Lip Service:
Derek Jones looks at the received wisdom surrounding tight mouths and whether it might be just a result of Chinese whispers.
Derek Jones looks at the received wisdom surrounding tight mouths and whether it might be just a result of Chinese whispers
A little bit of knowledge, they say, is a dangerous thing but compared to having a lot of knowledge based on hearsay and conjecture, it’s possibly less damaging in the long run. With a craft as old and as well documented as ours it’s not often you come across an opinion that flies in the face of convention. Over time anecdotal evidence can easily morph into wisdom and in so doing deter us from thinking any more about solving a particular problem. Much of what we accept as gospel has been handed down by generations of practitioners, and with that transfer of knowledge comes the inevitable, and I’m sure mostly unintentional, slant on things or complete spin.
Part of the problem I’m sure, is relating information to the here and now and the often very personal set of circumstances in which we find ourselves. In my opinion, repeat, my opinion, the most helpful ‘how to’ texts come with the caveat that there is generally more than one way to skin a tree and make a piece of furniture. And while some folk insist on absolutes to get them through the day, experience has taught me that in reality they are of limited use. I’ve also learned to embrace the subtle nuances of doctrine to expand my portfolio of useful techniques.
Open wide
What’s your understanding of a tight mouth? 1mm, half a mil or less or one that barely allows the light of day to pass through? Whatever it is I bet at some point you’ve considered it to be somewhere between advantageous and critical. What if I said it was neither and that edge and angle were more important and you could probably do the majority of your planing with a mouth resembling a permanent yawn? Which brings me neatly and rather tenuously onto the next most important factor for effective planing: the bed.
The best way to appreciate the relevance of all three criteria (edge, angle and bed) is to go and grab the nearest sharp paring chisel you can lay your hands one and use it as it is intended. Notice that there are only a couple of features missing that would otherwise turn your paring chisel into a plane, or are they? The frog, or bed in this instance, is disguised as the flat workpiece with the thumb on your non-dominant hand behaving exactly like a lever cap or wedge. And the mouth? Well obviously there isn’t one but miraculously you can still produce a very fine shaving.
I know what you’re thinking, how can that be possible? Angle, edge and bed, that’s, how and maybe a deft hand. I don’t want to dwell too much on the physics of either right now because what’s more important is to understand that if a tight mouth wasn’t the reason for the fine shaving then obviously something or a combination of things were and maybe they should be the focus of our obsession.
Badge of honour
I’ve quizzed a few plane makers about this and all agreed that there is a tendency to over emphasise the importance of tight mouths and many so-called improvements to mass-produced planes. The more sceptical among us might suspect a level of convenient misinformation to gain the upper hand and some serious bragging rights. Karl Holtey, for example, said that most of his metal bodied fixed mouth planes would perform just as effectively with a mouth opening twice or more than that expected by the customer but a tight mouth is very much regarded as a sign of quality, even if it does limit the planes use. A valid point indeed considering the difficulty (and therefore expense) of making a very fine fixed mouth BU plane; features that understandably effect perceived value.
All things considered
The business end of the most basic hand plane is a complex matrix of surface angles designed to cope with an equally complex and variable set of circumstances. The dynamics of a metal bevel-up blade vs those on a double iron woodie vary considerably and it’s here that I think the lines of communication may have become blurred. Plane maker Oliver Sparks is at home building either type of plane and enjoys challenging the wisdom of the old masters in his quest to produce planes that surpass conventional expectations in both appearance and performance. “In my experience if one wants to stop tearout it’s either a single high angle (55°)/tight mouth approach, or a wider mouth with a finely set cap iron. The single iron/tight mouth works faultlessly providing the set mouth is not much larger than the thickest shaving that will pass through it” he said.
