Broken angle grinder

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metiz
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Broken angle grinder

Post by metiz » Wed Jun 16, 2010 2:59 pm

I've got a small angle grinder, 900 watts, that I use on litterally every project. Next to my hammer it's the single most used tool I have.

And now it's broken.

It had some problems for a while. The head (part where the transmission is) got realy hot and today I noticed it whining down a bit now and again. It appeared it had a lot more rotational friction then it should have. I unscrewed the head to see if I could find anything. There was plenty of grease, no sign of any metal shavings in there either. I think the bearings were either dirty or partially broken because that's where the axle dragged. I reasembled it and gave it a try. dead. nothing at all. There was some increased friction but not enough to prevent the engine from spooling up. I then completely disassembled the grinder, cleaning and checking everything I could, also the electrics and could find nothing. Reasembled and tried it again. Nothing. I repeated this about 3 times without results.

Anyone recognise this problem or had a similar problem? Can anyone tell me if there's a likely culprit?
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PyroJoe
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Re: Broken angle grinder

Post by PyroJoe » Wed Jun 16, 2010 7:55 pm

I feel your pain,
Have two grinders, one fitted with a cut blade, the other grind only.
A break in the copper wire inside the cord happens about a third of the time, right at the position it goes into the grinder housing. Often this is because the cord tends to sag and load in that area. It is difficult to see, because it breaks inside the cording.

Check the other end that plugs into the receptacles also. Sometimes the plug will show minor swelling if it has exceeded its thermal duty cycle. A quick sniff test will also reveal a slightly sulfer smell as the plastic becomes charred.

Switches tend to build up abrasive dust and can break electrical contact. I usually flush/blast the switch out with a small amount of aerosol Ethyl ether followed by a blast of compressed air. It tends to result in a dry, clean switch (of course the unit needs to be unplugged and left setting for several hours before use)

In one instance a wire wasn't making good contact to one of the slide plates in the switch, resulting in sparatic on/off operation.

The brushes can also be a culprit if the unit has been disassembled.

All else fails check to see the outlet has power. A meter, or test light can save some time if available.

If the windings have surpassed their duty cycle, most often the lacquer / clear coating on the copper will pick up a dull brown, instead of the bright brassy look. The power will eventually decrease until it is useless as a tool.
If the windings have a burn through, there is not much to be done, call time of death and hunt another victim.

Life expectancy for my grinders are only about a year... two at most.

A few months ago I had a new grinder fail when the plastic fan inside lost its integrity and flew to pieces inside the unit, scared the bajeez out of me.

If the grind head is showing signs of decline it is a good idea to replace the unit, much cheaper than a hospital bill, and picking pieces of blade and gearing out of your arms for a few months. Be watchful of the screws in the grind head, as they have a nasty habit of loosening.

metiz
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Re: Broken angle grinder

Post by metiz » Wed Jun 16, 2010 8:22 pm

Hey Joe

Thanks for the comprehensive imput!

I had the problem with the cable break about two months ago. I managed to fix that one by cutting of about 10cm of cord and rewiring it on the inside. Worked fine since then...

After tearing the unit open for a fourth time (nerd rage) I think I might have found the culprit.

All the electrics (not much) are housed inside the handle. Two wires wich I asume are + and - run from the handle to both sides of the unit. They are attached to small metal "cages". on these "cages" are wound 2 metal coils with about 4 windings. the end of the coil reaches under the cage positioning itself between the cage and the motor. inside the cage is room for a small brittle metal block of about 8x4x4mm. this block rests upon the end of the coil. I asume this is to carry the current to the motor. One of these blocks has lost about 2/3rds of its length.

That's all I can find. I tried a bunch of things with coil and block but the results stay the same.
"The brushes can also be a culprit if the unit has been disassembled."

I think it's a brushless motor but that would be the best discription I think.

I guess there's no other solution then to buy a new one. they're pretty cheap and fathers day is comming up so lots of discounts but still, damn shame :)

Btw, how intensive do you use your grinders: 1 year is kinda...short :P This angle grinder was the cheapest I could find and it worked fine for more then 2 years...
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PyroJoe
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Re: Broken angle grinder

Post by PyroJoe » Wed Jun 16, 2010 8:53 pm

Those metal blocks are what I call the brushes, it is a term that is a throw, back to when brushes were used.

2/3 gone sounds like it is getting thin on the one block, sometimes it helps to stretch the spring/coil a little to give the block some extra pressure, but it is most likely used up.

The good news is if the brand is common, replacement blocks can be purchased and installed. In some models if the blocks sink to a point they dont contact the metal portion of the cage sides it will not transfer the electricity.

It is hard to let them go, they put out so much and ask so little in return. It is good to retire them before they hurt you, so the relationship can be looked back upon in a fond manner. :wink:

metiz
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Re: Broken angle grinder

Post by metiz » Wed Jun 16, 2010 9:11 pm

Hey Joe

Ah ok. I probably should have refraced "lost 2/3rds of its length" to one block is broken off (at the top) leaving only about 1/3rd, roughly. I tried bending the top of the cage inwards a bit to force the blocks down. This was actually necessary to keep the broken block in place. no luck... argh * (<---hey look! censor!)it. I'll buy another one. Some points you made about power loss and browning of the motor aplies on my grinder so it's had its best days behind it it seems
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tufty
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Re: Broken angle grinder

Post by tufty » Thu Jun 17, 2010 6:16 am

I treat the damn things as disposable. I have 3 "dead uns" in the garage. IME, "named brand" grinders last exactly as long as "no name" ones unless you start paying 10-20 times the "no-name" price, and even then they only last twice as long or so. As no-name 125mm grinders cost next to nothing, and 115mm ones are as good as given away - buy a new one.

PyroJoe
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Re: Broken angle grinder

Post by PyroJoe » Thu Jun 17, 2010 1:42 pm

In most cases the expensive ones will have the exact same components as the cheap ones, the housings will look a little more finished, with a brighter brand sticker, but the guts are the same. ha

I have a third back-up unit built by combined components of two dead uns.

A lifetime lived with only a hack saws and files make these things gold. If a good air compressor is available, a die grinder is a beloved item also.

My bother in law thinks I'm addicted to the things, as he brought a blown tire over to be changed and the first thing I did was go to the shop to retrieve my cut grinder. I pushed the bead of the old tire to the center of the rim and cut through it in a matter of seconds, flipped the rim and cut the other bead. The old tire came off in a matter of seconds, ready for the new tire to go on.
Last edited by PyroJoe on Thu Jun 17, 2010 8:26 pm, edited 1 time in total.

metiz
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Re: Broken angle grinder

Post by metiz » Thu Jun 17, 2010 6:31 pm

new grinder get. Same model, 50 watts more power and even with 20% discount 3 bucks more expensive. Oh well. funny thing is: I've got a 3 year waranty on this thing so if I manage to not loose the receipt I'll never have to buy another one :P
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