Spindle Encoder / Leadscrew stepper threading leadscrew idea

Marcus I have been thinking about your question about a small range of optimum cutting speeds. You have to consider what is optimum. In industry it is cost driven, time is money as they say but going too fast destroys tooling quicker. The optimum for industry is carefully calculated but is not the same for hobby use where time is not such a restraint, we are not working against a clock. Tooling costs override time costs. Often you see statements about recommended feeds and speeds being reduced for home use.

If thread cutting needed to be done at high speed for a good finish then how could anyone ever produce a clean and well finished thread using hand taps and dies? I put my lathe in backgear for parting off and produce shiny swarf and a good finish on the cut faces using a good dark cutting oil and carbide tips. I was parting off some 316L last weekend without problems with surface speeds well below "recommended" and getting a good finish.

But back to threading, the problem with threading large pitches at high speed is the rate the carriage needs to be driven. If you are using CNC then in order to accelerate the carriage up to speed quickly and then decelerate it again for reverse you need a very large motor to drive it. If you are using a conventional mechanical connection you may have a 2 hp motor working everything, most home CNC will not want a 2hp motor to drive the carriage. There usually has to be a compromise between speed of motion and size of Z axis motor and this will be true for the original concept in this thread as well. With CNC you have to be aware of the maximum speed you can move the carriage at so that you do not try to exceed this speed when threading.

As an example consider am M6 thread in mild steel. Recommended RPM is about 1250 for turning. If thread cutting at this RPM with a 1mm pitch you would need to move the carriage at 1250mm per minute. This could easily be greater than what the machine is capable of. It is much easier to drop the RPM to a value that is well within the machine's capabilities and also does not cause the machine to slow when cutting starts. If you consider how much time it takes to cut the thread then you are talking a few seconds longer for typical thread lengths cut at these lower speeds so that should not be a consideration for a hobby machinist. Additionally with CNC the rapid speeds on a lathe are not the same as the maximum cutting speed because as well as having to move the carriage or cross slide around the motor needs to overcome the forces due to pushing a tool into the workpiece.

I can produce a good finish on threads at low RPM so my optimum cutting speed range is clearly not a particularly small range and is also different from optimum in an industrial environment so there must be other factors affecting things beyond just surface cutting speed.

For someone looking to fit a motor to the Z axis they will have to decide for themselves what size it will be, can it be direct drive or will it involve pulleys and how fast they want the Z axis to move when cutting. A small motor with speed reduction through timing belt pulleys will give good torque but low speeds but may be the best option. It does require that the lathe is able to cut good threads at low speeds though.

Martin