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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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The sides of our houseboat are vertical, and make quite a good ‘dock’ for smaller boats. I have my gaff cutter moored alongside, and also we regularly bring boats alongside. Some kind of permanent fendering seemed like a good idea.

I happen to have in my garden a large stack of keruing, which I acquired a few years ago and have been using for various projects ever since. Unfortunately the pieces, although good section at 150mm x 45mm, are only 900mm long, so are often slightly short!

Keruing is a very durable wood, often used in docks, and so I have decided to make up some longer lengths, which will be bolted edgeways and vertically to the side of the houseboat. A horizontal carpet covered board, hung from the side of boat alongside and long enough to bridge two or more of these, will ride up and down these heavy battens.

This is not precision joinery, just a means of connecting two pieces with reasonable strength. I decided on a half lap joint of 100mm. The material was left rough sawn. I cut out the waste using half depth cuts on a chop saw, 100mm from the ends, and the bandsawed to approximate depth. The idea was to plane up the joints with a router.

Some years ago I made a scarphing jig for my router. This involved a base with two rails at a 1 in 8 slope. On this rode an extended baseboard for my router. So I decided to use this baseboard to finish the half lap joints, by building a different jig with parallel rails rather than a slope.

The new jig was cut from 18mm marine ply, 12 inches wide, on the table saw. Some spare 6×2 stock was edge planed, and then ripped into two 2.5” strips. These were screwed to the plywood plywood, ensuring they were parallel.

The extended router base, showing the stiffening battens

The extended router base, showing the stiffening battens

A small piece of plastic tube prevents the router from cutting into the rails

A small piece of plastic tube prevents the router from cutting into the rails

The router base was originally made from scrap 12mm ply. The holes for machine screws to fasten to the router base were located by using the detachable router base as a template. The hole for the bit to go through the base was cut with a hole saw to the outer diameter of a piece of plastic pipe, glued in place and extending 5 mm through the base, to serve as a stop and avoid routing the rails of the base by accident! The whole thing was made more rigid by screwing edge fastened battens along it – to avoid the router’s weight causing it to sag in the middle of a cut.

When making this router base, ensure that it is long enough – it has to be surprisingly long to ensure it is well supported and stiff across the whole cut.

Squaring off the router base

Squaring off the router base

Setting the end stops from the squared router base

Setting the end stops from the squared router base

The accurate positioning of the timber to be planed is simplicity. First, the location of the shoulder of the rebate has to be set. I did this by positioning the router on the jig in a suitable location for the length of timber I was planing. Then using a square, the router base was aligned exactly at right angles to the two jig rails. Two small clamps were fastened to the rails to serve as stops. I checked that this all worked by moving the router away and then relocating it against the stops, and rechecking with a square.

Here’s the procedure to align the stock:

Aligning the router cutter

Aligning the router cutter

Set the router bit by hand so that the cutters are parallel with the rails. Lower the router to a few millimeters above the depth of cut required.

The router base is aligned to the end stops

The router base is aligned to the end stops

Set the router base against the end stops

Two plastic spacers used to align the stock parallel to the rails

Two plastic spacers used to align the stock parallel to the rails

The stock is slid into position along the plastic shims

The stock is slid into position along the plastic shims

Take a parallel edged shim and, placing it against the back rail, slide the stock along it until the rebate touches the router cutter

Take a parallel edged shim and, placing it against the back rail, slide the stock along it until the rebate touches the router cutter.  Clamp the stock in place.

The rebate in the stock touches the router cutter

The rebate in the stock touches the router cutter

If your rails are parallel, and the clamp stops are set accurately, this will locate the stock in exactly the right place to plane off the surface of the lap joint, and clean the rebate shoulder square.

Then it’s just a matter of moving the router away, dropping it to the full depth of cut and carefully planing off the remaining waste.

I had twelve of these to do. Setting up for the next cut was dead easy, as the end stops and parallel shims make it a no brainer!

A zen koan?

No, a common problem, and one that came up today whilst fixing a circular table.

In a world of homogenous materials, which can be cut in pretty much any way with impunity, it is easy to forget that homogenous materials are really quite modern. In the past the basic materials used for building and making things were composite and directional.  Wood is a fibrous material that changes shape irregularly with changing humidiity.  Wrought iron is also fibrous, and has to be worked with due respect for the direction of those fibres.  And stone has fracture lines and grain.

Forgetting this leads to some interesting structural failures, especially self destruction.  This post explores one such type of failure.

The mahogany circular drop leaf table in question had a split in one of the leaves. Looking at it more closely, it was clear that the glue line between two boards had opened up. Looking through the crack with the light behind revealed a dowel in the centre and two steel pins and each end.

I pulled the two pieces apart, and tried to fit them together again. There was a gap of 2 millimetres, no matter how hard I pushed.

And here’s what had happened. The dowel was holding the two pieces apart, a dowel that had clearly been inserted when the table was made.

Dowel glue failure   mortisetenon failureThis is a common problem in gluing joints. In this case, the grain of the dowel is at 90 degrees to the grain of the wood. As the wood of the table loses moisture and shrinks, the dowel stays the same length. If the end of the dowel is glued in place, the glue line in the table is literally forced open by the dowel as the table shrinks.

The same thing happens with a spline joint or a biscuit joint in solid wood. The glued, un-shrinking spline or biscuit forces the shrinking wood apart.   It’s even worse in a mortise and tenon joint. By gluing the end of the tenon in the mortise, the mortised piece shrinks back from the shoulders of the tenon and the joint becomes structurally weaker, as the shoulders no longer support it – so it will probably rack.

In each case the joint self-destructs.

Dowel successWhat is the solution? Well the explanation for the problem is the failure to understand what the glue and the spline or dowel or biscuit is for. In the case of an edge joint, like this table leaf, the glue is to glue the boards together. The dowel, spline or biscuit is to locate the boards accurately when they are being glued. Indeed the traditional way of edge joining boards is to use no spline or dowel, but a very well fitting ‘rubbed glue’ joint which doesn’t even need to be clamped up! So don’t glue the dowel at all.

mortisetenon successIn a mortise and tenon joint, the glue is to hold the joint in place. The joint should be made to be strong enough to perform its duty without any glue – indeed many mortise and tenon joints are wedged or pegged, not glued. So the glue can simply be applied to the area adjacent to the shoulder of the tenon and just inside the mortise. Then when the mortised piece shrinks, it will shrink towards to shoulder, not away from it.

Basic principle – Wood shrinks and expands a lot across the grain, but very little along it.  Join the two orientations together in a rigid relationship at your peril.