tap and die set (AND SCREWS)

i picked one up on the cheap, quite a good one,

i intend to use it, i know what its for, but how do i use it to repair a bent thread.
just get the correct size tap, and screw it in forcefully ? or is there more to it than that.

also,how do screw sizes work ?


cheers

also, how do you use the screw extractors ? they look like drill bits that you drill into the screw ?

[Sid Squid]

If it's cheapy I wouldn't use it to cut a new thread, but it will probably be OK for cleaning and repairs to slightly damaged threads.

When you say 'bent' do you mean the screw/bolt is bent? If so then a die won't fix this, it's only for threads, you'll need to put the bolt in a soft vice or similar and bend it straight first.

To use a tap or die to clean a thread just screw it on/in, rotate forwards half a turn and then back a quarter turn, forwards half a turn back a quarter turn. In this way you'll advance up the thread a little at a time. The most importatnt thing is to go slowly, taps and dies are very hard and brittle, they snap/break stunningly easily. This is a very cautious way of course, you may find that if the thread is only a little distorted then you can go much further, remember to rotate backwards regularly though - this allows the flutes in the tap/die to clear out, if you don't do this the threads of the tool can get jammed with material and this can break things.

The best way to check if you have the right size is to use a screw pitch gauge and vernier to measure the parts, however a good alternative is take whichever component isn't damaged, so if you have a damaged bolt then the hole/nut it fits, and try your taps on that for a good fit.

Don't force anything, slowly is best.

[Sid Squid]

Forgot.

Screw extractors:

These are either straight or tapered screws with reverse threads, they are usually supplied with appropriate drill bits, which may similarly be tapered.

You drill a hole in the broken stud/screw/bolt/whatever and screw in the extractor, being reverse thread you screw this in anticlockwise. The plan is that the extractor will grip the broken fastener and, going the other way, will unscrew it.

Same deal as with taps though - very hard material is used, they break easily, beware when using them. If you snap an extractor it's bloody hard to get it out, and you won't drill it out that's for sure.

cheers mate.

i kinda understand, i got a vernier calipar and a pitch guage with the kit

i have bent threads that were caused by a clumsy man overtightening bolts on my bike.

how about thread restorer files ? ive seen them, are they mainly for cleaning up external threads ?

or can you get internal thread restorer's ?

[timwilky]

Overtightening isnt going to bend anything. it will strip the thread.

Having stripped the thread you cannot use a tap to recut as there is no material left. so you either have to drill it and tap the next size up or use a helicoil.

Next remember that the drill size is for the root diameter of the thread not the external diameter. you should consult an engineers handbook for the correct drill size for any thread.

Cheap tap and die sets tend to be made from brittle high carbon steel whereas the better ones are made from high speed steel. your taps should come with two possibly 3 taps to each size. the first is tapered to start the thread cutting and the second/third taps take the cut to the final depth. The third tap is usually a plug tap used for blind holes.

Every quarter of a turn back it off to break away the swarf. Always use a proper tap holder, and apply equal load with both hands. I wont go into the physics but I have seen people bang a nut onto a tap and attempt to use a socket wrench. This will induce a bending moment in the tap and lead to it snapping.

Thread restorer files are for small repairs to damaged threads. not to recut.

Also if you intend to use them on you bike bear in mind that Japaneese metric threads sometimes do not use the same pitch as european metric. 10mm being a good example.

[northwind]

Thread restorers are for fixing threads on bolts and studs I think. I've often been told that it's better to use a hard bolt rather than a tap to repair a damaged thread, where possible? But to be honest, no idea if that's a good idea.

[timwilky]

A damaged interal thread is usually as a result of cross threading. if lucky the culprit has realised before too late and taken out the offending bolt/stud etc before the damage becomes terminal. Under those circumstances a bolt with a taper ground into it to start it off may swage back the damaged areas of thread but be careful that you are not compounding the damage However for this to work the damage should really be limited and no stripping.

[embee]

Further to the above advice, when tapping threads it's advisable to use appropriate lubrication for the particular material.

There are purpose made lubes (oils/greases) often known as "RTD" (reaming/tapping/drilling) like here
http://www.lawson-his.co.uk/scripts/...g%20Lubricants
but for the odd one-off you can use other things.

As a guide
steel - RTD, or engine oil
aluminium - paraffin/kerosene is ideal, otherwise RTD or thin oil
brass - usually dry
cast iron - usually dry

For metric standard series threads a useful rule of thumb for drill sizes before tapping is to take the nominal thread size and subtract the thread pitch (round up to nearest decimal place), so
M6x1 thread uses a 5.0mm tapping drill
M8x1.25 uses 6.8mm etc