Always confused over threads and tapping

I have been told that I need to tap a hole 1/4" 20.

Am I correct in thinking this is BSW?
Is there another 1/4" 20?

UNC is also 1/4" x 20tpi.

What is the item or where do the drawings originate from as that will give a clue to the thread form

In the UK my guess would be BSW, in North America my guess would be UNC, I use M6. As far as I know the UK mostly used Whitworth (55 deg.) threadform up to the 1970s, US made items that were shipped to the UK during WW 2 was 60 deg., more here.

Thor

It's north American.
I have to drill and tap two thread sizes, 1/4" x 20 and #10-24. I'm pretty sure the latter is UNC so I guess the former probably is too. I need to find some used taps to fix the the item to a 3/4"x1/4" bar I'm trying to source.

I recently bought a BSW 1/4" tap and was hoping to save some money.

Since Brexit it has become very difficult and expensive to buy anything from the UK.

.

Whoever told you should have made it clear

If Whitworth thread-form is required, then it should be specified as 1/4” BSW

If [as seems likely] Unified thread-form is required, then there should be a ‘Tolerance Class’ appended

MichaelG.

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Edit: __ From reading your latest post , UNC does seem most likely, and in the absence of proper specification, probably class 2 … 

Edited By Michael Gilligan on 20/02/2022 08:37:29

They are written instructions, an email, from a US company. I'm sure they assumed I would understand.

They wrote

The thread sizes used are 1/4”-20 at the knobs and #10-24 at the pivot bolts, so the holes should be sized appropriately to accept those thread taps.  There is some leeway, but the ideal sizes are a #25 drill bit (0.1495&rdquo and a #7 drill bit (0.201&rdquo.  Bits sized to the nearest 1/64" should also work fine

Edited By pgrbff on 20/02/2022 08:47:17

For 1/4 x 20 UNC the tapping size is 5.20 mm

For 10 - 24 ANC the tapping drill is 3.90 mm.

Howard.

If you can find a local American car or motorcycle owner, or club, or an H-D dealership, you'll probably find someone who has access to, or knows where to get, UNC taps..

Likewise a British car / motorcycle club might do the same for BSW, BSF etc..

I'd have thought the automotive world might be your best allies for finding the kinds of tooling you need.

1/4" x 20 is UNC or maybe ANC, its American ancestor.

Although 1/4" Whitworth is also 20 tpi the designation is 1/4" BSW.

British practice has always been to name the common range of standard "tap'n die for fixing things together" threads so you get a diameter and a letter code that is a contraction of the name. In the UK if it a UK native specification thread has a x TPI suffix applied it is in greater or lesser degree specialist. Cycle is probably the only one that has an x TPI and can legitimately be considered common enough to be non-specialist.

Correct common UK practice would be 1/4" UNC for 1/4" x 20. This should also apply to the smaller number series, preferably with a No prefix. No 6 UNC for example. The potential for confusion with BA is obvious. American # prefix is probably clearer.

However most references tend to follow the Zeus book practice by tabulating the numbers as either ANC for coarse and ANF for fine. Which is what they are. The UN series imported the American specifications, as tided up for wartime use, wholesale for the smaller sizes.

American style number - TPI designation is common in the UK too and probably as correct as anything. As far as I know BSI is silent on this but there is probably a military standard that ought to be definitive.

American practice tends towards diameter x TPI or number - TPI, often with # in front, although applying ANC, UNC or one of the other less common specification identifiers isn't uncommon. Usually only done when it matters as the differences are of littile import for standard fastenings. Tables of sizes are generally headed NC, NF, ANC, ANF, etc as appropriate.

Tolerance classes are rare in normal bolt things together work whatever the thread. In practice equivalent to modern h6 / H6 class is assumed. Probably class 2 as Michael says.

Particularly when it comes to wartime production some of the tap and die designations can be - ahem - odd. If I've not broken it I have a BA tap lurking somewhere with the diameter in decimal inches and a pitch in mm! Also had / have some Whitworth and BSF sizes with the exact diameter in decimal inches. A few thou off nominal size. All American made from quality firms. I suspect that in the days before tolerance classes exact size designations were the American way to get tight or loose fits as needed. presumably for plated items.

