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20170626_121102_New Quay Court

The  knock down portable work bench

One of my favourite principles in doing pretty much anything is what I call ‘circuits’.  It’s all about getting the energy you are putting in, precisely to the place you want it.

Anyone familiar with metal cutting equipment like lathes and milling machines knows that a great deal depends upon stiffness.  The tool and the workpiece must be held rigidly.  When the tool is presented to the workpiece, any ‘slack’ in the physical path between the tool holder and workpiece will be taken up before cutting happens, causing wasted energy, inaccuracy and vibration.  The ‘circuit’ between the two must not have any energy leaks.

The metal worker, operating at thousands of an inch or less, knows that these ‘leaks’ are visible enemies, and will destroy his work.  They will cause the workpiece to deflect, set up vibration patterns, even cause ‘dig ins’ as the vibration pattern is superimposed on the force attempting to cut.

The woodworker is operating at bigger tolerances, so this problem is not so obvious. When a piece of timber moves as it is sawn, it is often just accepted.  When it bounces as it is struck with a chisel, it is accepted.  When the workbench wobbles as a piece is planed, it is accepted.

Well, not by me.  The point is that *any* slack in the system creates an enormous loss of power and accuracy.  I don’t really think many people realise how much energy is used in moving stuff around, rather than cutting, and what this actually means.

For example try this experiment.  Secure a piece of 1″ x 1″ wood in a vice so it is sticking out about 10″  Then try to cut it with a saw about 6″ from the vice.

You will instantly notice that the wood will vibrate as you cut it.  The energy you are using starts to fight against you.  The vibrating saw cut jams the saw.  It is impossible to cut a straight line because the wood always wants to bend.  The jamming of the saw causes the saw to vibrate and the wood to bend even more.

Now try the same experiment cutting very close to the vice.  All the thrust on the saw goes into cutting.  It is easy and accurate.

The difference between these two experiments is pretty obvious.  Cutting at a distance from the vice turns the wood into a spring that absorbs the energy of the cut, and then releases it in an unhelpful way.

If you properly secure a workpiece in an immovable vice, the effect is astonishing.  Cuts are effortless and take a fraction of the time. you can be stunningly accurate, paring off transparent shavings.  Everything is way, way better.

Securing your workpiece and cutting it close to the vice or clamp is second nature to craftsmen.  But how do you know your clamp is secured? What is going on between that clamp and the ground, and the ground and your feet? All you need is a slight wobble, a slight give in your bench, and all that sharpness vanishes.

A workbench is like a system of springs between you and the workpiece.  As you apply pressure to a workpiece the springs give, absorbing your cutting energy.  The more they give, the more energy is absorbed.  Only at the point that the wood you are cutting cannot resist the force of the ‘spring’, will the wood actually cut.

If there is any slack in the system, your workpiece will travel the distance of that slackness before it is cut.  So you are cutting a moving target, that springs back as you cut it.  The result is far lower accuracy and far less cutting power.  Even a small amount of spring makes a massive difference.

One question that came up some years ago was whether it was possible to construct a mobile workbench with very high stiffness, that could be taken apart easily.  Here is my solution, that is still going strong.  It could certainly be improved upon, but it is way better than most static workbenches, let alone mobile ones.

The basic principle that is essential in a mobile bench is that the circuit between the operator and the bench must be closed.  In practice this means that the bench must include a floor on which the operator stands.  The good old Workmate actually does address this, by having a platform you can put a foot on whilst using it.  But it is extremely springy, and the modern version is very poor quality.

My solution was to have a portable floor.  I made this from OSB, but I think a good quality plywood would be better,

The easiest way to describe it is to show you how I put it together.

Components are the floor, a base, three legs, the bench top/vice and three clamps.  The whole thing takes a couple of minutes to knock down and build, and easily fits in a car.  The bench part has only a single M12 bolt holding it together. The bench and legs are made from Keruing, a hard dense and heavy tropical timber, that I happened to have on hand.  I would recommend a heavy hardwood, because the mass definitely helps.  The base is softwood.

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The components.

The base that holds the bench is made from two pieces of timber in a ‘T’ shape. The  upright of the T is dovetailed into the long part.  The ends of each part are mortised to take the bench feet.

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Softwood base, dovetailed together

The bench itself is two pieces of hardwood, that clamp over the legs.  The legs are half-dovetailed into this, to that once the two pieces are held together, the legs and bench form a rigid structure.

The third leg is bolted through the bench.  This bolt is the only fastening holding the whole thing together.

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The third leg.  This bolt is the only fastening!

