# Cutting speeds

The subject of cutting speed often comes up when people encounter a problem of finish, vibration, chatter, etc or are just nervous beginners. They are then referred to a variety of websites, tables, diagrams etc.
They may then look something up, forget it next time, print a table or graph and lose it generally not have the information for next time and start the process again.

Firstly there is no need to be pedantic about it, as the ‘recommended’ speeds are just guides to get you in the general ball park. There are various hypothesis about the correct speed and where such tables come from. Some say they were laid down by Julius Caesar in 55 BC and apply to turning arrows, or the 18century for carbon steel tools knocked up by the blacksmith, or by Ford for optimising the tool life in his factories. There will have been some industrial or academic research but whether that was actually secret or passed through and what specific tool steel composition was used is not known.

For Metallurgists the optimum speed is that which equates to the speed of sound in the metal. This is because the tool doesn’t actually slice atoms apart, nor in fact even touch the metal at the very tip, but pushes the metal apart a little distance away causing a crack to form which what makes the ‘cut’ between atoms. Cracks move at the speed of sound and form when the stress build up enough at the atomic interface. The crack relieves that stress then stops while more stress builds up so as the tool must move fast enough to keep up making more stress at the same speed as the crack moves – and that is the speed of sound.
However all recommended speeds are well below this as there are also problems of heat generation.

If you arrive at a speed to use with HSS tools for steel then aluminium and brass can be cut at twice that speed. Carbide tools are run at 2-3 times the speed for HSS.

Milling cutters are the same as turning cutters. That is if you are using a 1 inch diameter milling cutter its speed would be the same as for turning a 1 inch dia bar.

If you double the diameter you halve the speed – pretty obvious. So if you can remember just one diameter / speed combination you can quickly estimate the speed for a different diameter, and as it doesn’t need to be at all accurate that can be primary school maths not anything difficult.

What is an easy reference combination to remember then?

For old Imperialists – remember 5/8 inch. That is 0.625 inches I hope you are able to work out in your head. And the cutting speed for 0.625 inches is 625rpm. Boom Boom,

For metricalists – it’s quarter past three for early tea. That’s 3:15 or 315 tenths of millimetres. And the cutting speed for 31.5mm is 315 rpm.
So there you are just remember that and you have the lot.