Windlass thread size?

[prv]

I have a Simpson Lawrence 1000 windlass. I want to be able to use the manual backup function, which goes on like this:

The boat came without the fitting shown being installed in the third picture, but a few months ago a kindly forumite gave me a similar part from the rope-drum version of the windlass. To convert it I need to make the piece that screws onto the central threaded shaft; I have a suitable chunk of brass and an elderly grandfather who is pleased that his model-making lathe will see a little more use in his lifetime, so the only thing remaining is to buy a suitable tap to cut the thread.

I've measured the outside diameter of the thread on the shaft as 19.73mm, and counted seven turns in the 16mm length that's visible above the gypsy. Obviously we can assume some minor inaccuracy in those figures. My best guess is that this is an M20 thread, based mostly on the diameter (slightly rounded-off peaks?). But I'm only really familiar with metric and BSP (I assume BSP is highly unlikely here!) so does this match up with a plausible imperial size? What do we think S-L would have been using in a windlass sold in the late 90s / early 2000s?

Cheers,

Pete

 

[colhel]

If the thread looks in good condition I'd go for M20.
If it was imperial the next *Standard* size would be 13/16" which is about 20.6 which would look pretty truncated at 19.73mm. 1/2" BSP is almost 21mm in dia and a finer pitch

 

[Boo2]

I've measured the outside diameter of the thread on the shaft as 19.73mm, and counted seven turns in the 16mm length that's visible above the gypsy. Obviously we can assume some minor inaccuracy in those figures. My best guess is that this is an M20 thread, based mostly on the diameter (slightly rounded-off peaks?).

There's a metric thread chart on wikipedia here http://en.wikipedia.org/wiki/ISO_metric_screw_thread which shows M20 threads as having either 2.5 (normal pitch) or 2 or 1.5mm threads (fine pitch). Your measurements of 16/7 ~= 2.28mm pitch don't allow you to decide which it is so if I were you I'd try to borrow a thread pitch guage like the ones ArcEurotrade have available here : http://www.arceurotrade.co.uk/Catalogue/Measurement/Thread-Gauges-and-Wires#Thread-Gauges (items 100-120-00100 and 100-120-00300).

It's often easier to tell a thread size from whether the pitch is metric or imperial than from the diameter but the one you quote can really only be M20 AFAICS.

Hth,

Boo2

 

[VicMallows]

If you'd like a standard M20 nut to try I'll happily send you one. (bought a bag of them before realising I needed M20 fine!)

 

[prv]

Cheers, guys.

I was confident enough in M20 to have ordered a single nut from eBay (thanks for the offer though Vic!) to test. But the pitch question remains. I ordered the nut very quickly and forgot to specify pitch - am I right in thinking that a normal M20 nut will be 2.5mm unless deliberately stated otherwise?

For what it's worth, given the function of the thing, I can't see any reason to have used a fine thread in the design.

Pete

 

[colhel]

Yes. Metric standard is coarse, 2.5mm

 

[prv]

Yes. Metric standard is coarse, 2.5mm

Cheers - I'm quite looking forward to making this thing. Haven't done any machining since I was at school.

(On which note, can I give you the gentlest of nudges about a certain autopilot tiller? )

Pete