Waara V-block

I usually check my strips for dimension (both correspondence to the design thickness from the taper plan and consistency flat to flat) by carefully taking measurements with a digital caliper. I know that this is less than accurate, since the apex, which is always one of the reference ‘sides’, is very easily crushed, resulting in an undersize reading (the actual crushing of the apex itself is not so much a problem).

There are better, more accurate ways, to measure dimensions of a (equilateral) triangular strip. The most common approach among bamboo rod makers is to use a so-called Waara v-block, named for the rod maker (and machinist and inventor) who popularised it and manufactured them for sale. The principle behind it is that you put a strip in a 60 degree v-groove in a block that has a cutout for a pair of calipers, supporting the apex side of the strip with the groove, rather than by its apex alone, and measuring the thickness of the strip-plus-block. If you know what the net thickness of the block is (i.e. from the ‘apex’ of the groove to the opposite side), you can subtract that number from the measurement and get the actual thickness of the strip, without ever touching the apex of the strip.

Establishing the net thickness of the block is done in a similar way to calibrating a planing form groove setting with a section of drill rod - if you place a round cylindrical object in a 60 degree groove, so that the object touches both sidewalls of the groove (not the chine-that is the center line of the object is below the chine of the groove) it always sits so that the top of the cylinder is 1.5 the diameter of the cylinder above the ‘apex’ of the groove. In other words, if you measure the thickness of rod plus block with the rod in the groove, the distance from the bottom of the block to the apex of the groove is the thickness just measured minus 1.5xdiameter of the rod.

If you then measure the thickness of the block with the strip in the groove, the thickness of the strip (flat to apex) is (thickness of the block with strip) - (net thickness of the block). If you can zero your calipers to that net thickness, you can even read the thickness of the strip directly.

So far, so good. However, there are few suppliers of Waara blocks, and none, to my knowledge, in Europe. Thinking about this, I realized that there is little need for actual accuracy in making a Waara V-block though. The most important requirement is a completely flat underside, and a V groove of uniform depth that runs parallel to this underside. I considered that with my Woodrat, or basically, with any router table, you can easily create such a thing from nice, dense, stable hardwood. In order to easily cut a V groove that is uniform and parallel to the underside on a router table or comparable machine, it’s imperative that the block has three completely flat, perpendicular sides, as the top side (the one that will hold the groove, and the rear side (the one that will lie against the fence of the router table) are the reference surfaces during routing, and should result in a groove that is parallel to the underside. For the accuracy we need in making and measuring bamboo strips, this is achieved adequately with small workshop and hand tools (jointer/planer and hand planes, preferably size 6 or larger); oh and a good quality try square. So I retrieved a piece of seasoned, stable hardwood from my wood storage attic, and flattened and squared 3 sides of it. I then routed a 6 mm deep V-groove in the top surface of the piece of wood, a 5.5 mm deep straight groove perpendicular to the V-groove at a number of intervals along the block, cut the piece of wood into individual blocks, cleaned up the V-groove and the perpendicular caliper groove, sawed a relief in the bottom (‘apex’) of the groove - those 60 degree cutters never have a truly sharp point, so the groove, as routed, has a flat apex that the strip will rest on. So you need that relief to ensure that the strip actually rests with its 2 sides on the side walls of the groove. 

After calibrating the individual blocks with a piece of 4 mm drill rod, I obtained the net thickness values of the individual blocks, which I stamped on the end of the block. This value can then be used to zero a digital caliper when using said block to measure strips.As said, the jaws of the caliper only rest on the (narrow section of) the underside of the block and on the strip or the drill rod (through the caliper channel), so as long as the underside of the block is flat and in the same plane as the groove, the block will be accurate.