TPI Threads

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TPI Threads

This topic has 33 replies, 15 voices, and was last updated 2 November 2010 at 11:25 by Anonymous.

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28 October 2010 at 08:08 #57635
Wolfie
Participant
@wolfie
OK heres another question.

In two places on my plans there is a call for threads of 40 TPI and 32 TPI. Now I’m sure TPI= Threads Per Inch.

OK whats this all about then?? Why are this not BA like all the others. Is it a standard size, what taps and dies do I ask for. Is this a scam so I have to buy yet more tackle
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28 October 2010 at 08:08 #5256
Wolfie
Participant
@wolfie
28 October 2010 at 09:01 #57636
Anonymous
Hi Wolfie,

Oeeer, where to start. Whole books can, and have, been written on thread standards. Rather than give a discourse on threads in general here are a few pointers:

1) Yep, TPI equals threads per inch, mainly applies to imperial threads, metric threads use pitch

2) The 32 and 40TPI threads almost certainly refer to ME (Model Engineer) threads. These are of Whitworth form but finer TPI. As a corollary of this the thread depth is smaller. Consequently small fittings can be made with internal and external threads without looking ‘chunky’. The original Whitworth series is pretty coarse, hence the introduction of BSF threads (British Standard Fine) within industry

3) BA (British Association) threads start at 0BA (just under a 1/4″, actually 6mm and 1mm pitch) and work their way down in size, and up in number, based on a geometric progression. So despite the name BA threads are actually based on metric measurement.

4) When all is said and done, don’t be afraid to change the design to use a nearby thread size for which you already have the taps and dies

Regards,

Andrew
28 October 2010 at 09:35 #57638
ady
Participant
@ady
A 1mm thread is 25 tpi

0.75mm thread 37tpi

0.5mm 50 tpi

simples!
28 October 2010 at 10:11 #57641
GoCreate
Participant
@gocreate
Hi

I believe BA threads are actually of metric origin and not imperial, their size being determined using a geometric progression going down from 6mm.

I think the French are to blame

????

Nigel
28 October 2010 at 11:10 #57646
wheeltapper
Participant
@wheeltapper
Hi

it really depends on the diameter of the material to be threaded.

for instance an M8 bolt is a standard size but you can put the same thread pitch on a piece of steel 3″ in diameter.

I don’t know what your’e building so I’m not sure what the thread applies to.

Roy
28 October 2010 at 12:00 #57648
Wolfie
Participant
@wolfie
OK thanks all.

Right then, so if 0.75mm = 37 TPI, then I can switch 1/4″ x 40 TPI for M6 x 0.70 and be close enough?

Its for the steam gland thing that goes into the front cylinder cover and carries the piston rod.

Edited By Wolfie on 28/10/2010 12:02:14

Edited By Wolfie on 28/10/2010 12:02:32
28 October 2010 at 12:37 #57652
Anonymous
Hi Wolfie,

Errr, no you can’t swap to M6 by 0.7mm, because that’s not a standard thread. Well, you could get a special tap and die made, but it’d cost an arm and a leg. Ordinary metric threads come in two forms, coarse and fine. If a drawing calls only for the size, then it’s understood that it refers to the coarse series. If the fine series is wanted then the pitch needs to be specified as well.

As an example, if a drawing calls for M6 it is understood that this is the coarse series, ie, M6 by 1mm. If the drawing says M6x0.75, then that’s the fine series.

So you could use M6x0.75 as a substitute.

Complicated isn’t it!

Regards,

Andrew
28 October 2010 at 13:32 #57658
Wolfie
Participant
@wolfie
Yes sorry I was referring to the standard M6 size, but I’m not at home and can’t look at my metric tap and die set haha.

So M6 x .075 will suffice to stop me buying 40 TPI x 1/4″ ?
28 October 2010 at 13:55 #57663
KWIL
Participant
@kwil
Yes if you are making the matching part, but if you need to buy something from the trade, probably not.
29 October 2010 at 21:15 #57791
Sub Mandrel
Participant
@submandrel
Hi Nigel

BA was based on Swiss practice, but the British Standard defined the sizes in inches!

