Thread Tapping Newbie Question!

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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.

Mart> Hi

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

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.

That's OK, if you're willing to wait months between posts.

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.

I never heard of "1ST, SECOND AND PLUG" In the US a set of three taps is Taper, plug, and bottom

The differance is how many threads at the tip are ground to form the cutting edges of the tap.

Taper taps have 5-6 threads lead. ( count the thread from the end of the tap until you get to the first full thread that's not ground off for the starting taper)

Plug taps have 3-4 threads lead.

Bottom taps have 1-2 threads lead.

New are semi-bottom taps with 2-3 threads, used mostly in CNC or tapping machines.

FYI -Never use a taper tap in stainless steel, as stainless is cut it leaves a work hardened surface behind, the next cut on a taper tap is so small it does not get under the hard surface but has to cut through it. tap will dull fast and break.

I'm late on this answer but maybe someone else will find it useful.

Randy