Oh god. This group *is* active

On or around Sat, 19 Jan 2008 01:05:52 +1300, Tom enlightened us thusly:

I think it was a table on the back of a steel rule. Hardly a guaranteed source, I admit. Presumably it represents 100% engagement, assuming Dormer et al have it right.

and yes, probably unnecessary. There is, though, the point someone made about inferior or worn screws - if I'm making a threaded boss for example with an M6 hole in it, then I don't know what quality or condition the screw used will be. If I drill with a 4.8 the chances are the hole is 4.9 anyway... and the tap seems to cope (in mild steel).

Tapping M5 into the head of a stainless A4 bolt is a bit more questionable. Luckily, I don't have to do that often.

Reply to
Austin Shackles
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Actually, he quotes this as the British Standard preferred size, not as his personal opinion. So the "well established reference source" in this case is a British Standard.

Regards, Tony

Reply to
Tony Jeffree

Whereas MH as regards: British Standard Tapping Drill Sizes for ISO Metric Coarse Pitch Series Threads [Supplement No. 1 (I967) to BS 1157: 1965) Quotes 6 x 1 @ 5mm with 81.5% as the recommended and 5.1 as the alternative @ 73.4%. % as the theoretical.

Interesting. :-)

Tom

Reply to
Tom

My thoughts entirely.

Allan

Reply to
Allan Waterfall

Oh dear - poor girl got more than she bargained for!

- but I'll repeat my ways of hand tapping M4 holes, which work for me.

For softer stuff like plastics and softer ali I use a 1/8th inch drill (which is probably less than the minor diameter), unless it is brittle plastic, when I'd use 3.3 or 3.4 mm.

For normal materials like mild steel and hard ali, I use a 3.3 mm drill.

For hard stuff like 300 series stainless, inconels and the like, unless it needs to exceptionally strong, I might use a 3.5 mm drill for rough work. If it needs to be strong, I'd use 3.3 mm and take extra care. A set of serial taps, which come in sets of three so the smaller two are undersize can help here, as can thread forming taps if the material is sufficiently ductile.

I'd never use a 3.5 mm drill for ali (unless I didn't have one the right size). 3.3mm or 1/8", depending on the hardness and the depth of the tapping - I'd go for 1/8" (and probably a forming tap) in ali up to 1/8 in thick.

Most times when I've broken a tap, which is infrequent these days, the reason is either that it has gone in askew or that the force applied is not in line/at right angles.

If at all possible I tap in the lathe, bench drill or mill in order to get the angle straight, and turn the tap by hand, to be able to feel the force used.

If I have to tap in the field, so to speak, I like to use HSS taps because they are stronger than carbon taps and take slight sideways forces better.

If it's getting extra hard to turn there is often a reason not related to the size of the hole. Withdraw the tap, have a look at the setup, clean off the chips from the tap and the hole, lubricate and try again. Never force it - it's a real hassle when you break a tap.

Lubricate always, except for some plastics.

-- Peter Fairbrother

Reply to
Peter Fairbrother

On or around Sat, 19 Jan 2008 22:53:28 +0000, Peter Fairbrother enlightened us thusly:

and cast iron...

Reply to
Austin Shackles

And brass...

Regards, Tony

Reply to
Tony Jeffree

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