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Thread cutting on the lathe

Questions about cutting threads

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Lawrie Alush-Jaggs06/12/2009 13:27:10
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118 forum posts
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Hi O Helpful Ones
 
I am in a quandry about cutting an internal thread. I have been looking all over the place, in the Engineers Black Book, in Machinery's handbook, on the web, all over.
I am trying to acertain the depth to which the thread should be cut.
The standard diagram lists the the major and minor diameters for male threads.  It also list the depth from the major diameter of the male thread to the minor depth of the female thread.
EBB lists the Basic minor diameter of the female thread but I don't know if the basic diameter is the same as the cutting depth.  Is the basic depth the actual cutting depth or is it the pitch depth (if I have the right term)
 
For example, with a 10mm thread the basic minor diameter of a female thread is 8.376mm.  Looking at an illustration that seems to indicate a diameter cut to a perfect "V" rather than an actual cutting diameter given that the form has a radiused root.
 
Can anyone enlighten me please?
LADmachining06/12/2009 14:35:31
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What thread are you trying to cut?  UNF/BSW/Metric/BA?
 
When I was cutting a 24tpi UNF thread recently, I used this link to calculate the depth of cut - http://walteranderson.us/hobbies/metalworking/microlathe/frog/threading.html
 
To calculate:-
 
p (pitch)= 1 / tpi  =1 / 24  =0.41666
 
depth of cut of external thread =.758 * p  =.758 * 0.41666  =0.00315"
 
HTH
 
Anthony
Tony Pratt 106/12/2009 16:10:45
2319 forum posts
13 photos
Hi Lawrie, as Anthony says what thread are you cutting ? As a general rule the thread depth is the difference between the crest dia [outside dia] & root dia[minor dia] divided by two but that tends to be a theoretical figure. In my experience the bore is machined oversize on minor dia. to make sure there is no fouling on the male screw O/D and the internal thread is cut to fit the male part [or gauge].
Are you using a single point tool screw cutting tool & have you a male part to fit the internal thread to?
Tony
LADmachining06/12/2009 16:15:12
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126 forum posts
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I have just seen that an extra 0 has crept into my posting above - the thread depth should read 0.0315" (not 0.00315" as shown).
 
.  Oops!
 
Anthony
chris stephens06/12/2009 16:23:31
1049 forum posts
1 photos
Hi Lawrie,
As Anthony says "what thread are you trying to cut". 
You should not worry too much about rounding the the cutting tool. When you find out what the depth of cut should be for your thread, cut to that depth even if you have a sharp point on your tool (stop sniggering at the back of the class). You then use the top slide to widen the thread till the bolt fits. This way saves you having to constantly adjust your cutting tool for different thread pitches. 
The is only a simplified explanation, this is not the place to write a long diatribe about thread cutting, but if someone wishes to, I wont stand in their way.

Hi Anthony,
I think if you have a 0.00315" depth of cut there must be something a tad wrong somewhere.
chriStephens
LADmachining06/12/2009 16:50:48
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Hi Chris,
 
There was a problem - a twitchy finger on the '0' key..

BTW, I was the one that was just sniggering at the back of the class...  Childish, I know
 
It won't happen again...
 
Anthony
chris stephens06/12/2009 17:10:17
1049 forum posts
1 photos
Hi Anthony,

Since you owned up to the sniggering, I hereby give you permission for the rest of the term.
chriStephens
KWIL06/12/2009 18:27:34
3681 forum posts
70 photos
All very well, you are talking about the depth of the thread but if the topslide is set over in the correct manner at one half the thread angle, the topslide movement for setting on the cut is different. There are charts of course for this. If L A-J sends me a member message with an email address I will send him the charts.

Edited By KWIL on 06/12/2009 18:29:01

chris stephens06/12/2009 18:51:04
1049 forum posts
1 photos
Hi Kwil,
First the original poster was  talking about internal threads where the offset top slide is rarely used.

The offset top slide is only one of many "correct" methods and should not be considered to be the only method. All the different methods have there pros and cons, the fact that you cannot move the top slide to give a wider cut in the offset method is one of the cons and one of the pros in the straight in method. 
Hope this clears up the matter.
chriStephens
Lawrie Alush-Jaggs06/12/2009 23:07:59
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118 forum posts
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Geez, what a pain in the neck.  I logged in, wrote out a reply, posted it and was then told to log in.  CTRL A then CTRL C before clicking the post button. so, once again......
 
Thanks everyone for your replies.  This is an excellent site populated by equally excellent people.
 
In reply to Ladmachinings' question, I am trying to cut a metric thread.  I am using a 60 degree TC insert in a threading boring bar.
I stuffed up the first attempt.  And the second.  I am down to minimal wall now and so the thread is going to be whatever diameter I can get rather than one that uses measurements.  The plug will be made to suit rather than the other way around.
Thanks Ladmachining for the link.  From this I understand that for a 2mm thread I would cut 1.33mm deep, P x 0.65.  That seems rather deep but then I haven't cut it and I am asking the question.
 
Thanks again for all the answers.
 
 
chris stephens07/12/2009 00:21:25
1049 forum posts
1 photos
Hi Lawrie, 
If you take too long composing your post, you might be asked to log in again. Go to top of the page you are on and log in, then back page and you should be on the page with you post, it might take a few moments for the words  to come back, but give it a try next time.
christephens
Richmond07/12/2009 00:32:19
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73 forum posts
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64 articles
Hi,
 
I never seem to have this problem......when I first logged into the site, I clicked on the "remember me" button. Even when I have cleared the cookie cache in IE8, and have relogged in, I have never been logged out, unless..... I have logged in on another PC that is on a different IP address.....but then again I use a router.
 
Basically, what Im saying is that the website, to my knowledge, doesnt log you out after a set period of inactivity. It is more likely to be something perhaps related to the users internet connection......some USB modems automatically log off from the ISP after a few mins of inactivity, and when they log back in they are given a different IP address.......the nett effect being that the website will clear the previous session..... and a "member" will loose any unsaved posts.
 
I beleive it IS possible to change settings on USB modems to prevent this...... but to give specifics is beyond the scope of this site.
 
Regards
 
Eddie07/12/2009 06:06:57
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56 forum posts
Hi,
I found the following website.
It has stacks of information of Thread data, both male and female threads.
E Potgieter
John Hall 624/03/2013 06:46:57
3 forum posts

I have a problem re angular depth of thread when screw cutting and indexing the cut using the topslide set at 1/2 the thread angle. ie 27.5 degres for whitworth threads. Many years ago ME carried a two page article outlining this simple method which allows the tool to only cut on one edge. Can any one help with the issue number which this was in, or even email a copy of the charts to me?

Thanks in advance

John Hall

Wingham

NSW

Australia

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