Once the bench is fitted to the legs and the bolt loosely done up, the whole thing is placed on the base and the tenons on the legs slotted into the mortises.

Then the base is located on the OSB floor.  The floor has captive ‘T’ nuts underneath that take the bolts that go through the base.  The bolts pass through timber clamps that fit the angle of the legs, and effectively create a kind of dovetail effect.

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The clamps bolts the bench to the base and create a dovetail effect on the legs.

This structure is amazingly rigid.  With forces along the vice, it is exceedingly stuff, better than most static work benches.  There is a small amount of lateral movement owing to the flexibility of the OSB but I have got around this by putting weights on the floorboard, or wedges underneath it at the back. An updated version might include stiffeners for the floor board.  I tend to use the centre of the vice for sideways loads, as the forces are resisted very well by the rear leg.

20170626_121102_New Quay CourtIn using the bench, one stands on the board.  Thus the circuit between the user and the bench is always fixed.  You can stand in front or behind.

The vice is fairly primitive, using captive nuts and 16mm studding.  At the time I had only basic metalworking equipment.  Ideally I would have sliding bars to prevent racking of the vice, and maybe handwheels.

Some kind of stop could prove useful. You could even put an end vice on it!

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In action, turning a chunk of oak into a Samson post

 

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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…

So having half lapped the rubbing post stock, shaped it and tidied it up a bit, it was time to bore some holes.

The posts were to be bored through the width (about 6”) with a 13 mm hole to take a length of 12mm stainless studding. The outboard edge needed to be counterbored so the nut and washer were below the surface.

Jig showing guide plate and 'legs', screwed together

Jig showing guide plate and ‘legs’, screwed together

The procedure I used for boring the holes is very familiar to boatbuilders, but with a couple of refinements.

Having set out the locations of the holes, I squared a line across the face and both edges. I found the centres of the edges, and punched the hole location on both sides. I almost always punch holes I’m going to drill. It ensures that the hole is spot on.

The principle of drilling a hole that is perfectly square to the edge and parallel to the face is to construct a guide jig that will support the drill some distance above the hole. This guide is a small piece of wood drilled to the same size at the hole. By positioning it exactly over the punched location of the hole, it will guide the drill along exactly the right path.

To construct this jig, I took a small offcut of 18mm plywood and ripped it to the thickness of the post. By sliding the post up to the circular saw blade, and the fence up the post, the exact thickness was captured on the saw. The offcut was ripped to this width.

Then I found the centreline on this guide piece, and then punched a mark in the middle on that centreline.

This I drilled out with a 13m drill.

I found two more long offcuts and screwed these to the edged of the drilled guidepiece. The jig is pretty obvious from the photographs.

The other requirement was two spacers, made from another offcut about 5 inches wide, with parallel sides, which I cut in two.

Jig initial set up

Jig initial set up

To align the jig, I lightly clamped it in place and slid the 13mm auger bit I was planning to use, through the hole, with the tip resting in the punched mark.

Spacers set up. The idea is that, because the spacer edges are parallel, the squareness of the square is 'transmitted' to the drill bit.

Spacers set up. The idea is that, because the spacer edges are parallel, the squareness of the square is ‘transmitted’ to the drill bit.

With the combination square resting on the edge, I placed the two spacers in position, one sitting in the edge and the other on the top of the jig. By pushing the jig towards the combination square (a light hammer tap maybe), the whole thing aligned perfectly.

A big gap - the jig and square are pushed together to square up the drill bit.

A big gap – the jig and square are pushed together to square up the drill bit.

A hammer can be used gently to tap it together

A hammer can be used gently to tap it together (terrible photo!)

The drill bit is now aligned to the spacer

The drill bit is now aligned to the spacer

The spacers press against the side of the auger bit and the edge of the square, the jig swivelling until everything is touching. Clamp up hard in two places.

Checking to ensure that the guide is on centre. As it was ripped to the same thickness as the stock, the straight edge should align with the the face of the stock and the edge of the guide.

Checking to ensure that the guide is on centre. As it was ripped to the same thickness as the stock, the straight edge should align with the the face of the stock and the edge of the guide.

To ensure that the guide plate was directly on centre, I set the straight edge against the face of the stock, which just touched the guide plate. If it hadn’t, I would have slackened the clamp and pushed it sideways slightly.

Rather than risk moving anything, I removed the bit from the jig to fit it to the drill, and then re-inserted it carefully. Then it was simply a matter of squeezing the trigger and letting the drill do the work.

Right through the hole...

Right through the hole…

I actually bored from both sides, first counterboring the outside with a forstner bit. But I probably didn’t have to because the holes were so accurate that you couldn’t see any join where they met.

