1/2" Coventry Diehead CHS type

Can anyone assist with some thread dimensions for the 4 off front plate retaining screws please?

This is the early type with non-standard screws and whislt I have sucessfully made the scroll retaining screws 0.155" x 48 TPI and the +/- adjuster 36tpi, I just can't work out what TPI the 0.155" dia retaining plate screws should be. I have tried 36, 48 and now 40 TPI. I have assumed that whatever it is is 55 degree due to the age of the head.

If someone is able to measure the O/D and TPI of an existing screw that would be a great help. In addtion can anyone offer advice on the correct tip width and thread depth for a nominall 0.155" Whitworh form ? It seems that the thread depth of 0.640327p and a radius of 0.137329p where p is the pitch. isn't working out for me I'm taking the calculation to be

0.640327 * (1000/40) = 25 i.e. 0.025" or 0.635mm single depth of thread and the radius to be 0.137329 * (1000/40) = 3.43 or 6.86" (0.14mm) across the dia.

I'm working on the basis that is the tip is a little narrow I'll need to dept a little more but trial so far are leaving a razor sharp crest if I depth anywhere above 0.022"

Matters are helped by havein no screw to work from and the threaded holes being so narrow and seemingly worn . I know I can buy them form eg Wiseman (cost as yet unknown) but as I need to make the front plate in for a penny ...

Thanks

I very much doubt that the crew holes are worn as they are through hardened. I will try to measure my screws and see what I come up with.

I can't remember where I found then, so can't provide a URL, but I've just uploaded a couple of scan/photos to my albums

Hi Bernard,

Thanks appreciate the help, its all a bit odd, had it not been hardened I would probably have re threaded them by now. I suggested wer as everything I have cut thus far tightens up the deeper the thread is wound in, hothing I have tried thus far seems 'right'.

Thanks Peak, I use this as my starting point, a real nusiance that they don't quote TPI.

It is not usual to quote tpi for a BA thread, primarily because it is generally assumed they would be die cut rather than screwcut; secondarily because BA is a metric pitch. Thirdly, the pitch progression of the thread series follows a geometric mathematical series leading to numbers that are difficult to produce with change gears.

The slotted front plate screw for the 1/2" head is given as 3BA.

3BA is 0.9^3 = 0.729mm pitch which is 34.8 tpi.

OK I'm struggling to measure the screws on mine, as they are a bit worn and gnarly.
From what I can gather, they used to use many of their own one-off threads, but may later have standardised, as per the table above.
My screws all measure about .01515" diameter and 0.57" long
I'm struggling a bit to measure the pitch/tpi, but keep on coming up with 0.024" or 0.61mm, neither of which match with a sensible tpi; 0.024" equates to 41.666 tpi

Just eyeballing the thread, it does seem to fall between 4 and 5 BA at 0.0260" & 0.0232" respectively, as far at pitch goes, but just undersize diameter for 3BA; that said, I'm on the waiting list for a cataract operation.

To try and measure the pitch, I rolled the screw on a piece of paper, counted 10 spaces between the oily lines and tried to measure it with both a caliper and a graticle magnifying glass

Rotagrip list them, but I can't tell whether they are the alleged 3BA ones, or those in a head similar to mine.
http://www.rotagriponline.com/index.php?page=shop.product_details&flypage=flypage_images.tpl&product_id=8823&category_id=889&option=com_virtuemart&Itemid=29 

Alternatively, try Wiseman
https://threadtools.com/diehead-parts-and-reconditioning/

Bill

Edited By peak4 on 17/06/2023 17:56:47

True, but this USA website does.

Small fasteners on British equipment are often BA. After finding Whitworth threads were unsuitable for Electrical and Instrumentation type work, the British Association set up a committee to define a good alternative and they came up with a logical metric thread. So far so good except they made BA dimensions a bit odd by converting them to inches!

Even numbered BA are more usual, so likely panel holding screws are 4BA=0.66 (38.5tpi), 6BA=0.53 (47.tpi), 8BA=0.43(59.1tpi)

How close your lathe can get to these pitches depends on what change wheels are available. On my machine I can do:

• 3BA - gear ratio 48:65, (34.4tpi), error about 1%
• 4BA - gear ratio 144:221 (38.98tpi) or 2:3 (38.1), error about 1%
• 6BA - gear ratio 9:17 (47.98tpi), error about 0.001%
• 8BA - gear ratio 3:7 (59.27tpi), error about 0.003%

Closer is possible by doing the sums. The required decimal ratio is converted to a series of increasingly close integer ratios by calculating converging fractions. For example, pi is approximately 3/1, 22/7, 333/106, 355/113, 103993/33102 etc. Then gears are made to implement the ratio on a banjo. More hard work!

Also, the thread angle is 47.5° not 55 or 60.

All very tedious so I'd just order some ready made BA screws off the internet and fit them!

Dave

Thanks everyone, sorry I missed the BA listings Inthink the chart I have is similar but missing that info. I have a fair range of BA taps & dies, my Dad a watchmaker, spent the last part of his career in instrument making, I know iI have 3BA taps not sure about dies. I agree it would be easier to buy them. Anyway I’ll have a look tomorrow.

Ok I couldn’t wait until tomorrow. I just ran a 3BA die over my best attempt a 40 TPI Whitworth approximation and it just removed a little material but it’s made a substantial improvement to the feel. Luckily the die is split so I can adjust the fit. I suppose the statement an out the high cost of spares for the CHS is based on the lack of readily available info on what is required.

Just re-reading my earlier comment;
"My screws all measure about .01515" diameter and 0.57" long"

seems to have acquired a misplaced decimal point, and should of course read
"My screws all measure about 0.1515" diameter and 0.57" long"
That's about 10 thou under 3BA

Bill

Edited By peak4 on 17/06/2023 22:22:26

Thanks Bill, I knew the screw should be about 0.155” and being a metric man I didn’t see the error. I do however need magnification to see what’s going on when cutting threads at that diameter. My Dad showed me a very fine fusee chain and told me they were made by children as they could see what they were doing.

Hi Tony, a similar question was asked in Coventry Die Heads

Regards Nick.

Maybe Wiseman Threading tools can assist try this link Wiseman