Thread Tapping Newbie Question!

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:
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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:
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But can't find a 3/4" 12TPI one, but then I am a complete novice to this
so make have missed it.
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under Series with Uniform (Constant Pitches - 12-UN has a value for 3/4 12-UN.
See if you have a Machinery's Handbook in the library. Or your shop.
First, second and plug - first tap into a hole. It is easiest to cut with. Doesn't cut deep and sets the thread.
The second deepens the thread and finishes the job.
The first two are for sheet / bar / etc. Hole through.
The plug is for a blind hole. Drill a hole part through a block and it is just a well. The plug cuts a thread into the hole but the process is constantly in/out threading - getting out bad metal shards. Then back in. The plug has cutting very close to the end.
The first two have a tapered lead-in and a pointy end to help center.
Hope that sorts out the taps and such.
Reply to
Martin Eastburn
Hi Martin
Many thanks for that explanation. This is a "hole-through" application, about 1/2 to 1", so the 1st tap is probably sufficient. The drill bit size is apparently 16.5mm.
Thanks Bigus
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I was pleasantly surprised to visit this group and see actual metalworking posts instead of having wade through a 30-50 total garbage threads per on topic thread over there.

Reply to
Bob La Londe
I have to confess that I am not metalworking - more like perspex/wood working! I managed to find some great help on a model engineering newsgroup though.
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