Thread: Screwcutting for beginners

Screw cutting for Beginners

Screw cutting can be an enjoyable task and it can also be the most exasperating task of all. I will endeavour to show how it can be fun to do and evade all the pitfalls. The key to understanding screw cutting is to understand the relationship between the lead screw, the job being done and that horrid little clock dial gizmo that is attached to the leadscrew. The units, metric and imperial will be mixed thru the discussion but don’t get too excited about that. For the discussion, I am using an Imperial lathe, the Leadscrew has 8 TPI.

The first part we will look at is how to set the tool up in the tool post. All the tooling used in this discussion is HSS, the tools being ground by hand using a thread gauge to set the angle. The thread we will cut is a standard UNC – 16 TPI form. Nearly all threads cut at home on the hobby lathe are usually never less than 10 TPI but increase to 32 TPI or even finer if making parts for a steam locomotive or steam engine. I like to shape the threading tool so the flank / edge of the cutting tool is never longer than about 3mm. or 4mm at the most. Take a stick of your favourite HSS and grind the required shape on the end. Keep trying the shape in the gauge and shape the “V” as accurately as possible. The sides of the tools must be straight, no facets are allowed whatsoever. When the tool is finished, it will have a sharp point at the end. This point must be removed and shaped to a small flat. But how much do we remove? Because of my experience, I use the TLAR technique (That Looks About Right) but a beginner will need an actual dimension. This can be determined from the formula Width of Flat = 0.25 x Pitch. See “Machinery’s Handbook” for more details.

The shape of the “V” cut in the bar is also affected (greatly) by how you set the tool. Accurate form and V angle can only be achieved by having the top face of the tool, flat and level and exactly on center. You cannot grind the tool to a perfect shape and then place it in a tool holder that sets the tool up on an angle, i.e., so the tool has any top rake. The angle of the “V” so cut will be formed to a wider angle and a poor thread is the result. Always pack the tool to its correct center height before starting any thread cutting. You can grind the top rake off the tool but then the thread angle will need to be adjusted, it’s best to have the tool clamped horizontally in the tool post from beginning the job.

There are two ways to set the Top Slide for screw cutting, it can be set at 90 degs. to the Cross Slide or at half the thread angle which in this case will be 30 degs. We will go the half thread angle route for this exercise. There are pros and cons for each technique which I will explain later.
Set the shaft to be threaded in the lathe. It is essential that the shaft run perfectly true. This is best done by machining the shaft to the finished size and then commence the threading in situ.
Loosen the Top Slide and swing the slide to the zero point. This is where the Centerline of the T/S is parallel with the center line of the Cross Slide. Now swing the T/S to the right to 30 degs. Did I say 30 degs? No, move it to 29 degs. and clamp it firmly. Now using the Thread Angle Gauge (also called Thread Gauge, Fish Tail, Fish Tail Gauge) set the tool to be perfectly square to the job. I can hear you calling “Hey Joe! What’s with the 29 degs?”
By setting the T/S to 29 degs, the right hand side of the tool bit will scrape a minute shaving off the thread flank each time the tool is advanced. This shaving-cut helps to produce a beautiful finish on the thread flank and also helps to stabilize the tool as it cuts. The finished thread form is still a perfect 60 degs. Included angle. Try it out and if you don’t like it, change the angle to 30 degs.
Position the tool somewhere along the job. Wind the Cross Slide handle so the tool almost touches the job. Now wind the cross slide out one full turn. Without the tool hitting the job, wind the cross slide in and stop when the handle that sticks out from the cross slide hand-wheel, is located at the 3:00 o’clock position. Set the cross slide dial to read Zero. This is essential and must be done as described. Do not touch the cross slide dial after this is set. Start the lathe and slowly and carefully, wind the top slide in until the tool just touches the job. Set the top slide dial to zero at this point and stop the lathe. Wind the tool away using the Cross Slide and position the tool about 1 inch past the end of the job. All tool advancement is now done with the Top Slide.

