Threading on a mini-lathe Step by Step
In my introduction to threading I outlined the basic principles of threading on a lathe as they appear to me. Next I will outline a simple step by step review of threading operations on a mini-lathe.
Please note this is what I do when thread cutting on my mini-lathe. It doesn't mean its right... it just works for me.
- First place the item to be threaded into the chuck and face it and turn it until everything is concentric and parallel. Lets assume we are doing an external thread. Turn the job down until its diameter matches the diameter you need for your thread. For an M42 thread, I'll normally work to 42mm, and then cut in deeper until the item fits.
- Securely mount the thread cutting tool in the toolpost, making sure its tip is square the the job.
- Set the correct threading tool advance angle.
- Change the change gears around to achieve the thread you require. My most common thread is 0.75mm pitch - which isn't on the instructions! However, lets assume we are doing a nice simple 1mm pitch thread.
- For a 1mm thread you will need to look at the chart. It says for 1mm we need 60,45,30,60. So adjust the change gears (being careful to not drop the woodruff keys into the swarf box) so that the 60 tooth gear is on the top gear on its own. Place the other 60 tooth gear on its own on the leadscrew gear. Then put the 45 and 30 tooth gears onto the centre spindle with the 45 gear on the inside.
- Now look at the indicator table. For a 1mm thread, this tells us we need the 16T gear in the thread dial. I have never actually cut a thread that needs any of the other dial gears. It also tells us that valid start positions are 3,6,9 and 12.
- Wheel the carriage up the far end away from the job, and engage the hlaf nut lock lever. With the lathe off, or running dead slow, engage the change gears. You should engage them so that the carriage starts to move slowly down the lathe towards the chuck. There are two positions.
- Turn off the lathe and sit down and catch your breath.
- We are now in a position where turning the chuck makes the carriage move. The important points now are DO not move the tool using the sideways adjuster at the top of the carriage (the one next to the angle markings). The best thing to do with this is unscrew its little handle and forget it. I advise making one unscrewing turn on it to take up the slack in the backlash.
- With the carriage unlocked, move the tool in so that its tip is JUST touching the job. Adjust its hieght so that it is on the centre line. Never adjust the hieght after we have started threading.
- Wind the carriage back down the lathe away from the chuck until its clear of the job.
- Start the lathe on DEAD SLOW speed.
- Watch the thread dial go around. When the marker lines up with the 3,6,9 or 12, smartly engage the half nut lever. Hold it down a bit to make sure it has properly engaged the half nut around the leadscrew.
- Now the carriage starts to move down towards the job. Scary!
- Speed the lathe up a tiny bit. Keep one hand on the stop button and one hand on the half nut lever.
- When the tool reaches the job, it will cut a spiral down the job.
- When you reach the end of the portion you want threaded smartly pull up the half nut lever to stop the motion of the carriage. Its normally worth cutting a small slot at the end of the thread to give the thread somewhere to end.
- Now note the reading on the cross slide (back and forth) dial. Wind the tool out a few thousandths so it is clear of the job and wind the carriage back to where you started.
- The lathe is still running. Put it back on dead slow.
- Wind the cross slide back in to eactly the same reading you did before.
- Watch the thread dial and when it reaches a 3,6,9 or 12, engage the half nut lever and the carriage starts to move again.
- When it reaches the job, it should, all being well, follow around the spiral you cut previously. If it trys cutting a spiral in a different position, you are doing something wrong.Threading on Mini Lathe - illustrated guide
- When it reaches the end of the threaded portion, disengage the half nut lever.
- Start breathing again.
- Wind out the tool a bit, and wind back the carriage to the start position again. Wind the tool in again, but this time, a few thousandths closer, and make another run down the thread.
- Repeat until you have the thread as deep as you want.
The important key is to always engage the half nut when the dial indicator is on one of those numbers listed for your desired thread on the thread indicator table.
It is equally important to remember to wind the tool out clear of the job before moving the carriage back to the start.
Remember to use lots of cutting fluid.
The finish up the thread, i use a thread pitch measuring tool to perform a sort of "lapping" process on the job, this helps clean the burrs out.
Threading is a bit of an art. You need to balance the speed of the lathe and the depth of each cut. The motor needs to be fast enough to prevent stalling, but slow enough that you can see what you are doing. Cut to deep and it will bend the job or worth. It takes some practise to become proficient at thread cutting.