Holding an electrical screw steady...

[GMM]

I have this heating controller to wire up and screw onto a standard electrical back box. Instead of a standard hole straight through for each screw, it has a curved 'keyhole' shaped slot (presumably to allow a bit of rotation to get it nice and straight). The wide end of the slot take a standard 3.5mm screw no problem, but the curved slot is too narrow, so tightening it up puts the whole thing significantly out of line. There weren't any screws supplied but it looks like it needs a screw with a narrow shank for the first 5mm from the head. It should be simple, in principle, to stroke the threads from a spare screw using a file but it needs to be held tight while filing and clamping it in a vice will just wreck the working part of the thread. If it was any other size, I would put a couple of nuts on it, but electrical screws are 'special' and nobody sells that size nut (M3 and M4, ok, but not M3.5). So....How would you resourceful people do it? I have thought of drilling the slot it goes through to widen it, but I'm reluctant to start carving up a 70-quid controller - screws are, at least, cheap enough to wreck one or two! Clearly, there's no point in spending ages (well relatively, I know they're pretty soft) filing away to end up with a screw that won't go into the box because I've cream- crackered the thread.

[OG]

Could you screw it into a wallplug and hold the plug in the vice? Unscrew or cut the plug off when filing completed.

[RW]

At the size mentioned the actual thread pitch won't be that different from the next size up (M4) Cut an M4 nut in half using a junior hacksaw and use the two halves as a support in the vice wit the original screw between them.

[Harry Bloomfield]

GMM expressed precisely :

Put the screw thread end into the chuck of a drill, tighten then start the drill. Use a file against the thread to turn the thread down a little. It will not damage the threads unless it slips in the chuck.

[ARWadsworth]

Buy a couple of screw longer than you need, clamp the end in a vice, file down what you need to and then cut off the threads knackered by the vice to leave you with the correct length screw.

Adam

[The Wanderer]

Was this a new purchase, or something you've acquired? Very surprised there weren't any screws enclosed.

Have you tried a 'phone call to the manufacturer or electrical wholesaler? If it came from a shed, take it back and ask for a replacement with screws. Sadly, it's a fact of life that stuff gets opened in the sheds so that all the novices can take a gander to see what it's all about, and bits go missing or get damaged. Oh, FWIW, I always open stuff like that when I'm buying from a shed, but that's only to make sure everything's there that should be there.

[Cicero]

========================================== If you have enough length to work with clamp the screw at 45 degrees in one side of the vice jaws so that the screw is held mid way between head and working end. Use a small 'rat tail' file or a riffler to do the filing.

Cic.

[Dave]

Or use a piece of wood and drill the right size hole in it to allow the screw to be pushed in with a bit of force. Now, using as fine a saw as you can find, make a saw cut down the wood so that it cuts the hole into two sides. Put the screw in and clamp in a vice to file the screw.

Dave

[Tim Lamb]

In message , GMM writes

Use a bit of soft aluminium between the vice jaws and the screw thread.

regards

[Andrew Mawson]

thread.

Grip in vice using 'soft jaws'. Two bits of aluminium angle, or a bit of bent lead.

AWEM

[meow2222]

Wrap some 0.5mm iron wire round the screw thread, many turns, so it sits in the screw thead. then you can put it in the drill chuck and the wire takes the force. Still need to go easy on it force wise, but it should work.

NT

[YAPH]

Oh, those!

Take it back and get a different make ;-)

[Appin]

The message from "ARWadsworth" contains these words:

At least one person has a sane answer.

One additional point -- since presumaby no die is readily available to clean up the thread and since no M3.5 nuts are likely to be readily available, take a metal backbox and remove one or more of the tapped lugs -- if you've a four-lug box it will still be usable afterwards -- and run the removed lugs up the thread before cutting off the surplus length below them. Then when you unthread them they'll clean up the thread nicely for you.

[The Wanderer]

No, mine was the sane answer 6 days ago.

I'm truly amazed that everyone else came up with all sorts of weird and wonderful ways to cut back the thread, when the logical thing to do is ask the supplier or manufacturer if screws should be supplied with the fitting. If not, then and only then is it time to prat about.

[dennis]

The cheap crimp tools have threaded holes for you to cut bolts, you screw through one of these first and then you chop the end off, Unscrewing it reforms the thread if needed. You could leave it dangling from the vice while you file the end and then chop it off.

[Appin]

The message from "dennis@home" contains these words:

The question is whether the said cheap crimp tools have a M3.5 threaded hole. It is rather a specialised size.

[dennis]

They screw into the one I have quite well but the numbers have rubbed off so I don't know what size it says.

[Appin]

The message from "dennis@home" contains these words:

Well, you have a nice simple answer to the problem, then :-)

[Alang]

It will be 3.5mm or 4ba depending on the age. I have one somewhere from 1980s. Difference in thread is so minor they are interchangeable.