Single point depth of cut

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Yes indeed

As I wrote earlier : Again, a telecentric lens is most useful, but ‘needs must, when the Devil drives’

MichaelG.

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Dell

Forgive me, please, if I have mis-understood, and therefore exaggerated, your requirements.

I thought you were seeking a way to accurately measure the pitch and profile of an historically important screw thread which had probably been produced with a unique set of tools.

Checking the pitch of a threaded hole, using peg wood, is fine … but it will not properly identify the profile.

At the risk of stating the obvious:

One cannot grind a single-point threading tool to the correct shape until one knows the required shape.

MichaelG.

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P.S. __ How obsessive you need to be about this would obviously depend upon the importance of the clock and whether you are doing restoration or renovation.

Just as an example of what can be achieved with modest optics and free software:

This is the lead-screw on a very small stepper motor

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The image is sufficient to identify the two-start left-hand thread … but not to define its profile.

MichaelG.

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Edit: __ the blue text is barely legible here … it’s 1.4 mm

Edited By Michael Gilligan on 22/05/2023 08:57:24

Three wire thread measure.

You can check the flanks of the thread with three wires and a micromiter. I use the shanks of three drills as they are hard and ground usually. You can then check the part you are making against the original or new screw. If using a worn part use a piece which has least or no wear.

David

Three wire will not help if you do not know the form (vee-angle) of the thread you are cutting.

I can cut a Whitworth form thread of same nominal diameter and pitch as a UNC thread and make an identical three wire measurement on both.

Nor will three wire help you easily determine depth of cut (the title of this discussion). Without doing a lot of quite complicated maths, it will only tell you 'I need to go a bit further' or 'I have overshot the mark'.

Two measurements with different size wire should allow you to calculate the angle.

Going back to Dell’s comment in the opening post:

”Some of the clocks pre 1900 I do the manufacture had there own thread forms and they were made using thread plates … “

Here is a useful page: **LINK**

https://www.chiark.greenend.org.uk/~alanb/martin.html

MichaelG.

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Edit: __ it is also worth looking at the ‘parent’ page:

https://www.chiark.greenend.org.uk/~alanb/

Edited By Michael Gilligan on 23/05/2023 07:54:21

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On the off-chance that anyone is interested … here is a real-world example of the ‘perspective problem’ found when using non-telecentric lenses:

Taken this morning, with my iPad … it shows an age-related glue failure:

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The relevant point being that the trim-ring is further away, by the height of the mouse, and therefore appears to be significantly smaller than the face to which it was previously attached.

Taking measurements from such images can be deceptive !

MichaelG.

Edited By Michael Gilligan on 23/05/2023 08:37:33

Does Dell need to worry about thread profiles?

I often single-point threads on my lathe with a 60° V HSS cutter for metric, creating sharp peaks and valleys. Although the threads have the correct pitch and angle, metric form calls for a flat top and rounded bottom. The rounded bottom is for strength rather than fit, and isn't essential. The flat top is easily produced with a file, again the dimensions aren't critical.

Whitworth and BA call for rounded tops and bottoms, but again not strictly necessary. In the US, Sellers decided Whitworth's round form was OTT, and defined an easier to make 60° thread with flat tops and bottoms. Sellers thread morphed into UN, which has flat tops and round bottoms.

If I thought a round-bottom was important on one of my sharp threads, I'd round off the point of a V tool for a finishing cut with a stone. For strength, the round bottom doesn't have to be particularly accurate - the main point is to remove the sharp stress-raiser. Lathe cut threads are weaker than rolled, making commercial fasteners a better bet when safety matters, but safety isn't a concern in clocks.

I suspect traditional threads made with thread-plates were crude by modern standards, like as not with plates producing sharp threads when new, gradually rounding off as the plate wore out.

Bearing in mind I'm a bodger, I'd make the odd thread as close as I could with a sharp cutter (concentrating on angle and pitch, not the form) , flatten the top, perhaps with a pot scourer rather than a file, and ignore the bottom. Then fettle to fit. When a thread screws in and holds tight, 'good enough', I'm happy,

Dave

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Dave … Please see my post at 08:37:45 yesterday particularly the footnote

MichaelG.

As I nunderstand it, the peak of a thread is either rounded (Whitworth form ) or truncated (Unified form ) to prevent root / crest interference..

So in practice, there would be little difference in function betwenn a rounded creast or a flat topped one.

(The objective is the same )

Measutring the thread with wires will ensure that the thread is the correct depth, but won't prevent root / crest interference..

Cutting an external thtead, the top is truncated by turning the raw material very slightly undersize (The proportion will be listed in various places, and is somewhere in my Apprentivce notes for Unified threads )

Presumably, an internal thread is drilled /bored slightly oversize, also to produce a flat topped thread.

In both cases, the infeed needs to take account of the truncation, or an oversoze thread will result.

