Mods 17




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Peter Smith has sent me a detailed description of his repeatable method of cutting joints on the WoodRat. This is a scale system  which is set to an individual cutter and once set up is very accurate. It takes into account any cutter tolerance and glue line requirement and does not depend on pencil lines on the channel. Designed for finger and dovetail joints and is adaptable for mortise cutting too. Take your time to read it through and refer to the diagrams at the end which make things clear.

So a big welcome to Peter who presents his ....................

 Alternate method for cutting Finger joints with the Woodrat

View of scale board and cursor in situ and ready to go (Turbocad).


"Methods described in the manual use part of the project timber as a template to follow when setting up and cutting the joints. 

A ‘pattern’ of the cut made is drawn onto the Woodrat channel and is followed or referenced against while making subsequent cuts into the project timber.  Sometimes this “marker” board is removed and re-mounted several times per job.

The fact that this method works is not being contested, but what is offered here is a more precise and reproducible method that doesn’t need so much work in setting up, still leaves you the freedom to put things where you want them and “lives” again another day to provide a template for further joint making - without having all the rigmarole of setting up and drawing shapes on the channel anew.

Drawn cut outs on the Woodrat beam give the impression that we are matching shapes – which is re-confirmed every time the Marker board is moved along to the next cut out – whereas we are really referencing position instead.

The cutter itself will take care of shape; all we have to do is get the bit to the correct position on the board being jointed before we make a cut.

To get accuracy your reference marks, once set up, should not be moved from their mounting on a machine (like the WoodRat) for the duration of the job in hand – to prevent inaccuracies due to re-positioning errors creeping in.

This means that the ‘Marker/project’ board as described in the manual is replaced by a permanent reference scale as described here.

The reference scale ‘lives’ in the left hand (Marker) cam of the Woodrat and is set up according to the job in hand.  It's dimensions are “long enough” for the job, “deep enough” to fit in the cam-lock and “thick enough” to bear a white card “scale” ruler strip which is glued to the top edge of the board.  One also glued onto the bottom edge would double its efficiency.

To refer to this scale a transparent acrylic cursor is mounted on the Woodrat beam (channel ed) so its engraved reference line overlies the cammed in scale rule.

If used for finger jointing this method is far less confusing to do than the method given in the manual, gives a more consistent joint – and there’s no swapping of boards around; and you have only one simple line as point of reference.

It is, perhaps, an extension of the method given in the manual for stacked comb joints but is more refined and used in the E to W orientation.

Practical details

The cursor can be adjusted E to W by a generous amount so it can zero in to the ‘First cut’ line with room to spare.   There’s also a little ‘play’ so it can be ‘squared up’ to the marked scale lines on the ruler. The scale for a particular job is drawn onto this card ‘ruler’ and the lines will indicate the centre lines of the cuts to be made – whether it is to be a dovetail or a finger joint job  - although setting up for finger joints is rather more precise in nature.

The type of card used is illustration board and is quite thick so it won’t stretch when glued into place; what it will do however is take divider marks more readily than a laminate – which, in turn, helps to keep things precise – and it is preciseness we’re looking for.   The card is replaced with clean stuff easily if something like Copydex is used as adhesive.

It is possible that you may have to do more than one scale before the joint produced is just right but thereafter the scale will give accurate, reproducible, joints with no more setting up to do than zero in to the first cut line as described in this article.

One of the things to remember is that the nominal 8mm cutter may cut a slot that is marginally more, or less, than 8mm so it IS important to use the actual CUT as the unit of measurement for the dividers to step out to.  

It may be possible to protect the rule surface by using a lacquer varnish so it keeps longer in service – especially if it refers to a finger joint.


Drawing the reference scale must be precise – use dividers that can be locked and draw the scale lines square to the rule’s edge.

Start cutting each board from the same edge – e.g. from the top one.

Finger jointed corners have TWO components – side A & side B (just as dovetails have tails and pins).   Only side B slotting into a side A will make a corner joint. In practice for a box this means having two opposing sides as ‘A’ and the other two as ‘B’; this will give a more balanced end product - in much the same way as a posh dovetailed box would do.

Making sure it fits first time

A ‘unit’ of finger joint is one tongue and one slot.  The slot is made by the cutter and the tongue is what’s left after the cutter has passed by.

The tongue should slide easily into its slot but without being sloppy and should allow just enough space for the glue line.  This means that the tongue is narrower than the slot cut by the router bit.  It isn’t much – a mere wiff waff is all but it IS important – so allow about, 0.1mm or so when making the measurements.  It has to be consistent throughout the whole joint or it looks awful – hence the dividers.

How to do it:-

Woodrat is set up with the straight cut aluminium guide rails in place.

Make a cut into a small piece of timber using the (bottom cutting) straight bit (a spiral bit would be good  ed) that will be used for the joint.   Go just deep enough to get an accurate measurement of the width of the cut that has been made – it IS necessary to measure the CUT made and not rely on labels or being able to measure the bit itself – use dividers.