Neil
29 October 2010 at 23:25 #57797
KWIL
Participant
@kwil
Same applies to plumbing fittings the threads are imperial, but the continent defines them in mm.
30 October 2010 at 08:17 #57800
Bogstandard
Participant
@bogstandard
You shouldn’t really play about with mixing up metric and imperial threads on the same job, as metric is defined by pitch (the amount the screw moves forwards in one full revolution) and the imperial system of threads per inch (TPI).

But when attempting to find a somewhere near compatible metric thread for replacing an imperial one, it is fairly easy.

Divide 25.4 by the pitch of the metric thread.

So for a 6mm course (1mm pitch) = 25.4÷1= 25.4TPI

and

6mm fine (0.75mm pitch) = 25.4 ÷0.75=33.8TPI

So looking at the results, the 6mm fine pitch would be a fairly good replacement for

1/4″ x 32TPI.

Normally, the main reason for all the different pitches and TPI’s is actually to do with the strengths of the parent metals.

You would use a coarser pitch in softer materials as there would then be less risk of stripping the threads.

You also have to be aware that most imperial threads have a 55º thread angle whereas metric and the US system use a 60º angle, then you come to the famous BA system which for some reason uses 47.5º.

I hope this has helped to clear up a few points, and I am sure there will be some who can make it as complicated as you would ever want.

Bogs
30 October 2010 at 09:20 #57803
Terryd
Participant
@terryd72465
Hi Wolfie,

it should also be pointed out that unlike other thread systems ME (Model Engineer) threads use the same tpi on all diameters of thread. So a 1/8″ 40 tpi thread of a given length will have the same number of threads as a 1/2″ 40 tpi thread of the same length. The same applies to 35tpi of course. This has the advantage that a necessarily short stub thread such as a gland or a screw cap will have the maximum number of threads and often look more in scale.

Also if linear movement is required using a thread (as in a lathe leadscrew) the same can be gained using a 1/8″ thread as with a 1/2″ diameter thread e.g. a 40 tpi thread gives a convenient 25 thou linear movement per rev and depending on the robustness demanded by the application you can choose your diameter accordingly.

However in most other thread systems there is a relationship between the tpi and it’s diameter. hence the larger the thread, the coarser it is .Although metric fine is a bit different as one pitch covers a range of diameters (e.g. 3mm to 6mm metric fine uses a pitch of 0.5 mm and comes in 0.5 mm increments in diameter etc) and is closer to the ME series. So you can get away for most purposes with a set of Metric coarse and fine threads, most other high quality engineering countries do pretty well with these for most purposes.

As I personally feel closer to my Continental cousins rather than my New World ones my work is mostly Metric if I can possibly avoid imperial although I can work easily in both (often at the same time!) , I use Metric fine and coarse for most purposes, but I do supplement these with a set of ME for special applications. I have dumped Whit, BSF, BA, ANC and ANF thread systems and all the other special purpose systems which go on ad nauseum and find that these 4 systems provide all I have needed for years.

In answer to your other question it is not a deliberate attempt to simply empty your pockets just that these systems have developed according to needs over many long years and are just now being standardised as the ‘preferred’ systems.

Terry
30 October 2010 at 10:21 #57804
Gordon W
Participant
@gordonw
Don’t forget that whatever thread system you use ,you might need to buy compatible parts, in model eng. these could be in ME, BA, metric or anything in between. I use mostly metric coarse, because that’s all I can easily buy locally.
30 October 2010 at 12:18 #57814
Anonymous
But what about the metric constant pitch series; isn’t that just the same concept as the ME threads?

Regards,

Andrew
30 October 2010 at 15:14 #57827
Terryd
Participant
@terryd72465
As far as I understood Andrew, constant pitch metric threads are only available in larger sizes Could you point me to some references so that I could do some more research, my efforts with Google etc have drawn a blank on this subject.