This method is pretty good. For boring really long holes there are better ways, as any bit will tend to wander off course, diverted by the grain of the wood. A good way is to use a boring bar rather like you would bore out a cylinder on a metal lathe. This is often used for boring the stern tube hole in a boat, which has to be very accurate over a long thickness of timber.

One thing that became apparent to me quite early on in learning about boatbuilding was how everything is built in layers, one process upon another.

It occurred to me that being really really aware of this can make a huge difference to the ease in which the multiple operations necessary to make stuff, can happen.

What if it was possible always to be mindful of a process in which the tool performing each operation “ran” on the surface created by the previous tool, like a railway running on tracks? This would surely increase the precision and speed of working.

Well it is possible, with a bit of foresight. Perhaps not in every operation, but in almost every case where something is designed, rather than freeform.

To think about this more clearly we need to work backwards, looking at the properties of each tool, and seeing what is needed for it to run on.

And this is the thing: tools are designed to have specific properties that you don’t have to think about. Even the simplest tool. All we are doing here is acknowledging those properties and using them to our advantage.

It is possible, reading this, that you might wonder whether I’m slightly mad at the attention to detail. But I assure you that if you take these principles and apply them, it will make everything so, so much easier.

I would recommend taking some scrap wood and experimenting with all this.

Chisels

A poor chisel point.  The bottom edge has been dubbed over by bad sharpening, so the cutting edge is above the plane of the flat back.

A poor chisel point. The bottom edge has been dubbed over by bad sharpening, so the cutting edge is above the plane of the flat back.

The most important things about a chisel are a completely flat back, parallel edges, and a square cutting edge.

By ‘completely flat’ I mean really really flat. There should be no dubbing over of the edge. This is an exceedingly common sharpening mistake, and it destroys the main useful property of a chisel, which is that it cuts exactly on a plane with its back.

Furthermore, the corners should be square and sharp, and not rounded over.

A good chisel like this will run on an edge and create a surface. Like for example cutting a housing, if I have a clean edge on each side, I can run the back of the chisel on the edge to level the surface.

A good chisel point - the cutting edge is on the same plane as the flat back

A good chisel point – the cutting edge is on the same plane as the flat back

It will also run on a surface to extend the surface. So if I am cutting a mortise and want to trim up the sides, I can cut a part of it perfectly square and then use that to ‘rock’ the chisel along the mortise, running on the surface and the top edge.

This property is regularly used in cutting joints. The prerequisite is a flat surface or a straight edge to run on. How do we create those?

Turning a line into an edge

How do you turn a line into an edge that a chisel can run on?

Well you can use the chisel to do it. But a chisel cannot ‘run’ on a pencil line. It can however run on a groove.

One technique for accurate chiselling to a line is to create a groove that the chisel’s tip will run in and will locate the chisel’s final cut precisely. Because that nice sharp edge on the chisel will slide up and down a groove like a monorail train.

So how do we turn a pencil line into a groove?

With a knife.

Grooving and the properties of knives and rulers

The lightweight scribing knife picks up the line and the rule pivots on it into position

The lightweight scribing knife picks up the line and the rule pivots on it into position

A few light strokes from the scribing knife

A few light strokes from the scribing knife

The  heavier utility knife runs in the scribing knife's groove without the need for the ruler.

The heavier utility knife runs in the scribing knife’s groove without the need for the ruler.

Knives are pointy cutting edges. All they do is sever fibres superficially, crushing them back on either side of the cut and leaving a ‘V’ shaped groove.

Fortunately, we generally have available a surface that a knife will run on – a ruler.

What you need is a nice, stiff, square cut metal ruler. Not plastic, because the knife runs up onto it, and not one of those wafer thin ones, as the knife runs up onto it on even underneath it..

This may seem all very simple now. Line up the ruler on the stock, cut the line with the knife.

No. Still too much hassle.

We still need to be thinking about doing things easily, not compromising. How are you going to line up the ruler? Fiddling around peering at it?

There is a much easier way. A knife doesn’t just cut. It can be used as a temporary pin. Touch your knife point at the beginning of the line. Then push the ruler up to it. Then rotate the ruler parallel to the drawn line.

You are essentially ‘running’ the ruler on the knife point. And this will end up with the ruler being exactly where it needs to be. It is very easy to position it, because only one end is moving and the other is in exactly the right place. Then you can run the knife on the ruler.

Now score a nice deep line?

NOOOOOOO!

The chances are if you do that , you will push the ruler off line, or the knife will follow the grain rather than the ruler.

