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TiggerToo

TiggerToo

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what do you call an instrument to make a "thread" in a hole? Is it possible to do this with a hand-operated instrument in as 3.5mm hole in mild steel?
 
It is called a tap. You also need the correct size pilot drill for your chosen thread. Go to an engineering suppliers and ask them. Yes you should be able to do it by hand.
 
M3.5 is not a common size ... my set of taps does not include one. Not non-standard by any means but you may have to go a tool supplier or buy on line rather than a DIY store.


They come in three main types. A tapered, or first cut, tap... which is very tapered to make an easy start. A second cut which is only very slightly tapered and a plug tap which is not tapered at all and is intended for the final cut in a blind hole.

A tapered tap will probably be OK for you without needing the second cut one.

The correct size drill is 2.9mm ....... you will need a decent tool suppliers or to buy on line as I'd doubt very much if you will find one in a DIY store.

Don't forget to buy a wrench (handle) .. It'll probably cost more than the tap !

Look up some instructions for tapping holes on line somewhere. A thin oil will be a suitable lubricant
 
If you are going to tap a hole M3.5 be very careful as I is very easy to break a tap that size inside the hole, (hen you are screwed (sorry for the pun)

If you don't have a lot of experience and still wish to do it your self use a high carbon steel tap not high speed steel as the high carbon is softer but will not break as easy
 
FlyingDutchman said:
If the turning requires some force, turn back to break the chips.
Click to expand...

At school I was taught to go forward half a turn, back a quarter turn, forward half a turn, etc. The chips should escape into the flutes cut along the length of the tap, hence why the first tap has wider flutes as it cuts the most material. I think at school we used tallow for lubrication.

As an aside, it's always puzzled me why we say "straight as a die", when it's the tap that is (and needs to be) straight, the die is just a block.

Pete
 
The force needed to tap a 3.5 mm hole in mild steel should bot be that great but clearing the chips is a very good approach it keeping the tap straight once you start tapping is IMHO where care is needed.

The word "die" in engineering is used to describe lots of different tools, wire and tube is drawn through a die to reduce its diameter. A lot of press tools are also refereed to as die's.
 
Good advice above if the material is mild steel (presumably not on a boat?).
Worth noting that if tapping stainless, cheap taps will not suffice: it's tough, sticky stuff and easy to break them. Definitely worth paying the extra for good, branded gear.
 
prv said:
As an aside, it's always puzzled me why we say "straight as a die", when it's the tap that is (and needs to be) straight, the die is just a block.
Click to expand...

Nothing to do with die as discussed here, because words change meaning, cf the complete illogicality of "it's the exception that proves the rule". The origin seems to be die as the singular of dice.

Several sources quote variations on this theme:
...straight means correct and true, rather than 'as the crow flies'.

The phrase originated as 'as smooth as a die', no doubt referring to the smoothness of the bone that dice were made from. This dates back to at least the 16th century. For example, Jehan Palsgrave's Lesclarcissement de la langue françoyse, 1530:

"Make this borde as smothe as a dyce"

It migrated to 'as true as a die' in the 18th century, as found in John Gay's Songs & Ball., New Song on New Similies, 1732:

"You'll know me truer than a die."
 
VicS said:
Good thinking

However 3.5mm is a bit large for M4 .... should only be 3.3mm

M4.5 requires a 3.7mm hole
Click to expand...

I think Macnorton's point was that lots of people are assuming the OP is tapping an M3.5 thread (cf electrical boxes, Westerly windows, etc). He's not, he has a 3.5mm hole and will be tapping a larger thread into it.

I was lucky the other day in that the bolts I was tapping for happened to be M6 - because the proper size for M6 is a 5mm hole, and everyone has a 5mm drill bit :)

Pete
 
prv said:
I think Macnorton's point was that lots of people are assuming the OP is tapping an M3.5 thread (cf electrical boxes, Westerly windows, etc). He's not, he has a 3.5mm hole and will be tapping a larger thread into it.

Pete
Click to expand...

Yes I realised that.. like others I'd assumed he wanted to tap M3.5
Trouble is that a 3.5mm hole is a bit on the large side for an M4 tap if he wants a decent depth of thread but too small to tap out to M4.5
 
TiggerToo said:
what do you call an instrument to make a "thread" in a hole? Is it possible to do this with a hand-operated instrument in as 3.5mm hole in mild steel?
Click to expand...

Lots of excellent advice here, if I may add something also, most taps come in 3's a starter which is tapered, a second for obvious reasons, and a bottom tap which is to get maximum depth if the hole is 'blind'

I always use a guide block with a starter, that is a piece of hard material, (mine is bakelite) about 1/2 to 3/4" thick in which a hole is drilled through at 90deg. the same size as the tap dia. place and hole in the block over hole to be tapped and the tap will stay at 90 to the work being tapped.

My block is about 1.1/2 square, which has many holes in it now for different size taps, a lazy but accurate way to ensure tap enters correctly.
Also use a tallow or cutting oil, and take your time until you get the hang of it.

Good luck
 
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