Tap/Die sets - BSW/BSF or UNC/UNF?

Good afternoon gents,

I'm starting in my model engineering journey. All things being equal, is there any meaningful distinction between the two standards mentioned above? I have to 'pick one' to start my collection, the part I'm working on has no other interfaces so I can pick either. I would prefer not to use metric components as I have everything else in imperial.

It is my understanding that I should probably go with UNC/UNF due to the wider availability, but wanted to check.

Cheers,

Unless you intend to make a lot of American designs I would not get any of those, if you are the UNC is the norm over there.

In the UK Older designs would mostly have BA and newer Metric coarse. Metric in Europe

Hi William, assuming you are in the UK, for model engineering proper, as opposed to messing around with old machinery, both standards that you mention are fairly useless.

For model making, the smaller iso metric thead standard would be much better. British Association, BA standard can also be useful. For fine threads, Model Emgineer , ME threads at 32 tpi and 40 tpi still have a place.

Rather than buy sets, buying as and when you need for you initial projects has some merit.

Perhaps different if you are not UK.

Stand by for disagreement.

Clive.

Hello William - might I enquire in which part of the world you reside?

If it's somewhere on the North American continent - then I suspect UNC/UNF will be a better choice for you than Whitworth...

IanT

I wouldn't buy a multi size set, I have over the past couple of years bought three taps and a matching die as I need them. It needs a bit of forward planning but I've found I can buy lightly used quality equipment at a fraction of the cost of new, and I don't have to take a chance on unknown brand quality.

Invariably if you but the one sort you will need the other if your projects are many and varied. I've majored on metric but recently have needed some UNC and UNF and BSFP.

I am in the UK, and yes typically model engineering going forward. I'm hopefully going to be coming in to ownership of a myford ML7 sans screw cutting gearbox so I figure it was best to go with imperial threads. Maybe then, the 32 and 40 TPI ME? I specifically need the equivalent of an M5 or 10-32

If you don't actually need M5 or 10-32 then use 2BA. It's within a gnats whatsit of 3/16" and 32tpi. Taps and dies are readily available from commercial suppliers. I'm not a fan of ME threads. They're useful when fine pitch threads are needed in larger sizes and I use them on the smaller fittings on my traction engines. But I screwcut most external ME threads and internal ones where possible as I've found the modern taps and dies available are not great quality or particularly accurate on diameters.

Andrew

So, my only thought was that the Myford doesn't have a screwcutting gearbox and a 8TPI lead screw, so I can only 'natively' cut 24/32/40 TPI threads so I figured it would make sense to stick with those kind of imperial threads - i..e I can screw-cut ME/BSF/UNF but can't cut BA or Metric threads as features on parts.

I think the majority of us would use a die for threads below 1/4" and not bother screwcutting unless there was a specific need.

A better idea may be to choose a couple of things that you want to make and then see what threads they use.

Edited By JasonB on 23/10/2020 15:04:08

Double check the requirement William! If fasteners are needed go Metric UNLESS in the USA or there's a need to make threads compatible with older gear.

BSW means big threads on old equipment. Restoration work on traction engines rather than small models. BSF extended BSW with finer and smaller threads, but they're still too big for most models. Useful for car restoration and older machinery.

BSW and BSF are fading away. Obsolete, not used on new work.

BA provided small threads for electrical and other instruments. Also fading, but more available, and popular for model work.

UNC/UNF replaced BSW/BSF after WW2 so nuts and bolts on British and US military equipment would be interchangeable in the event of another European War. Still clunky, and also fading from the British scene. Found on last century cars rather than new ones. Again, too big for most models.

All these thrread systems in the UK are being displaced by metric, which has the advantage of providing sizes from tiny to huge. Cheap because common as muck, while Imperial sizes are turning into commercial specials. If you can find them at all, perfectly ordinary BSW bolts these days cost a bomb because so few are sold. Using expensive fasteners just to bolt two bits of metal together is insane unless they have to be compatible with the larger assembly.

I decided my general-purpose workshop would be metric because it's cheaper and easier in the 21st century. I would have done different if:

• I was in the US, or
• My main interest was restoring old equipment, or
• I wanted to build models from Imperial Plans, or
• A kind person had given me a workshop full of imperial tooling.

Even so, in the UK it makes sense to use metric fasteners unless there's a strong reason not to. As most small threads are cut with taps and dies, it doesn't matter if the lathe is imperial.

For modelling, BA or better still metric. There are also Model Engineering threads. These are designed to be correct to scale, which is important on fine models where inappropriate fasteners ruin the visual effect.

Dave

The only problem with buying just the sizes you need now, is how to store them. Rattling about in a tin or box do not do them any favours.

