BSW threads on fobco drill

[Daniel BrannanParticipant @danielbrannan48897]

[Daniel BrannanParticipant @danielbrannan48897]

Hi

Now this might be a stupid question but here goes. I have just acquired an old fobco star pillar drill and the information plate is held on by some small screws, half of which are missing so I want to buy some. I measured them using my calliper as 2.75mm across the thread crests which according to my conversion chart makes it closest to 7/64 (2.78mm). However I can’t find 7/64 fasteners for love nor money. I’m relatively new to old British fasteners so am I measuring it wrong? Has literally nobody in the world ever used a 7/64 bolt?

Thanks

Dan

[Martin ConnellyParticipant @martinconnelly55370]

The TPI/pitch of the threads often gives a good idea of what it is. If you do not have thread gauges to measure this then estimating against both metric and imperial scales can help.

Martin c

[RedsetterParticipant @redsetter]

6BA?

[SillyOldDufferModerator @sillyoldduffer]

Posted by Redsetter on 15/05/2023 22:14:50:

6BA?

+1. BSW dates from the cast-iron era and covers larger fasteners. BA was introduced later to provide the small nuts and bolts needed electrical gear and so forth. On British Imperial kit, small fasteners are often BA.

Dave

[Daniel BrannanParticipant @danielbrannan48897]

Thanks for the replies.

I don’t have any imperial thread gauges but I’ll try count them

I had never actually heard of BA threads until just now but that does seem a good match. Would fobco really have used a mix of standards though?

[Clive FosterParticipant @clivefoster55965]

Agree with Resetter that it's likely to be 6 BA. Not uncommon for nameplate holding and similar duties.

Nominal outside diameter is 2.8 mm and pitch is 0.53 mm.

As you already have screws one way of estimating thread pitch is :-

1)  start the screw by a couple of threads

2) measure the stick out from top of head to the drill body

3) screw in by as many full turns as is convenient

4) repeat teh measurement from top of head to body

5) subtract the second measurement rom the first and divide by the number of turns

6) result will be close enough to true pitch to identify the thread.

Clive

Edited By Clive Foster on 15/05/2023 22:28:36

[Daniel BrannanParticipant @danielbrannan48897]

Posted by SillyOldDuffer on 15/05/2023 22:21:09:

Posted by Redsetter on 15/05/2023 22:14:50:

6BA?

+1. BSW dates from the cast-iron era and covers larger fasteners. BA was introduced later to provide the small nuts and bolts needed electrical gear and so forth. On British Imperial kit, small fasteners are often BA.

Dave

I have learnt something today. Thanks!

[DMBParticipant @dmb]

Daniel,

Fobco not the only ones using a mixed bag of threads. Myford lathes use 2BA and BSF and the leadscrew is probably something else but I dont know what.

John

[Mike PooleParticipant @mikepoole82104]

Posted by Daniel Brannan on 15/05/2023 22:22:05:

Thanks for the replies.

I don’t have any imperial thread gauges but I’ll try count them

I had never actually heard of BA threads until just now but that does seem a good match. Would fobco really have used a mix of standards though?

It was quite common practice to use BA for sizes below 14” . 0BA is 6mm x 1mm pitch the thread angle is the big difference between standard metric. The seemingly odd sizes in the BA series are because the next smaller size is 90% of the preceding size in diameter and pitch which throws up the odd sizes.

Mike

[Clive FosterParticipant @clivefoster55965]

Further to what SillyOldDuffer says 0 BA and 1/4 BSF are dangerously close in size. 0 BA screws into 1/4 BSF but is very slack so the joint is weak.

Whitworth goes down to 1/8" diameter, 40 tpi in once common small sizes so its not just for big stuff.

Maybe I'm a minority of 1 but I like the small Whitworth sizes.

Clive

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Mike Poole on 15/05/2023 22:33:27:

It was quite common practice to use BA for sizes below 14”

.

Not only ‘quite common’ but ‘BSI recommended’ practice

Sorry, I can’t recall the standard number or its issue date, but I know it was clearly stated.

MichaelG.

[PeroParticipant @pero]

Whitworth and BA together – not unusual. My 1970's Series III Land Rover features almost every known thread, including BSF, BSW, BA, UNF/UNC and metric.

On the plus side I do now have a very well appointed mechanics workshop, including the range of hammers needed to hit anything which refuses to behave – not an uncommon occurrence.

