Tap Threading Newbie Question

If you look carefully at the ends of the taps, you should find that the 1st tap has a long taper ground on it, the second tap has a somewhat shorter taper on it and the plug tap has virtually no taper at all. This is so that the first tap can guide itself into the drilled hole easily, but the later taps can finish cutting the threads to the bottom of a blind hole when needed. If the first tap goes all the way through the material, that's the job done. If it's a blind hole, or too deep for the first tap, then follow up with the others.

3/4"x12tpi is 3/4" BSF (British Standard Fine)

The recommended tapping drill would be 16.75mm, but you could get away with

16.5 or 17mm in this application. possibly even 16mm in wood.

If you decide to use wood, you're better off drilling into the side of the grain, rather than the end-grain. The end-grain may well just tear out when you tap it. 1/2" plywood might work fairly well, Perspex might work, or it might crack. Hard to tell. Aluminium would work well. but don't go below

16.75mm on the drill size or tapping will be hard work.

Have fun. Regards

Yes I see, in this case I just need the first tap then.

Thanks for the tips aswell. I've read the flat wood drills can do quite clean hole edges on perspex and I think I have a 17mm bit so will try that on an offcut of perspex first.

Thanks Bigus

Except in shallow stopped holes, you cut with the taper, which makes a clean cut. This should *not* look like the other two.

You then screw in the chaser (second) and finish the job with the plug

Scratches head - isn't ¾ × 12 t.p.i. BSF?

If it is, look under 'Imperial'

If you don't get sorted, I'll look out my Intel book. Oh, and fire-up the other box and get the URL of a good tool supplier - who sends immediately (!)

IIRC you drill Perspex with water as a help to a clean finish. Drilling it under water is said to be best, but I've never needed to drill a small enough piece to try it.

Oh, and the same goes for threading - keep the cut wet.

The correct tap is a slightly unusual 18mm by 1.5mm. Can sell you a quality new HSS one for a fiver plus postage (I had the same issue, bought 3 taps for £15). 16.5mm drill.

So I bought some, in particular these ones:

I don't much go on RDG tools, don't like the quality.

The first is somewhat tapered at the cutting end (over about 5-7 threads), the second less tapered (over about 3-5 threads), the plug not tapered at all.

If the hole is blind I use the first to start the thread, then the plug to finish it. Otherwise I personally just use the first.

There is another type of set of taps, called serial taps, where the taps get slightly bigger over the set, but they would be horribly expensive in that size. They go rougher, intermediate, finisher.

-- Peter Fairbrother

When you tap Perspex, which is an acrylic plastic, you should not use any oil at all on the tap. Same goes for drilling. The oil will wick into the tiny cracks which will eventually propagate into large cracks. This can happen overnight. I found this out the hard way. Eric

Hi Peter, thanks for the response.

As a matter of interest, did the 3/4" 12TPI tap on a 16.5mm hole work at all? What kind of material did you try it on?

I might be tempted by your offer, although having already got the got the taps & drill bit (just waiting for the tap wrench to arrive) I want to give it a try first.

Btw, noted about the serial taps - the price of some tooling never ceases to amaze me!

Thanks for that tip. That fits in with Rusty's suggestion of using water as a lubricant.

Oh, just realised you are going into perspex. I'd use a 17mm drill for

66% engagement then, much easier to turn the tap and much less likely to crack the perspex.

As others have said, use water when tapping, not oil.

If possible use a drill stand, tapping stand, or similar to ensure the tap is going in absolutely at right angles.

-- Peter Fairbrother

Presumably without getting electricuted in the process? ;-)

Yes, only closer inspection the ends of each tap have differing tapering.

It is BSF yes. 16.5mm, although it sounds like I haven't quite got the right taps for the Dremel application.

Thanks for your reply

I have never tried a 3/4" 12TPI tap on a 16.5mm hole, sorry.

I try and avoid anything imperial if possible, and stick to metric. There are enough metric thread sizes for most purposes without adding the double gazillion imperial ones!

I have in the past used similar-sized taps in superalloys, copper, steel, aluminium, brass. bronze, delrin, PEEK, PTFE... not perspex, it's not something I use often, and only for its transparency. I have tapped smaller threads, eg m6, in perspex though.

For Perspex I'd go with a larger hole rather than a smaller. gives a bit more wiggle room, makes the tap easier to turn, and most important it will be much less likely to crack the perspex.

17mm should be fine for 3/4" 12TPI, I'd think, but I don't know, and I know 17mm would definitely be fine for m18 x 1.5mm.

-- Peter Fairbrother

I have never tried a 3/4" 12TPI tap on a 16.5mm hole, sorry.

I try and avoid anything imperial if possible, and stick to metric. There are enough metric thread sizes for most purposes without adding the double gazillion imperial ones!

I have in the past used similar-sized taps in superalloys, copper, steel, aluminium, brass. bronze, delrin, PEEK, PTFE... not perspex, it's not something I use often, and only for its transparency. I have tapped smaller threads, eg m6, in perspex though.

For Perspex I'd go with a larger hole rather than a smaller. gives a bit more wiggle room, makes the tap easier to turn, and most important it will be much less likely to crack the perspex.

17mm should be fine for 3/4" 12TPI, I'd think, but I don't know, and I know 17mm would definitely be fine for m18 x 1.5mm.

-- Peter Fairbrother

It's not necessary to immesse the eclectic drill...

Recently, I bought three adjustable reamers.

The mortgage...

Sorry but this is probably a bit late - I've only just got back from holiday.

In a later later post you say that it is a BSF thread so you should be able to get a suitable nut that will fit the dremel's threaded collar. I would not be happy cutting a thread in wood and perspex might get strees fractures so I would turn down the nut on a lathe to make a "top hat" shaped insert and then glue that into a suitable slot in the wood or perspex.

Alan

Very droll

Cliff Coggin.

Hi Alan, thanks for the reply.

Peter mentioned the correct tap for the dremel collar is actually 18mm by 1.5mm, so that throws into question the source I found that stated

3/4" 12TPI. However, since I already have the 3/4" 12TPI tap, I want to give it a try in some scrap wood first and see how it holds. Bit of a c*ck up on the wrench front, so am still waiting on delivery of that to try it!

That's an good idea finding an existing nut that fits on the Dremel - not sure I've seen them that large before but actually, on a similar note, I have a cheap Dremel copy which appears to have a similar collar and since that is effectively going in the bin, I could pinch the collar off that and embed it into some wood.

By the way, the perspex I am using for the router basue and was also considering using for the bit the Dremel screws into is 10mm thick, so quite hefty stuff, i.e: it doesn't look like it'd readily crack!

Oh yes, I didn't realise I had spelt electrocute wrong and my brain's lingual autocorrect overlooked the subtle irony of Rusty's response.. double doh!