To fit any accessories (such as a chuck or faceplate) to your lathe, you will need to know the headstock spindle thread size. Bear in mind, these can be manufactured in Imperial (inch) or Metric (millimeter) sizes.
Using a pair of calipers - ideally digital calipers - measure the thread diameter across the maximum diameter of the threads, peak to peak. This will normally be marginally under the nominal thread size due to wear and tear and essential operating clearances as well as manufacturing tolerances.
Measure the thread pitch (the distance between successive thread peaks).
2.1. For an imperial (inch) thread, this is measured in 'threads per inch'. If the thread is imperial size, place the zero point of an accurate rule, marked in inches, on the peak of a thread close to the headstock. Count this point as 'zero' and count how many more peaks you find up until you reach the half-inch point on your rule. (NB: This will almost always be a whole number - very few threads have an odd number of 'threads per inch' - though nine does occur very occasionally). Take the number of threads you have counted and double this number: the value you calculate is the number of 'threads per inch'. (You counted the number of threads in half an inch, so the number in a full inch is double the number in half an inch!). Typical pitches found on lathe spindles would be: 6tpi; 8 tpi; (rarely, 9tpi); 10 tpi; 12 tpi and 16tpi.
2.2. For a metric (millimeter) thread the approach is similar excepting that you use a rule which has metric scale markings. For a metric thread we need to find the size of a single thread pitch, measured in millimeters. Measuring a single pitch is almost impossible on most threads so it is easier for us to start with the end of the rule on one thread peak (our zero point) and then keep counting threads until we have another point where the thread peak exactly coincides with another marking on our rule. Usually, counting four, five or ten threads will make life easier, if that fits in with where your rule is placed. To find the pitch, divide the distance you have measured between your chosen start and end thread peak by the number of threads you have counted. So if, for example we counted 4 threads in the space of 14 millimeters, the pitch equals 14 divided by 4 = 3.5: the pitch in this case is 3.5mm. Typical pitches found on lathe spindles would be: 1.5mm; 2mm; 2.5mm; 3mm and 3.5mm.
If your lathe is 10 years old or less, and supplied for the UK market, it will almost certainly be one of these 4 sizes: 3/4” x16tpi; 1” x 8tpi; 1.1/4"x8tpi and M33 x 3.5mm
Older lathes could have a much greater variety, as may those manufactured for sale in other markets. If you cna find the 'makers plate' on you lathe and supply us with the lathe make and model we cna probably guide you to the most likely spindle size for your lathe. Other, less common, thread sizes include: 3/4” x 10tpi ; M18 x 2.5mm ; 1” x 10tpi; 1” x 12tpi; M25 x 2mm; 1.1/8” x 12tpi; M30 x 3.5mm; 1.1/2” x 6tpi - but there are many others! Some lathes spindles have a 'plain' section of spindle behind the threaded section - this is known as the 'register'. Its existence (or not) and its dimensions can also be important. If your lathe had a register, then the dimensions of that feature may also be important in ensuring a correct fit for the spindle thread. Please refer to the following diagram, which shows a typical spindle nose, including a register. If you can provide each of the dimensions noted, then we have a better chance of supplying correct equipment. The various features you need to measure are: T: the overall diameter of the threaded section of the spindle, as outlined in paragraph 1 above; P: the thread pitch as explained in paragraph 2, above; L: the overall length of the spindle, including any register, measured from the back face of the register - or the equivalent if there is no register; D: the diameter of the register (the maximum diameter ignoring any locking groove that may run around the register); and, finally, S: the depth (length) of the register.
If all else fails (!) give us a call and we'll talk you through it and provide whatever advice we can...