Selecta Grinder Refurb

[Dieselman 63]

My latest adition to my workshop is this 7" Selecta bench grinder. I got it for $90, which (thanks to Brexit) is £48. The halving rule doesn't seem to work anymore. But anyway, I have it and that's all that matters.

The wiring was really shonky - the seller said it ran well which it probably did, but I didn't want to touch it with electricity as they'd obviously cut the British plug and spliced a kiwi one leaving loads of copper exposed, wrapped up in masking tape! I took it in to the industrial people at uni and they fitted me a new plug and wire for free! Here's a photo of the beast running:



When I turn the power on it makes a heavy electrical 'GNNNNNN' humming that doesn't go anywhere. If you spin the stones a little then apply power it makes the same humming followed by a few thumps which even out to a really smooth action. I haven't used one of these before, so my experience is nil. Is this normal? The machine does also vibrate a bit in action, is that normal?

I think I will repaint and clean the grinder - there's a tonne of caked crap inside the wheel cowlings that I'd like gone, and I also need to make some eye-guards. I don't know why the previous owner has cut notches into the tool rests, it's annoying me. God knows what the right one's purpose was and why it needed cutting short. I dislike people 'specialising' tools for one job!

Should be a quick job as it all looks in pretty good nick, but someone can no doubt fill me in on what needs doing etc.

Cheers

 

[Ton-up]

Nice old grinder but get some gaurds though! It should start straight away. If you look inside the base there should be a capacitor. This will need replacing as that gives the motor its starting power.

 

[Dieselman 63]

Hmm. There's a grey metal box with three points coming out, I'll take the cover off again to take a pic and write down the symbols on it. Looks pretty knackered.

 

[Dieselman 63]

Well I'm going to need some help here as I have no clue what this stuff means. Transcribed, the plate reads: 10 C/4618 4-2 μF 400/250v. DC Working. Just in case you need to copy and paste it, should make it easier.

Then on top, only the ports marked 2 μF and 4 μF are connected (the 4 has come off needs soldering and I've since colour coded it to make it easier). Why does 'C' not get a connection? I understand this is merely the casing, I'm not actually in my workshop so I don't have access to my tools to get it off and look at what's inside.

Any help is going to be great.

Thanks!

 

[Maker]

That's the capacitor Ton-up was talking about. Usually they only have two points with a set capacitance. Yours can be wired as 2 or 4 microfarad depending on what is required for the motor. The point labelled 'C' is 'common', using C and 2uf will give you a 2uf cap, using C and 4uf will be a 4uf cap. You could probably also connect to the 2 and 4 point to get a 1.3uf capacitor.

I don't know which your motor will need, hopefully someone will be along soon.

 

[Gareth J]

Was the 4uf terminal disconnected before? Or did it come off when you removed the capacitor? If before, try soldering it back and see how it goes. Don't open up the capacitor.

 

[rtbcomp]

It looks like the 4 uF may be 400 v DC and the 2uf 250, if so I'd use the 4uF on AC. The fact it doesn't start by itself and the 4uf is disconnected may be significant.

 

[Dieselman 63]

The 2uf and 4uf were connected - the 4uf only just, so when I tried to get a better angle on the box it snapped off. C never had a connection. It was running this afternoon.

When I took it in for testing the electrical guys said they reckoned the capacitor was on its last legs, but I don't really know what they meant at the time. Looks like this is what they were talking about, it needing a spin to get it going. I had a feeling buying it blind wasn't a great idea aha...

I have a feeling the box is just a casing so there might be a capacitor inside - why should I not open the box?

I don't own a soldering iron but the uni does. I have to book online for a tutorial before I can use it. It's such a joke. Apparently the health and safety legislation has changed for every workplace after the mining disaster at Pike River a few years ago.

 

[brightspark]

buy a new capacitor they only cost peanuts

 

[rtbcomp]

Some capacitors have toxic substances in them, some may even have oil in them.

That capacitor looks like a replacement. I can't see a manufacturer wiring two capacitances in series when a single cap. would do. It's not as if it's a critical application where a precise value is required. The arrow on that box suggests it may be an ex-military component.

 

[Dieselman 63]

buy a new capacitor

Not exactly helpful as I have no idea what's going on and what to buy.

I wasn't sure if it was the broad arrow or something electrical related! If it is a replacement it could be that the box has some completely different markings on it that are unrelated to my motor. I'll have to check tomorrow. Thank you for helping!

