Numpty metalwork question
 

I was the kid from the council estate who went up to the 'posh' grammar school where we conjugated Latin verbs instead of doing metalwork (the nearest we got was woodwork - where I made my mum a coat rack).

As a result I now find myself, decades later, needing to put some threaded holes in some metalwork and I realise that I have no idea how to do it properly. I only have one shot at this so it needs to be right (although I might practise a bit first).

Now I presume that all I need to do is go out and buy the appropriate sized tap, drill a hole then screw the tap into it, but how do I get it the right size? I presume the hole I drill has to be slightly undersized? But by how much? Does the tap need lubrication? Does it sir, does it? oooh suit you sir. How much force should I use? Do I just wind the little bugga in or is it easy to snap the thing off...

Told you it was a numpty question

Don't you watch American Chopper Jools?

:lol: sorry, I have no idea - not helping much today am I?

Jools, you will indeed need to buy the correct sized tap, but you may need a couple of different ones, depending on the job in hand.
There are three basic types of tap. A taper, or 1st tap for starting the thread off. A second tap and a plug tap.
If you are tapping a through hole, and it's not too deep you could do the job with a taper tap on it's own. You may need a 2nd tap if the hole is deeper.
If you are tapping a blind hole, then you start with the taper and finish with a plug tap. The plug tap is cut with a full thread right from the nose of the tap.
You'll also need a tap wrench to turn the tap with.
The hole needs to be drilled according to the thread size. Yes it will be undersize, but the actual diameter difference depends upon the thread size.
A M4 thread for instance will be drilled 3.2mm, a M10 will be drilled about 9mm off the top of my head.
Lubrication is always a good idea :devil: It'll stop the tap breaking the metal away rather than cutting it, so as to give a good sharp thread rather than a rough one, which ultimately will be weaker. Use a cutting or specific tapping lube.

Cheers Nelly,

Ya see, I knew there was more to it than meets the eye. I'll giv it a go.

for a given thread size you use a drill that is the pitch size less than the bolt diameter

so Bolts are usually 8mm X 1.25m, 6mm X 1.0mm, 5mm X 0.8mm etc

8mm tap you use a 6.8mm drill as 6.75mm are very hard to find
6mm tap you use a 5mm drill
5mm.......................4.2mm drill
4mm.......................3.3mm drill

10mm bolts usually come in two varieties on bikes
ISO standard 10mm X 1.5mm so use a 8.5mm drill
and Metric fine 10mm X 1.25mm so use a 8.8mm drill although a 8.7mm drill will be OK
Brake banjo bolts are either 10mm X 1.25 or 10mm X 1.00mm pitch

[Edited on 3-12-2004 by skidlids]

Taps and all the bits aren't cheap so if it's only a one off, you may be better off taking whatever you need tapping to a freindly engineering shop who'll do it for the price of a couple of beers ;)

Being one of those boys who went to the local borstal^H^H high school and then went on and did a mechanical engineering course, I've done this a few times myself.

As Nelly says, there are 3 types of tap, but you should be able to get away with just buying one - the taper tap - if all you need to do is thread a couple of holes in a bit of metal.

You can even get away without using a proper tap wrench - I just use a pair of quick-grips, but then I've done it plenty of times! The tap wrench will make sure that you get the tap level and apply even pressure so you don't tap at an offset angle, but with a tapered tap you shouldn't have this problem anyway.

When tapping, remember that there are three size holes - the size of the hole you need to drill first before cutting the tap, then the hole that is cut with the thread in it, then finally the size hole you need to drill out the thread.

If you look at this page, it gives you the sizes you need.
http://www.usq.edu.au/users/grantd/m...us/TAP_DIE.HTM

For example, you need to drill a 5.1mm hole for an M6 thread, but if you want to drill out an M6 thread you need to drill it out to 6.4mm. However, not many people have a 5.1mm drill bit, so a 5mm should be fine.

Depending on the metal, it's always a good idea to cut a bit, then back it out. Keep doing this to free out the swarf you create otherwise this can get jammed and ruin the thread. Stuff like aluminium you can get away with just banging it straight through in one go!

You can pick up some very cheap tap and die sets, but you may be better off just buying the size tap you need. I've got a couple of Draper taps in common sizes and these are fine. Taps are a bit like drill bits - cheap ones go blunt quickly if you use and abuse them.

Take your time, use some scrap to test and you won't go far wrong.

Everything above and; if you do do it yourself, when tapping, for every full revolution of the tap do 1/2 turn back. Lubrication is very important and most oils will do for mild or stainless steels but if you tap aluminium parafin is much better.

5.1mm for a M6 X 1.0 tap they jest
and 6.4mm is a bit over kill when a 6.1mm drill would be all that was needed if you wanted to use a 6mm nut and bolt in place of a stripped thread

I got a cheap (£4.99 from memory) tap & die set in Aldi of all places; I guess that would be one of those that gets blunt very quickly but good to practice with at that price.