Also, most fan speed controls start on high, to allow full voltage on startup when current demand is greatest, while the standard light dimmers can usually be turned on at any point. Also, reducing the voltage to a motor will cause a great amperage draw under load, that is just plain physics at work, and there is no way around that. When the current draw exceeds the supply heat develops in the motor, and heat will shorten the life of the motor. A Variac (basically a variable voltage transformer) is probably the best method of providing a reduced voltage while maintaining sufficient amperage, but that really depends on the capacity of the variac, not all are created equal.
I'd read a Q&A which says this too: A. "There is currently only one way to control the speed of a single speed AC motor. Using a variable frequency drive", in response to Q. Can I use a dimmer switch to control the speed of my fan? Well, I don't think there would be multi-fan controls or variable speeds on tools if a larger frequency drive was the only way to do that well enough (maybe 3-speed fans are more quirky according to the description). Why don't they make plug-in versions of VFDs, like dimmers or variacs, if nothing else could be agreed upon by those who know what they're doing? I think it's down to matching the capacity of any switch made for motors, and not running them at low speed all day (as far as this applies to power tools). Another article rates switches as to how well each might work: Slowing Fans Down... no mention of frequency drives there. I've only come across videos of those installed on huge drill presses, in the DIY category. The other thing is, using a tool shortens the life of the motor, regardless of whether it's single or variable speed, so if the controller costs much more than the tool, then there you go (using the controller shortens its life too, and everything wears out either way). It's still something, in and of itself, to think of how sophisticated such things may be (I was considering a custom box with flush fitting inlets and outlets too, it just wasn't worth putting together on a whim). Coincidentally, all the Radio Shack stores had closed down around here as soon as I got into this stuff, and others seem to be charging three times more than average for shipping obscure little electric components (not sold in hardware departments). So I didn't go for the rpm meters either, just figured that whatever the lowest speed a bit does its job at would be safest, as they could break apart even if they were rated for the highest speed that a tool could turn them, and the lower the velocity of flying fragments, the better (therefore, I don't want to maximize it with a meter). It's alright though if my switch box is as big as the power tool (as they say, there is no such thing as a heat sink that's too big)! Well, I didn't read all of that, but someone was addding a heat sink to the router speed control, so apparently these things can get rather hot when you shrink them down (and some of those have a belt clip there, of all things). Now I'd imagine the smallest VFDs could clip on a belt though (kind of), and so I've read more about temperature issues with those: "A variable frequency drive develops heat during work and to avoid overheating ventilation is required." http://www.engineeringtoolbox.com/variable-frequency-drives-d_656.html "A motor is also a heater and requires cooling. If run at low speeds for too long it will overheat. The low range of operation in a VFD is really only to provide a soft start when/if needed." http://www.vfds.in/be-aware-of-vfd-running-in-low-speed-frequency-655982.html So it seems that all of this stuff, if used for this particular purpose, is fundamentally the same (sans the funds). Who says sans though? Yeah, I don't even know what I'm talking about. What I presume to know, after all, is that only some of those knobies are designed for this single purpose (which would be closest to having a speed control that's built in, or even more adjustable). The rest are either larger and heavier, smaller and underpowered, or expensive and complex, for doing something else primarily, or something like this intermittently (and that's really what they claim to be more efficient at). The simpler dedicated ones can still be somewhat costly, as they get into higher capacities. These fan types are common enough to be sold in various ranges (except the 5 amp switch that I found marked down was the last one on the shelf... who has five ceiling fans?)! Wait, who has an electric die grinder?! You know, they only officially recommend about two different grinding bits for use with that, and as far as I can tell, nobody who makes controls recommends anything to do with this whole topic. Same here, so don't blame me either (advice is okay though). Well I think it can be safer that way myself, and more versatile, etc. Instead of tinkering with a clamp on the trigger of a drill that's in a vise, to run it slower sometimes, this ultimately simpler (and some drills are under 5 amps). Alrighty, there's my hokey ass message, then.
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