Oh god. This group *is* active

I meant "Oh *good* of course!

How long is the piece of string ;)

What are you wanting to tap into? I tend to use a bigger drill when working with softer aluminium as I find the tap tends to bind less, and pushes up the tops of the thread anyway. And for a hard material like steel, while you will get better strength with a smaller hole, it still depends a bit on how clean the tap cuts.

It also depends on how accurately your drill cuts as well, a 3.3mm drill may well cut a 3.5mm hole if slightly miss ground.

So it's a little difficult to give an accurate answer on the information available.

Depends on

1. If the quality of the finished thread isn't too important. it'll do
2. How does your supplier define a 'standard' tap? Is at second? If so it's a bit of a compromise - but see next comment. Beware that there are some really nasty 'bargain' taps around.
3. Is it a blind or through tapped hole? - if its blind you wont necessarily get a decent thread as deep as you want.

hth Mike

In article , Suzy writes

3.3 mm would be near-suicide (for the tap) in hard materials.

3.5 mm is what I would use for steel - 65% thread engagement, which is fine. For Al or other softer materials I might use 3.4 mm - 80% thread engagement.

Do bear in mind drill bits tend to drill a little oversize if used alone. Best to drill small - say 3.0mm - then finish to size. Trust you are using a drill stand, not freehand, but if you have to, use a guide block to get started all square.

I strongly recommend you get a copy of Tubal Cain's "Drills, Taps and Dies"; it's only about £6 and is worth every penny.

Oh, and find yourself a decent supplier, you will find it interesting trying to tap a blind hole with only a second tap. Then you can get a

1.0 - 6.0 mm x 0.1 mm drill set, which all metal bashers of any pretensions will have.

David

You were right the first time ;-)

Regards, Tony

3.3 is on the tight side for M4 - Tubal Cain's book lists BSI recommended sizes as 3.5mm for 65% thread engagement and 3.4mm for 80% thread engagement, so 3.3mm sounds like somewhere around 95%. That would be fine for tapping in soft butter, but would be a recipe for broken taps in anything sticky (Al) or hard.

As Lester observes, drills often don't cut to size, so that may save you, but I would choose 3.4 or 3.5 depending on the material.

Regards, Tony

On or around Tue, 8 Jan 2008 21:19:48 +1100, "Suzy" enlightened us thusly:

yes, for pretty much any normal application. You'll be 0.1mm or less short of maximum thread depth.

Hi Suzy, There are a lot of naff taps about these days, even from well known suppliers. If you want good ones at reasonable prices try Tilgear. They sell metric taps in the "normal" taper, second and plug/ bottoming, and also spiral point and spiral flute taps, both of which cut cleanly and work very well. See Tilgear catalogue for their uses. Highly recommended. Usual disclaimer except as satisfied customer. Can also highly recommend Tubal Cain book, I have a photocopy of tapping sizes pages on the bench at all times. T.W.

Thanks Lester. I'm drilling aluminium and plastic (I know -- which plastic? Answer: I don't know but it's stock moulding from a hardware store, 15mm X

20mm section). 3.5 mm has worked perfectly in practice but as I'm writing an article on the project I did not want to mislead my (non model engineering) readers

Hi Suzy, There are a lot of naff taps about these days, even from well known suppliers. If you want good ones at reasonable prices try Tilgear. They sell metric taps in the "normal" taper, second and plug/ bottoming, and also spiral point and spiral flute taps, both of which cut cleanly and work very well. See Tilgear catalogue for their uses. Highly recommended. Usual disclaimer except as satisfied customer. Can also highly recommend Tubal Cain book, I have a photocopy of tapping sizes pages on the bench at all times. T.W.

Thanks. I have two other questions. Here in Australia an M4 screw (as I'll now call it, having no shank (it - not me!)) actually has an overall diam of

3.9 mm. Thus it fits through an M4 clearance hole (M4 drill). Is that the UK experience please? Second question. The Australian word is centrepunch and the automatic (sprint-loaded) type is very popular. What is the UK position? Reason for all these questions: left UK 30 years ago and not sure of current usage.

Plastic mouldings are a bit variable on 'density' but certainly on the softer materials the slightly larger drill is safe enough. I suspect you could probably simply screw an M4 bolt into it. In practice on the plastic boxes I tend to NOT tap all the way through, so that the bolt or spacer IS cutting the last couple of threads itself. But on the softer materials, being a little over enthusiastic fitting an M3 or M4 screw can easily strip the threads anyway so it is worth ensuring that the drill IS cutting 3.5mm and that the tap is cutting clean.

In message , Suzy writes

[snip]

M4 is an International Standard. Bought screws are usually 'rolled' rather than 'cut' so the 3.9 is not unduly surprising.

A centrepunch is a centrepunch (or preferably a centre punch) and a spring loaded automatic centre punch is a spring loaded automatic centre punch.

Thanks, and I meant spring-loaded and not sprint-loaded of course!

Hi. The tone here is so different to that other ng that shall be nameless...

73 OM!

58% surely.

Mark Rand RTFM

One thing for sure is that you won't find this info on the Model Engineers Workshop recent data sheet on drills etc they chose to ignore the metric screw system Regards Alan

Nope...80%...and his name's not Shirley

Regards, Tony

Quite right too. Metric threads are just Unified threads made on French lathes and Unified threads were a crap idea compared with BSW, BSF and BA.

Mark Rand RTFM

Core diameter for M4x0.7 is 3.141mm...

3.5mm looks like 58% to me :-)

Mark Rand RTFM