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Thread Pitch Info.

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Alan .20418/05/2019 17:14:21
304 forum posts
14 photos

i want to make a ER32 collet holder to fit on to my dividing head, the spindle thread measures out to be 1.5 inch OD with 8tpi it’s a bit of an odd ball I think any one have any info on the thread data for this, internal data I mean ?


Bazyle18/05/2019 17:27:06
5288 forum posts
201 photos

You need to measure the thread angle, whether it is Whitworth or UNC. All the parameters are defined then against the diameter and pitch. When you look up a table of thread pitches that is just he common ones for ease of buing compatible parts but the thread standard definition can be used to calculate the dimensions for any 'odball' diameter or pitch.

Alan .20418/05/2019 17:46:12
304 forum posts
14 photos

Should have said it’s UNC

david sanderson 318/05/2019 18:06:46
7 forum posts

Alan is it not BSF 1.5 inch 8 tpi same as a boxford spindle nose UNC would normally be 1 inch 8 tpi

Alan .20418/05/2019 19:18:02
304 forum posts
14 photos

Hi David, no mate it’s defnatly UNC 8tpi, said it was an odd ball, I’ll see what I can find on the net, i would like to start it tomorrow just need the thread info.


Alan .20418/05/2019 19:32:46
304 forum posts
14 photos

I’ve got 1.498 for the bolt and 1.349 for the nut should be ok with that hopefully.


Speedy Builder518/05/2019 19:35:20
2027 forum posts
144 photos

What was the Southbend thread ? My 'Southbend" backplate fits my BOXFORD and as the Southbend is American, I assume the thread to have been 60 degree 1.5 x 8 tpi.

Bazyle18/05/2019 20:16:40
5288 forum posts
201 photos
Posted by Speedy Builder5 on 18/05/2019 19:35:20:

What was the Southbend thread ? My 'Southbend" backplate fits my BOXFORD and as the Southbend is American, I assume the thread to have been 60 degree 1.5 x 8 tpi.

that's odd. I have a Southbend backplate that came on a chuck and it doesn't fit my Boxford though I think the other way round does work.

Alan .20419/05/2019 14:13:12
304 forum posts
14 photos

Ok chaps now I’m struggling with this thread I’m trying to cut, I’ve checked things again, OD of the screw thread is 1.5 inch I’ve checked the thread pitch again as I’ve found there is a 1.5 inch / 8 tip in BSF but it’s not that, it’s defiantly UNC, how can I work out the bore size for the internal thread, I know some of you chaps minght think it’s easy to do but it’s not when you don’t have a clue, can anyone explain how u do it, instead of knowing getting the bore size I would like to know how to work it out for myself for future refrance.


XD 35119/05/2019 14:34:15
1430 forum posts
1 photos

3. The formula below will give you the single depth for undertaking unified threads:

d = P x 0.750

Where d = Single Depth

P = Pitch

n = Number of threads per inch (TPI)

Infeed Depth = .75 / n


To cut a correct thread on the lathe, it is necessary first to make calculations so that the thread will have proper dimensions. The following diagrams and formulas will be helpful when calculating thread dimensions.

Example: Calculate the pitch, depth, minor diameter, and width of flat for a ¾-10 NC thread.

P = 1 / n = 1 / 10 = 0.100 in.

Depth = .7500 x Pitch = .7500 x .100 = .0750 in.

Minor Diameter = Major Diameter – (D + D) = .750 – (.075 + .075) = 0.600 in.

Width of Flat = P / 8 = (1 / 8) x (1/10) = .0125 in.

Edited By XD 351 on 19/05/2019 14:35:08

Alan .20419/05/2019 14:54:35
304 forum posts
14 photos

Thanks for that, I get a bore size of 1.314 inch am I right?


SillyOldDuffer19/05/2019 15:33:23
5915 forum posts
1280 photos


This German site might help. It gives dimensions for the 8-UN series, all of which are 8TPI, and might well be what you've got. Machinery's Handbook describes 8-UN as 'a uniform pitch series for large diameters. Although originally intended for high-pressure-joint bolts and nuts it is now widely used as a substitute for the Coarse-Thread Series for diameters larger than 1 inch." It's a valid Unified size, just not bog-standard.

For 1½" 8TPI the German table suggests a 35.1mm tap drill, which is about right for cutting an 8tpi internal thread to a depth of 0.06766" (Source: Machinery's)

When tackling confusing threads, I usually do a trial run on an Aluminium test piece first. Checking the result against the other thread will show if you've got the dimensions about right or need to think again.

The Unified spec says the internal thread should be truncated (flat) at the bottom rather than cut to the full sharp depth of 0.10825". It should also be rounded at the top. Beyond blunting the sharp top of external threads with a fine file I don't worry too much about either detail. If male and female mate I'm happy.

Making a new thread, I usually cut to a little less than theoretical, test for fit, and then take a succession of smaller cuts until the thread engages smoothly.

