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15 November 2011 at 18:29 #32446
Colin Bishop
Moderator
@colinbishop34627
Just a few comments which some people may find of interest..

Many of us have stuff in our bits box such as old prop shafts, propellers and couplings etc. which have the old BA thread system. Most modern items you buy are now metric which can be a bit of a headache if you want to use your old, perfectly serviceable running gear.

I am currently building a kit for review which comes with a prop shaft fitted with an M4 thread,. now common practice. Some years ago I was told that M4 and 4BA were effectively interchangeable for practical purposes so I thought I would be able to use one of my old 4BA couplings or a pulley threaded 4BA depending on which option I wanted to use for my drive train. However it seems that this is not quite correct.

While you can screw a M4 propeller on to a 4BA shaft it seems that you cannot do it the other way round! The M4 thread is slightly bigger than 4BA.

In the end I bought a new M4 coupling and also bought an M4 tap which I used to put a thread into a plastic pulley. I still haven’t decided which option to choose. The traditional double coupling should be the most elegant engineering solution but it does seem to take a lot of work to get it set up accurately and is still quite noisy. On the other hand the pulley drive is much less critical to set up, quieter, offers the bonus of adjustable gearing to help match the motor speed to the slower propeller and makes it easier to withdraw the shaft into the boat for servicing without having to dismantle the rudder or unmount the motor.

On balance I think the pulley drive has it, as is my usual practice but we will see.

Any other comments on matching BA to metric would be welcome.

Colin
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15 November 2011 at 18:29 #3780
Colin Bishop
Moderator
@colinbishop34627
Compatibility between Metric and BA
15 November 2011 at 19:12 #32448
Bob Abell 2
Participant
@bobabell2
Hello Colin

The thought of mixing BA and metric threads is not music to my ears!

It’s ok as an emergency measure, but not as a design feature

The different threads are a real nuisance to builders with lots of surplus gear.
On completed models I always make a note in the model file, which thread has been installed

I like the idea of the pulley system, for scale models especially, as it offers a few options

You mention tapping out a pulley? Not really a good idea, good chance of running out of true.
Better a plain shaft with a retaining nut.

Bob the engineer…….lol
15 November 2011 at 19:39 #32449
Colin Bishop
Moderator
@colinbishop34627
Bob,

You are quite right, it’s easy to run out of true – but I’ve got a lot of pulleys….!

The particular issue I have here is that the thread on the shaft isn’t long enough for a pulley and a couple of lock nuts on the inboard end. So I had to be a bit devious.

A long time ago I acquired a full BA set of taps and dies which have been quite useful over the years. I have now just bought a cheap set of metric taps & dies starting at M3.

As an engineer Bob, you’d best look away now…

So,,,what I did was to take a plastic pulley and carefully apply an 8BA tap to it which ran true in the existing hole. Then I worked up through 7,6 & 5 BA taps, each of which ran pretty true to the one before. The M4 tap wouldn’t go into the resulting hole so I had to first use a 4BA tap on it. I am sure that the interior of the hole is now a miniature engineering nightmare but it does give a good interference fit on the shaft against which I can secure a M4 locknut. The pulley doesn’t quite run true but it is only a very small fraction out and is quite serviceable.

The other thing is that normally I like to fit double pulleys and the best way to do that is to bore an oversize hole in the second pulley and bolt the two together using 12 or 10BA nuts and bolts. Then it doesn’t matter if the second pulley is proud of the end of the shaft.

No, I’m not an engineer, but I do possess a degree of low, practical cunning!

Colin

Edited By Colin Bishop, Website Editor on 15/11/2011 19:41:27
15 November 2011 at 20:09 #32451
Bob Abell 2
Participant
@bobabell2
Well, Colin…..That sounds horrendous!

How scratch builders manage without a small lathe, I can’t imagine!
Not to mention a vertical drill, bandsaw and a linisher!

I know they cost money, but tools of the trade do make for satisfying solutions to mini engineering problems!……and they are not that expensive these days?

Just wondering now, how much flak is coming my way!

Where’s me tin hat!

Bob the Engineer (ret)
15 November 2011 at 21:01 #32452
Colin Bishop
Moderator
@colinbishop34627
The end justifies the means Bob, the end justifies the means! We think of ourselves as unacknowledged artists.

Yes, you can certainly go out and buy the items you mention – but then you need the skills to use them. And anyway, the ones you mention wouldn’t fit in my tiny little ‘workshop’ and my wife would not take kindly to sharing the dining table with a lathe however small its dimensions.

