Boring head speed.

[Michael Butler 6Participant @michaelbutler6]

how do I determine the correct speed for a boring head in the mill?

[JasonBModerator @jasonb]

You need to look up the cutting speed for the particular metal and then work out the circumference of the circle that the tip of the cutter is rotating at. Divide the cutting speed by the circumference.

Example cutting speed for steel 100ft/min or 30m/min with HSS. Say you are boring a 2″ or 50mm hole that will be pi x dia so 6.28″ or 157mm.

100ft (1200″) / 6.23 = 192rpm or 30 (30000mm) / 157 = 191rpm

Select the next lowest speed your mill has if belt reduction or wind the pot to set speed. if you have hi/low and variable speed then use the low range

On thing to watch is that a boring head can get out of balance so you may need to run a bit slower

[HopperParticipant @hopper]

It depends on the diameter of the hole you are boring. Use the usual formula, same as for a milling cutter of the same diameter as the hole you are boring.

As a rough guide, for a HSS toolbit boring a 1″ diameter hole in steel, 400 RPM. For a hole half that diameter, double the RPM. For a hole twice that diameter, halve the RPM.

For carbide tooling, double the RPM you would use for HSS. For material such as aluminium or brass, double the RPM. For cast iron and tool steel etc, halve the RPM.

That’s the theoretical cutting speed. In practice, if you get tool chatter when boring, try dropping the RPM.

And be aware that many of the sets of  brazed carbide tool bits that come in boring head kits from China need sharpening on a grinder before they are any use.

The formula I use is 4 x Cutting Speed/diameter

So cutting speed for mild steel is 100 ft/min x 4 = 400.

Divided by the diameter in inches, eg 2″ = 200 RPM.

[SillyOldDufferModerator @sillyoldduffer]

I use the metric equivalent of Hopper’s formula:  10000 / diameter in mm.

It gives much the same answer, 400rpm for a 25mm diameter hole.

But, exactly as Hopper says, be prepared to adjust the RPM.  Quite likely I’d bore a 25mm at about 200 rpm to avoid chatter, faster if the tool behaves.   Although I always start with a calculated rpm, it’s also worth fiddling with depth of cut and feed rate too.   Done by ear: if its sounds as if it cutting correctly, and the motor and drive train are working short of labouring, then I’m happy.  (I worry less about the swarf: chips are ideal, but ribbons are OK too.  Not wisps though.)

Dave

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