Oh Dear, I’ve blown the chop saw…

[Speedy Builder5Participant @speedybuilder5]

Before the chop saw, I used 10" x 1/8" cutoff discs in the 3/4 Hp circular saw for cutting through steel plate. You had to be careful not to set the workshop on fire with all the sawdust around !!

[not done it yetParticipant @notdoneityet]

Cutting speeds for wood tend to be far higher than for metal, so most are not appropriate. OK with a cutting disc as SB5 indicates.

[Iain DownsParticipant @iaindowns78295]

I've taken the saw to bits and extracted the motory.

One of the bushes is chipped on one edge.

The armature was dirty and there was carbon between the copper.

I scraped this with a small knife and then brushed with isopropyl alcohol.

I measured the resistance between the copper rings. Adjacent rings had a resistance of about 1 ohm. each successive ring added about 0.1 ohm – so copper on opposite sides of the armature had a resistance between them of about 2.2 ohms. I didn't test all of the pairs but all I tested seemed to show the same.

The resistance between live and one of the bushes was about 4.3 ohms and similarly between neutral and the other bush.

I don't know if this is normal or not.

I have pictures, but I didn't see anything untoward – if they would be useful then I can upload them.

Iain

[Phil WhitleyParticipant @philwhitley94135]

Check between the comm bars and the armature spindle for leakage, preferably with a megger, armatures usually fail to earth when they overheat and the varnish melts. also check continuity through the field coils, and to the metal laminations of the field coils.

[Perko7Participant @perko7]

Before you consign the chop saw to the scrap-head, don't forget to retrieve all the useful bits (bearings, gears, shafts, and anything else that might come in *handy* one day).

[Iain DownsParticipant @iaindowns78295]

I don't really know what I'm doing here, but I checked various resistances and capacitances and it all seemed OK. So I thought I would put it back together and give it another go – who knows, it might work.

When re-assembling I noticed this

It's not the best photo in the world, but the windings are clearly burned and the shell has also been burned.

So it's bye byes for this little chap.

I will pop into Aldi and see if they can do spares. The interweb appears not to.

Iain

[john fletcher 1Participant @johnfletcher1]

it's unusual for the field coils to fail. You may find a reader who has a set of coils as it is usually the armature which fails when they overload the motor/ saw. These devices such as angle grinders,and table saws need the speed to keep them cool. John

[John RuddParticipant @johnrudd16576]

Field coils can be rewound……not too much trouble…

But I stand to be corrected by more knowlegeable folk…..

Edited By John Rudd on 26/05/2019 19:21:39

[Iain DownsParticipant @iaindowns78295]

Thanks, All.

Guy (and others) – the Rage range of chop saws look good. I know you said that it would go through solid bar, but the pictures only show tube. I'm a bit concerned that I will blow one of these as well – though at least spares exist. Will it really cope with that thickness of solid steel?

Also, the size range is up to (say) 65mm and I'd like to cut some bar thicker than that. In this case I have some 90mm cast iron which I got in the minimum length and need to cut in half. It's not that long so I anticipate needing to make some jig to hold it, but will if fit under the blade of one of these chappies?

And yes I will take the bits out that are useful – i have boxes of bits which might be useful someday!

And, John. I don't think rewinding field coils for this beasty is within my competence or skill level. Sad, really.

Iain

[Jeff DaymanParticipant @jeffdayman43397]

It could be that when that coil was wound the magnet wire got chafed or the insulation coating cracked. Also could be the protective insulator paper or plastic sheet between wire and coil laminations was not placed correctly or was split. After a bunch of hot / cold / vibration cycles the conductor became exposed and the distance between wire and laminations got short enough to allow an arc to start. It can happen. Some years ago people were having exactly the same fault on Weed Eater brand line voltage grass trimmers. Must have had the fault in many thousands of them during a production run – there were hundreds brought in for service and hundreds more visible at the curb on garbage day for months.

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