|Greg H||05/12/2016 09:19:58|
46 forum posts
I need to make some boiler fittings for my loco and was wondering about the preferred thread type. The plans state 1/2" x 26 TPI, but I have noticed that some suppliers sell safety valves with a 1/2" x 32 TPI thread.
So before I make the fittings I was wondering what the pros and cons of these two threads was?
I have taps and dies in both 32 and 26 TPI, so could go either way.
|732 forum posts|
Both will take the pressure if that's your concern assuming done to size.
Have tested the 26TPI cycle thread/Brass threads to 10000psi near on metric 1mm pitch.
General rule if tapping, finer the pitch the easier to turn due to less metal removal.
Finer pitch can leave more material thickness over the thread so safer and usually over looked.
|Speedy Builder5||05/12/2016 15:44:53|
|1413 forum posts|
Sometimes the finer pitch will allow you to position the fitting more accurately i.e.: backhead sight glass fittings etc
|julian atkins||06/12/2016 00:09:25|
1179 forum posts
I use 1/2" x 26 tpi ME thread for boiler bushes and fittings of this size. I go down to 32 tpi ME thread for 3/8" and below till I get to 1/4" when I use 40 tpi and for smaller sizes.
I have 32 and 40 tpi dies and taps for 1/2", but are not used on boiler work.
|Neil Wyatt||06/12/2016 07:23:29|
13062 forum posts
As a fully paid-up lover of imperial sizes it pains me to suggest this, but perhaps in the future 1.0mm (25.4 tpi) and 0.75mm (33.8 tpi) will become the preferred sizes? 0.6mm pitch is ~42tpi but only easily available get as M3.5.and 0.8mm (31.75 tpi) is just M5.
0.75 has the advantage of being available for M4.5, M6,M7,M8, M10 and M12 giving a relatively fine constant pitch series in the most useful sizes.
The problem will obviously be people trying to screw metric fittings into imperial holes and vice-versa.
Just a thought....
|Greg H||06/12/2016 09:55:59|
46 forum posts
So 26 tpi is a little stronger as expected but not really needed. A finer thread as mentioned could make alignment easier on my turret, but then I would probably tighten the turret base in the bush, then work out the alignment before soldering the top on.
I might just stick with the 26 tpi.
I am a metric man, but just as good working in imperial. I dismissed your thought very quickly because my loco is a scale model of British locomotive, and a build of a 1958 Martin Evans design (264T) so I couldn't possibly have any metric threads on it, but my mix of BA, ME, BSW and UNF threads sits just fine with me
|Nigel Bennett||06/12/2016 10:03:04|
|231 forum posts|
If you're in the habit of removing the safety valve at every steam-up in order to fill the boiler, go for 26TPI.
|Greg H||06/12/2016 10:18:35|
46 forum posts
Good advice. Due to one of our more experienced members telling me in my earlier days that it was not a good practice I don't do it and so I pump away on my hand pump each time for some time. A a bit of effort, but at least I know my hand pump is working.
777 forum posts
When having fun as a stress engineer at work we had one customer who needed proof that the thread would not strip on every nut & bolt as well as proof the bolt would not snap under load. So, hundreds of times over I had to go through the same calculations with different loads. It hammered into my brain that:
That was fun. Summarised 3 years of calculations in 4 points. 4 points the customer would not accept as generally applicable cases.
1 forum posts
Jon you mentioned "26TPI cycle thread/Brass threads" do you know which is used as they are differnt thead forms
|duncan webster||25/09/2017 23:01:53|
1408 forum posts
BA is about the most metric thread you can get! Intensely logical in that the threads are in a geometric progression (I think that's the right term). The pitch in millimeters is 0.9^k where k is the BA number, and the OD is 6*pitch^1.2, again in mm. This is why they are so ridiculously difficult to screwcut. I very much doubt you'll find a BA thread on a full size Stanier 2-6-4
|Brian G||26/09/2017 00:08:38|
|218 forum posts|
Is there a reliable way of telling which is used for an internal thread? I have bought individual BSB taps and dies where needed, but on second hand models there is always a fear that they will have used cycle thread - after all, there are plenty of sets of "BSB 26 TPI Cycle Thread" tap and die sets on eBay.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
Love Model Engineering? Sign up to our emails for the latest news and special offers!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.