The Tap & Die Company sell MDF boxes with spaces for taps and dies along with the appropriate tap wrenches and die stocks . I have sorted all of my M.E. taps and dies into one of these and can now lay my hands on the ones that I want and not have any damage caused to them.

Brian

Edited By Brian H on 23/10/2020 15:18:46

I don't think that the capabilities of your machines are a significant influence on your choice of fastening. For model making in the UK then BA is still the choice since most designs and Stuart specify these threads and you wont normally cut any threads smaller than 1/4" / 6mm. I find that for general use, repairing stuff, making fixtures and tooling, then metric coarse is my choice since these fastening are most readily and cheaply available from Halfords, B&Q, Amazon or Ebay. Any way, cutting metric threads on an imperial machine is pretty straightforward, the only downside is that you cannot release the clasp nut when returning the saddle. A set of metric taps and dies in 3, 4, 5, 6, 8 and 10mm plus an few specific BA sizes for a particular model will probably suffice for all you needs. I'm currently making a Farm Boy which is a US design but it is straightforward to substitute the threads to something more readily available in the UK.

Rod

When the lathe turns up have a look at the change gears it comes with. If there's a 127 or 63 toothed gear, it will cut metric threads. It's also possible to cut some metric threads, or their close approximations, with imperial gears. Ask again when you know what the lathe actually has.

And welcome to the forum!

Dave

If your interest is steam you will find most commercial steam fittings are in an ME thread.

Some BA sizes are available using a one size smaller head as SOD says to avoid the out of scale look

I favour getting quality taps and dies as they are needed rather than sets. Some sizes tend to sit in the box and never get used.

Pretty much with Dave on this one, which is hardly surprising as I suspect he's more experienced in many aspects than me.
The only reason I'm echoing his sentiments is to emphasise that the lack of gearbox isn't a limiting factor at all.
Having one just makes swapping between different imperial pitches faster.
In some ways it can work against you if you want to cut Metric, BA (which is bases on metric), or something weird.

Hopefully your new lathe will come with a full set of changewheels, which will allow you to cut pretty much anything you will need.
Strictly speaking a 127 tooth would be needed for a perfect thread match in metric, but other combinations are close enough for almost all our purposes. (I'll stand to be corrected, but I don't think Myford ever supplied a 127 as standard.)
Not advising which thread forms to pick though, as I'm not a modeller; more a tinkerer with older vehicles, and repairer of whatever I've just broken.
Hence I regularly use BSW, BSF, UNC, UNF, BA, Cycle, BSP, Metric (coarse, fine, superfine and weird), as well as using a couple of ME threads to re-work things built by other folk.

Bill

UNF/UNC did not really make a big inroad into British industry, the motor industry dipped its toe in the water and then went metric shortly after. Whitworth and BSF were rarely used below 1/4” as BA was largely used here. As you will not have a gearbox on the Myford then with a set of change wheels you will be able to cut almost any thread you might need including metric. Even without the famous 127 tooth wheel you can cut threads that are so close to the correct pitch you would need some high end metrology equipment to measure it. Apart from making some parts for my early 70s Triumph my UNF/C stuff has had little use. Many of the classic British models will call up BA and ME threads but the extended metric series can make some very close equivalents if you don’t mind deviating from drawing. Bought in metric fastners will look wrong in some situations but if you are making them from scratch then you can put whatever head you like on them. I suspect that like most people you will eventually have odds and bits from most thread standards.

Mike

A genuine question, Mike, if a little off-topic:

A large choice of head shape is clearly something any lathe can offer, but what about the "slot"? A simple line slot will surely look wrong in some situations, but what other realistic choices does the lathe owner making fasteners from scratch have? Hex, Torx, Pozi, Philips et al. are surely not practicable.

I have been known to use dies; just made some custom D-type connector lockscrews for a client and I used a die to cut the external 4-40 UNC threads as I already had it. Albeit using a quick release holder on the repetition lathe so I could thread up to a shoulder at 500rpm. I prefer to use Coventry dieheads for more than one offs. They cut really nice thread forms, are fast, don't wobble or wander and are simple to adjust to size to fit mating parts.

Andrew

I was rather thinking of getting the proportions of a hex head to be pleasing. A rotary broach or wobble broach could probably do hex socket and Torx but cross heads are forged I think so not a practical proposition for most people, the broaching is quite an ambitious method but it has been discussed on the forum and in the magazine. Of course the plain slotted screw is easy to make if they are appropriate for the job. Unless you have some sort of facility to do repetition work I think making more than a few of something will soon tax the will to live.

Mike