Did you notice that to bring the tool away from the job, the cross slide handle was lifted up to start it moving (or rotating). By setting the handle in this position, any panic attack to suddenly get the tool away from disaster should result in rotating the handle in the correct direction. Think of it this way. When the half-nuts are disengaged, the handle is lifted upwards. When you take a tool off the bench, you lift it upwards. When you take the threading tool away from the job, you lift the handle upwards and continue winding anticlockwise. Only when you have screwcut at least eleventeen hundred threads will you be able to rotate the C/S handle the correct way from any other position. This is why the above technique is best for beginners.
If you set the T/S at 90 degs. and screwcut, you must be able to wind the tool out from any position of the handle. Both of the above cutting techniques can be made easier by cutting a groove at the end of the thread to form clearance for the tool to slide into. Such a groove is not always possible or practicable to cut. For a Newbie with no experience, I recommend machining the .625 dia bar as shown to practise on. The tool can proceed into the relieved section with total safety and will help you develop your reflexes without tears.
And now for the next exciting part, ……..

The Horrible Clock thing on the Leadscrew
Actually, this is your very best friend, he’s a bit simple but you can’t do without him/it. Take a good look at it and count the number of longer and more prominent lines on it. Most “clocks” have four heavy and well defined lines and are also numbered. Wind the carriage up or down until one of these lines is aligned to the datum mark displayed near the edge of the “clock”. At this point, the halfnuts should be able to engage fully into the leadscrew, so push the handle all the way down. Get a pencil and draw a line on the lathe bed at the front of the carriage. Lift the handle up (disengage the halfnuts) and wind the carriage towards the Tailstock until the next heavy line on the “clock” aligns with the datum once more. Engage the halfnuts fully once more. Measure the distance from the pencil mark to the front of the carriage. It should be 1 inch. Are there any smaller lines between the heavy 1” marks? There may be one mark between the heavy lines and these are the “1/2 inch” distances. Depending on the diameter of the “clock”, there may be an extra line displayed to represent “1/4 inch” distances.
The thread we are cutting has 16 tpi. The Leadscrew has 8 tpi. Note that the thread we are cutting (16) is a multiple of 8.
Look at the drawing of the thread with the ruler above it.
A thread of 16 tpi has 8 whole threads at the 1/2” position. This means we can engage the halfnuts at this point and the cutting tool will follow exactly in the groove (thread) being cut. At the 1/4” position there are 4 whole threads and again we can engage the halfnuts. At 1/8” there are 2 whole threads so you can engage here also. The 1/8” position also corresponds with the pitch of the Leadscrew and this means that you can therefore engage the Leadscrew at any place displayed on the “clock” or anywhere the halfnuts will “drop in”. The drawing shown in the article will display this feature. Look at the thread on the RHS of the drawing. This thread to be cut has 10 tpi. Note there are only two places where you can engage the halfnuts. They can be engaged at any 1” mark and any 1/2” mark. One inch equals 10 whole threads; 1/2" equals 5 whole threads. At the 1/4" position (2 ½) threads, shown by the RED toolbit, the tool would cut into the thread you are trying to make and totally destroy it.

Now the fun part
Cutting the thread
With the tool about an inch from the end of the job, wind the T/S in 2 divisions. Start the lathe with the speed set to the slowest it can go and engage the halfnuts on any line shown on the “clock”. Run the tool to the end of the thread length required and in one motion, lift the handle of the halfnuts and the C/S handle to lift the tool away from the job. Stop the lathe and start to breathe again. Get a Thread Pitch Gauge and check that you are cutting the correct TPI that you wanted. Wind the carriage back to the original start position and wind the C/S in to the original zero point. This point must be set very precisely every time. Wind the T/S in another 3 divisions and start the process over again. Watch the video on Screwcutting for more details. If you get stuck, you can ask me here for any info you need. Unfortunately, I was unable to get the drawing to display within the thread as I wanted. Sorry for the awful long article, it’s hard to write this any shorter. I hope all the Newbie’s can understand the above and that the experienced can glean a few tips.

Kody