Maximizing Power for High Speed Hog-outs

A reader recently used the “Ask an Expert” feature of our High Speed Machining Zone to ask a series of questions aimed at better understanding high speed machining.

Question

We run a lot of high speed hog-out operations. If the spindle is rated at 15,000 rpm, how long can we run it at that speed heavily loaded? Is there general rule of thumb for a duty cyle?

Response from Tom Delio, co-founder of BlueSwarf

Typically, spindles are rated for S1 (continuous operation) and S6 (30 minutes).  Assuming that the operation is running in a stable fashion without any large forced vibration, and within its design constraints  related to factors such as bending moment and pull-out load, you should be able to run up to the limits specified in the operator's manual.

But please note that the cutting power is not going to be the drive power.  There are losses and an efficiency factor to deal with.  I find many shops run the cutting power at about 80% of the "continuous" rated power to ensure they don’t overtask the spindle drive.  Doing this also provides a decent safety margin for areas where the tool might see a hard spot in the material or temporarily exceed the programmed cut width or depth.

To repeat a point above, one final caution is to make sure that you are not chatter limited because of unstable cutting.  In hog-out operations, this is normally the constraint that prevents use of full power.