Myford lathe/drill and thread charts

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Handy drill and thread charts for use with the lathe.
Remember for all drilling operations to use a suitable RPM and drill fluid.

Drill RPM calculations

Cutting Diameter Mild Steel Stainless Steel Brass/Bronze Cast Iron Aluminium
mm inch R.P.M R.P.M R.P.M R.P.M R.P.M
3 1/8 * * * * *
6 1/4 * 2400 * * *
10 3/8 2200 1430 * * *
13 1/2 1700 1100 * 2200 *
20 3/4 1100 700 2500 1400 *
25 1 890 570 2000 1150 *
32 1.1/4 700 450 1600 900 2000
38 1.1/2 580 380 1300 750 1700
50 2 450 290 1000 570 1300
63 2.1/2 350 230 800 450 1000
75 3 300 190 680 380 850
100 4 220 140 500 280 640
125 5 180 110 400 230 510
160 6 140 90 320 180 400

Where an asterix is used in the table, the required speed exceeds 2500rpm. Simply use a value as close to 2500RPM as possible in this case.

Credit: https://www.greenwood-tools.co.uk/shop/technical-information/cutting-data.html

Center Drills

For any drilling operation on the lathe the Center drill is used first to make the small accurate pilot that the next drill will follow.
They are also use for drilling the required hole when using the live or dead centres.
Use the Highest RPM setting on the lathe!
General rule of thumb is to drill no more than 2/3 up the 60 degree angle
We have 4 Center drill sizes to chose form, BS1 to BS4
For general piloting work the BS3 or BS4 are recommended as the smaller ones are easy to snap.

When supporting work with a live or dead centre lookup the work diameter in the following table to determine the correct drill size and size of the hole needed.

Gauge Size (Inches) Size (mm)
BS1 1⁄8 in 3.175 mm
BS2 3⁄16 in 4.763 mm
BS3 1⁄4 in 6.350 mm
BS4 5⁄16 in 7.938 mm
BS5 7⁄16 in 11.113 mm
BS5A 1⁄2 in 12.700 mm
BS6 5⁄8 in 15.875 mm
BS7 3⁄4 in 19.050 mm

Threading Charts

Metric

The Hackspace has a tap and die set with the following metric coarse sizes

M3 M4 M5 M6 M8 M10 M12

For tapping internal threads use the chart below to select the correct drill size and if needed clearance size for the tap.

Threading Diagram
Nominal Diameter Pitch Root Radius Pitch Diameter Minor Diameter Thread Height Drill Diameter
d = D P r d2=D2 d3 D1 h3 H1 mm
M 1.00 0.25 0.036 0.838 0.693 0.729 0.153 0.135 0.75
M 1.10 0.25 0.036 0.938 0.793 0.829 0.153 0.135 0.85
M 1.20 0.25 0.036 1.038 0.893 0.929 0.153 0.135 0.95
M 1.40 0.3 0.043 1.205 1.032 1.075 0.184 0.162 1.1
M 1.60 0.35 0.051 1.373 1.171 1.221 0.215 0.189 1.25
M 1.80 0.35 0.051 1.573 1.371 1.421 0.215 0.189 1.45
M 2.00 0.4 0.058 1.74 1.509 1.567 0.245 0.217 1.6
M 2.20 0.45 0.065 1.908 1.648 1.713 0.276 0.244 1.75
M 2.50 0.45 0.065 2.208 1.948 2.013 0.276 0.244 2.05
M 3.00 0.5 0.072 2.675 2.387 2.459 0.307 0.271 2.5
M 3.50 0.6 0.087 3.11 2.764 2.85 0.368 0.325 2.9
M 4.00 0.7 0.101 3.545 3.141 3.242 0.429 0.379 3.3
M 4.50 0.75 0.108 4.013 3.58 3.688 0.46 0.406 3.8
M 5.00 0.8 0.115 4.48 4.019 4.134 0.491 0.433 4.2
M 6.00 1 0.144 5.35 4.773 4.917 0.613 0.541 5
M 7.00 1 0.144 6.35 5.773 5.917 0.613 0.541 6
M 8.00 1.25 0.18 7.188 6.466 6.647 0.767 0.677 6.8
M 9.00 1.25 0.18 8.188 7.466 7.647 0.767 0.677 7.8
M 10.00 1.5 0.217 9.026 8.16 8.376 0.92 0.812 8.5
M 11.00 1.5 0.217 10.026 9.16 9.376 0.92 0.812 9.5
M 12.00 1.75 0.253 10.863 9.853 10.106 1.074 0.947 10.2
M 14.00 2 0.289 12.701 11.546 11.835 1.227 1.083 12
M 16.00 2 0.289 14.701 13.546 13.835 1.227 1.083 14
M 18.00 2.5 0.361 16.376 14.933 15.394 1.534 1.353 15.5
M 20.00 2.5 0.361 18.376 16.933 17.294 1.534 1.353 17.5
M 22.00 2.5 0.361 20.376 18.933 19.294 1.534 1.353 19.5
M 24.00 3 0.433 22.051 20.319 20.752 1.84 1.624 21
M 27.00 3 0.433 25.051 23.319 23.752 1.84 1.624 24
M 30.00 3.5 0.505 27.727 25.706 26.211 2.147 1.894 26.5
M 33.00 3.5 0.505 30.727 28.706 29.211 2.147 1.894 29.5
M 36.00 4 0.577 33.402 31.093 31.67 2.454 2.165 32
M 39.00 4 0.577 36.402 34.093 34.67 2.454 2.165 35
M 42.00 4.5 0.65 39.077 36.479 37.129 2.76 2.436 37.5
M 45.00 4.5 0.65 42.077 39.479 40.129 2.76 2.436 40.5
M 48.00 5 0.722 44.752 41.866 42.857 3.067 2.706 43
M 52.00 5 0.722 48.752 45.866 46.587 3.067 2.706 47
M 56.00 5.5 0.794 52.428 49.252 50.046 3.374 2.977 50.5
M 60.00 5.5 0.794 56.428 53.252 54.046 3.374 2.977 54.5
M 64.00 6 0.866 60.103 56.639 57.505 3.681 3.248 58
M 68.00 6 0.866 64.103 60.639 61.505 3.681 3.248 62

Credit for table: www.tribology-abc.com

For the external threads turn down the work piece to the major diameter.
It is advisable to add a slight chamfer to the right end of the work, to just under the thread minor diameter using the left edge of the cutting tool.

British Standard Whitworth (BSW)

Reference Sites

http://www.tribology-abc.com/calculators/metric-iso.htm