Step the dividers two ‘paces’ onto card (= twice the cut width), then reduce this by the amount to be allowed for the glue line fitting (about 0.1mm).   Adjust the dividers to this new dimension for transfer to the card scale.

Make the first line on the scale rule ½” in from the leading (left hand) edge then, with the dividers, step from here along the rule making pointed impressions along the way.  As this scale is re-usable for the same cutter you can mark as much of the rule as you feel will be useful.   Reiterate the divider marks with fine drawn lines square across the whole width of the rule.  

These are the lines to be referenced by the cursor when cutting all the slots for the sides of a box and can now be numbered, from the left edge, from 1 to whatever.

The A sides are made first after setting up and ‘zeroing’ the cursor / scale / cutter relationship. 

To make the B sides we have to ‘shift’ the reference points to one side by just enough for the wood left on a side B to fill the gaps left by the cutter in side A, and the gaps cut in B sides to correspond exactly with the wood still left on the A sides.

A short line this time to set it apart from the ones used to guide the cuts – call it “shorty”.

So now set the divider points to precisely the cutter’s width apart and use them to mark a point to the LEFT of the 3rd ruled scale line.  Draw a “shorty” line over this marked point.

Shorty is precisely one cutter’s width to the LEFT of the 3rd ruled line – remember that.

It is important to make sure that Shorty cannot be confused with the other lines on the rule.

 Zeroing the Cursor and Rule

  1. Cam the first work piece in the cutter position, zero the router bit and set plunge depth  to thickness.
  2. Cam the scale rule board in the ‘marker’ position with the ruled surface at the top and just barely skimmed by the cursor.  The ruler’s start end at the LEFT.
  3. Move the carriage until the depressed router cutter is poised to cut not quite its full width from the leading edge of the work piece – i.e. the cutter overhangs the leading edge of the board by a small margin.  This is the ‘first cut To affect this we need a further line scribed onto the ruler – position’.
  4. Without moving the boards now adjust the cursor until its engraved line overlies the whole of the first line on the rule.  Lock cursor in this position.
  5. This is ‘Zero mark’ or ‘first cut line’.

For the first use of a scale and cutter combo it is wise to do a test corner joint first using as much of the drawn scale as convenient – the more positions you reference the more thoroughly you check the scale.   Once this has been done satisfactorily the scale can be made permanent and used with confidence for a new job, using same cutter, without need for any more setting up and drafting – or drawing anything on the aluminium channel.

Cutting side A

Do make a point of ignoring “Shorty” line when making cuts at all stages.

  1. After zeroing the scale take first cut at the ‘first cut’ line on ruler, in steps or a single pass to full depth.
  2. Move carriage until cursor lies over next line on the ruler and make the next full cut.
  3. Repeat step (b) until side A is completed.

To repeat a side A on same timber somersault the board and re-cam it into the cutting position then start at first cut line again and repeat the sequence.

Cutting side B – or why “Shorty” exists

a.       Cam side B timber in cutting position. Wind carriage along until cursor covers the 3rd line on the rule.  Stop.

b.       Unlock the cursor levers and move the cursor BACK to the Left until it covers “Shorty”.  Lock the cursor down.

c.       Wind carriage until the FIRST CUT ruled line is under cursor and repeat the cutting sequences done for side A – but this time the product will be a side ‘B’.

To produce side B we have, in step ‘c’ above effectively moved the CUT FORWARD by the distance of one cutter’s width. 

This action removes, on side B, the timber left standing on the previously machined side A.

Side B should slot into side A without problem.

For a box cut all sides A before resetting to cut sides B – in the interests of accuracy.

The carriage is now wound on so cursor covers “Shorty”.   Unlock the cursor and move it FORWARD (to the Right) to cover the 3rd scale line.   Lock it down and the reference scale is “zeroed” to start cutting sides A again

In every case of making reference the cursor should lie superimposed over the line drawn on the scale ruler.   If both lines are fine ones there should be no difficulty about precision.

If there is a problem with joint fitting then it means back to the drawing board on the following remedial path.

1.      If joint is too tight then the scale lines are too far apart.

2.      If joint is too slack then the lines are too close together

Re-measure the cut made and repeat the divider job

With care and a good eraser another scale can be drawn on the old rule.

 Can errors still occur in this method?

Surely they can if:-

  1. The cursor line, or scale lines are drawn too thick
  2. The cursor is too far above the scale rule – (parallax problems).
  3. The cursor line is drawn on the top surface of the acrylic instead of lower one.
  4. Viewing is done from the side of the cursor line rather than from directly above it.

However, because the positioning of the scale and cursor allows (or demands) a more physically comfortable viewing angle (than a stooping squint) these problems should be easy to circumvent.

Dovetails by scale and cursor

This is less involved than the finger joint procedure but requires more thought on pin positions for a given width of project timber.

