Cutting screw threads in the lathe is one of the more advanced techniques you can try in your home workshop.
But how do you know if it has been done correctly?
Although it is tempting to just try fitting the internal and external threads, that risks damage to one or both. In addition different designs may have different tolerances, for example a screw that moves a slide vs one designed to clamp parts together.
Thread Pitch Gauge
So some form of instrumentation is necessary for testing your work. At the simplest end of the spectrum you have the thread pitch gauge. These are copies of the thread form on a slip of metal, typically with multiple different pitches on a single gauge.
However these only give you part of the picture and don’t tell you if your parts diameter is incorrect. Typically these gauges are used for thread identification rather than checking dimensional accuracy.
Three Wire Technique
A technique of checking thread pitch diameter is known as the three wire technique. This method uses a micrometer to measure the thread without damaging it.
Two wires are placed on one side and one on the other. The wire thickness is used with micrometer reading to check the size of the thread on it’s active surface and hence can even be used when there is damage to the crests or where the major diameter is intentionally under size.
Go and a NoGo Thread Gauge
The three wire technique can be fiddly to perform so for production quality control a knife edge gauge or hit and miss pair of thread gauges can be used. These are thread forms precision ground to the relevant standard and are hardened or coated for durability.
To use a thread gauge:
A thread gauge incorporates a Go and a NoGo plug. In simple terms if the Go plug can be inserted into the threaded hole with no excess movement and the NoGo can’t then the object being measured is generally considered within the limits of tolerance and therefore acceptable. When using a ring gauge follow the opposite rules. When using a thread gauge it’s essential never to force it as this can distort the object and lead to inaccurate measurements.
Different gauges are available for tapered, multistart and different thread forms. You can also get plain gauges for ensuring holes are drilled for clearance fit or transition fits.
When using gauges, be aware that thread gauges cannot be used for any of the following:
- Thread cleaning
- Measuring actual size
- Lapping tool
Looking after your inspection equipment
Follow these tips to properly maintain your thread gauge:
- Never force a gauge into an object being measured
- Use the gauge as you would any other precision tool
- Mishandling may result in damage that will render the gauge useless
- Where possible store your gauge in an individual box or compartment in your tool box
- Prior to storage coat your gauge in anti-rust solution or dip it in a wax or oil based seal
Auditing and Calibration
Like any engineering or scientific equipment it’s important to ensure your thread gauges are not worn or damaged. Periodic audits and calibration will ensure long term success.
Inspection and gauging
Screw cutting by the Master Screw method
Thanks to TheGauge for their suggestions and help in writing this article.