Take a deep breath, maybe grab a cold one, and settle in for a long story about calibration. I just finished this, after an hour and a half or so of writing, and I apologize in advance for the thesis I’ve written here. I couldn’t help but be as thorough as possible. It’s not glamorous, but it helped me, and it might help someone else. Away we go…
I’ve had an on-again off-again conversation with Routerisstillmyname (gotta love that name) about calibration, specifically squaring my Incra 1000se miter gauge. This post is really wordy, I know, but I’m hoping at least some of you will find it helpful.
A bit of history…
I bought the gauge to supplement the bare-bones factory gauge that came with my Rigid TS3650. Honestly, I don’t remember doing much testing for factory calibration – in hindsight probably a big mistake – I’ve heard lots of great response about the Incras being set up pretty well right out of the box.
I went right to a manual calibration routine, and prior to much of the insight I’ve picked up from “Router-man” and other LJs, I used my $15 framing square to set the fence perpendicular to the blade. My first 45-degree mitered panel frame was nothing short of a disaster.
I bought a set of “precision squares” from Rockler a couple of weeks later. A huge difference from my framing square, and the calibration using the largest one was pretty darn close, although not close enough for my conscience.
Research on LJs (http://www.thewoodshop.20m.com/five_cut_method_swf.htm – thanks, Jon3), emails with Router-man and a couple of calls to the really great customer service at Incra helped me see the beauty of fairly simple geometry as a much more dependable tool than any machine square.
Finally, another couple of weeks ago, I got what I think is a pretty good setup for calibrating my fence. I’m sure it’s not too much different from the technique many folks use, but I thought I’d post it anyway. The thing with the Incra is that there are 3 allen bolts that fasten the fence assembly to the protractor. By loosening them, adjustments can be made to the zero orientation of the fence itself. Tighten, and (ideally) you’re locked in and good to go.
The Squaring Procedure:
I have an analog dial caliper, accurate to .001” (really better, as a physics teacher I rail on my kids to “estimate the next significant digit!”). With jaws open, the extended segment serves as a depth gauge.
Each time I ran the 5-cut test and determined the error (really 4x the error, per the 5-cut procedure), I followed the following steps:
1. Clamp the protractor to the table (I positioned my fence at a location that I could fit a clamp through a cavity milled into the TS table – see pics – you can click on the pic to see the full-size image that shows all of the caliper)
2. Clamp the caliper to the table. (I actually cut a short shim to fit under the caliper so clamping wouldn’t pitch the caliper upward with clamping pressure applied. You can see the shim right under the caliper where the clamp is applied)
3. Extend the caliper’s depth gauge to meet the fence (or fence extension, as in my setup).
4. Use the locking screw on the caliper (just left of the dial, on the near side of the caliper) to lock the caliper jaws & depth gauge in place.
5. Zero the caliper by turning the dial face to match 0.000 with the dial position.
6. Loosen 2 of the set bolts on the gauge to allow the fence to rotate (relatively) freely with one tight bolt acting as a pivot. (more discussion of this later)
7. Loosen the locking screw on the caliper, reset the depth gauge to the appropriate depth (1/4 of the error determined by the 5-cut test), and re-tighten the locking screw.
8. Rotate the fence until flush with the caliper’s depth gauge, and tighten the set bolts.
It took me a while to think to do this – particularly because each time I thought I should really be getting down to tiny tiny errors, I seemed to be off by about the same pesky 0.002” (per 8” length or so)
After step 8, with gauge tightened down and still clamped in place, loosen locking screw on the dial caliper, and “pull-push” the caliper (closing the jaws so the depth gauge is well short of the fence, and then extend it to it meets the fence again – just to be sure the 0.000 you thought you locked it at is still 0.000. Here’s where I figured out the real pain – on initial tightening of those set bolts, 0.000 stayed at 0.000. However, when I did my final serious torquing of the bolts, what I can only describe as “frictional effects” actually torqued the fence itself – each time by about 0.002” – hence the pesky error that wouldn’t seem to go away. Eventually I accounted for this in my initial zero-setting procedure (step 5).
Mistakes I Made (there are more than space allows, I’m sure, but here’s a big one I’m sure of)
The geometry of my setup is questionable, in hindsight. I was using the right-most screw as my pivot – I didn’t loosen that one – only the middle and left-most. With the depth gauge so much farther away from this pivot than the workpiece is, it would seem I made smaller adjustments than I should have at the actual cut line. Adjusting a point 12” away from the pivot by 0.005” wouldn’t produce the same change at a place 4” from the pivot. Hmm… that explains a lot…
Using perhaps the left-most bolt as my pivot, and clamping the depth gauge at an equal distance away from this pivot (compared to the cut-line) seems like a much more appropriate process.
The Bottom Line
I’m within 0.001 after my last 5-cut test, which means down to 0.00025” over the final length I cut (that square of 1/4” MDF kept getting smaller and smaller…) I’ll admit I’m hesitant to say it’s set forever – I’ll be sure to run another test before I use it on good material – but I’m confident that I have a good procedure to get it really finely tuned, particularly when I don’t make the same mistakes I made before.
Hope you’re still awake.
-- happiness is a sharp plane iron