Tap Threading Newbie Question

Hi
I have decided to try making my own Dremel precision router base where the Dremel's threaded collar screws into a piece of thick perspex (or
wood, depending on what I choose to use).
A bit of googling came up with suggestions that 3/4" 12TPI taps would closest suit the job. So I bought some, in particular these ones:
http://www.rdgtools.co.uk/cgi-bin/sh000001.pl?WD tpi&PN=bsf_TAPS_AND_DIES.html#a451
In the description it says "A FULL SET OF TAPS INC 1ST, SECOND AND PLUG (3 TAPS)"
Coudl someone tell me what it means by "1ST, SECOND AND PLUG"? Are they for different purposes, only all three look the same to me!
The other thing is - does anyone know what the ideal drill bit size, in mm, would be for these? I have had a look here:
http://en.wikipedia.org/wiki/List_of_drill_and_tap_sizes
But can't find a 3/4" 12TPI one, but then I am a complete novice to this so make have missed it.
Thanks Bigus
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
--

Mark Rand
RTFM

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/2013 00:27, Mark Rand wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/13 10:20, Bigus wrote:

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.
--
Rusty Hinge

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/2013 12:10, RustyHinge wrote:

Presumably without getting electricuted in the process? ;-)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/13 16:57, Bigus wrote:

It's not necessary to immesse the eclectic drill...
--
Rusty Hinge

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Very droll <g>
Cliff Coggin.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 07/07/2013 09:24, Cliff Coggin wrote:

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!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 02/07/13 20:32, Bigus wrote:

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 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 (!)
--
Rusty Hinge

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/2013 12:04, RustyHinge wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 02/07/13 20:32, Bigus wrote:

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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/2013 13:49, Peter Fairbrother wrote:

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!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/13 16:53, Bigus wrote:

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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/13 16:53, Bigus wrote:

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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/2013 17:25, Peter Fairbrother wrote:

Hi Peter
The 12TPI 1/2" and 17mm flat blade worked very nicely and the Dremel fits snug and secure in the 10mm perspex.
I see what everyone meant about water though - I did a trial run on a offcut and it doesn't half melt the plastic, although I was able to clear the hole easily with a stanley knife.
Thanks
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/13 16:53, Bigus wrote:

Recently, I bought three adjustable reamers.
The mortgage...
--
Rusty Hinge

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/13 13:49, Peter Fairbrother wrote:

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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/07/2013 16:07, snipped-for-privacy@whidbey.com wrote:

Thanks for that tip. That fits in with Rusty's suggestion of using water as a lubricant.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
--
snipped-for-privacy@argonet.co.uk
snipped-for-privacy@riscos.org
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.