router fence - precision question

From all of the discussions here and from my review of the $$ one can spend for a router lift, it seems clear that the precision of the height adjustment can be very important. To better than 0.01". I've also seen the $$ that can be spent for a router table fence, e.g. Incra. However, I have also seen some fences for sale that have no more than jig screws/knobs in T-tracks. And the same type is used on "ultimate" tables. This _suggests_ that setting the fence is not so critical -- or, it is much easier to set without a micro-adjust mechanism. Even on the small-shop shapers, the fence seems to be a t-track set-up.
Of course, some bits use bearings, so that seems to solve a problem with them. And other bits seem to involve setting part of the bit in line with the fence - so a straight edge might make that easy.
I am planning on using a locking miter box bit, and there both height and fence setting do seem critical.
So, before I set out to build my table for my new Bosch router, I'd appreciate some insights on router table fence precision -- both "up and back" setting as well as keeing it "square" -- i.e., parallel to a diameter. I _can_ use my DW TS fence, but it would be better if I could use a T-track fence effectively so that I can route, cut, route again, as needed. TIA. -- Igor
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"igor" wrote in message

If you mean to a bit, a router fence doesn't have to be "square" or "parallel to a diameter", unless your fence is curved, it always is perpendicular to the diameter of the bit no matter how you position it ... think about it. :)
IME, the biggest plus in a router fence (and setting bit height for that matter) is not necessarily in the "precision" of the setting (that can achieved by simply using a simple system like a ruler), but in the ability to LOCK it down securely.
Be wary of those designs that rely on cheap plastic parts to _secure_ the fence. Once you've ruined an expensive piece of wood because the fence moved, you'll appreciate that fact more than ever.
--
www.e-woodshop.net
Last update: 11/06/04
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fence when it's locked. 'Precision' can be achieved with the right setup procedure and a simple screw lock down (assuming the fence doesn't move when you lock it down). The Incra system offers one additional advantage that is important only for some applications: Repeatable and precise incremental positioning of the fence. So if you are routing finger or dovetail joints precisely spaced, for example, Incra's mechanism allows you to quickly and confidently adjust to the next position (assuming it is on 1/32 inch increments).
TWS
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Igor,
I posted a pic of my router fence and lift combo in abpw for you to look at and maybe consider building one yourself and save a bundle. The combination is capable of repeatable tolerances of .001 if needed. I don't need that kind of precision but the ability to dial in the cuts sure is nice.....
It is not a novice project to build and it will present some challenges - looks are deceiving. Pat was selling these for around $400 last time I looked. I also purchased the Jessem Rout-R-Lift when it first came out but there have been numerous improvements from different manf so check them out. The lift I have is solid and accurate so I have no need to look for another.
Bob S.

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BobS wrote:

Please forgive a newbie to rec.woodworking, but what's "abpw"? I'm in the middle of building my first router table, and will be getting started on the fence sometime in the next few weeks.
Thanks Charlie
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alt.binaries.pictures.woodworking
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I've had 2 or 3 router tables and the one I use the most and keep "coming back" to is basically a 4' wide table with a straight fence made of 2 layers of 3/4 ply... cut out under the fence using largest bit so I know it clears.. The only adjustment is that it's held to the table with 2 carriage bolts set in slots in the underside of the table.. I find that the height of the bit is much more critical than the fence setting.... and that the thinner the stock, the more critical the bit height becomes..

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As others have pointed out - the fence need only be flat since it's tangential to the bit. If the fence is tall, then it should be perpendicular to the table.

Yeah - but within reason. I see no need to define critical as "0.0000001in."

I used an 3/4" MDF "aux fence" that slid ontop of my table saw fence. Flat and perpendicular. I liked it very much. The only thing that annoyed me as dust collection - there wasn't any.
So, I just finished up the fence that's described in last month's Wood magazine. It think it's a small step up, the dust collection now works very well, but its not as easy to use as my old MDF one that just slipped onto the TS fence.
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Those were some of my wasted $$ 8-(

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

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wrote:

miter gauge in a table slot, it would seem.

Wow. Thanks for all that. I've read through your post twice and will read it again. I appreciate both the "academic" points as well as the practical. Very helpful. -- Igor
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On Thu, 18 Nov 2004 03:31:49 +0000, Andy Dingley
<snip>

I would add one comment about the swing type of fence. If the router bit is positioned half-way between the pivot point and the measure point then you get a 2X gain in measurement resolution. If you swing the fence 1/32 of an inch at the measurement point it adjusts the depth of the cut 1/64. This is the poor man's micro adjustment.
TWS
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