SCrew size mystery

[bionichamster]

I have a pack of tiny drawer knobs, the sort of thing you might find on a watchmakers toolbox. they come with small countersunk machine screws to attach them from behind through the drawer. Unfortunately the screws are too short for the wood I have in mind so need to find some longer screws of the same thread gauge. But it's not obvious to me what that gauge might be.

Using a callipers the outer thread diameter is 2.83mm. which doesn't match with any metric or imperial thread I can find.

I did find a general chart that listed two threads very close, at 2.84. but these were listed as UNF 4/48 and UNC 4/48, problem is that those appear to have different pitches to each other so I have no idea which might be appropriate.

Just wondering if anyone has any ideas as to what the thread could be in case I've missed something obvious.

I suppose I could order a few of the UNF or UNC, because at that tiny size they'll probabbly mold themselves to the female thread anyway....

Any suggestions welcome.

Bh

[Urbandreamer]

I can't help for certain with the screw size.

However, have you thought of drilling a 2mm pilot hole then taking a 3mm bit and, using a depth gauge, drilling a deep countersink hole.
The screws would then fit.

Alternatively you could use a M3.5 tap to create new threads in the handles.

[BullDog]

I have a pack of tiny drawer knobs, the sort of thing you might find on a watchmakers toolbox. they come with small countersunk machine screws to attach them from behind through the drawer. Unfortunately the screws are too short for the wood I have in mind so need to find some longer screws of the same thread gauge. But it's not obvious to me what that gauge might be.

Using a callipers the outer thread diameter is 2.83mm. which doesn't match with any metric or imperial thread I can find.

I did find a general chart that listed two threads very close, at 2.84. but these were listed as UNF 4/48 and UNC 4/48, problem is that those appear to have different pitches to each other so I have no idea which might be appropriate.

Just wondering if anyone has any ideas as to what the thread could be in case I've missed something obvious.

I suppose I could order a few of the UNF or UNC, because at that tiny size they'll probabbly mold themselves to the female thread anyway....

Any suggestions welcome.

Bh

It would be unusual, but they could be 6 BA screws. Diameter is 2.79mm, a little smaller than you measure but by a small difference.

Link to BA screw sizes -

https://www.britishmetrics.com/images/pdf/technical/ba(1).htm


Edited to add that URL link is correct, but the forum doesn't display it correctly.

Moderator Message:
Link fixed (chas49)

[jaizan]

Counterbore the timber.

Or, if you really want longer screws, measuring the thread pitch would be helpful for the investigation.

[U962]

BA thread sizes continue on down from below the minimum BSF thread size: which is around 4mm dia in metric units
So indeed for an old thread in imperial sizes times at that size BA is indeed quite a possibility.

UNF (fine) and UNC (coarse) are indeed different thread pitches just like BSW (whitworth) was the coarse thread and BSF (fine) was the fine threrad in imperial times when British Standards ruled the world

[bionichamster]

I can't help for certain with the screw size.

However, have you thought of drilling a 2mm pilot hole then taking a 3mm bit and, using a depth gauge, drilling a deep countersink hole.
The screws would then fit.

Alternatively you could use a M3.5 tap to create new threads in the handles.

Certainly thought of something like that.There’s only 6mm of thread, I reckon the female thread on the knob needs about 3mm to get a good secure grip, so I’d have to drill out the 9mm thick oak drawer fronts I’m making down to a 3mm depth (because contrary to my previous post the screws aren’t countersunk but flat heads!

Yes it’s doable but a bit of a faff with little room for error, I just feel the simplest thing is to buy longer screws of maybe 15mm for it, if only I was sure of the thread!At least if the screw is a bit too long it can be cut or filed down.

I may just take a gamble and buy a couple of different packs of screws online, provided They aren’t too pricey,

Bh

[bungeejumper]

I may just take a gamble and buy a couple of different packs of screws online, provided They aren’t too pricey,

You can buy a boxed set of small bolts and set screws in various sizes for a couple of quid. Although they'll almost certainly be metric, unless otherwise specified.

It's at times like this that I turn to my old dad's tin of ancient nuts and bolts from the fifties and sixties. To which I have appended various weird and wonderful bolts and studs from the decidedly non-metric Austins and Morrises that I used to run.

And if all else fails, there's always my trusty tap and die set. I wonder how many young'uns would know how to use one of those?

BJ

[jfgw]

I thought of BA too. Another possibility is cheaply made M3 (the minimum major diameter of an M3 male thread is 2.874mm).

I would not expect them to UNF. Coarse threads are the norm for most things.

If you have an M3 screw (or tap) lying around, you could compare the pitch.

#4 UNC has a pitch of 40 tpi.
M3 has a pitch of 0.5mm (about 51 tpi).
6BA has a pitch of 0.53mm. (about 48 tpi).

M3.5 screws (the size used for electrical accessories such as sockets and switches) have a pitch of 0.6mm (about 42 tpi).

