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Eric Cox06/03/2010 11:54:32
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557 forum posts
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As an engineer I am used to referring to threads as 1/4 BSF, 3/16 UNF , M4 etc.
Why do model engineers refer to threads as X TPI.  When following a drawing the last thing I want to do is keep referring to thread data charts to find which thread is being used.
Circlip06/03/2010 12:21:31
1723 forum posts
It's so the Colonials don't get confused.
 
  Regards  Ian

Edited By Circlip on 06/03/2010 12:22:06

Jeff Dayman06/03/2010 13:38:46
2356 forum posts
47 photos
well since the first "colonials" comment offered no real value I thought I should chime in.
 
For the standard engineering threads generally a pitch is related to the diameter and is usually rather coarse, for strength and ease of cutting, tool longevity etc. If these coarse threads were used in the scaled down walls of model parts, there would be very few engaged threads.
 
In ME designs often a special finer -than-standard thread is used, ie 40 threads per inch or 32 threads per inch, which give more engaged threads in thin walls. it also helps keep diameters small and closer to scale in things like piston rod glands, because the fine threads are also shallower than coarse ones.
 
Hope the info helps.
Geoff Sheppard06/03/2010 13:46:47
80 forum posts
1 photos
'Model Engineer' standard threads are not used in unique diameter/t.p.i. combinations, so it is possible to find different threads on the same diameter. For instance, I have taps and dies for 3/16in. x 40 tpi, 7/32in. x 40 tpi, 1/4in. x 40 tpi, 9/32in. x 40 tpi and 3/8in. x 40 tpi. The same applies to many other sizes in the 26, 32, 40 and 60 tpi series, so it is necessary to specify diameter and thread pitch for a given application.
 
Regards, Geoff
Rob Manley06/03/2010 13:47:49
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71 forum posts
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Regards stating pitch it is very similar to the metric system, example: M4 as you could have M4x0.7 and M4x0.5.  I was told to keep things on a drawing standard so if all the ME sizes are written 3/16x40 the larger sizes which may also be BSF should be written 1/4x26. 
I have a chart on the wall with each diameter and then the thread options and hole sizes available etc, extremely useful and time saving.  I believe one was given out in the ME a while ago.  
Personally I very much like the system. 
Geoff Theasby06/03/2010 15:35:08
615 forum posts
21 photos
I agree.
As a beginner, I needed a 5/16" BSF tap and some studding.
I had to do some research before I found that all 5/16" BSF threads are 22 tpi.
 
Regards
Geoff
Niloch06/03/2010 16:42:02
371 forum posts
Posted by Rob Manley on 06/03/2010 13:47:49:

I have a chart on the wall with each diameter and then the thread options and hole sizes available etc, extremely useful and time saving.  I believe one was given out in the ME a while ago.   

 Could it be that the ME chart was based on the four pages of charts(+ an explanatory page) given in MEW 36?

KWIL06/03/2010 16:58:35
3681 forum posts
70 photos
The one thing about BSW and BSF is that they ONLY have ONE thread pitch per diameter, metric can have up to SEVEN pitches at one diameter, and I thought that the move to metric was supposed to be an advance.
chris stephens06/03/2010 19:42:14
1049 forum posts
1 photos
Hi Eric,
As an engineer you should not need to be reminded that all UNF/UNC, or should that be ANF/ANC, threads under 1/4" have the pitch as part of their designation, for example 10-32 or 6-40. The confusing thing about those threads , though, is the diameter given as a number rather than a measurement.
I am sure that a lot of the oddities are there merely to show up newbies.
chriStephens 
 

 
Jeff Dayman06/03/2010 20:33:31
2356 forum posts
47 photos
Just in case any newbies don't know, there is a simple rule for UNC/UNF numbered threads. The base number 0 is .060" OD. the numbers indicate what multiple of .013" to add to the base dia .060" to find OD.
 
example #4-40: 4 x .013 +.060 =.112" OD 40 TPI
 
                #10-32: 10 x .013 +.060=.190" OD 32 TPI
 
check em on yer chart if you don't believe me...
 
easy now you know the rule eh?
 
