Tapping imperial to metric screws

[Deleted2797112]

Hi,
In my loft I've got old round steel conduit boxes with no lids. They're full of dust, cobwebs and insulation so I want to clean them out and put new lids on. I've bought one new lid and it fits and the screwholes are centred with the lugs inside the box. However, I can't find screws that fit, presumably because the lugs are sized for imperial screws.

I've been though my own box of old scavenged and saved screws and I've found a single one that fits and taken it round everywhere I can think of but I've drawn a blank. Brass electrical screws are too thin as are B&Q conduit box screws. The closest I can get is an M5 which is a fraction too large - I can get a bit less than a quarter turn before it won't go any further. M4 are a fraction too small.

It was suggested on another thread that I tap the lugs for metric but I'm unsure how to go about this. The screws don't go into a threaded 'socket', it's just a thin lug that the screw goes through. Can I just use a drill with a metal bit to widen the hole a tiny amount or do I need a tap and die set? Or something different?

Many thanks!

 

[Burnerman]

I guess you're talking about the old 'slip' tubing here......using a 5mm taper or second tap, carefully recut the thread on the lugs.
Failing that, you may be able to use a self tapping screw to the same effect.
John

 

[Dave54]

The old boxes usually used 2 BA. I just went on Wikipedia to check the size, (and my memory from 30 years ago), and conduit boxes are mentioned on the page.
http://en.wikipedia.org/wiki/British_Association_screw_threads
You can get 2BA screws off eBay.
Otherwise do as Burnerman says.

 

[Burnerman]

Aaah the good old British Association thread....much loved by the electrical and electronics industry in the days of yore!
Could I hell remember BA when I posted this morning - thanks for the memory jog!
A soft thread if I remember, rounded in profile.
John

 

[Dave54]

You're obviously coming at it from an engineering POV. I just used 'em when I was in electrical maintenance. . .
Any which way, you had me reaching for my ancient "Machinery's Handbook"
You're right. It is a soft thread. A lot of the ones we used were brass. Would that be the reason for the rounding?

 

[big-all]

wood screws will often do in low load applications in steel as long as you dont try to wide a screw

 

[Burnerman]

You're obviously coming at it from an engineering POV. I just used 'em when I was in electrical maintenance. . .
Any which way, you had me reaching for my ancient "Machinery's Handbook"
You're right. It is a soft thread. A lot of the ones we used were brass. Would that be the reason for the rounding?

Hell I'm really stretching the brain memory cell now.....the crest and root of the thread were rounded, and this meant that the screw and nut would mate together with minimal risk of cross threading, and on the smaller size (6 BA) it wasn't necessary to hold the nut when tightening. Naturally it was a fairly weak system, often in plated brass and very low torque.
It was prone to vibration failure which is why the Military, in their radio sets and the like, used shellac to lock the threads together.
You know - I really must try to get out more, and why I had to learn all this stuff is beyond me
Methinks, Dave54, there's something in your user name that suggests we share the same vintage
Regards!
John

 

[Dave54]

Yes you're right about the vintage being in the name.
I learned a lot of stuff like that as well. In fact I had a vague memory when looking at the drawing of "doing" BA threads in the training school after I'd chosen the electrical side of things. I never thought (or could have thought) then that it would come up 40 years later on the internet. Computers were big machines that filled rooms!
I need to get out more too!

 

[swbjackson]

http://uk.rs-online.com/web/c/tools/threading-tools/rethreading-screwdrivers/?searchTerm=rethreader

 

[Deleted2797112]

A big thank you to everyone who replied and especially to Dave54 for identifying the screws and finding a source too. I ordered them from one of the eBay sellers yesterday, they arrived today and they're now fitted - job done.

I've also learned a bit about something I've never heard of previously - British Association screws, found a new tool - a rethreading screwdriver and how to go about using a tap and die - I love this forum!

Thanks again to everyone!

PS I too remember computers that filled rooms and ones that read punched cards!