The annoying this about this sort of job is I usually mess up the first attempt in a muck sweat of time wasting anxiety, then make a poor second attempt that fits. If I go to the trouble of making a third, it will be made in half the time, be closer to size and have a much better finish. Make four and it becomes almost trivially easy to churn out good results...


Jeff Dayman19/05/2019 15:45:28
1827 forum posts
45 photos

Machinery's Handbook states that for 1 1/2"-8 thread ( 8UN series, pg 1271 in my old 21st edition) the minor dia / bore should be 1.3647" for internal threads.

By the way from same book a 1 1/2" UNC thread would be 6 TPI, 1 1/2" UNF thread would be 12 TPI.

If you are going to be doing a lot of unusual-size thread work it would be a good idea to get a Machinery's Handbook (any edition). They are an invaluable reference for all mechanical work.

JasonB19/05/2019 15:59:27
18296 forum posts
2024 photos
1 articles

Do you have anything else that fits the same spindle nose? If so just measure the hole in that.

Roderick Jenkins19/05/2019 16:06:46
1894 forum posts
486 photos


Just a point for clarification: UNC, UNF, Metric coarse, Metric fine, BSF, BSW and BA are all standards that are really meant to define sets of fastenings - basically nuts and bolts.

Threads for things like lathe noses do not necessarily follow these standards so your thread is not necessarily a UNC thread. If it is American (unified) it will have a 60 degree thread angle (as will metric) and if it is a British (Whitworth) thread then it will be a 55 degree angle. So looking at a set of thread tables for a near match will not necessarily give you the right answer. I think you really need to measure the thread angle: A couple of bits of card cut to 55 degrees and 60 degrees will tell you what you need to know.

Having said all that, It us usually the register rather than the thread that is important in getting a good fit so a slack thread of the correct pitch but wrong thread angle will probably be fine.



SillyOldDuffer19/05/2019 16:14:02
5915 forum posts
1280 photos

Confirm my 20th edition agrees with Jeff's 1.3647" which is a smaller hole than the German 35.1mm ( 1.382" ).

This is probably due to a different view about how tight the fit should be. Matching internal and external threads made to exactly the same dimensions would be impractically tight, prone to jam, and made worse by tiny manufacturing errors or dirt. Thread specifications allow for this, which is why the internal and external dimensions of threads don't quite match.

If the internal thread hole is bored a little oversize accidentally or deliberately, the two parts will screw together more easily, less metal is cut, but the join will be slightly weaker. Make the starting hole much too big, and the fit becomes inaccurate and seriously weak. If you do a trial run, I'd go with the German suggestion and see if it's 'good enough'. Cutting the thread into the tighter American hole will be more work and - perhaps - not much benefit.

Ordinary commercial nuts and bolts are a distinctly loose fit.


Edit: pesky smileys.

Edited By SillyOldDuffer on 19/05/2019 16:14:42

JasonB19/05/2019 16:19:10
18296 forum posts
2024 photos
1 articles

Dave, diameters don't really determine how tight the thread is, just the engagement. It is the fit of the flanks that determines tightness hence the use of thread wires to accurately measure a thread as these contact the flanks not OD or ID.

Edited By JasonB on 19/05/2019 16:19:53

Howard Lewis19/05/2019 16:42:47
3370 forum posts
2 photos

You are talking about the thread on the nose of the Dividing Head? Are any other threads on it Imperial? What make is it? If UK, probably 55 degree, Whit form threads. If American, possibly Unified with 60 degree threads. If European, or more recent Far Eastern most likely totbe Metyric, 60 degree thread, but the pitch may be (Older far eastern can be direct copies with Whit form threads, like my RF25 Mill/Drill)

The thread for the ER32 Clamp Nut will be 40 x 1.5mm, so your adaptor will stick out a little way from the Dividing head.



JasonB19/05/2019 16:50:44
18296 forum posts
2024 photos
1 articles

The other easy way to find the size is to look up another 8tpi UN thread and simply add or subtract the difference in major diameters.

1" x 8tpi UNC is a standard size and has a basic minor diameter for internal threads of 0.8647 on many charts found by Google.

difference between a 1" thread and a 1.5" thread is 0.5"

Simply add 0.8647" and 0.5" and you get 1.3647" that is confirmed by jeff's sizes simples.


Edited By JasonB on 19/05/2019 16:54:11

Alan .20419/05/2019 17:55:56
304 forum posts
14 photos

1.364 is what I've gone with and all seams OK, it's a Vertex dividing head and the thread is 8 unc I know it's uncommon for this head as I bought a back plate for this module and it didn't fit the supplier couldn't belive the size and thread either till I sent him some photos of it, it really is an odd ball, maybe it was made at the end of the day on friaday, Jason I have a female thread to measure but the fit really is shite hence cutting a new one. I wanted to get it right, with the help received from you guys I'm nearly there so thanks every one, by the way I did measure it with a Whitworth pitch gauge, close but the imperial thread gauge is pretty much a perfect fit not pretty much it is.

Thanks Alan.

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