Lathe & Mill

Lathe and Drill Cutting Speeds

To find the required RPM, look up the material, operation and tool type in the table. The data relates to 20 mm diameter work piece turning on the lathe or a 10mm drill. The table needs to be scaled inversely with work piece diameter or drill size. For example, if the work piece is only 10 mm in diameter, the turning RPM needs to be doubled; if it is 40 mm in diameter, then the turning RPM needs to be halved.

Cutting Imperial Threads on a metric lathe and vice-versa

Cutting Imperial Threads on Metric lathes 

Most common metric lathes have a leadscrew of 3 mm pitch. This pitch would translate to 25.4/3 or 8.466666 TPI. This is not a convenient number from which to derive the standard Imperial TPI threads. Therefore we use the Imperial transposing gears to convert this awkward TPI value to a more useful value , such as 8 TPI which can be divided or multiplied easily using common gear ratios. The ratio between the (25.4/3) TPI and 8 TPI is 25.4/3/8 = 25.4/24. Multiplying by 10 to remove the decimal point since gears need to have a whole number of teeth we end up with 254/240 or 127/120. This ratio can be obtained exactly with a compound gear consisting of a 127 and a 120 teeth gears and would provide accurate TPI threads in combination with the other gear set. In certain small lathes, this 127/120 compound would still be too large to fit and hence other fractions nearly equal to 127/120 (or 1.0583333...) are used to give a more workable gear size at the expense of accuracy. For example the 36/34 pair gives a ratio of 1.058824 (0.0463% out)  while the 73/69 compound gives a ratio of 1.05797 (0.0343% out) and can be used and should be good enough for short length threads.

Naturally, if your lathe has a different leadscrew pitch, say 2 mm, then the ratio would be 25.4/2/8 giving 254/160 or 127/80 = 1.5875. If the 127 gear would not fit, we can try  (27/17 = 1.58824 which is 0.0466% out), or  (73/46 = 1.5862 which is 0.08189%).

Cutting Metric threads on Imperial lathes

Most common Imperial lathes have a leadscrew of 8 TPI. This pitch would translate to 25.4/8 or 3.175 mm pitch. This is not a convenient number from which to derive the standard metric threads. Therefore we use the Metric  transposing gears to convert this awkward pitch value to a more useful value , such as 2.5 mm which can be divided or multiplied easily using common gear ratios. The ratio between the (25.4/8) mm pitch  and the more suitable 2.5 mm pitch is 25.4/8/2.5 = 25.4/20. Multiplying by 10 to remove the decimal point since gears need to have a whole number of teeth we end up with 254/200 or 127/100. This ratio can be obtained exactly with a compound gear consisting of a 127 and a 100 teeth gears and would provide accurate metric threads in combination with the other gear set. In certain small lathes, this 127/100 compound would still be too large to fit and hence other fractions nearly equal to 127/100 (or 1.27) are used to give a more workable gear size at the expense of accuracy. For example the 80/63 pair gives a ratio of 1.269841 (0.01252% out)  while the ratio 47/37 gives a ratio of 1.270270 (0.02126% out) or even 33/26 which gives a ratio of 1.269231 (0.06055%). Any one of these compound combinations can be used and should be good enough for short length threads.

Quick Gear Train Look-Up for common threads (Metric, TPI, or BA)