UNC dilemma

Hi - I'm after some advice regarding UNC threads....

A couple of years ago I ventured into model engineering and ended up acquiring a decent selection of metric taps and dies. I subseqently went on to machine Stuart S50, 10V and half beam engines which resulted in me acquiring a decent collection of BA taps and dies.

I have recently decided to make a Jerry Howell Farm Boy hit and miss engine and see that the majority of threads are UNC. Rather than buy a whole load of UNC taps and dies, my initial thoughts were covert to BA. I then came across a old thread (Nov 2011) on the ME forum which discussed this type of dilemma and I see JasonB made a very useful chart showing UNC with BA and metric equivalents. So, I was convinced that converting to metric would be a good move as I would only need to purchase taps for M2.5 and M3.5 (I already have M3, M4, M5, M6). However I have just noticed that the availability of M3.5 screws (in particular countersunk) appears to be non-existent. I'm also a little concerned that the look of metric on an old engine might not be great either.

So, I'm back to thinking that I should just bite the bullet and purchase a load of UNC taps and dies. Maybe one option would be to purchase second-hand UNC in order to keep the cost down?

I guess this must be quite a common issue and I would really appreciate any views/advice on this.

Cheers

Andrew

You might have a look at EKP supplies for fasteners, they might have what you want.

**LINK**

http://www.screw-shop.com/index.php?route=common/home

Edited By old mart on 18/04/2020 15:46:20

The countersunk screws that hold mains electrical sockets and switches together are M3.5. I don't advise removing them from electrical fittings that are in use though.....

Try AccuScrew (Accu Group), they list over 1000 items at M3.5. I find them reasonably quick but probably not the cheapest but they sell in whatever quantity you need.

John

I can't think that there are many M3.5 CSK screws on the Farmboy so easier to just go up or down to M4 or M3, if they are hidden ones that hold the engine "castings" together then counterboring for cap heads would also be an option.

Commercial mass produced metric hex will not look ideal on a period engine as the hex is usually a bit large an A/F and too flat. You can get Smaller hex metric fixings that are fully machined so also nice and crisp unlike forged cheapies, Polly Engeneering or GHW do them upto about M4, over that I find imperial hex makes good proportioned heads if you work to 1/16" to 1mm of thread dia eg 5/16" A/F for M5, 3/8" A/F for M6 etc.

Common as muck in electrical sizes, otherwise M3.5 is unusual rather than unobtainium. I had no trouble finding M3.5 from ebay like this example, and others too. Got Loads in a cheap Lidl boxed set. Is it a particular head that's difficult? - they tend to be posidrive pan-head or counter-sunk rather than hex, Allen or straight.

A model maker is needed to comment on how they look, I'm not sure any of the small standard screw fittings scale perfectly, or if some are uglier than others? The small UNF screws found in computers don't seem better looking than Metric or BA to me, but I'm an oik!

Dave

Commercial small UNC can look worse than metric but then again you can buy "model fixings" with historical looking heads such as these

If you do need a few CSK screws then easy enough to make in M3.5 with a slotted head, same goes for Square headed fixings which were common on a lot of old hit and miss engines, I've made my fair share of those too, typical requirements for one engine, all metric though UNC on the drawings

Edited By JasonB on 18/04/2020 16:46:58

If you already have metric taps and dies, just use the nearest equivalent size. I wouldnt bother with 3.5. As Jason says, go to either 3 or 4, depending on what fits and what the loading requirements are if in critical applications, which most are not.

Andrew,

Buying second hand taps and dies is a gamble as you don't know if they are sharp and will cut as required. Unless you have a tool and cutter grinder to re-sharpen.

Paul.

I have long suspected that 3.5 mm was chosen for light electrical fitting duties, such as holding switchplate panels and the like on, because it is "bodge interchangeable" with 4 BA. At commercial mass production standards the two will screw together, especially if you put a bit of welly on the screwdriver, and hold in a manner adequate to the intended function.

Even when the proper screws are used fit on the common "threads in a thin bent over metal tab" system found on wall boxes and the like is tends to be on the poor side of crappy anyway. Most of the brass insert in plastic types tend to be distressingly slack at the best of times.

Not always true though. I have some very high quality and probably eyewateringly expensive when new 4 13 amp socket steel cased distribution boards which have proper, close tolerance, 4 BA inserts for every mounting thread. As the sockets can be mounted either way round thats about 20 per box. Everything else was to similar standards. I rescued about 20 after management condemned them as being too old and "not meeting PAT testing regulations". The former I could believe as they were certainly past their 20 th birthday the latter no way. Was not impressed by the bulk ordered "high quality" plastic lumps supplied as replacements. Especially after half of them had failed within two years withs witch or contact issues.

Clive

Many thanks for all the advice and suggestions. Lots of useful information and links.

After considering all the options I've decided to stick to the drawings and go with unc. I think it should make my life easier. In the past I've purchased hss taps and dies from Chronos and they seem reasonable quality at a decent price. So, I've just purchased a variety of unc from there. I also think AccuScrew (thanks John) and modelfixings will be able to provide all the fixings I need.

Thanks again and stay safe all. 👍

Andrew

And a no6 3/4" woodscrew was fitted in several places in this house when we bought it!

Compare prices with Tracy Tools. They specialise in Taps and Dies of all sorts, BSW, BSF, UNF, UNC, BA Metric fine and coarse, so you should be ablt o find waht you want. SErvice, under normal circumstances, is pretty good.

Howard.

When I saw this, I was just about to post to the effect that to me it would be a no-brainer. A sufficient subset of the correct taps/dies is not really that expensive in the scheme of things and to me would totally outweigh the hassle involved in switching threads.

Different strokes for different folks of course. (it also helps if you are on this side of the pond where UNC/F is ubiquitous ).

Ha ha - nice one Nick.

Thanks for the suggestion Howard. I did check out Tracey Tools and their prices were similar to Chronos. Having had good experiences with Chronos it was hard to change, but I must give Tracey a try sometime.

Eh up Bandersnarch, I'm really glad you think I made the right choice. At least it should simplify the build and I do like to machine stuff to spec.

Cheers!