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In accurate large scale work, long straight edges are often essential.  But how do we get to an edge that is really straight over a long distance?

We can use chalk or string lines or lasers, but here is a low tech method that does the job very well and quite fast.

This came up in a project involving constructing a 12 segment yurt floor in plywood, and I wanted a straight edge 8 feet long to draw along, and also as a fence for a circular saw.  I happened to have some 6mm ply in 300mm widths, so I used a piece of this.

A good width for a straight edge is very useful.  It makes it much stiffer, and also offers clamping opportunities well out of the way of the business edge.

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Laying out the stock on a surface

To check whether an edge is straight or not is really quite simple.  Lay the piece on a surface and draw down the edge.  I used the back of one of the plywood floor sheets. I use weights to hold down the ply, otherwise the pencil can creep underneath and give a false edge.

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Draw tightly down the edge.  Use weights to avoid any gaps

Then flip it over and match it up with the drawn line.  If the edge is straight, it will perfectly match the line.  If not, the actual edge and the drawn line will be mirror image curves.  My edge was not straight, but convex.

Now I can use the drawn line to help mark a proper straight line on the stock.

I aligned the stock with the drawn line, closing the gap as far as possible.  In this case the edge was slightly convex, so the stock touched the line in the middle, leaving gaps at the ends, which I equalised.

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Transferring the line

Then I set up a gauge to the widest gap.  Moving along the drawn line I transferred the drawn line at regular intervals onto the stock.  You don’t need anything fancy, a piece of wood with a 6mm notch cut out of it would do.  The main thing is to have something set up to the fixed distance. This is much better than repeated measurement.

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Near the centre at the widest bulge

This left me with a line of marks on the stock that was a mirror image of the actual edge.

Then at each transferred point I marked, by eye, half way between the point and the edge.  This new line of points had to be a straight line.

IMG_0157Then it was simply a matter of joining the dots with a batten,

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The straight line is half way between the edge and the transferred points

and planing down to the line.

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When I retested it, the edge was spot on straight.

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The drawn line fits the flipped straight edge exactly

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A clearer view of the flipped line, shifted sideways

A simple but effective way of creating an accurate straight edge!

Points to remember are:

  • make sure the edge of the stock is firmly down on the drawing surface
  • use a sharp pencil for a clean line
  • use some kind of gauge to transfer the line.  A piece of wood with a cut out the thickness of the plywood would do.  The great thing is to have a fixed reference.
  • a long soled plane will be quicker and more accurate.  Clamp the stock firmly.  I just clamped it to the stack of plywood, because it was more convenient than taking it to a bench.
  • Label the straight edge!  It is easy to forget which edge you have straightened…
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Traditional rig fans w20150613_180956ill recognise this immediately.  No, not some strange Aleutian Islands carving of a marine Beast, but a jaw for our sailing dinghy Tiny Tim’s new boom.

The last boom for Tiny Tim had no gooseneck or jaw, and it always blew away from the mast on port tack.  So I thought the two hours that this project would take would be be worth it  For land lubbers, and those strange people who like bermudan rigs, the jaw is fastened by the straight edge to the boom.  The jaw and the protruding boom then form a collar that fits aft of the mast, creating a kind of universal swivel joint.  A slightly fancier vesion has a string of parrel beads around the forward curve of the mast to complete the circle.

I designed this to cut out of a piece of keruing.  Ideally the grain should be chosen to follow the curve, but I didn’t have the right shape.  I do however have loads of keruing, a tough, dense tropical harwood, so I used that and cut it on the diagonal.  For a small spar it should easily be strong enough.

The fun thing about making bits for boats is that you can to some extent make up some of the lines, and here patterns become fun things to make.

For repeat items, plywood patterns are worth making, but for one offs, a paper pattern can be quick and very useful.

In this case, the diameter of the mast is a fixed factor, as is the curvature of the boom.  But the rest is up for grabs.  On a piece of paper a compass gave me the circular space for the mast, plus quarter inch clearance all round.  The curve was drawn freehand until it felt good. So now I had my paper pattern.

The principle I have found most useful in turning an idea into a physical object is a simple one, which involves the gradual transformation of a conceptual line into a physical one.  The way this is done depends upon the tools used.  Ideally each tool rides on the surface left by the previous one.  So working out the strategy backwards is a good idea.

The finished jaw, before final shaping with abrasive paper, would be routed with round overs running on a bearing.  The router would run on a square cut ‘blank’ already satisfactorily curved.  The circle is cut with a hole saw in a drill press.  The curve will be cut roughly with a bandsaw and smoothed with a spoke shave.    The straight edge will be band sawn or cut carefully freehand with a circular saw, and then planed to the lines.

So how to transfer the lines from the paper pattern?  Easy.  Glue it on with a stick glue like Pritt.

This gluing on strategy is brilliant.  You can also use PVA glue, although Pritt dries fast and doesn’t cockle the paper much.  If you use PVA, paste the paper and let it expand before stucking it on the timber.

For a job like this, simply line up the sticky pattern with the desired grain direction and rub it on.

First I drilled the big hole on the drill press.  Then I then sawed the straight edge on the circular saw and planed to the line.  I bandsawed off the waste around the curves and tickled then up with a pair of spokeshaves.  A stationary sanding disc or clamped belt sander also does a good job of the convex curves.

The pattern is now redundant, so can be pulled off.  A light rubbing with sandpaper gets rid of most of it, the rest will come off in finishing.

I thought of routing the roundovers on the router table, but it just felt too small to work with safely, so I clamped it to a bench and used a laminate trimmer with a 3/8th roundover to get the curves on the edges.  Here I made a slight error of judgement, as I predrilled the fastening holes when it was square, thinking it would be easier and more accurate (which it was).  But of course the roundover bit bearing dipped slightly into the drilled holes.  It’s a tiny wobble in the curve, but a good reminder for the future.

Then I used 40 grit abrasive really to round out everything.  Don’t muck around with anything finer.  80 grit to rub out the marks, and 120 pre-finishing.

The straight section had to be hollowed to take the curvature of the spar.  I do have a set of hollows and rounds sitting up in my workshop loft.  But I used my handy rasp, which for such a small job was just as easy and avoided climbing a ladder.

The hole drilled in the face has a double purpose.  It serves as a fastening point for a downhaul; and whilst I am experimenting with where it fits on the boom (before permanently fastening it with copper nails and roves), I will lash it through this hole.

A coat of oil rubbed in with wet and dry, more to come.

An enjoyable little project, which I am looking forward to using.