Identifying small threads

[TakeawayParticipant @takeaway]

Like a lot of you I have accumulated over the years lot of small – to very small – screws/bolts of various types. Problem is, and always has been for me, is identifying the thread type. The smaller they get the harder it gets.

A "for instance" :- I just came across a packet of small set screws with nuts and washers. The diameter of the threaded screws is .0928 and they have a fairly fine TPI. I am making an early assumption that they are 3/32" something or other. My problem starts with my Moore & Wright thread gauge which covers MM and UNF. None of the gauges match these screws TPI. Worse, at this age the old hands shake and the eyes won't focus as they used to.

I want a universal thread gauge which can check all small threads from MM – BA – NF – Model Engineer – everything!

Something like a thick drill gauge but with appropriate threading enabling tactile feedback rather than dodgy eyesight to identify the thread.

Am I looking for the Holy Grail or does someone sell something that will do the job?

Stuart

[TakeawayParticipant @takeaway]

Small screw thread gauge?

[David LittlewoodParticipant @davidlittlewood51847]

Stuart,

Sometimes the only way to gauge small threads is to use a toolmakers' microscope; do you know anyone who has one?

David

[Les Jones 1Participant @lesjones1]

Hi Stuart,
As you have the screw and the nut you could measure how far it moves for one revolution of the nut. Put a scratch on the screw head and the nut an line them up. Measure the distance from the top of the screw head to the nut with a micrometer or digital calipers. Rotate the nut one complete revolution so the scratches again line up. Repeat the measurment. The difference in the measurments is the pitch of the thread. You will get a more accurate result if you rotate the nut several revolutions and divide the difference in measuments by the number of revolutions.

Les.

[Sub MandrelParticipant @submandrel]

You might be able to use a tap as a thread gauge. Or try a USB microscope.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by David Littlewood on 29/09/2012 17:23:05:

Stuart,

Sometimes the only way to gauge small threads is to use a toolmakers' microscope; do you know anyone who has one?

David

Stuart

Although no substitute for "the real thing", the modern USB microscopes are very useful, and typically include measurement software.

There was also a recent series in MEW, describing a Poor Man's ShadowGraph, that is well worth studying.

MichaelG.

[Les Jones 1Participant @lesjones1]

If the USB microscope was mounted on the toolpost of the lathe you could just use a reference mark on the microscope to line up with the crest of a thread then move the toolpost so the reference mark was lined up with another crest. This would not then rely on the accuracy of the microsope. I would just rely on the accuracy of the leadscrew (Or DRO) on the lathe.

Les.

[Michael GilliganParticipant @michaelgilligan61133]

Good idea Les … although strictly speaking, the microscope resolution is still relevant, because the "hairlines" can never be less than one pixel in width. [and most of these microscopes have a native resolution of only 640×480 pixels … the rest is usually "smoke & mirrors" interpolation.]

By comparison, my old Gaertner ToolMaker's Microscope [currently under restoration] has what are effectively adjustable-pitch leadscrews on the stage axes.

MichaelG.

[Sub MandrelParticipant @submandrel]

Don't underestimate the piotential of thes microscopes. i have a £30 Aldi 'traveller' one. These three pictures are at 200x zoom. I have to up the contrast at this level of zomm, but I am pleased with the results.

Neil

A lovely flexible stainless steel rule with 100th inch graduations – so one black + 1 white section is 10 thou – ample resolution to measure screws to <1 thou accuracy:

A tool tip at 200x – not the best bit of hand grinding ever, but it cuts well!

Finally, can you guess what this is?

[Charlie,Participant @charlie18171]

Hi Stuart,

I happen to have a Ba thread gauge picked up cheap at a car boot sale, It is numbered among my most usefull bits of kit and i use it a lot, Like yourself i have a large quantity of smallish and tiny nuts n bolts and my thread gauge will probably grade over 50% of them,My eyesight is not what it was so i understand your problem, Another route you could take if you have enough taps is to make one using gauge plate, That way you could cover some of the other usefull threads as well,

Best regards Charlie,

[Ian PParticipant @ianp]

Stuart

Quite a lot of useful suggestions have been made already and I have one more.

If you have a large collection surely you must know what some of them are so you can check the ones you are unsure of by using your known examples as test gauges. If you have any taps and dies you can use those as someone already said.

If you are really stuck, let me know the range of diameters you want to cover and I could send you a nut and screw of BA, Metric, UNF/UNC etc from my collection (I would mark them before sending).

Ian

[Ian PParticipant @ianp]

Posted by Stub Mandrel on 29/09/2012 21:03:13:

Finally, can you guess what this is?

It looks like a full stop?

Ian

[I.M. OUTAHEREParticipant @i-m-outahere]

I have seen thread plates that you can screw the offending fastener into to find the size but only in metric and sae – there may have been a whitworth one as well i can remember .

I know little machine shop in the USA do have one part #3459 but shipping will cost you .( usual disclaimer applies )

I use a similar method to Les which only works if you have a nut that fits the screw you want to check .

I use a peice of scrap as a holder so it leaves my hands free to operate a height gauge to measure the diffence in height the screw moves after 1 turn of the nut .

Bascally i measure the screw dia then find a drill a little larger than this but as close to the screw dia as i can get .

Then drill a hole in the scrap and set it up in the vice of my mill on some parrallels and get a height measurement with a height gauge , i screw 1 turn and re measure .

The difference is distance travelled per turn .

Divide 1inch by this and see what comes up if it looks odd then convert to metric to see if there is a match.

Sounds long winded but is useful for small screws and only takes a few moments to do but if you have some scrap flat bar large enough to sit the height gauge on then the set up for the milling vice is not needed as it can be clamped in the bench vice .

I have found the thread gauges that you get with a tap and die set are useless for anything with a fine thread and the fine metric units hard to see for small screws .

Ian

[Michael GilliganParticipant @michaelgilligan61133]

Neil,

It looks like one of these

MichaelG.

[Mark P.Participant @markp]

There is an imperial thread plate on flea bay at the moment £25.

Regards Mark P.

[TakeawayParticipant @takeaway]

Thanks to all who have responded to my request for help on thread measurement. Because I bought a digital vernier height gauge only three weeks ago I decided to give Les’s method a go.

I took a small countersink head set screw and nut from an unmarked packet. The mass produced anodized screw measured 2.3mm over the threads. Gripping the screw vertically in a small toolmakers vice I wound on a nut and took a height measurement and zeroed the readout. I wound the nut down six complete turns and got a reading which when divided by six indicated a thread pitch of .445mm. Double checking I added a further four turns making a total of ten. This time it showed a thread pitch of .448.

Reaching for my thread data tables I reckon that on balance my mystery packet of not very well made screws and nuts are 2.5mm dia. metric coarse fixtures.

The only observation I would make is that using this method on something like 14 BA might become a little fiddly but I think that others suggestions of tapping the range of BA sizes into a piece of gauge plate seems to be the way to go – certainly for BA.

Notwithstanding, the height gauge method worked this time and my thanks to Les Jones for his tip.

I think though that I will resist the dubious temptation of calibrating ALL of my mystery fixtures on the basis that life’s to short – apart from which I want to get started on a little garden steam loco which is beginning to take shape between my ears.

ATB

Stuart

[Clive HartlandParticipant @clivehartland94829]

I think Gauge plate might be a bit too much for fine taps to cut, we all made Brass plates to check threads and for shortening screws to length.

Clive

[TakeawayParticipant @takeaway]

Thanks Clive – on reflection I agree with you about using brass instead of gauge plate for making gauges for small taps. I already have a nice thick brass letter box flap lined up for the job.

Stuart

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