Terry
30 October 2010 at 16:33 #57833
chris stephens
Participant
@chrisstephens63393
Hi Guys (Terryd),

Some of us are asked to make replacement parts for old equipment be it car, motorcycle or washing machine. Having sets of “old” threading equipment is quite useful.

If making new stuff the first thing I reach for is a Metric tap and die, for convenience not for some moral crusade.

At present I am making some bolts for a friend’s pre-war Humber, the heads are Whit size but the thread is metric fine, and recently I made some bolts for a Lancia that had metric spanner sizes but Whit threads, how do they fit in the equation I wonder?

chriStephens
30 October 2010 at 17:16 #57839
Anonymous
Hi Terry,

Yes, you are correct, Machinery’s Handbook lists constant pitch metric threads from 8mm to 300mm.

I have used them once, to tap a M10 x1mm hole in the brake system for my glider for a replacement part. The original M8x1.25 bolt kept breaking.

Regards,

Andrew
30 October 2010 at 22:59 #57846
John Olsen
Participant
@johnolsen79199
BS3643 as quoted in Tubal Cains “Model Engineers Handbook” lists three metric constant pitch series in sizes that would be useful to us:

4 mm to 5.5 mm with 0.5 mm pitch

4.5mm to 12 mm with 0.75mm pitch

6mm to 18mm with 1mm pitch.

I have no idea about general availability of taps for these ranges, although I have a few of them.

regards

John
30 October 2010 at 23:23 #57850
Wolfie
Participant
@wolfie
I have used them once, to tap a M10 x1mm hole in the brake system for my glider for a replacement part.

Gliders have brakes??
31 October 2010 at 04:21 #57858
John Olsen
Participant
@johnolsen79199
They usually have air brakes, since it is embarrassing to just clear the fence on approach and then not be able to get onto the ground before you hit the fence at the other end. Maybe I exagerate a little, but with the very flat glide angle they acheive, plus the influence of ground effect, you need some means of steepening up the glide angle. They often have a single wheel and a skid, so I am not sure if they go in for wheel brakes as well.

regards

John
31 October 2010 at 04:46 #57859
GoCreate
Participant
@gocreate
This may be stupid but isn’t an air brake not actually a brake in the real sense of the word when related to aircraft, is it not a spoiler to ‘spoil’ the airflow over the wing to reduce lift and put more weight on the ground more quickly so the wheel brakes are more effective more quickly?

Is it just a case of terminology?

Or is it the case that by reducing lift the plane slows thus effectively brakeing?

Nigel

Edited By tractionengine42 on 31/10/2010 05:16:06
31 October 2010 at 07:00 #57860
Terryd
Participant
@terryd72465
Hi Chris,

Of course i am in full agreement with you, sometimes when making parts for existing equipment it is necessary to use the appropriate threads whatever they may be. I had a good range of threading tools including Imperial, metric and US, unfortunately I suffered a serious fire which destroyed my workshop garage, including all of my tools and machinery.

When I replace my equipment I will only buy Metric (coarse and fine), but purchase imperial etc piecemeal if and when the need arises.

Hi John,

Those constant pitch ranges you describe are very similar to metric fine which have a constant pitch over a given range as described in a earlier post of mine. The important thing about ME treads is that the constant pitch covers a much wider range i.e. 1/8″ to 1/2″ which makes it very useful in certain applications.

Terry
31 October 2010 at 08:05 #57865
Terryd
Participant
@terryd72465
Posted by Bogstandard on 30/10/2010 08:17:38:

You shouldn’t really play about with mixing up metric and imperial threads on the same job, as metric is defined by pitch (the amount the screw moves forwards in one full revolution) and the imperial system of threads per inch (TPI).

Hi Bogs,

Can I start by thanking you for the illuminating and interesting posts, especially on the other forum (HMEM). I quite agree with you about mixing threads, it causes all sorts of problems. For example, I don’t know if you have ever tried to work with modern PC computer equipment, building or upgrading them, but some components were developed in the USA and some in the Far East. Hence there are US based thread systems on some components and metric on others. It can be very confusing as the small screws, around 3mm dia, are almost identical looking.

Terry
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