CNC SPINDLE TIR:
Total Indicator Runout
What is TIR:
 TIR is an acronym for “Total Indicator Runout” : This is an industry measure of spindle accuracy, a measuring stick of how true the rotation of the shaft is, ... in other words is the bit going to run in concentric circles around the center or is it going to have a wobble??
For obvious reasons, this is key if high precision is on your agenda and often referred to as the CNC spindle runout. Ask for a specification on any CNC spindle you're thinking of buying, except of course for ordinary routers or rotary tools, they're too low end to have been even measured :)
So, the smaller the number the better! It quantifies the amount the router bit deflects from the true center in free running, as measured by say a dial indicator as indicated in the picture above.
Longer tool life, better finish!
The lower the TIR the less chatter and vibration during cutting so you end up with longer spindle bearing life, thus longer tool life. I think I read somewhere in the neighborhood of 2-4 times longer life which might explain why some people seem to be able to cut a lot more wood than others, using the same router bits and under the same conditions. As a bonus less vibration gets you a better overall surface finish.
Does Speed Make a Difference?
With the very high rotational speed reached when machining softer material like wood, the greater the need for a lower TIR spindle head since the amplitude of vibration increases greatly with increase in rpm, making the chatter worse at high speeds than at low rpm’s.
So if your only planning on cutting aluminum at say 100 rpm, TIR may be almost imperceptible
Well that should give you enough definitions for now and some suggestions on how these basic characteristics might be used for CNC spindle buy decisions. Rpm, torque, power and TIR are all metrics that dictate whether a particular motor will work in your specific application.
Now let’s have a REALLY critical discussion on the next page about the difference between Brush or Brushless Spindle Motors, in a long list of CNC spindle options.
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