Tap and die question

[Cordless Crazy]

If I want to form a M8 thread in a piece of sheet steel, what size hol

do I initially drill?

I am thinking 2mm less or is that too much?

Would that make it a 6mm hole?!

-- Cordless Crazy

[Jason]

Sorry it's not an exact answer, but from what I remember from my distant days at BT Fulcrum, it is certainly no greater than a mm. But don't take my word for it - try it out, but don't force the tap if the hole is too small, as they are very brittle.

-- JJ

[John McLean]

Why don't you first do an internet search: -

e.g.

[The Natural Philosopher]

Its not hugely critical. The tap will be more or less tapered for a thread in sheet..I'd guess at around a 6.5mm or 1/4" hole meself.

In use, the taper tap will remove as much metals as it needs to, but go slow, use oil, and work it backwards and forwards till it runs freely before advancing it the net half a turn or so. If it binds, rotate backwards and forwards till it frees up and proceed.

[John McLean]

or even better: -

[Frank Erskine]

6.8mm is the correct size, according to "Machinery's Handbook".

[Cicero]

=================== My chart shows some variation depending on the pitch of the thread.

Fine thread (0.75) - 7.2mm drill, medium thread (1.00) - 7mm drill, coarse thread (1.25) - 6.8mm drill. If you're not sure what the pitch is it would be sensible to start with the smallest hole and if the tap binds go to the next larger drill hole.

Cic.

[Tim Lamb]

In message , The Natural Philosopher writes

1/2 turn forward and then a 1/4 back to break off the swarf, according to my apprentice supervisor, nearly 50 years ago:-(

regards

[Dave Baker]

The rule for any metric thread is the nominal size minus the thread pitch which for a standard 8mm thread is 8 - 1.25 = 6.75mm

[Jeff]

and if its a different tap.......

standard M8 tapping drill size is 6.8mm

Regards Jeff

[Dave Fawthrop]

|In message , The Natural |Philosopher writes |>

|>In use, the taper tap will remove as much metals as it needs to, but go |>slow, use oil, and work it backwards and forwards till it runs freely |>before advancing it the net half a turn or so. If it binds, rotate |>backwards and forwards till it frees up and proceed. | |1/2 turn forward and then a 1/4 back to break off the swarf, according |to my apprentice supervisor, nearly 50 years ago:-(

Bi Hek that brings back memories! The first thing we had to do was to file a cube with all sides within 1 thou of each other from a bit of round bar ins dia by 1 ins long.

[nightjar]

I would guess that an EP grade would work best. The tapping lubricant I buy is more like a grease than an oil though.

Colin Bignell

[nightjar]

If the sheet metal is thin, you are better off using a clinch nut:

ones in the top row are applied with a pop rivet gun, for which you need one with an appropriate mandrel. The ones in the bottom row are inserted using a press.

Alternatively, you can drill out a small hole and use a tapered drift to enlarge it. That will throw up a heavy burr on the back of the sheet, which increases the length of thread you can cut. You can also use a special punch to press a hole of that form.

Colin Bignell

[Tim Lamb]

In message , Dave Fawthrop writes

Crumbs. I was an *electrical technician* so that sort of precision was not reached.

There was the occasion when one of the craft lads filed the master to suit his test piece which put the rest of us out.

Leaning on the saddle of the only lathe in the dept. took another 10 thou off the work.

Happy days.

regards

[Dave]

I'm glad that some one brought this up in the thread (sorry about the pun).

Pitch is the determining factor in the drill size, as metric threads have symmetrical pitch/ thread depth.

Dave

[Rob Morley]

If you're not sure what the pitch is it would be sensible to measure it.

[Cicero]

==================== If the OP had been aware that there are three possible pitch sizes and knew how to measure his tap then he probably wouldn't have asked a question in the first place.

I supplied good data which the OP could match against the information stamped on the tap he intended to use. In the unlikely event that his tap didn't show full information then a common sense thing to do would be to start with the smallest tapping hole and change to a larger hole if necessary.

Cic.

[Rob Morley]

Surely it's better to explain the differences, and how to measure them?

That's a good way to break a tap if you're not familiar with the torques that they can withstand. Adnittedly not a problem with 8mm and mild steel sheet, but with smaller taps and tougher metal it can end in tears. There again, while he said it's sheet steel it would have to be pretty thick to take an 8mm thread. Nobody mentioned the difference between taper, second and plug either.