For most of the standard threads encountered, Taps and Dies should suffice , but truncation needs to be considered when forced to screwcut an "oddball"

Very often, one reads that drilling slightly oversize reduces the actual percentage of thread engagement, but with littlem practical effect on thread strength or clamping force.

To produce the ultimate in clampingn force, the fasteners need to be tightened JUST into yield.

For a clock this is probably not needed.

Howard

In the context of his original question then Yes.

A pointed tool will need a greater depth of cut than one with a rounded or flat end. So if he goes and looks up the DOC for an M8 thread but uses a sharp pointed tool he won't have cut deep enough

Any fudging it by playing about with the starting diameter will also affect the DOC so should be taken into account

Edited By JasonB on 25/05/2023 08:23:13

It is not really practical to use a screwcutting tool sharpened to a true point. The end of the point invariably breaks off under load. This is one reason (among several) that thread profiles use a small flat or radius in the root.

The depth of thread formulas as given in places such as Machinery's Handbook usually take into account the radius or flat on the end of the screwcutting tool. And the flat on the crest, achieved by turning the OD slightly undersized to make it meet the thread spec.

In practice in the home workshop, a HSS toolbit is usually sharpened to a point at the correct angle and then the tip rounded off by rubbing on a bench oil stone. It may not be the exact radius the standards specify, but as Howard says above, crests and roots are clearance areas with no contact with the other thread so it is not ultra critical. Good carbide inserts of course come correctly proportioned out of the box.

Also in practice on home hobby type machines, you will be hard pressed to achieve exact thread depthing repeatably to a perfect accuracy, due to lack of rigidity in machines etc etc. Even in commercial jobbing shops, the final fit of the thread is mostly done by "try and cut" with a test nut (Or bolt). Relying solely on theoretical depths etc is not considered reliable in reality.

Modern commercial ultra-rigid CNC machine centres are a different story, using precision insert tooling under very controlled conditions. And mass produced commercial threads are deliberately made with much looser clearances than model engineers like to use so that any nut will fit on to any bolt out of the 100,000s made that day or the millions that year. The thread depth could be all over the place within a very large tolerance, but the generous clearance between the male and female threads will allow for it.

So don't make the mistake of mixing up the specified ISO or BS Standard dimensions with the actual mass produced dimensions or even the practical home hobby dimensions.

Edited By Hopper on 25/05/2023 08:55:31

Edited By Hopper on 25/05/2023 08:57:31

Good point, what suits me in my workshop may not be good enough for others. My larger screw-cuts are always fudged, because I DOC to about the right depth, test the fit, and then take a little more off as necessary until the screw takes. Most DIY store type nuts and bolts are wide tolerance and don't need much fettling. The tighter fitting nuts and bolts found on cars etc need more care.

Just a thought, when cutting Whitworth or BA threads, do hobbyists finish them correctly with rounded-crests? Being metric I've only cut 2 Whitworth studs in 10 years, and I flattened the crests, which might be heresy for all I know! Thought the studs fitted, they aren't to Whitworth specification. Where is Inspector Meticulous when you need him?

Dave

Thanks for all the replies, I think I have opened a can of worms , I have received my copy of Trubal cains book and this is helpful.

but it still doesn’t answer the question of unknown thread forms pre 1900 , could be Thury or could be something developed by individual clock manufacturers, all the clocks I restore I like and try to keep them as original as I possibly can and as much as possible use the original fixings but sometimes I have to make copies of screws but they need to be to original specification ( if possible) , yes BA, metric, and Thury, are all very similar because they are all based on metric measurements and it’s possible to re tap but then the next person to dismantle the clock has one original screw and one non original but both look the same , what one goes in what hole but if both or more are identical then no problem, so it’s not just for my benefit.

Dell

Nothing does! Up until about 1850 workshops and craftsmen made their own threads, to whatever approximate diameter, form, flank angle, and pitch they thought best. Using standard threads caught on slowl, and British clock makers were ultra-conservative.

The only way I know is to measure them. With luck they will be found to match to a standard for which taps and dies are available. Otherwise they have to be bespoke. Not difficult in principle, but fiddly. Female threads can be imprinted on a soft wooden dowel by screwing it into the hole. The professionals use shadowgraphs and other gizmos to do this accurately. However, males can be measured at home with a caliper, v-angle gauges and a loupe. A USB microscope and pixel measuring software would speed the process considerably if more than a few need to be investigated.

Once the thread's properties are known, a tap can be made in the lathe. And from the tap, a die.

I guess (but don't know!!!) that pre-standard clock threads will be similar. For practical reasons, thread applications have a limited range of useful pitches and diameters. - for example, fasteners all have thread angles between 48° and 60°. Thus, a fairly small set of home-made taps and dies might cover most clock repair requirements.

Dave

Nicely re-stated, Dell

Yes, it's both important, and tricky

MichaelG.