Us artists must do the best we can but we shall not be daunted.

Colin
16 November 2011 at 08:08 #32456
ashley needham
Participant
@ashleyneedham69188
Ah, a thread to me own heart !!! (pun)

I have tapped out to M4 several old props with no bother, and if they are slightly out of true I think it makes not much difference unless you are running at very high revs.

in any event, double couplings and pullies are what was invented for these “situations”. I have bought several brass inserts for couplings over the years which were slightly out of true.

I would love a small lathe but just know that without a degree of expertise the machine would not fulfill its function properly, especially as we are talking of very small items here. I have a large pillar drill which is usefull for drilling holes in BSA motorbike bits but less usefull for drilling out toddy plastic pully centres to 3.7mm or whatever !! (it is the holding of these bits that normally defeats me)

Ashley
16 November 2011 at 09:23 #32457
Bob Abell 2
Participant
@bobabell2
Ashley

With your inventive genius…..You definitely deserve a lathe!

You will love it!

I once had to make a special nut with a crazy thread of about 1/1/4″ x 38 TPI for the top forks of a Tiger Cub scrambler…..Great fun at the time!

Such a great feeling of triumph, when it fitted!

Bob

Edited By Bob Abell on 16/11/2011 09:31:23
16 November 2011 at 12:58 #32460
Colin Bishop
Moderator
@colinbishop34627
Bob, Ashley,

My philosophy is summed up in an article I wrote some years ago which you may perhaps remember:

http://www.modelboats.co.uk/news/article.asp?a=471#

Colin
16 November 2011 at 18:06 #32463
Bob Abell 2
Participant
@bobabell2
That’s some story, Colin!

I am generally in agreement……..My Great Eastern was built in stand well back mode and I’ve always said, it resembles on oil painting.
Stand well back and it looks brill, but in close up mode, it is rather rough. In fact, Jeanie Deans at War was so rough, it’s gone in the dustbin!
I’m highly delighted with the plating and rivets, they impress the public at Etherow immensely……even though your story disapproves for some reason
The bodged up winch looks the biz too!
The skylights and cabins are only made from card but have never attracted any adverse criticisms

Now then, we come to the motor drive!
You seem to have shied away from engineering quality?
I say that the best engineering quality is essential with all working mechanisms!

I think you have to agree, really?………Bob
16 November 2011 at 19:27 #32465
Colin Bishop
Moderator
@colinbishop34627
Bob,

As regards plating I didn’t say I was against it, only that you don’t necessarily need it in detail when working to small scales. GE looks fine as it is a big model but I have often seen models at smaller scales where the builder was determined to show the plating and maybe rivets as well but has overdone it and lost realism thereby. Even on large ships, the plating is rarely more than one inch thick and frequently a lot less, so if you put plates on your model which are the equivalent of six inches thick with the rivets represented by mushroom sized protrusions then it isn’t going to look right.

I do agree with you that working mechanisms should ideally be well engineered and in most cases modellers get over that one by buying them! You have an engineering background and to do other than ‘proper’ practice is anethema to you. Those of us who have pursued different careers don’t have the training or experience and maybe not the time to invest in skills which they may only need to use very occasionally for hobby purposes and so we compromise!

The pulley I treated so dreadfully with a series of taps actually runs very sweetly and is probably a better fit than is the shaft in the prop tube which is a commercial item!

Colin
17 November 2011 at 07:09 #32467
Bob Abell 2
Participant
@bobabell2
Hello Colin

Resorting back to your original post……..M4 and 4BA etc

Just to tidy things up…

4mm = 0.157″

4BA = 0.142″

Diff =0.015″

15 Thou` is a lot of slop!……….No wonder it won`t go on the other way!

Verdict……..Not recco for prop shafts!

Bob
17 November 2011 at 07:59 #32468
ashley needham
Participant
@ashleyneedham69188
Bob, you know, only just a few posts ago SOMEONE (not you ) called me a “tightwad” for not purchasing brand new pinstripe tape…as I was re-using old but perfectly serviceable if, a tad too wide tape.

True colours will out, eh…Colin?

Ashley
17 November 2011 at 08:24 #32471
Colin Bishop
Moderator
@colinbishop34627
Ah, but Ashley, you were suggesting that maybe the adhesive had ‘gone off”.

Colin
17 November 2011 at 10:03 #32473
Bob Abell 2
Participant
@bobabell2
It`s bad enough getting a 4BA nut onto a 4milli screw, but I can`t work out what Ashley`s on about!

What`s he saying, Colin?

Bob
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