Just score the surface lightly with the knife a couple of times. Then press a little more firmly. Now the knife will want to run in the light score mark.

Take the ruler away and now run your knife down the scored line freehand. See? It stays in the groove.

Some people will use different knives here. A fine pointed light craft knife makes a good scoring tool. Then you can move on to something beefier and deepen the groove – if you want. This can be very useful if you want to create a groove for a saw to run on (more on that later).  What you use depends upon how accurate you want to be.

To go deeper: a Stanley type knife, or a marking knife. A wide bladed chisel is a very good option. If you are deepening the groove significantly, you want a blade with only one bevel, so it leaves a vertical face at your line, not a ‘V’ groove.

So now you have turned your pencil line into a groove, and you have a mechanical way of following your line.

This may sound like a real hassle. But it’s a lot quicker to do – about ten seconds – than describe, ensures precision, and you hardly have to think. The tools do what they are best at.

Chiselling to a groove

The struck chisel is driven back over the line, because the wood cut at the bevel face has no where to go

The struck chisel is driven back over the line, because the wood cut at the bevel face has no where to go

Stick the chisel vertically in the groove and bash it.

Oh dear. You notice that the cut has shifted backwards over the line?

The problem here is that a chisel is essentially a sharp wedge. Drive it into wood and it crushes the fibres on both sides of the wedge.  The fibres on the bevel face have no where to go, so the chisel is driven backwards.

To ensure that the chisel cuts bang on the line, you have to stop it from being driven back. So you have to remove most of the material on the waste side; then when you bash the chisel in the groove, the waste falls away with minimal resistance. This will prevent the chisel from being forced back across the line.

With nothing but a small amount of waste to cut off, the struck chisel stays bang on the line, and the waste breaks off.

With nothing but a small amount of waste to cut off, the struck chisel stays bang on the line, and the waste breaks off.

The amount of waste you need to remove depends upon the type of wood, and the direction of the grain. Dense wood generally presents a much greater resistance, so you have to take it right back to less than a millimetre to be absolutely sure.

Experiment!

The other thing you must be very careful about is the grain direction. Paring away end grain is easy. But if the grain is parallel with the groove, you have to work from the correct end. There will usually be at least a slight angle between the grain and the line, so the grain runs out over the line. You have to work in the direction of the run out, so the cut chip splits out into the waste. If you have a wavy grain line, you may have to work in different directions. If you are uncertain, draw arrows on it. If you don’t, the grain may split across your line.

Chiselling a surface from an edge

The first thing to think about here is do not destroy your edge. If you have a nice sharp edge and apply leverage against it with the chisel, you will have a round edge instead. This will happen if you drop the handle of the chisel below the plane of the surface you are cutting.

Creating a shelf to run on a short distance above the groove

Creating a shelf to run on a short distance above the groove

The solution is never to go back to your groove until most of the waste has gone. The groove is used to give you perfection. Instead work to a line a millimetre or two so on the waste side.  Cut a shelf to work from.  Then clear away the waste.

Now you are going to turn the groove into a shelf.

The waste is mostly gone.  Now a shelf is cut using the grooveto locate the chisel

The waste is mostly gone. Now a shelf is cut using the grooveto locate the chisel

Once you have removed the waste to this point, take it back to the point where there is no danger of being forced back across the line. Then tap the chisel in on the line with a mallet at exactly the angle you want. One firm, decisive tap. You don’t need 5 inches of penetration. Just five millimetres will give you a ‘shelf’ at exactly the right position. Now, resting the chisel on the shelf, pare off the rest of the wood. Running totally flat on the shelf, the tip of the chisel simply extends the plane of the shelf.

Using a 90 degree block for an accurate right angle cut

Using a 90 degree block for an accurate right angle cut

If you feel a bit uncomfortable about your accuracy with the angle of the chisel, make up a hardwood block with the right angle, or use a small square. Put the chisel tip in the groove and slide the block up to it, pivoting the chisel in the groove so it is exactly seated on the block. Then take the cut. All you are doing is extending the face of the block into the wood. For a 90 degree angle really you should get used to judging it. For other angles this can be a very useful jig. Make it even better by pasting some 80 grit abrasive to the bottom surface, to stop any danger of slipping.

Using an angle block to run on

Using an angle block to run on

Hopefully this will have demonstrated why it is essential not to dub over the back of a chisel even slightly. It simply will not do the right thing unless the cutting edge is exactly on a plane with the back.

Next episode..

There are many ways of using this idea of building a succession of processes that run on the previous one.  As a thinking exercise, every time you pick up a tool ask yourself what it runs on.

In the next episode I’ll talk about planes.