The Art of Smoke Fitting

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The Art of Smoke Fitting
Plane maker Oliver Sparks explains how the use of soot helps to seat an iron and wedge

When it comes to wooden planes, I have read many times that having a correctly seated iron and wedge is vital to a plane’s performance. I agree, and yet there seems to be little instruction out there on how to achieve this. In this article, I will be using the ‘smoke fitting’ technique to do just that on a matched pair of wooden thumb planes. Whether the smoke fitting technique is applied to a shop-made or a vintage plane, the method set out here will transform that temperamental tool into  a well-behaved instrument. Smoke fitting is a very old process used extensively by gunsmiths to fine-tune the fit between parts that must mate exactly. The theory is a simple one – a layer of soot is deposited onto a component using a smoke lamp. When the part is offered up to its mating component, soot is transferred between the two, but only where they touch via high spots. These high spots are then removed and the process repeated until a satisfactory fit is achieved. This is a simple way of achieving a high degree of precision.

Equipment required

The smoke lamp consists of an oil reservoir, wick and burner

Grinding the cutting face of a bench chisel at 90° turns it into a flat scraper

You can see how effective this tool is by looking at the shavings!

These are the two irons that I’ll be fitting into their respective bodies

A smoke lamp or ‘smoke pot’ is effectively an oil reservoir, wick and burner. My burner is put together from copper plumbing fittings; a thin section of copper pipe carrying the wick passes through a cork and the copper cap sits on top. It’s as simple as that. A candle or dry board maker can also be used, but I prefer a lamp for the visible coverage it provides, even on dark timbers. My bottle is filled with (ironically enough) indoor smokeless lamp oil. How much soot is produced depends on the length of the exposed wick, around 6mm will be fine for our purposes. The most accessible non-specialist tool used for this kind of work is a re-ground chisel. By grinding the cutting face at 90° instead of the usual 25° turns a regular bench chisel into a flat scraper. Repeated regrinds have left my cutting face the entire thickness of the chisel but yours need not be, even a 1mm flat is sufficient. Grind a slight camber across the width. Lap the back as usual, but the face can be left straight off the wheel. Use moderate pressure and repetitive strokes to get a feel for the tool, practising on a piece of scrap hardwood first. I will be fitting two irons into their respective bodies. My irons are made from new tool steel so are generally pretty flat, however you will often find antique irons in much worse shape. If the back of your iron looks like a miniature mountain range, try to get it as flat as possible using whatever you have to hand. If it is a double iron, aim to reduce the hollow often found between slot and edge. A coarse file is preferred using the same principles as flattening a piece of wood. Coarse (120 grit) abrasive paper stuck to a flat surface will also work well.

The process

STEP 1 Wipe the iron clean before igniting the smoke lamp. Lick the flame up from the middle towards the front in a sideways zigzag motion until a uniform black colour has been achieved. I find the best method is slightly raising the business end to ensure the smoke flows upwards, away from your hand. It can help to raise both lamp and iron above your head to get a clear view of where the soot is being deposited

STEP 2 Note the stark difference between smoked and unsmoked surfaces, it’s so black it appears to drink the light itself! Remember you’re in a wood shop so practice fire safety at all times

STEP 3 This technique is a useful one but you have to be quite disciplined regarding the handling of smoked items. If you are not careful, everything will end up very grubby in short order. Wipe away any excess with a dry cloth, paying particular attention to the sides

STEP 4 Now carefully manoeuvre the iron into the body. You are aiming to position the iron just shy of the mouth, floating above the bed. When in position, bring the iron down in contact with the bed. While applying firm downward pressure as shown, slide the iron forward and back in strokes of around 6mm

STEP 5 Observe where the iron has made contact. In this plane there is a bump down near the mouth and some contact up top

STEP 6 The rosewood (Dalbergia spp.) twin has fared better, with surprisingly good contact for an initial fit. The marks left on the iron tell me that the top portion is fine, but towards the throat contact tapers away, forming a high central ridge. With darker coloured timbers I use a strong desk lamp to help light up the soot marks

STEP 7 Enter the scraper! Using short strokes, remove only the sooted portions. With each repeated smoking/scraping the area  of contact will grow. Be patient, it can feel like a slow process but a little care will pay dividends

STEP 8 When the bed is about 40% covered in contact marks, smoke fit the iron’s face side to the wedge’s back surface. The rosewood wedge shown here is completed. I used my miniature mitre plane, but a scraper chisel or cabinet scraper will perform  the same task

STEP 9 So, the iron is fitted to the bed, the wedge is fitted to the iron and now it’s time to fit the wedge to the abutments (the two narrow faces that hold the wedge in place). My method follows the same principles of smoke fitting, but utilises friction rather than soot. Lightly abrade the wedge’s front face with a small square of 240 grit abrasive paper. Light pressure is key here as it will yield a dull surface sheen. Seat and remove the wedge a few times

STEP 10 This is a good representation of a well-fitted wedge. Contact between wedge and abutment have caused friction to burnish a line, showing up nice and shiny against the dull background. The aim here is to get equally burnished lines running the length of both wedge arms. Same rules as the smoke fitting apply here; only remove the contact marks, ignoring unburnished areas. A cabinet scraper is ideal for this task

STEP 11 A completed bed. This final process is the crescendo because you must now adjust the wedge/abutment fit, and that  of the iron/bed as part of the same process. The correct order is as follows:

• Clean then soot the irons back, wiping the irons’ edges afterwards. Place the iron carefully onto the bed just shy of the mouth.
• Press the wedge into position. Use a suitable light hammer or mallet to secure with a moderate tap.
• Tap the iron’s top to advance it out through the mouth by a centimetre or so. Unseat the wedge with a sharp tap on the plane’s heel and lift out the iron, once again being careful not to slide the iron out.
• Note contact points from both the bed and wedge arms. Remove these high spots until the wedge is as previously shown. Work until the bed has approximately 50–60% overall contact, paying particular attention to the bed portion a couple of centimetres above the sole.
• If you are using an old double iron, chances are even after flattening it will still have a slightly arced back due to cap-iron pressure. In this scenario imagine splitting the bed into thirds, horizontally. Aim for good contact in the first and third portions.

So there you have it, a step-by-step guide to seating plane irons – the professional way.

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