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Oh god. This group *is* active Suzy 01-08-2008
Posted by MikeH_QB on January 8, 2008, 5:31 am
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> I know 3.3 mm is the preferred drilling for a 4 mm tap, but in practice is
> 3.5 mm OK? My supplier only stocks "standard" taps (no starters or
> bottomers)

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

Posted by David Littlewood on January 8, 2008, 5:39 am
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 writes
>I know 3.3 mm is the preferred drilling for a 4 mm tap, but in practice is
>3.5 mm OK? My supplier only stocks "standard" taps (no starters or
>bottomers)
>
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
--
David Littlewood

Posted by Mark Rand on January 8, 2008, 3:49 pm
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wrote:

>writes
>>I know 3.3 mm is the preferred drilling for a 4 mm tap, but in practice is
>>3.5 mm OK? My supplier only stocks "standard" taps (no starters or
>>bottomers)
>>
>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.

58% surely.


Mark Rand
RTFM

Posted by jackary on January 8, 2008, 5:20 pm
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> wrote:
>
> >writes
> >>I know 3.3 mm is the preferred drilling for a 4 mm tap, but in practice is
> >>3.5 mm OK? My supplier only stocks "standard" taps (no starters or
> >>bottomers)
>
> >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.
>
> 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

Posted by Mark Rand on January 8, 2008, 7:04 pm
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wrote:


>
>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


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.

<G>


Mark Rand
RTFM


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