As far as where this system came from I assume that it was started in a big USA hardware maker's factory in the 19th century and was gradually adopted as a working standard (and still is in North America although Metric is creeping in slowly-cars and office equipment are designed in metric and use metric fasteners. Steel and other metals are still made and sold in inches. Fasteners are available in both systems but metric are often more money. Domestic plumbing is all inch including threads)
chris stephens07/03/2010 14:26:31
1049 forum posts
1 photos
Hi Jeff,
Thanks for that, it just goes to show that the Yanks are not as completely unthinking as we are led to believe. 
If you know the reasoning behind  number and letter drills, as well, I for one would be all ears, but I wont loose any sleep if you don't.
chriStephens
 
 
Jeff Dayman07/03/2010 14:55:07
2356 forum posts
47 photos
Sorry I don't know about the origins of the letter and number drill rationale. Could be just another adopted working standard from a particular company.
 
I forgot to mention the below 0 size threads in UNC/UNF. These go down to 0000-160 that I know of. Again it is based on .060 as number 0, and you subtract the multiple of .013 as you go downward from number 0.
 
ie 0000-160: .060-3 x .013 = .021" od x 160 tpi
 
I have some 00-90 screws used on a couple of my models and that is as low as I want to go (I can hardly see if they're threaded!)
 
There is also the UNEF series of threads which have finer pitch for a given OD similar approach to the ME threads. You can still buy UNEF taps and dies from industrial tool suppliers  though which are generally far less money than ME suppliers due to their higher sales volume. 
 
Charts for UNEF and most other threads are of course in Machinery's Handbook and probably on the web if you seacrh around a bit.
Versaboss07/03/2010 15:00:28
512 forum posts
77 photos

KWIL, I would like very much to know which metric diameter has seven pitches as a norm or standard. Sure with your statement you don't mean that a given diameter can be threaded with any pitch your lathe is able to do; the same misuse would also be possible in the imperial system.

Btw, the common metric threads are simply called Mxx; all others are MFyy x pitch. Isn't this the same as BSW versus BSF or UNC versus UNF?

Greetings, Hansrudolf

KWIL07/03/2010 15:22:46
3681 forum posts
70 photos
M18
Terryd07/03/2010 22:35:15
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1946 forum posts
179 photos
Eric, Jeff Dayman had it just about right in his first post.  There is a set of specialist threads specifically known as ME (Model Engineers) threads which have a fixed pitch across all diameters in which they are available.
 
These allow for finer work on small components than the normal imperial threads allowed.  The pitches are 26, 32 and 40 tpi. and are available in a range of diameters.  For example on a short steam gland, say 5/16 diameter using a Whitworth fine you would get very few threads whereas using 32 or 40 tpi would ensure good thread engagement.
 
So if this is the case on the drawings and you see these pitches e.g 3/16 x 32 tpi then the odds are it is an ME thread.  Sets are available from most good tool suppliers who deal with model engineers such as Arc Eurotrade, Chronos and RDG.  Look here for the specs and range available:
 
 
http://www.rdgtools.co.uk/acatalog/Standard_Right_Hand_Taps_and_Dies.html
 
Of course American threads are a different thing, but if it is ME threads that are being referred to there is no need to refer to any tables.
 
By the way, you get used to 'the colonials' type comments, just ignore them.
Terryd07/03/2010 23:26:40
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1946 forum posts
179 photos
Hi Kwil,  I didn't know that there could be so many pitches in metric, please enlighten me before I go fully metric so that I know what I am getting into.
 
Terry
KWIL08/03/2010 11:17:57
3681 forum posts
70 photos
There are only so many common pitches in everyday common use, model engineers would only need a single pitch per diameter of use [probably].  Look at the common sets of taps and dies to see what they are. The common ones are the metric coarse series, even then they are not that coarse and to my mind are still too fine for a decent thread in some casting applications.
Circlip08/03/2010 12:02:51
1723 forum posts
And for steam connections in Brasses and Copper the Mertic fines are still too coarse so it looks like ME and BA sizes will still be needed for the forseeable future.
 
  Regards  Ian.
Rob Manley08/03/2010 12:04:07
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71 forum posts
14 photos
I would avoid going totally metric at all costs, there are generally so many imperial threads for a reason.  I couldnt imagine being limited to a 1mm pitch (as i dont have that many metric fine stuff) for a diameter as small as 6mm for model use. 
 
JasonB08/03/2010 12:32:23
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25215 forum posts
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TerryD, this will give you an idea of whats available in metric
 
 
While we are on about pitches can someone tell me if the small 1/4 x32 spark plugs are threaded ME or the not very common UNEF?
 
Jason
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