RequiredOutside Diameter to Cut 5/16 BSF Thread

[James AlfordParticipant @jamesalford67616]

I am making a stepped stud for the water manifold on an Austin Seven and wish to cut a 5/16 BSF thread on one end. I can find charts showing the size of hole to drill to tap a thread, but nothing for the diameter for studs and bolts. Would it be the same as the tapping diameter or something else?

Regards,

James.

[James AlfordParticipant @jamesalford67616]

[blowlampParticipant @blowlamp]

0.3125" or 5/16"

Martin.

[JasonBModerator @jasonb]

5/16" or 0.3125"

[larry phelan 1Participant @larryphelan1]

Would it not be 5/16" ?

Just asking.

[James AlfordParticipant @jamesalford67616]

Thank you both.

So, showing my utter ignorance: is the outside diameter always the same as thread size eg a 7/16 BSF thread would need a 7/16" shaft?

Regards,

JAmes.

[Brian Sweeting 2Participant @briansweeting2]

Sometimes known as the major diameter IIRC.

Table here **LINK**

[JasonBModerator @jasonb]

In most cases it is but you also get threads that are specified by number eg 2BA, 10-32 UNC both of which are approx 3/16" diameter. Pipe threads are also different in that they tend to go by the bore, for example 1/4BSP has a OD of 0.518".

[James AlfordParticipant @jamesalford67616]

Thank you.

[Paul KempParticipant @paulkemp46892]

James,

All above answers correct. Some other considerations for you though depending on how you plan to cut the thread. If you measure commercially made fastenings depending on the tolerance and thread engagement you will find they have a major diameter slightly smaller than nominal.

if you are going to screw cut then starting with the full nominal is a good idea as your depth will be related to that. If you are going to use a split circular die then being a few thou under the nominal won't hurt as there will be a degree of extrusion in the die cutting process and it will be easier to cut.

Paul.

[old martParticipant @oldmart]

There is a site with some of the most common thread charts : http://www.motalia.com

[James AlfordParticipant @jamesalford67616]

Thank you for the information. I have turned the shaft down and tapered the end a little to ease the die. The die is now cutting nicely.

Regards,

James.

[HopperParticipant @hopper]

In the commercial machine shop world it is common practice to make the OD about five thou under the nominal size, so somewhere around .307" (in practice, about .305" will do).

This provides a bit of tip clearance on the threads but does not reduce thread strength measurably. You die will thread the job much easier. With screwcut threads, parts will fit together better without riding on the thread tips.

Larger threads are usually made ten thou or more undersize.

You only need 65 per cent thread depth engaging to have about 95 per cent strength of a full depth thread so there is no danger at all in going a few thou undersize..

[JasonBModerator @jasonb]

I can't see that making any commercial sense. If a jobbing shop got an order to make say 1000 studs it would just about double the cost if they first had to turn down each end rather than going straight at them with a die head or even a die.

Also beware of generalisations of" 5thou less". What if you were cutting 40tpi, by the time you have taken 5thou off that's almost 20% thread depth and then you start doing the same with the tapping drill you won't have much % engagement.

[Danni BurnsParticipant @danniburns84841]

Hi all

I have a related question.

Is there an industry standard (best practice type of thing) for outside diameters for split dies? (i.e. If I need to buy a 1/2″ BSW (or maybe an M12) Split Die – should I be able to determine what the outside diameter will be and therefore what size of die holder I will need).

Does anyone have links to charts, etc?

Thank you

[JasonBModerator @jasonb]

When you buy a die it usually says what the diameter of the die is, if it does not move on to a supplier that gives the info.

Imperial dies mostly come with imperial OD but metric can more asily be had with both metric or imperial OD

Depending on what you want you could buy an M3 die with 15/16″, 1″, 20mm or 25mm OD, possibly more!

[Roderick JenkinsParticipant @roderickjenkins93242]

I’m not sure I can see an advantage in split dies but I’m happy to be convinced of one.  Almost all my large collection of BSW, BSF, ME, BA and metric dies are split.  I do however have a collection of M2 to M12 dies from ARC which are not split and cut lovely threads on nominally sized stock.   What am I missing?

Rod

[Danni BurnsParticipant @danniburns84841]

Thanks Jason

So it’s a manufacturer preference.

I’ve just bought a 1/2″ BSW with 1″ (25.2mm actually) diameter, and it looks disproportionate. Maybe, because it’s (supposed to be) Tungsten or just manufacturer cost saving. I don’t mind as long as it cuts the thread well – for £6 I’d be a fool to moan. It’s half a day (turning and milling) wasted if it makes a pig’s ear, though.

My main point is – I know I won’t be using the holder/wrench that I was expecting to use, and it could be hard/uncomfortable work with a smaller handled wrench.

FYI – My quick search for wrenches (online shop) showed 25mm for a max. M9 die; with 38mm for M12 and bigger.

Thanks

[Danni BurnsParticipant @danniburns84841]

Hi Rod

I think split dies are better (more expensive as well). They give some control of tread cutting, using the middle cone screw to expand the cutting diameter.

But if your tool does that job you want, it seems you must have the best tool for it.

cheeers

 

[bernard towersParticipant @bernardtowers37738]

Danni, Use your larger holder with a split sleeve, these are quit common on larger sets.

[Danni BurnsParticipant @danniburns84841]

Thanks Bernard

I hope you are well.

I have seen mention in the description of die sets, but I have never seen one.

I can make one easy enough.

[Danni BurnsParticipant @danniburns84841]

Maybe I have led a sheltered life, but I have also never seen one of these inserts to make a die wrench into a tap wrench

[DiogenesParticipant @diogenes]

Tap Snappers

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Danni Burns, I bought one of those sets of taps and die’s, with the adapters for holding the taps in the die stock from Chronos, a few years ago, Mini tap and Die’s needless to say they were rather fiddley to use, and I didn’t think it held the taps that good, but I did buy a size 0 tap wrench from Tap wrench size 0 which holds better and controls the use well.

Regards Nick.

[Danni BurnsParticipant @danniburns84841]

I found that I had a 25mm die holder/wrench (actually, it’s double-sided with 20mm) that I had made as an apprentice. So, I just opened it up to 1″.

I’ve just done the job with that (manually, but the part held in the lathe chuck) and made a real mess of the part and my hands. It undercut approximately 30% of the length of the thread – this can only be due to swarf build-up, really, so entirely my fault.

BTW the thread is only 2″ long, and Mild Steel at that!

Anyway, it’s too much hard work without the leverage of a large die wrench and I’m not convinced that a sleeve wont just create a point of weakness i.e. that would mean holding the die with only 1 screw.

In conclusion, a 1″ dia Die for cutting a fresh 1/2″ thread is way too small. Lesson learned.

Nick, maybe those kits are OK for little taps/dies that don’t need any real torque. You could easily make your own tap wrenches using square bar and some cap-head allen screws. Maybe even rubber/nylon pipe on the handles. They’ll last your lifetime.

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