Router Runout Problem

[Bruce Thompson]

Bosch 1618 EVS router with Rockler 1/4" two-flute straight bit; 1/2" shank, in Jessem Rout-R-Lift

Incra LS system router fence with right angle stock holder for making box joints. Accuracy of spacing of cuts 1/1000"

Making small boxes for my travel chess set project with sides 1 1/8" deep, using 1/4" box joints with 4 full cuts and one 1/2 cut per corner, so pins and slots must all be .250" to fit snugly together.

I made a trial cut on two pieces of scrap stock and found woefully sloppy joints. Measured with digital calipers, the slots measured .261" wide and pins .239" wide. A clear sign that there is significant runout at the bit flutes. I set up a dial gauge against the flutes and found .010" difference from one flute to the other as the bit is rotated by hand past the gauge, so there is apparently .005" runout; .005" under at one flute and .005" over at the other.

So, where is the error coming from; the bit, the collet, or the motor shaft? To try to find out, I marked one flute on the bit and one side of the shaft, then loosened the bit and rotated it 60 degrees while holding the shaft in place, and tightened it down again. I did so five times, advancing 60 degrees each time and measuring again with the dial gauge after each re-positioning. The results were as follows, counting the initial position as 0 degrees:
0/360º .010" difference between flutes
060º .008"
120º .0025"
180º .001"
240º .0005"
300º .0025"
So, I succeeded in finding the sweet spot to complete this project, but that makes installing bits for future box joints or dovetail joints a tedious process. I would rather fix the problem, but I don't really know how to define it based on the evidence so far. I think that if the shaft/collet were running true, then the fault would be with the bit only, but that runout should remain constant no matter how it is installed. If it is a combination of bit runout and collet runout happening to combine for the big error, I would have expected the 300º value to be close to its neighbor the 360º value, but instead, it was the same as its opposite 120º position.

What have I missed in my testing regime?
What could explain the odd distribution of test results?
What would the cure be?

[Lee Schierer]

It is very difficult to get repeatable readings off a cutting edge. I would suggest that you get a 1/4" drill rod and put that in your chuck. This will tell you if the rotating parts of your router are running true. No offense to Rockler, but their bits in my experience are not the best quality. I buy nothing but Freud bits for my router. Others like Whiteside bits.

[Michael Schuch]

I have a Porter Cable 690 that had quite a bit of runout until I replaced the collet with a new Porter Cable OEM collet. I think the collet that was in the router when I bought it used was a bit abused.

I would start by cleaning the collet and the hole in the arbor that the collet sits in thoroughly with acetone. If that doesn't improve the runout I would setup your dial indicator to read on the inside of the arbor hole with the collet and collet nut removed and rotate the arbor to distinguish between arbor runout and collet runout. Drill rod for measure the collet runout is also a really good idea!

[Lisa Starr]

I'll second Lee's input. Before you do anything else, you need to get a couple of drill blanks. As you're using this in a router table, I would assume it has both 1/4" and 1/2" collects. Purchase both a 1/4" and 1/2" drill blank. Mount each drill and record your measurements as you did. If the readings follow one another, it is most likely the router spindle. If the measurements are erratic from one collet to the other, then it is likely the collect(s).

[Curt Harms]

I don't have any Rockler bits but I'd expect them to be sourced from Asia, same as Woodcraft's house brand or MLCS. If the bit turns out to not be the problem, that leaves the router. So it could be the motor shaft itself or it could be the collet. There are after market collet suppliers that may have tighter tolerances than the OEM collet. A couple that come to mind are Elaire and musclechuck. I'm sure there are others. What I'm not certain about is how to determine if it's the collet or motor shaft. If you could remove the collet and measure the shaft directly that should tell the tale.

[Robert Engel]

More likely to be the bit. Whiteside and CMT are two of the most reliable brands.