“These two factors work very well together – some of my best smoothers are high angle woodies with a mouth so tight the light barely passes through them” he added. He also mentioned that the drawback of having a 55° pitch is that it’s noticeably harder to push than one pitched at 45° and will therefore tire the user quicker. Incidentally, pushing harder can also lead to a more erratic planing style bordering
on frantic. “It also generates more heat near the cutting edge, though I’m not sure what (if any) effect this has on performance, as it will never reach 175° which is about the lowest point for tempering an O1 iron. ”For the ‘absolutes’ I mentioned earlier he had some more bad news: “If the mouth is opened up a little one must take lighter cuts on the same piece of wood to avoid bad tearout, but the high angle on its own is a
big improvement over standard pitch (45°) and still viable on many woods, just not proper nasty stuff like burr yew.” To summarise that point, if the angle is high enough the mouth can be quite wide – think bevel-up low angle jack. A typical bevel up jack has a bed angle of 15° so to achieve a high angle you’d need to be grinding a primary bevel on your blade somewhere around 40° to constitute a high angle. However, a secondary bevel of more than a couple of degrees might well render the plane a monster to use. Note a low angle bevel up plane is bedded at 12.5°.
Scraper zone pitfalls
There’s a definite correlation between steep angles and wide mouths. The steeper the angle, the wider the mouth can be but anything above 55° will be extremely hard to push, bevel up or down and puts you into scraper territory where a mouth is no longer a consideration. HNT Gordon produce specialist planes with a steep pitch and with HSS irons to cope with the more truculent exotic timbers native to Australia.
Cap iron conundrum
When it comes to cap irons and tight mouths, Oliver doesn’t believe it’s strictly necessary to have both on the same plane. “My main workhorse around the shop is an LN No.5 with an A2 iron. I set the mouth years go at around 1mm and that is pretty much where it’s stayed. I don’t use it like a true jack, I grind the iron with only a hint of camber because for the most part I will be working very small stock pieces with a close set cap iron. I have plenty of woodies should the need arise for a ‘true’ jack.” The only time he would want it wider would be to take big cuts with a more pronounced cambered iron where the easily adjustable bedrock frog comes in handy.
Oliver’s tip to avoid tearout on a fixed mouthed plane with a cap iron is to set it ludicrously close to the edge, maybe 0.1 or 0.2 mm. “This works very well, and if the iron is bedded nice and firmly it will handle 90% of difficult stock no matter what the mouth is saying.” This is why the cap iron was born, and continues to be used to this day. “I think all the old conjecture about it originally only being there to stiffen/bulk up the thin iron is nonsense and has been disproved many times over. It does stiffen up the iron assembly, but that is not its primary objective. “ he added. There are consequences, however, of setting the cap iron close to the edge as it will increase friction and requires more force to push the plane.
Richard Arnold has the mouth on his Norris panel plane set very fine and although it’s a double iron plane, he finds taking a full width shaving with the cap iron very close to the edge too much effort. To counter this he sets the cap iron further back than normal (for example 0.6mm instead of 0.2mm) and lets the tight mouth do the rest giving him the best of both worlds; a plane that is easier to push while still controlling tearout. Though funnily enough he finds the best performing plane in the workshop with regards to handling tearout is one of Oliver’s Slipper planes, which is bevel up at 19º with a super fine mouth.
Top tip
It sounds obvious but for best results when using a two-piece iron it is imperative that you prepare the cap iron (chip breaker) with as much attention to detail as your blade. An hour in front of David Charlesworth’s recent DVD Plane Sharpening will set you and your planes up for premium results for life.
Adjustable mouths
Planes that present the best case for having an adjustable mouth are the ones that require the least amount of grunt to get them going with the most obvious example being the block plane, low angle or regular. “Chamfers are a good example”, says Oliver. “What’s the first thing you do when chamfering, before you even touch the wood? Set the iron for a heavy cut. If it only had a set fine mouth, you could only take fine shavings and that would take forever, so you open up the mouth, crank the blade right out and remove the bulk quickly without the mouth jamming up. When you are getting close to the layout lines, retract the iron and close up the mouth for a more predictable/reliable cut.”
To conclude
If at the start of this article you were a little confused about the merits of tight mouths and why they are held in such high regard, I hope we’ve managed to dispel a few myths and present a stronger case for considering the importance of the other factors surrounding the set up of your planes.
For the ‘absolutes’ there’s no silver bullet for this particular problem and neither should there be. After all every piece of timber is unique and making it shine like the diamond it is, is all part of the challenge.