It is very flexible in that you can put the pins where you want them of course and to hang with aesthetics or you can use the parallelogram for positioning the cuts.

Once again the pins are marked by their centres and not by the shape they are to be.

Having sorted that out the lines are marked onto the scale rule with a line for the first (fat) pin marked somewhere about 3/8th to 1/2inch in from leading edge of rule.    The reference scale for dovetails will be more specific to timber width than that for finger joints since the first and last pins must be sturdier than their inner neighbours and occur at the very ends of the compound joint.

We don’t need a “Shorty” for dovetails but we do need test sticks to complete the setting up.

A line on the rule to represent the test cut position is made about 3/8” to ½” in from the first, numbered, line on the scale and, when set up for test cutting the timber dimensions will be sized to suit where the router bit will cut when the cursor indicates ‘test cut position’.

It is not possible to draw other lines on the scale once it has been zeroed – which HAS to occur before we are ready to prepare the test sticks – so we must dictate where the test cut will be made on the ruler before we start.

The test stick line should, again, be easily distinguishable from the other reference lines.

Test stick components are cut using this mark only.

Zero the cursor and scale to the first cut line by poising the router over its first cutting position in much the same way as done for finger joints but this time you have to ensure that the end pin(s) are sturdier than the inner set as described in the manual.

All following dovetail processes are as described in the manual apart from the fact that you will match position by cursor and scale line rather than drawn outline and cut out: you won’t be shifting the timber from cam to cam, and you won’t be squinting to see if you are in position yet – just a comfortable observation of two lines coming into conjunction.

 Scales that are more or less ‘universal’ in application – without too much dependence upon width of timber to be processed (like finger joints) can be inked in for re-use.

Those for ‘one off’ applications should be pencilled so they can be cleaned up and re-drawn for other projects.

Mortise Rail

The mortise rail could also have a similar scale fitted along the back of the rail construct to mark out – in a ‘rod’ like manner – the extent(s) of the mortises to be cut into the project timber clamped into the rail.

Although we can – carefully – watch the cutter at work when mortising it is still more precise to be able to travel from line to line as the cursor and scale would allow – and you don’t have to mark out each piece of project timber before you cut – the rod will control the process.  

Some means of having timber support and end stops on the mortise rail would enable easier reproduction of jointed parts without having to measure out each one.

Mortise rail with adjustable timber support and adjustable stops is shown on separate pages.

Scale and Cursor Construction for Finger Jointing



Made from 6mm acrylic sheet, routed to shape and with a 5 or 6mm wide slot routed for mounting bolts. The cursor line is 'engraved' by a sharp awl, needle, or marking tool on the underside. The scratch made is filled with black ink from a marker pen and the surplus ink wiped away to leave a fine black reference. There is a little waggle allowed to enable squaring up of cursor line to ruler line.

Method of mounting on the LEFT hand side of the winding handle when at it's right hand limit and where the cursor can easily overhang the end of the ruler when it is mounted in the marking cam at it's left hand useful limit. Holes for the M5/M6 studs are tapped into the upper surface of the beam at a suitable point to allow the maximum amount of travel E or W for adjustment and zeroing.

Small Bristol levers through the slot supply the locking mechanism-- Trend bits

border="0" Scale board with rule top edge and showing "shorty" line position.
Scale rule for finger joints is specific to the router bit and can be re-used with the same bit as many times as the scale remains readable- or the bit produces the same cut. The only setting up needed for a repeat jointing operation is to perform the 'zeroing' technique after mounting scale and project timber into the WoodRat as directed in the description. Distance between the numbered lines is (2 x cutter width) minus an allowance for the glue line - which means that, for each unit of joint, slightly more wood is taken away than is left in situ. Shorty is precisely 1 cutter's width to the LEFT of line 3 - thus by the logic of contrariness, pushing the cut FORWARD to the RIGHT by the distance of the width of the cutter.

No more drawing on the channel, swapping boards round and other time consuming activities before getting on directly with the proper joint.

Another scale on the opposing end of the 'marker' timber doubles it's usefulness."

Well thank you peter very well presented.

Come on folks I think that deserves a round of applause  ...   ....  ....  ... ... ... ... ...


I had to select a quiet time and  sit down with a hot cup of tea to properly digest all that. I have to say that is an excellent way to get a really good joint as it is not only the tolerance of the cutter but any run-out of the router that is taken care of here. I used a good spiral bit and cut a gauge into some acrylic to measure the true cut diameter. I keep the gauge and label it with the bit description for further reference. If you do not want to drill your 'Rat then I think this would work fine by just clamping the plastic cursor to the channel.

Instead of using dividers I measured the gauge with digital calipers and printed out a card scale using Corel Draw on the PC. (For a 1/2" bit which did cut a 1/2" slot I set a grid with 0.95" spacing and printed that out with very fine lines). I plan to set this up and post some photos later.


Peter thank you for taking the trouble to write this up and share it with us.


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