1/2″ Coventry Diehead CHS type

This topic has 13 replies, 7 voices, and was last updated 18 June 2023 at 09:47 by JohnF.

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17 June 2023 at 13:57 #34233
Tony Ray
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@tonyray65007
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17 June 2023 at 13:57 #648827
Tony Ray
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@tonyray65007
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
17 June 2023 at 14:35 #648833
bernard towers
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@bernardtowers37738
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.
17 June 2023 at 14:54 #648836
peak4
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@peak4
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
17 June 2023 at 15:28 #648839
Tony Ray
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@tonyray65007
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.
17 June 2023 at 17:08 #648847
DC31k
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@dc31k
Posted by Tony Ray on 17/06/2023 15:28:56:

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.
17 June 2023 at 17:44 #648850
peak4
Participant
@peak4
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
17 June 2023 at 18:46 #648858
SillyOldDuffer
Moderator
@sillyoldduffer
Posted by DC31k on 17/06/2023 17:08:15:

Posted by Tony Ray on 17/06/2023 15:28:56:

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, …

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
17 June 2023 at 20:55 #648878
Tony Ray
Participant
@tonyray65007
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.
17 June 2023 at 21:22 #648879
Tony Ray
Participant
@tonyray65007
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.
17 June 2023 at 22:21 #648890
peak4
Participant
@peak4
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
18 June 2023 at 07:30 #648907
Tony Ray
Participant
@tonyray65007
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.
18 June 2023 at 08:28 #648911
Nicholas Farr
Participant
@nicholasfarr14254
Hi Tony, a similar question was asked in Coventry Die Heads

Regards Nick.
18 June 2023 at 09:47 #648916
JohnF
Participant
@johnf59703
Maybe Wiseman Threading tools can assist try this link Wiseman
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