Clive

Edited By Clive Foster on 20/02/2022 09:22:35

.

That’s all quite reasonable, as far as it goes

But my comment about ‘Tolerance Class’ still stands

MichaelG.

Wikipedia explains it quite well: **LINK**

https://en.wikipedia.org/wiki/Unified_Thread_Standard#Tolerance_classes

If the bolt and hole are both new, there's no particular reason to follow the instructions exactly. They're probably intended to make life easy in North America and being confusing outside the USA is an accidental side effect.

As Metric fasteners are available almost everywhere, it may be appropriate to substitute the nearest metric size because taps, dies, and fasteners are all easier to source in metric. 1/4" is close to M6 and #10 is close to M5.

The exception is when a bolt has to fit into an existing thread, or a new hole has to be threaded to fit an existing bolt. Happens a lot when repairing machines. In that case the original thread specifications should be followed.

But when the need is just to join two lumps of metal together, you can use whatever thread system is convenient locally. 1/4" BSW isn't ideal because it's an obsolete British thread. Could be substituted for UNC 1/4 - 20, but then you have to find a BSW bolt to fit the thread. Though the internet is helpful, it may not be easy to buy obsolete British fasteners.

Dave

I don't kow why anyone would even mention ANC since it's just an early designation for what has been called UNC for many years and is just going to confuse people..

Given that the hole size is actually in the email the best policy would be to drill that size hole. 3.8mm and 5.1mm would be perfectly adequate.

I may be legitimately accused of ‘reading between the lines’ Dave … but I think the holes need to be tapped to suit already ‘bought-in’ items.

[quote] The thread sizes used are 1/4”-20 at the knobs … [/quote]

MichaelG.

Hi there, pgrbff,

On this forum, ALWAYS type a space before typing a right-hand bracket - that prevents the smiley gremlin from waking up!!

Also, always press right arrow before typing a follow-up to a quote - that avoids the quote marker line from also extending over your part of the post.

Best regards,

Swarf, Mostly!

You can use your 1/4 BSW tap and a 1/4 UNC bolt or other thread will screw into the resulting thread just fine. They are the same TPI, ie 20. The slight difference in thread form angle is negligible in most real world applications and the two are commonly screwed together without problems the world over.

For all practical purposes in the home workshop, BSW and UNC threads are interchangeable on those sizes that use the same diameter and TPI, which is most of the ones commonly used such as 1/4, 5/16, 3/8 etc, but annoying not all. Check online thread charts for the crossover sizes.

Edited By Hopper on 20/02/2022 10:37:19

Unfortunately, I haven't got the item yet that I need to attach to the bar, so I'm not sure if the through-holes are threaded or not. I doubt it, but the screws will have to be a tight fit as I need to eliminate any possible movement. Maybe I need to buy the item first and then decide if metric will do.

I can buy UNC taps from Germany but they are £10 each and postage on top. The problem is that living in Italy I will probably never need them again. I bought an Italian bandsaw that turned out to have all BSW fixings so I had to buy several taps and dies to get it back into shape as many of the threads were damaged.

It is nice to have all of these tools just in case but it does get very expensive when I have to order from the UK.

Thank you all for your help.

Whilst on the subject of threads what is the difference between BSB and BSC both are 26tpi ? does one have a different thread angle ? Noel.

[This posting has been removed]

Yes it might be wise to use 6mm if you can make both halves of the job and fit them together.

I go through very similar ructions here in Australia. BSF in particular is getting very hard to source locally and shipping costs from UK have gone through the roof, plus the two week wait for it to arrive.... So I have resorted in some instances to making up new accessories for my Myford but using UNF threads, but making my own hex nuts with UNF threads but BS sized hexagons so I only have to use the one set of spanners to work on the Myford. Some poor future owner will have fun trying to work that one out!

But once again, the one will screw into the other in most practical instances. There is sufficient tolerance in a standard thread to allow for the slight difference in flank angle. And most mass produced fasteners will be all over the place so are deliberately made slack so no two will ever refuse to screw together.