 

[Lufbramatt]

Have a google for "motor run capacitor" really common for use in washing machines, lawnmowers etc.

 

[RichardM]

the arrow is most likely an MOD mark

http://www.mwrforum.net/forums/show...the-%91Pheon%92-(Broad-Arrow)-Government-Mark

which would mean the capacitor could be years old and probably not OE. Electrolytic capacitors usually have a shelf life of twenty years so I am not surprised it is breaking down.

http://www.capacitor-sales.co.uk/

if it is a starting capacitor there should be a centrifugal switch in the motor to disconnect it when up to running revs.

 

[the snooper]

I just bought a 12.5 uf 450v capacitor from ebay 2.99 dirt cheap

 

[Dieselman 63]

if it is a starting capacitor there should be a centrifugal switch in the motor to disconnect it when up to running revs.

I've not opened up the motor. Like I say I'm not an electrical wizard so unless I'm salvaging the copper I don't really like taking things apart I don't know how to fix again

If the capacitor is just a charge bank to start the motor, could I just cut it out and attach the two lines together? Would that just mean I have to spin the wheel to get it going or could I cause a short circuit and burn the motor out?

Thanks for all the help everyone, it's invaluable!

 

[Dieselman 63]

That's the capacitor Ton-up was talking about. Usually they only have two points with a set capacitance. Yours can be wired as 2 or 4 microfarad depending on what is required for the motor. The point labelled 'C' is 'common', using C and 2uf will give you a 2uf cap, using C and 4uf will be a 4uf cap. You could probably also connect to the 2 and 4 point to get a 1.3uf capacitor.

I don't know which your motor will need, hopefully someone will be along soon.

Went to the electrical supplies shop today - boy were they excited about my old military capacitor! They reckoned it must be out of something like a vehicle but something pretty heavy duty. Anyway, @Maker, you were right on the money for a 1.3uF Cap. it was only $5 so worth a gamble if it popped, but it didn't. I decided to buy a soldering iron too instead of taking the test to use one at uni. I've a few old model railway locos that need rewiring so I'm going to fix them up. Thank you so much!

For anyone interested I'm using a 1.3uF 250v AC capacitor which will give me 400v DC, apparently. They guy said he could hook me up three beefy looking 2uF caps which would produce the same in series - that's probably what was in the grey box, but again I'd be left with an unused common point and a whole $60 lighter of pocket.

Now that it runs properly I'm going to take it back to my house and I can begin the more conventional refurb; paint stripping and re-painting.

 

[Dieselman 63]

Went into the industrial design centre at uni today and asked if they had any 6mm clear perspex offcuts I could have for some guards - not only did they have some, but the guy whacked them on the laser-cutter and cut me two, even engraved one for me, lovely job.

For mounting I think I'll steel epoxy the holes in the cowling and drill a new one central each side, then get some thin metal to form a Z shape to hold the guard on.

 

[Dieselman 63]

The right hand rest looks close to drill bit angle - pop a little bit of plate upright in that slot as a reference/rest.

Plastic guards are a good idea in theory, but they don't half keep getting in the way and get hot grinding particles melted into them

Pardon me for not quite understanding what you mean about the right hand rest? And yeah, glass would be lovely but placcy will have to do for now! I'm not planning to do a hell of a lot of grinding, so hopefully it'll hold out

 

[Dieselman 63]

Not best pleased. Opened it up tonight to look at the wheels, one is rated at 3157 rpm, the other is rated at 2000 m/min. The machine runs at 2800 rpm, so the first wheel is fine but some quick calculations tell me that 2800 rpm is 3200 m/min. Bit scary as I've heard the horror stories of them shattering etc. So now I have only one wheel on the machine. Can you run it like this, or will it make the motor load uneven?

An interesting thing about the rests, too. One is made of a ferrous metal, presumably iron, and the other is made of aluminium. Why would this be the case? They both look to be original castings and have the same factory paint on the insides. Quite a puzzler...

Lastly, does anyone know where I can buy 7" wheels? There seem to be loads of 6" and 8" ones but none in the middle! I'd rather buy in NZ but I could justify a spend to ship one from England if it was good.

 

[Dieselman 63]

Using this handy calculator:

http://www.endmemo.com/physics/rpmlinear.php