Cutting Internal threads from First Principles

[GregLocock]

I just watched a Stephen Fry doco on the Gutenberg press.

I hadn't realised how many things he had to hack together, but the one that interested me most was cutting an internal thread.

To do this the TV team carved an external thread, then built a box of pegs to engage with it, which was used to drive a cutter through a wooden blank, to create a mating internal thread.

I like this idea. but.... does it pass the sniff test?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

The box of pegs? I don't think that's "authentic".

I'm under the impression that the 'traditional' way is to cut off part of the male thread and turn it into a tap.



Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

I can imagine that working reasonably well with, say, an iron or steel screw and a brass or bronze nut, but would it work with wooden components? The chances are the nut would be lignum vitae or something equally hard.


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If we learn from our mistakes I'm getting a great education!

 

[GregLocock]

Yeah sorry Mike it was a 150 mm OD 2 start wooden thread, say 20mm pitch. They chiselled the external thread.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ScottyUK]

Greg,

By 'box of pegs' I assume you mean a block fitted with dowels or something similar on a pitch to engage the male thread, like a crude forerunner of a half-nut and leadscrew?

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If we learn from our mistakes I'm getting a great education!

 

[GregLocock]

Yes, it was a square box with a clearance hole for the thread through it. They then drilled say 40 radial holes and inserted pegs with pointed ends into the holes to engage in the thread.

I haven't seen anything quite like this before, and although it is ingenious I'd have thought that at least one of the many museums I've been to would have something similar, not that it would necessarily make a great exhibit.

Reasons I don't like it - the geometry would be a bear to work out, I haven't seen one, and a half nut chased by hand would be easier.

Reasons I like it - it is adjustable, robust (if the pegs wear, replace them, if the holes are in the wrong place retract the peg a bit, or carve a custom peg). In the spirit of Whitworth's approach to things, you could start with a single peg version, then use that as the register to move a box under a pillar drill and that way the box would automatically have the correct geometry (sorry that's a bit garbled).

Well according to google whatever it might be called it isn't a pegbox.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[drawoh]

GregLocock,

I strongly recommend you read One Good Turn, A Natural History of the Screwdriver and the Screw, by Witold Rybczynski, Harper Books.

JHG

 

[plasgears]

The peg design is rather old and evident in windmill drives. The thinking behind it may have been easy replacement of pegs during maint. We have a working Dutch windmill in Holland, MI near Lake Michigan. The pegs evidently wear out before the mating parts.

 

[GregLocock]

That's pegs as teeth on a gear wheel? and I suppose the other use of the idea is as a pinwheel. Good bit of lateral thinking, but Gutenberg was there ~ 200 years beforehand, we need prior art!




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.