Woodworking Blind Part 2: Measuring & Marking


In the second instalment in the Woodworking Blind series, we will discuss measurement and marking. The age-old adage “measure twice, cut once” is especially applicable when you have no sight, and I would vehemently encourage you take this to heart from the get-go. Accurate measurements are key to almost any project and having limited or no sight can make accuracy especially difficult. I’d go as far as to say that with safety practices accounted for, measuring is in many cases more challenging than the application of any tool.

Tapemeasures

The most common measuring device is of course the traditional measuring tape, in which a steel tape unwinds from the tool’s casing to the desired length (up to about 10 metres or so) and retracts when released. Measurements are marked along the tape with reference to the hook at the end, with classes of tape denoting the accuracy of those markings.

A good tape measure can be a highly accurate tool in the hands of a sighted user, but the markings are indistinguishable by touch and any significant tactility would impede the tape as it coiled back into the case, or make the tape measure too bulky to be practical. Braille tapemeasures do exist as do tapes with tactile dots along their length, though usually in short lengths of a metre or so. The size of a braille cell (the space in which a braille character is formed) or the tactile dots limit their accuracy to plus or minus five millimetres at best.

And so we have talking tape measures. These devices closely resemble a standard tape measure but contain electronic circuitry that senses the position of the tape and speaks the readings in an audible voice in the desired unit. By far the best of these currently on the market is the Tape King from Caretec. The Tape King is no larger than an ordinary 5-metre tape measure yet can also be used as an accurate digital level and inclinometer. It speaks in a natural, assured British voice with adjustable volume and the ability to read measurements in both imperial and metric and various units of measure. Accuracy is +/- 1 mm.

The other 5-metre tape is the ProTape Talking TapeMeasure from Cobolt Systems. It is a more basic unit with similar reading functions to the Tape King. It’s much less sleek in design and lacks a volume control. Without modification its loud, booming voice obnoxious, especially as its auto power-off mode can’t be defeated and is announced after a short period of inactivity; 5 seconds with the tape retracted, and 30 seconds with the tape extended. A strip of duct tape over the speaker is a quick solution to this problem, or a small variable resistor in series with the speaker if you’re handy with a soldering iron and don’t mind voiding your warranty.

Both tape measures operate by the same principle. Their tapes have tiny holes positioned every ten millimetres spanning their length. A sensor encases the tape within the tool. This sensor consists of lights at one side and photosensors at the other. When the tape is pulled through the sensor, the lights alternate between being blocked by the tape and shining through the holes in the tape thereby triggering the photosensors. Mathematics enable the tape to calculate the position of the tape in reference to the 0 point, that being its fully retracted state.

Both tape measures can measure from the rear of their casings, which is useful when you’re measuring the internal width of a door frame or cupboard for example. You can also use this function to position a saw fence. The hook at the end of the tape slides to give an accurate measurement from either face. Both can store values in their memory, and perform basic addition and subtraction of measured values within their memory.

Both also carry a significant price tag. An ordinary tape measure from a quality brand can be purchased for around £10 delivered. The Cobolt Talking Tape Measure costs £65, and the Caretec Tape King has a street price of around £120. These are eye-watering prices for devices, which, on the surface are standard tape measures with basic optical photosensors and electronics, plus a bit of custom software and a custom case. But as with all specialist devices they’re not mass-produced so economy’s of scale have yet to bring costs down, and specialist devices always command a premium price regardless.

Unless you are abnormally careful in your ownership of either model, it is likely that your first won’t be your last, as tape measures all wear out eventually. Neither model is particularly well made with the tape king by far the better of the two. The smallest accidents are enough to damage them.

A fall from the workbench (700 mm to the floor) finished off the the casing of my first Cobolt tape measure, simultaneously breaking every screw mount and the internal spring retainer. My Tape King went back for repair with a cracked display that I am convinced was caused by changes in workshop temperature that occurred during a particularly cold winter, as it had never been dropped. The repaired unit lasted only weeks before developing significant inaccuracy, and remains unfixed; something which I will attempt myself on a rainy day.

To my knowledge, these are the only commercially available offerings. I am working on a DIY-friendly design but it is a complex device to tackle with off-the-shelf components. I am, however, determined that an accurate DIY talking tape measure can be made possible, and will pursue the idea until proven otherwise.