Re Clive's comment on Whitworth, I acquired many years ago from a ( real ) auction, and never used in anger, a set of 1/16" BSW taps and dies. I also understand that 3/32" BSW were also made. They have not been seen in my workshop for many years but I remember they were made by a reputable Aus or UK manufacturer. It's a pity I have since gone metric and am unlikely to ever use them. Perhaps a museum item!

Pero

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 15/05/2023 23:44:08:

.

Not only ‘quite common’ but ‘BSI recommended’ practice

Sorry, I can’t recall the standard number or its issue date, but I know it was clearly stated.

MichaelG.

.

Found it

BS 93:1951

In the spirit of ‘fair use’ … Two brief paragraphs quoted:

.

.

MichaelG.

[DC31kParticipant @dc31k]

Posted by Clive Foster on 15/05/2023 22:22:39:

As you already have screws one way of estimating thread pitch is :-

…to purchase an inexpensive BA thread pitch gauge

https://www.ebay.co.uk/itm/352474288510

(from RDG but I could not find it on their website. Chronos also sell one. Maybe Proops as well).

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, in all my days in industry I have never come across any standard BSW/BSF threads or lists with any size that has a 64th increment. Below is the best list that I have.

As you can see, they go up to 9/32 in 1/32 increments, and from 5/16 to 15/16 in 1/16 increments and then from 1-1/8 to 1-7/8 in 1/8 increments, then the rest is in 1/4 increments.

Regards Nick.

[Jim GuthrieParticipant @jimguthrie82658]

I think I remember from my dim and distant past that there was a preference in electrical engineering to use even BA sizes and in mechanical engineering to use odd BA sizes- or it might have been the other way round.

Jim.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Jim, the electricians that I used to work alongside with, used mostly the even BA sizes from 0 to 8 up to the time when metric equivalents became popular.

Regards Nick.

[Clive FosterParticipant @clivefoster55965]

Posted by DC31k on 16/05/2023 07:20:49:

Posted by Clive Foster on 15/05/2023 22:22:39:

As you already have screws one way of estimating thread pitch is :-

…to purchase an inexpensive BA thread pitch gauge

https://www.ebay.co.uk/itm/352474288510

(from RDG but I could not find it on their website. Chronos also sell one. Maybe Proops as well).

Hmmn. I seem to recall Geo.H Thomas warning against relying on inexpensive thread gauges as "some are pretty ropey".

Probably more about the thread form than actual pitch count.

A good gauge will seat fully into and accurate thread essentially blocking light transmission, or at least leaving an even line of light should then thread be shallow, when studied under magnification. A not unimportant asset when trying to distinguish between 55° and 60° threads. Way back I had a couple of cheapies that were merely just about OK. Didn't realise how poor they was until I could afford to upgrade to a combination of M&W and Starrett ones.

BA is now an odd ball thread so the gauge market is declining so one wonders if makers find it worth producing quality gauges as quality costs more and small market drives the price up even further.

For what is likely to be one time job it doesn't seem worth dipping into the inevitably inadequate funds available for Home Workshop duties to buy a gauge when a simple process is adequate to do the "sheep or goats" level identification needed here.

Clive

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Jim Guthrie on 16/05/2023 08:51:12:

I think I remember from my dim and distant past that there was a preference in electrical engineering to use even BA sizes and in mechanical engineering to use odd BA sizes- or it might have been the other way round.

Jim.

5 BA and 7BA were favoured in model engineering because they happen to be be near enough 1/8" and 3/32" OD respectively. LBSC used them a lot in his designs.

Rod

[bernard towersParticipant @bernardtowers37738]

My mates fobco has the nameplate fitted with drive screws!

[Daniel BrannanParticipant @danielbrannan48897]

I’m glad i asked this on here now. I’ve ordered a pack of stainless 6ba screws – I will let you know if they fit when they arrive

Edited By Daniel Brannan on 16/05/2023 09:24:28

[JasonBModerator @jasonb]

#4-40 or #4-48 is also not a million miles away if they had access to UNC and UNF

[Howard LewisParticipant @howardlewis46836]

If the screws are BA, the pitch will be Metric 0BA is 1mm, and the pitch decreases by a factor of 0.9 for each smaller size, so 6 BA will be 0.53mm.

Being BA the thread form will be 47.5 degrees, rather than the 55 of Whitworth form threads (BSW, BSF, BSP, BSB) or 60 of Unified or Matric.

Howard

[Daniel BrannanParticipant @danielbrannan48897]

6BA were a good shout. Fit perfectly

thanks again

[Author]

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