One of the leading causes of bit breakage
on CNC routers is excessive runout or eccentricity at the cutting tool
TIR - Total Indicated Runout)
As recently reported in the trade press, excessive runout can have a significant effect on the life of carbide tools.
In a test of of solid carbide drills, improving TIR from 0.0006 " to 0.00008 " tripled tool life. Ironically, in the survey,
the average tool user considered 0.0005" (0.0125mm) TIR to be acceptable.
While researching the source of the runout, a surprising
fact emerged. Most of the low-cost (<US$150.00) routers and laminate trimmers
that we tested were actually quite good (TIR < 0.0003 in. measured
inside the hollow collet taper). However, most of the the
stock 1/8 in. collet adapters were just plain junk. Our measurements of
stock adapters ranged from 0.0030 in. TIR to 0.0100 in. TIR. We did not find a
single adapter with less than 0.0030 in. TIR, enough to snap off a 0.0313 in.
(1/32 in.) carbide cutter, even under a low to moderate chip load.
The aluminum collets found in many hand
grinders were the worst of all with TIR measuring as bad as 0.0150 in.
The situation improved substantially with more expensive routers,
although some models did not offer collets for tools smaller than 1/4
To test our carbide cutters using a couple of
these low cost routers we designed our own high-precision
collet with run-out less than 0.0002".
Before measuring the TIR, make
sure that your spindle bore and collets are clean and free of any
Measuring spindle TIR is quite straight forward if you have access to
a "Dial Test Indicator" (NOT a Dial Indicator) with a resolution of at least 0.0005 in. (0.0001 in. is
preferred). If you don't have one,
go buy one. If you
are on a budget and can wait for a couple of weeks, go to eBay
and search under "test indicator". Starret, Fowler and Minutoyo are brands to look for.