Over the past few weeks, I’ve been working up a project that’s been on the ‘to do’ list for well over a year now: prototyping an M2 mitre . The plane itself- shown above – is a new size designed around a 1-1/4″ blade, placing it dead-center between the two sizes I’ve offered to date. Overall I’m quite pleased with the size, which is an excellent all-round compromise for small-scale work.
The other development I incorporated in the plane, which is the real subject of this post, is a new (to me) blade holding mechanism. The technique, which I’m referring to as a screw-drawn wedge, employs a traditional wooden wedge and metal bridge, but with the addition of a screw through the bridge to aid in securing the wedge. This is not the most common method for securing a blade, but it is a tried-and-true technique that I believe was first used by Norris in some of its thumb planes, chariots, and shoulder planes.
One thing that I think worth noting is that in this mechanism, the screw has a very different function than the screw in a lever cap does. Where the lever cap screw is the primary means of forcing down-pressure to secure the blade to bed, in the screw-drawn wedge it is still the wooden wedge that provides this primary force. The screw’s role here, at least as I implement it, is actually to locate and secure the wedge in its position. In order to facilitate this, the screw is terminated with a cone, which in turn sits in a precisely mated bronze seating point inset in the wedge. When placing the seating point, it is actually offset by a small amount (on the order of .030″, or 1/32 of an inch) such that as the screw is tightened, the taper of the screw draws the wedge further down to ensure the wedge is set quite securely. In this regard, the screw’s function is somewhat analogous to a drawbored mortise and tenon joint.
While it may at first seem that the rationale here is increase the holding power compared to a conventional wedge, that’s really not the case – the simple wedge has been adequately holding blades on its own for centuries just fine, thank you. The screw in this system is effectively performing the same function as the final tap of the wedge with a mallet: setting the wedge.
So what are the advantages? Well, as with most things it’s a bit of a tradeoff. As I’ve said, it’s not as though the traditional wedge and bridge design needed any ‘fixing’, and in terms of actually securing the blade in place the two methods work more or less equally well. To my mind, the primary rationale for using the screw-drawn wedge is that it allows the user to adjust the blade without needing to reset the wedge. The tradeoff part is that once the plane is built, the wedge tension cannot be properly adjusted on the screw-drawn wedge by any means other than altering blade thickness – and in this application, a thickness difference of 1 or 2 thousandths of an inch is quite significant. In practice, this ‘disadvantage’ has much more to do with plane-making than plane-using, but it does make replacing blades a trickier proposition in the long term.
So do I have a feeling as to which system is ‘better’? Well, personally I probably prefer the aesthetics of the traditional design just a bit – and since I’m quite used to using a mallet for all the adjustments and setting needed, the screw isn’t much of an advantage for me. Having said that, though, I do think that for many users the screw-drawn schema will be more convenient in both the short and long term. Over the next few months of using this plane and my others I’m sure I’ll develop some more opinions about it, but for now that’s about it.
Finally, just to be clear here — there is absolutely nothing unique or new in this system. As I’ve said, I believe the technique originated with Norris long before my great-grandparents were born. Additionally, several modern makers have been using the technique for years: Karl Holtey, Konrad Sauer, and Wayne Anderson have all been making planes with it for years. However, I do think that there’s relatively little information about how it actually functions out there, and since it is not necessarily intuitive I thought it worth a blog entry.
Besides – it’s the first actual planemaking content I’ve had in six months now. Man oh man it’s nice to have a shop again.