Reamer for Conical Dowels

 This is my experience on making a wooden reamer suitable for making mortices for chair legs etc. The advantages of this joint, in comparison to a straight cylindrical mortice, is that the leg cannot push through the joint because of its conical shape, the joint does not need gluing and can be dismantled for transporting. Also the joint does not need a wedge hammered across the tenon to tighten the joint. They were used routinely by Windsor chairmakers.

I made one by following some of the instructions on the "South Fork Timber" internet site with a few modifications. I started with an old keyhole saw blade and filed the teeth off using a hand file to give a metal blade about 24.3 mm wide at one end and sloping to 12 mm at the other. The length of the blade was 15 cm and gave an included angle of 5 degrees (i.e. the internal angle at the tip if originally pointed and hence a slope of 3 degrees off vertical). This will allow holes to be shaped to a maximum of near an inch at the wide part and 1 cm at the narrow. The choice of the included angle was determined by the blade available and how much filing I wanted to do. If the angle is too steep, e.g. 12 degrees, the surface contact area is reduced making it easier to rotate the tenon and making a less snug fit. If the angle is very small, e.g. 2 degrees, there will be greater contact area and hence force to break the tenon open especially if making a tenon in soft wood with a hard wood reamer. Six degrees seems to be a popular angle used in the past by chairmakers. There are more explanations on reamers on the blog of Tim Manney and also Elia Bizzarri, including some videos.

Having prepared the shape of the blade, making sure the sides were straight, I filed a 70-degree angle to each edge of the blade with the high points on opposite sides of the blade. The choice of sides will determine in which direction of rotation the reamer will cut. I hadn't realised this so my reamer cuts on a anticlockwise rotation. I honed the edge to 1200-grit at 70-degrees. (some advice the same angle as card scrapers, i.e.45 degrees). A cutting edge will be applied later using a hard chisel steel using the same technique used for card scrapers. 

The wooden part of the reamer was made from a piece of  oven dried beech dowel about 35 cm long and 2 inches diameter (I could have used a narrower dowel but this is what I had available). This was mounted on the pole lathe and the top handle area shaped as shown. The exact dimensions are not important the only criteria is that the handle must be of sufficient diameter to mount a cross-bar and the blade area must match the profile of the blade.

The reamer being shaped on the pole lathe. Note that the "pointed end" ,on the right, has a cylinder left on to aid later adjustments after the slot for the blade has been cut. This allows the reamer to be mounted on the lathe after cutting. The reamer blade is also shown,

 The profile of the blade portion was guided by using the blade itself with many minor alterations using the skew-edged chisel and finally coarse sandpaper. I found this difficult to get exactly right and so erred on the side of leaving a little too much wood on the sides.

A tricky part of the job is to mark the centre line on the blade portion and also where the handle holes need to be bored for the "T" bar. To do this I used the pole lathe bed with the reamer mounted. My attachment points on the lathe are exactly parallel to the bed. I place a board across the bed and with callipers measured the distance from the bed to the attachment points. I then attached the reamer tightly to the lathe and marked dots along the reamer blade portion of the wood using the pre-set calliper. It is important to get the calliper tips vertical. The other side of the reamer was also marked in the same way ensuring that the lather was not turned. The handle holes were marked in a similar way.

Marking the saw-cut line using the lathe bed and callipers

Maybe marking the saw-cut line is tricky but not as challenging as making the saw-cut itself. I checked my tenon saw and small panel saw for the width or kerf and checked this by sawing some waste wood and inserting the blade of the reamer. Both were fine making a tight fit for the reamer blade. I found the best method of sawing was to have the reamer at about 30-degrees from the horizontal and cutting short sections from each side and then mounting it vertically and cutting down. I did as much as possible with the tenon saw and finished with a panel saw. The pole lathe mounting cylinder was cut through but otherwise left intact so that the sawn reamer could be shut with a small cable tie and remounted on the lathe. The kerf of the saw-cut was cleaned-up using some 120-grit sandpaper backed with a thin card scraper. After checking the fit of the blade, the reamer was mounted on the lathe and fine adjustments made by sanding. The skew-chisel was not used. This was repeated a few times so that the blade protruded a fraction, less then 0.5 mm, from the reamer body. Finally, a narrow channel was carved with a chisel to accommodate waste wood cut in the tenon. This was about 1/8-inch wide cut on the high side of the blade edges.

A "T" bar was shaped from some dry hazel and gauged on the pole lathe to 5/8 inch diameter. A mounting hole was drilled through the reamer using the marks previously made using the lathe bed. After finishing the blade, the reamer was successfully tested and as expected, does need to be removed often from the tenon to release waste dust. 

Finished beech reamer with blade mounted and ready for use. The tool will be treated with linseed oil.

  An adjustment screw was added to aid the positioning of the blade. I used a short M5 bolt and drilled a hole 4.2 mm diameter and threaded this. Drilling the hole was tricky, so I used a small drill and made the hole from the bottom within the saw-slot upwards and at an angle of about 60-degrees. I then enlarged this to 4.2 mm diameter, drilled from the top, and gouged out space for the bolt head. The screw must be adjusted with the blade out to reduce stress on the beech threads.

Adjuster screw in position. 

I thought I'd add a few words on the geometry of the tenon. The shape is determined by the larger diameter of the mortice and the length of the hole i.e., if a through tenon, thickness of the wood. For mine, with an included angle of 5 degrees, this leads to a formula:

 

Small diameter side of tenon = Large diameter side of tenon - length of tenon (i.e. wood thickness) x 0.088

So if the wood is 30 mm thick and the maximum diameter is 15 mm, the smaller diameter will be 12.4 mm. The tenon could then be cut on the pole lathe or alternatively, you could make a rounding plane to match the profile of the reamer.