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Single point threading on the lathe

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spuddevans:
Ok, I ( foolishly ) said I would put up a post on how I do single point threading on the lathe. To start off with let me say that this is just how I do it, there are many who are more experienced / qualified than I am, and I am really hoping that they will all chime in with their insights and experience.

I have just a (relatively) small lathe, a Sieg C2, and depending on the size and rigidness of your lathe you may be able to take deeper cuts than me ( or you might have to take shallower ).

I'll start of by apologising if it comes across like I'm speaking like you are all are beginners, I dont mean to patronise, it's just that I am still learning and this is how I understand all this. This isnt "The" idiot's guide, it's just "This" idiot's guide.  :lol:

The 1st thing to do is to mount and prepare (bring to size) the material you want to thread. In this example that I am demonstrating I will be cutting a M6 thread on a part for my Paddleducks build. On this part the threaded section is quite short, only about 3.2mm long, but if you are wanting to practice single point thread cutting I'd suggest threading about 5-10mm length of whatever thread you choose.

So to start off with, here's what my material looks like with the end section turned down to the required size ( easy for the metric system as M6 is 6mm diameter, I prefer to go a little under 6mm, about 5.85-5.95mm as it makes for an easier fit. )



The next stage is to make a little groove where you want the thread to end. Unfortunately I dont think there is a way to single point a thread all the way up to a shoulder, so we need to make a groove for the tool to stop at the end of each pass. All you need to do is grind up a tool to look something like this.


Just grind it up to whatever width that you want the groove to be.


So now you can make the groove at where the thread will end.



You will also need a tool to cut the threads, there are different angles for different types of threads, the metric system uses a 60 degree angle, and you can get carbide tools already formed, or you can grind up your own tool which is not hard at all. I ground my own tool using a grinding gauge shown below.



That threading angle gauge is your very good friend, not only useful for grinding the right angles, but it is also an essential tool to set the tool at the right angle to the workpiece. The pic below shows what I mean, you hold the angle gauge on the tip of the tool and the opposite edge of the gauge rests close to the workpiece. This will show up if the tool is not at 90degrees to the workpiece and then you can adjust the toolpost to allign it.



Once the tool is angled correctly and set at center height we can get the cross-slide zeroed on the OD of the workpiece. I do this by winding in the cross slide until the tool just contacts the workpiece enough to leave a little score like this,



Now the tool is all set you can zero your cross-slide, or if you have dro's you can zero those now.

Depending on your lathe you will either have a gear selector to select the different ratios for the leadscrew, or like mine you will have a little bag of gears and a chart that tells you where to put what gears to give you the required cutting pitch.



Now you can position the carriage clear from the workpiece and engage the half nuts and advance the cross-slide by 0.1mm (for brass, ali ect. Steel might need a shallower cut depending on your lathe)

Then select the slowest speed you can do on your lathe (this is where having variable speed really helps) and start up the lathe. Watch carefully and stop the lathe when the tool tip has got to the groove we cut earlier.


Then withdraw the cross-slide until it has backed out past the zero point, then select reverse, do not release the half-nuts, start the lathe and run it back past the workpiece, advance the cross-slide past zero and to 0.2mm this time (0.1mm deeper) and repeat the above paragraph.

Then you keep doing this until you get to the desired thread depth. Now, what is the right thread depth for each thread? I cant tell you as I dont know, but I can tell you that I seem to go to a depth of 0.650-0.700mm for M6. I have some tables that explain all about thread pitches, angles and all sort of things I dont understand, yet!! But the easy way to get the thread depth right is to use a commercially made nut of the thread you are cutting and when the thread form starts to look like it should, try the nut on it after each pass until it fits.








And that's all there is to single point threading, well that's the basics of it, there is a lot more but I'll leave it up to those better experienced and qualified to tell you about that.

By the way, in the time that it takes to read this you can single point thread literally feet of thread.

Hope this helps someone, if any of my ramblings do not make sense please tell me so I can explain or correct it.


Tim

Darren:
Very nicely shown Tim,

That's almost how I do it, only difference I make is I don't back the tool out after each pass. I can see others throwing their arms up in horror here.... :doh:

I leave the feedscrew nuts engaged all the time and just advance the top slide with each cutting pass.

I have read that you can't remove the work between starting and finishing as you'll loose it's placement. I'm not sure why that is as I have done it several times with no apparent ill effects? Just make sure the tool tip is firmly located in a groove before re-clamping the chuck. Seems to work....well so far anyways....


 

raynerd:
 :thumbup: :thumbup: :thumbup: :thumbup: :thumbup: :thumbup:

Great job, Tim. I`ve been doing quite a bit of single point threading but my problem is my Boxford and Clarke cl300 are both imperial and being in the UK, I only ever use metric - I think this has inspired me to order the metric conversion kit for the clarke in the next few days (ideally would do the boxford but don`t know how to go about it) - anyway that is all  :offtopic:  :D

Good job mate, would love to see you do an internal thread as well - what size internal thread do you think would be achievable? I presume the limitation is in the size of the internal thread cutting tool? Am I going  :offtopic: again .... lol

Good job mate.

Chris

kvom:
I'd add a couple of minor points.

After you cut the first pass with a scratch cut, it's a good idea to use a thread gauge to ensure you've set the pitch correctly.  If not, you can fix it before continuing.

If your lathe has a thread dial (and cutting imperial threads), it's easy to back out the cross slide and back up the carriage.  You can then use the dials or DRO to set the depth for the next pass.  For very short threads like above, I think it's faster to reverse the lathe with the leadscrew engaged, assuming there is not enough backlash that the threadform will be affected.

As for thread depth: if you assume a 60-degree thread with pitch P, then the depth D is the height of an equilateral triangle.  So P/2D = tan(30) or D = P/(2*tan(30)) or .866*P.

Darren:
I'm glad you added the last bit.......866*P........ :lol:

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