Small Stationery

There are other devices including a popular tactile ruler with small raised lines in five-millimetre increments, larger raised lines in increments of a centimetre and raised numbers every centimetre for the first ten centimetres and then every five centimetres thereafter. They come in useful for smaller projects though cannot be relied upon where accuracy is important.

Their limited readings leave you reliant on guesswork for any measurement that can’t be rounded to a half or whole centimetre, though they do have a line of indentations alone one edge in half-centimetre increments which fit perfectly the tip of a pencil, scribe, awl or centre punch. They can be purchased from various retailers, occasionally on eBay and via the RNIB, though my personal opinion of the charity holds back my recommending the latter as a source to purchase any product.

There also exists a tactile compass of the same design, with a 2.9-millimetre hole every 0.5 millimetres through which you can push the tip of a pencil, awl or punch. Combined with some three-millimetre metal dowel pins however they make excellent, accurate centre finders.

Across the pond where blind woodworking is more prevalent, and arguably more widely accepted there are devices such as the Click Rule or Rotomatic. Both devices are designed for the imperial measurement standard, though I’m sure a metric version could be produced with accurate machine tools. I’ve never seen either device in person, nor do I know of either device ever having made it to the UK. Interested parties can read extensively on both in the WW4B (woodworking for the blind) woodworking manual.

Finding Centres

One common measurement that presents a challenge is finding the centre of a workpiece. It is possible to do so with a talking tape measure with experience, but your accuracy is limited to the accuracy of your measuring device and the size of your measuring device combined. A centre finder is the best tool for this, especially where small pieces are concerned.

A centre finder is a flat tool much like a ruler, but with three holes drilled at an equal distance from one another. The outer-most holes hold a pin, which protrudes from the underside of the device. The tool is placed on the workpiece and rotated until the pins are touching either side of the piece. The hole precisely between the two pins then marks the exact centre of the workpiece.

Commercial centre finders are available, but it is also possible to make them using common materials providing you have an accurate means of measuring and drilling your 3 holes. The aforementioned tactile compass also works well for this purpose. It is worth knowing that many commercial centre finders have a centre hole that is offset slightly to compensate for the width of a pencil mark and are not suitable where a punch, awl or small drill are used to create the mark.

Marking

Once measured the workpiece usually must be marked. The sighted woodworker would use a pencil, pen or engineer’s paint for this purpose. None of these devices give us any tactile reference, and they present plenty of opportunity for inaccuracy even if the user has 20/20 vision. The thickness and sharpness of a pencil determines the thickness of the mark, and thus any cut or hole made on a pencil mark has the potential to be ‘out’ plus or minus the pencil thickness if not positioned correctly on the mark.

There are many tools that can be employed to produce a tactile mark. The centre punch is particularly effective at marking holes, as it has a thin tip that produces a dot that while sometimes difficult to feel should accept the tip of a drill bit with a firm ‘bump’ that is easily felt. If a deeper mark is acceptable, the tactility of a pin punch mark can be increased by striking the punch a few times with a hammer to sink it into the wood. The tapered tip enlarges the hole as it goes and the results are marks that are highly tactile, but will still guide a drill bit into the correct position.

To ‘draw’ lines, one can use a scribe or an awl, both sharp tools that will produce a raised line on the material surface. A knife also works wonders, in particular the traditional folding Stanley knife or a small craft knife. The thin blades of either tool, if used with care, can produce a highly accurate line, with the advantage that the line will ‘catch’ the tip of a small pilot drill or a thin saw blade such as that of a hacksaw, coping saw, jigsaw or band saw.

A small set of needle files is an indispensable tool for marking. Triangular and flat files can be used to ‘notch’ the edges of a workpiece. I often use a triangular file or the edge of a flat or half-round file to mark pieces for sawing. I place the mark on the piece that will become the offcut, or in an area that will be within the kerf of the blade so as not to leave a mark on my final piece.

