On Wed, 17 Nov 2004 14:41:27 GMT, igor wrote:

I've also seen the $$ that can be spent for a router table fence, e.g. Incra.

Those were some of my wasted $$ 8-(

However, I
have also seen some fences for sale that have no more than jig screws/knobs
in T-tracks.

A fallacy in fences is that they have to slide. A swing is _much_
easier to arrange, and just as good.

There are different sorts of "accuracy"; precision, accuracy and
repeatability. For physical guides such as fences, we can also add
rigidity. As a user interface question, there's also adjustability.

Precision is the size of the calibrations. It's a good thing if
they're actually in the positions they claim, but that's not what's
measured by precision.

Accuracy is the question of whether the calibrations are really where
they claim. As most fences are uncalibrated, this is simply
irrelevant.

Repeatability is again only a matter for a calibrated fence. It's the
question of whether the same labelled position is in the same place,
no matter whether this is accurate or not.

Most fence designs are uncalibrated and continuously adjustable. This
renders all three accuracy measures somewhat irrelevant. We manage
because the (should) have good adjustability and most craft
woodworking is a one-off process. By "sneaking up on" a measurement,
we can get a cut where we want it. Of course, to cut another piece
the next day (after the fence has been used for other things), we'd
have to re-make the same set-up process from scratch. There is no
repeatability.

The first Incra jig was notable for it's _lack_ of precision. It used
a pair of meshing plastic racks and could only be adjusted in 1/32"
steps. In contrast it had good accuracy (between steps) and was
notably good for repeatability.

The current Incra tries to solve the precision problem by using a
micro-adjust knob. This allows infinitessimal adjustment and fine
precision. Unfortunately the clamp mechanism is such that
adjustability is very poor - the fence is simply not in the right
place whilst the clamp is released for adjustment.

Another downside to the Incra (very much IMHO) is that it's just not
rigid enough. I don't care how accurate the datum mark movement is, if
the ends of the fence can wobble.


So all this leaves us with two sorts of fence. Make yourself an
L-girder of MDF and hinge one end. That's a simple and easy to build
fence that has good adjustabilty within its precision, and can have
plenty of rigidity. If you want more precision, add a horizontal
adjusting screw and even a dial gauge, so that you can adjust a small
range without slackening the main clamp.

If, OTOH, you're doing repeated box making or dovetailing, the Incra
approach is good. It gives you the repeatability and best of all the
speed of adjustment that you need. You may not need accuracy to a
datum at all (the box parts are cut to match themselves, not some
external standard) and the accuracy between cuts is superb. Provided
that you use imperial dimensions and not metric, the 1/32" precision
is adequate.

I wouldn't buy an Incra again. Too flexible for the cost. If I was
doing a lot of finger jointing in this way, I might well make a simiar
fence myself, with some form of rack-positioning (probably 1/16" or
1/8" spacing).

--
Smert' spamionam