Different threads have different thread forms. 0BA and M6 are both 6mm with a pitch of 1mm but the BA thread appears bigger if you try to mix them.


Julian F. G. W.

[Urbandreamer]

Different threads have different thread forms. 0BA and M6 are both 6mm with a pitch of 1mm but the BA thread appears bigger if you try to mix them.


Julian F. G. W.

On that point thread gauges are relatively inexpensive. Just difficult to justify if you are only to use them very occasionally.

[BullDog]

Just to add, when considering different screw threads, thread pitch isn't the only variable. The angle of the thread form varies too. Metric and unified threads have a 60 degree angle. British Standard threads a 55 degree and BA threads 47 degree angle. So even if the screw size is similar, they're generally not interchangeable.

[bionichamster]

I may just take a gamble and buy a couple of different packs of screws online, provided They aren’t too pricey,

You can buy a boxed set of small bolts and set screws in various sizes for a couple of quid. Although they'll almost certainly be metric, unless otherwise specified.

It's at times like this that I turn to my old dad's tin of ancient nuts and bolts from the fifties and sixties. To which I have appended various weird and wonderful bolts and studs from the decidedly non-metric Austins and Morrises that I used to run.

And if all else fails, there's always my trusty tap and die set. I wonder how many young'uns would know how to use one of those?

BJ

Thanks for all the suggestions.

I have several jars and trays of old bolts and other fixings but nothing in there has been suitable (and I need 10 of them! ). something that thin and long is pretty uncommon.

I am considering trying a cheap M2.5 in hope that the thread will be enough to grip but that there's enough free space due to the size difference to allow deformation of the non matching threads.... and maybe a tiny dod of epoxy or super glue in the hole before fitting. Also thought about an aluminium machine screw of nearest size as that would maybe deform to fit.

It's not a big issue, there's always a solution but the correct fittings of the riequired length is obviously the simplest. The knobs were very cheap at a car boot, so buying an expensive pack of screws for them kinda defeats my aim, might as well buy new knobs with the correct length screws then!

Cheers
Bh

[servodude]

You can buy a boxed set of small bolts and set screws in various sizes for a couple of quid. Although they'll almost certainly be metric, unless otherwise specified.

It's at times like this that I turn to my old dad's tin of ancient nuts and bolts from the fifties and sixties. To which I have appended various weird and wonderful bolts and studs from the decidedly non-metric Austins and Morrises that I used to run.

And if all else fails, there's always my trusty tap and die set. I wonder how many young'uns would know how to use one of those?

BJ

Thanks for all the suggestions.

I have several jars and trays of old bolts and other fixings but nothing in there has been suitable (and I need 10 of them! ). something that thin and long is pretty uncommon.

I am considering trying a cheap M2.5 in hope that the thread will be enough to grip but that there's enough free space due to the size difference to allow deformation of the non matching threads.... and maybe a tiny dod of epoxy or super glue in the hole before fitting. Also thought about an aluminium machine screw of nearest size as that would maybe deform to fit.

It's not a big issue, there's always a solution but the correct fittings of the riequired length is obviously the simplest. The knobs were very cheap at a car boot, so buying an expensive pack of screws for them kinda defeats my aim, might as well buy new knobs with the correct length screws then!

Cheers
Bh

Thread tape, or loctite, can help if things are just a bit out

[jfgw]

My money is on them being M3. A bolt thread measures undersize even at the upper end of its tolerance range. Cheap rolled thread screws could well be out of tolerance - undersize will fit but oversize will not. There is a distinction to be made between cheap imported fasteners and prescision engineering.

The knobs themselves may not be properly threaded. I have had to run a tap down the screw holes in kitchen handles before in order for the supplied screw to screw in more than about 4mm.


Julian F. G. W.

[jfgw]

Could you count the threads and calculate the pitch? You may be able to photograph (or scan) a screw and enlarge it.


Julian F. G. W.

[bionichamster]

My money is on them being M3. A bolt thread measures undersize even at the upper end of its tolerance range. Cheap rolled thread screws could well be out of tolerance - undersize will fit but oversize will not. There is a distinction to be made between cheap imported fasteners and prescision engineering.

The knobs themselves may not be properly threaded. I have had to run a tap down the screw holes in kitchen handles before in order for the supplied screw to screw in more than about 4mm.


Julian F. G. W.

Well done, you are correct. My M2.5s arrived and were useless, never even connected with the female thread.

But the M3 fits as well as the original screws… better probably given it is slightly greater diameter.

Interestingly the thread diameter on th M3 is around 2.91
(And on the M2.6 : 2.41) there is a bit of variation of .02mm depending on where it is measured but that might be me holding either screw or callipers inaccurately at a slight angle.

Useful learning experience on ACTUAL thread sizes!

Will def use thread lock or equivalent

Thanks all
Bh