When cutting, I align the edge of the blade tooth with the edge of the mark. This process relies on touch and experience, as there are many factors in cutting accuracy including the size of the mark you make and your blade kerf. But with these methods, I am able to achieve accuracy to within a millimetre. In situations where absolute accuracy is critical, I will achieve the final measurement by planing or sanding or use a method whereby the cut face is rested against the blade. This method, known as ‘kissing the blade’ removes a fractional amount of material each time and is useful for refining a mitre or a straight cut to produce two pieces that are identical in size.

Using Reference Pieces

One important means of measuring that is often overlooked is the use of a reference piece. If you’ve cut a piece to the desired length on a mitre saw, for example, there is no need to measure to cut subsequent identical pieces. With the piece to be cut laying on the saw bed, place your cut piece on top of it to either side of the blade, and flush with the end of the material to be cut.

Lower the saw head until the blade is touching the piece to be cut, raising the blade guard as necessary. You may need to hold the saw head down with an arm over the top to use both hands to position your workpiece. You can then push both pieces along the back fence of the saw, ensuring you keep the flush ends aligned until your cut piece touches the blade. Ensuring that the piece to be cut doesn’t move, remove the cut piece from the top and make your cut. As your cut was measured with reference to a piece that was already prepared, the resultant piece should be identical.

This method works with many tools, including the table saw, band saw and any other tool with a fence. With practice, it can also be used with sliding horizontal fences and on mitred pieces to replicate angles or make accurate frames. Never underestimate the usefulness of pre-cut pieces that are known to be accurate, or even pre-prepared stock in known sizes as measuring aids.

If you wanted to rip an 18 mm strip from a board using the table saw, for example, resting the face of a strip of 18 mm MDF or plywood against the fence, and sliding the fence until the opposite face touches the blade is much faster and usually more accurate than trying to measure such a small distance with a tape, even with sight. Most modern saws have a measurement scale, but they are only accurate if calibrated such that their zero mark ice perfectly aligned with the face of the cutting tooth of the blade.

Many blind woodworkers have fabricated small blocks of material in a pre-determined measurement, be it a fraction of an inch or a centimetre. These blocks can be stacked or arranged in countless ways to give accurate and repeatable measurements when adjusting or operating a tool, or even in positioning pieces in assembly.

Digital Tools

While we’re on this subject, and in closing, I’ll touch on digital readouts and also cover some of the tools that are still scarce in an accessible form. Digital readouts aren’t accessible to us, with only one notable exception that I am aware of. As far as I know, an American company has produced an adapted talking DRO for a lathe, but from the references to it I have found it looks to be a one-off. You can see the devices at work in the videos by blind machinist Dan Parker.

The market also lacks a pair of speaking vernier calipers or a micrometer. There are tools made by Mitutoyo which have data output ports and can interface with a speech module by Captek and a module by Mitutoyo. Both modules are scarce and prohibitively expensive.

I have approached Barry Wixey of Wixey with the suggestion of adding Bluetooth to more of his tools. Wixey manufacture a superb smart Angle Gauge and Protractor, both of which have Bluetooth connectivity and can interface with a purpose-designed iOS Android app to speak their readings.

Short of devices with integral speech synthesis, this is an ideal solution as it brings us accessibility in mainstream devices at a significantly lower cost than specialist kit. Barry was enthusiastic about accessibility and welcoming of my feedback. Knowing how the mind of an engineer works, I can only hope I have started the wheels in motion and that we will one day have access to some of the many digital readout systems Wixey offer, which are reputedly excellent.

The aforementioned WR300BT angle gauge is a tiny device with a magnetic base. It is zeroed on the bed of a tool, at which point any angle such as that of the blade or drill bit bit can be measured and set with reference to the tool bed. It is the ideal tool for squaring up or setting bevels on a saw or drill press without being able to read the scales on the tools themselves. The device is well constructed, and I have happily used it in the workshop without fear that it will be damaged by occasional heavy-handedness or a moment of “where did I put that?” senility.

Future instalments in this series will cover tips and techniques for sightless operation of specific tools. I also hope to detail various tools and jigs that can be made or adapted to better aid the blind woodworker. I welcome feedback, questions and suggestions as the series progresses. I hope that this series can go some way to proving that woodworking can be accessible to all as a hobby or tradecraft and that having no sight should not inhibit your success or enjoyment of it.

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