Keep That Mouth Shut

UPDATE: Make sure and read the comments. Kees and I have an interchange on the subject of high bed angles and chip breakers. We discuss there effect on mouth openings. Good stuff. I will let you make up your mind what matters most. Original post follows.

Last year some time I posted plans for a coffin shaped smoothing plane. Of course, to be made out of wood. I asked anyone to point out any errors in the plans. I got some feedback but no one seemed to question the reference to the mouth opening being 1/32″. That is quite large actually. It is about double what I would consider as a size to shoot for.

So what is the size I shoot for. I aim for as tight as possible. That means about 0.006″-0.008 is excellent but I am happy with anything up to around 0.012″. Now, that is for planes I sell. As for planes that I use, I have some with mouths much larger than that.

Here are a couple of photos of examples of mouth opening on my planes. The first is a smoother at about 0.004″ and the second is a miter plane at around 0.010″. Click to zoom in.

Of course we are talking about smoothing planes. For jack planes I don’t get bent out of shape on the mouth size. About a 0.030 is fine with me or around a 1/32″. That is how I think that I ended up with that measurement in the plans. I should be more careful when I copy and paste.

So essentially if you make a smoothing plane with a mouth opening of about 1/64″ (0.015″) then pat yourself on the back. You did an outstanding job. If you can get it to half of that, then you are ready for a new profession. 🙂

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9 thoughts on “Keep That Mouth Shut

  1. Caleb, thanks a lot for all the interesting information about wooden planes. I like your blogs a lot.Regarding the shaving aperture, I wouldn't sweat it too much. Just put a double iron in that plane and be done with it. Tight mouth size is the least effective of all measures to reduce tearout. Even your 0.15 mm mouth isn't half as effective as a higher angle or a close set capiron. And trust me, I tested them all rigourously. Reasonably tight mouths for wooden smoothing planes are about 0.5 mm. Less is just cumbersome. With a tapered iron the mouth will open anyway as the iron gets shorter over the years.

  2. Kees,I appreciate your input. I am sure you have tested these things quite a bit. I have no doubt that a cap iron can be effect as you say. My approach is mostly that a closed mouth is effective at least at reducing the chances of such a large chip levering out that one couldn't go back and clean it up with a card scraper. I really don't think any of these things should be intentionally leaned upon in place of keeping a really sharp blade and choosing the best stock we can get our hands on or using the proper skills etc. I am sure you feel the same way. There is of course lots of room for debate on methods which I get little joy in. Everyone chooses there methods which have advantages and disadvantages and each has to choose which they prefer to adopt or reject.I may have to take another look at the use of the cap iron though. I did see that video several years ago. My biggest question after I saw it was…What do you do to match the radius of the blade to the cap iron when you have to have the iron that close to the edge? The .004" (.1mm) is really close to the edge. Do you shape your cap iron to your radius or are you using a straight across iron? I guess I don't like the thought of fussing with the cap iron. I have always hated them compared to popping out the iron in a wooden plane and just keeping it really sharp. I think I have preferred to just focus on choosing really good stock, keeping my iron sharp and of course working with the grain. If those don't solve it then I will use a toothing plane followed by a finely set smoother and fall back on the card scraper if I need to. Those have always worked for me. I do have lot of experience with that method because working the quarter sawn face of beech is the absolute worst for tear out. I have never encountered anything worse.By the way my tapered iron is not an extreme taper. It is 0.5˚ taper which means for every 1/2" it is only 0.004" inch thinner. I of course realize this is a draw back with a wooden plane but like I said we have to choose our methods and live with the drawbacks. Any how good stuff. Feel free to comment as you like. I do welcome good food for thought. 🙂

  3. Hi Caleb, thanks for your reply. First, I can completey understand if you don't like to fuss with the chipbreaker. It is a bit finicky. I've gotten used to it and setting at about 8 thou isn't very difficult, but needs a bit of practice. You only need 4 thou in the most extreme cases.To answer your question about the cambered blade, no I keep my blade straight. I t doesn't matter if it touches the edge in the corners where it won't cut anyway. On a jackplane I set it a little further away at 20 thou or so (difficult stuff all these inch measurements when you're used to metric!). It doesn't matter even if it overlaps the corners a little bit. I haven't thrown out my card scraper either. I love the simplicity of that tool. Use it a lot less though.I have documented some experiments with anti tearout mesurements on my blog. The tight mouth was disappointing. http://seekelot.blogspot.nl/2014/02/research-project.htmlA reasonably tight mouth is still a good thing to have because it makes setting the plane for a fine shaving easier and it plays a role in guiding the shaving, preventing it to quickly curl up and clog the mouth.

  4. Kees,Interesting study. I understand where you are coming form.I guess I should mention something that I took from granted that I assumed would have been presumed. (Is that clear? :)) I alway use a high cutting angle. All my smoothing planes are 55˚ or higher. I have always had the theory that the high cutting angles essentially mimicked the effects of the chip breaker. I personally don't like a plane bedded at 45˚ unless I am hogging away at some material and I will have a lot of room left to clean up the remaining surface with other high bedded irons.So am I to understand the study correctly that a high angle of attack (bed angle) is just as good as a cap iron used on a lower bed angle? I may have not looked at it closely enough to have figured that out. It seems that you say a 0.3-0.2mm cap iron setting from the edge of a 45˚ bedded iron is the same as a 55˚ bed blade without a cap iron? The same goes for the 0.2-0.1mm to a 60˚ bedded iron?If that is the case I think we are on the same page but just approaching it from different angles. 🙂 Pun intended.

  5. As far as preventing tearout, Yes. But there are more factors playing in the total "handplane experience".When you have a 55 degree bedding angle, you sure don't need to fret too much over the mouth size. But because you are a professional planemaker, I can see why you still have to. It's what is expected.

  6. Kees,I hear ya'.By the way this has been much more productive a discussion than I would have guessed. I feel like I have a whole lot better understanding of the chip breaker or at least how it relates to why I do what I do.And as far as the last part goes, you hit the nail on the head. I do it because it is expected. Like I mentioned in the post that personally I have planes that I use that have gapping mouths and they work fine with my (probably) odd methods.Seriously, I do appreciate your input. I hope others read the comments. They will learn more down here than in the post. 🙂

  7. Hi CalebYou ask how wide the mouth can be? My question to you was going to be what bed angle you are using, which you have answered to Kees (Hi Kees!). I discussed this with Terry Gordon (of HNT Gordon planes) some years ago, and his belief is that the mouth size is irrelevant once you have a cutting angle of 55 degrees. And he is correct – I've played around with mouth size with high angle planes, both BU and BD. Nevertheless, he still builds his planes (which have either 55- or 60 degree beds) with a very tight mouth. I guess that there is a need to respond to the public's perception of how a quality smoother should be constructed.Regards from PerthDerekwww.inthewoodshop.com

  8. Derek,Yes that is basically how I feel too. Though as I said early in the comments is that my approach is mostly that a closed mouth is effective atleast at reducing the chances of such a large chip levering out that one couldn't go back and clean it up with a card scraper. So in that case I wouldn't say it is totally irrelevant but just a bit of a backup if I happen to be being careless ect. By the way I wouldn't actually suggest a 0.004" mouth opening is desirable. That small is likely to be problematic for general use. I don't want anyone to expect that it is something that is needed. Anyhow I should update the post to direct folks down here to the comments section so folks can read the different thoughts on the subject so they can make up there own mind as to what they want to do. Thanks for your input too.

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