Robert Paul Gurney explains how to use ordinary tools to get extraordinarily accurate layouts.
The key to accurate marking and layout is to use common tools in an uncommon way. With common marking tools you can achieve an accuracy of one hundredth of a millimetre or better. If you don’t have these tools already they are inexpensive and commonly available.
Marking & layout tools
The tools you need are:
• 200mm engineer’s square – you don’t need to spend a fortune, it just needs to be square and stable. You can extend its range with a straightedge if you need to.
• Saddle square – you’ll find out why later.
• Rulers – you’ll need three rulers to get you started: 150mm, 300mm and 500mm. You can add a 1000mm ruler later. These rulers should have square ends and should have no extensions beyond their stated lengths.
• Machinist’s 1-2-3 blocks – you’ll need two that measure 25 x 50 x 75mm.
• Set-up blocks – these blocks start at 0.5mm and allow you to go in 1mm increments to 25mm.
• Dial calliper – a 150mm dial calliper can be used to transfer the measurements from mating parts or hardware. It can also be used as a ruler – use the thumb roller to get a desired dimension and then lock
it in place.
• Marking gauges – you will need two marking gauges: one with the bevel towards fence and one with the bevel away from fence.
• Marking knife – a single bevel knife should be fine.
Caring for your tools
Before you use them, run your finger along the edges to feel for burrs and to wipe off any bits of dust or particles. You can gently run your finest whetstone over the burrs to take them off and rub them with a light oil when you’re finished with them. Take good care of your tools and protect them from damage and they can last you a lifetime.
Making a ‘stack’
Most layout errors involve the ruler. The ruler’s markings or etchings can be quite wide, maybe 0.3mm or so. Trying to get your marking knife in the same place each time is next to impossible. If you want to align the etching with the end of a board or another mark, you are building in another error. The ruler can also slip while you’re trying to make your mark. You get an error called an alignment error if the ruler is not parallel with the edge of the board. Also, if you are not sighting the ruler mark from the same position, you create a parallax error. It’s the error you get when you’re in the passenger seat of a car looking at the speedometer – you get a different reading than the driver. All these errors accumulate and you then find yourself with problems at glue up.
The solution is to use rulers in a different way than intended by placing the rulers end-to-end. Three rulers end-to-end can make measurements up to 950mm. Add a 1000mm ruler and you can make measurements up to almost 2m. This is more than enough for most furniture making and this is why I use square-ended rulers. Putting the measuring instruments together end-to-end is what I call a ‘stack’.
The stack can include set-up blocks, machinist’s blocks and dial callipers. Machinist’s blocks add dimensions in 25mm increments. Set-up blocks offer dimensions in 1mm increments and the dial calliper to one hundredth of a millimetre.
No matter how accurate your measuring tools are, they cannot be accurate if they move during layout – your work and tools must be stable. By clamping your workpiece and clamping your tools (wherever possible) you will take out movement and increase the accuracy of your layout. A little bit of clamping can help you when you’re trying to scribe a line against a square. The V-shape of your marking knife can actually deflect the blade of the square and leave you with a slightly angled line.
Even with movement eliminated you can only get a true measurement when the workpiece and the measuring instruments align exactly. It isn’t difficult but there is a trick. Two blocks clamped to either end of your workbench (square and true to the front edge and bench top) are the arcane solution. But first you need to add a mark to your usual method of doing witness marks. To your face-side and face-edge marks, you need to add a face-end mark. I use a check mark at the left end of the board or a circle on the end grain. Like your other common faces, this face should be true and square. This is the end you index against one of the two blocks on your bench. The block on the left is where you index the face-end when the face edge is to the front and the face side is up. The other is for when the face end is down. These blocks must be thicker than your workpiece because you will be indexing your stack and your workpiece against them.
A marked line of zero width is impossible but you can use your marking knife in a way that comes close. A marking knife leaves a V-shape with one face vertical and the other at an angle from vertical. The vertical face forms the shoulder of a joint – the rest of the groove lies in the waste portion of the joint. The shape of the marked line creates two situations when using the stack method. Situation 1 is when the bevelled portion of the marked line is under the stack. This is easy to mark: just slide your square up against the end of the stack, clamp it, remove the stack, and mark your line. Situation 2 is when the bevelled portion of the marked line is beyond the stack. This requires you to add another step. Instead of sliding your try square against the stack, you slide your saddle square against it and clamp it. Now it’s just a matter of sliding your try square against the stack, clamping it, removing the saddle square and marking your line. If you are near the end of a board, you may not have enough board left to place the saddle square but you can place your marking knife against the stack (lightly engaged in the wood) replace your stack with the try square and mark your line.
Transferring a scribe line around a piece of wood always creates a slight offset. I know this because I’ve checked it with a magnifying glass. Using the stack on all four faces solves this problem. If the boards aren’t too thin you can use this method on every marked face.
A common problem with traditional marking techniques is laying out a through joint. When trying to transfer measurements to the opposite side of the board we would normally switch to a pencil. Unless you are extremely lucky your knifed lines won’t line up due to the thickness of the pencil line. By using the stack, this problem is eliminated.
A common solution of having to set and reset marking gauges is to have lots of them. However, now you can use the stack to set two marking gauges the same each time.
We have only two marking gauges: one with the bevelled portion of the blade facing into the fence and one with it facing away. You will use one depending on where the waste is in relation to your reference face. Two methods are needed for setting them.
With the bevel facing away from the stack just place the stack between the fence and the pin and tighten the fence.
With the bevel towards the fence, there are two solutions depending on what type of marking gauge you have. These are shown below. With the stack, you will find it easy to recapture forgotten dimensions without any loss of accuracy.
It is a good idea to lay out everything in pencil before you begin as a double-check. This will be helpful when you use the stack with your hand and machine tools – but that’s the subject of another article.