Myford lathe/drill and thread charts

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.

Reference Sites

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