bench planes – how flat does the sole need to be?

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Given the main purpose of bench planes is to smooth and flatten wood it is unsurprising that the sole of your plane should be flat, but views differ on exactly how flat it needs to make them work optimally.

To understand why flatness is important we can consider the mechanics of the plane: if you imagine the sole of a plane that is significantly convex – in other words it has a “belly”- then you can also see how it would tend to rock from side-to-side or end-to-end as you pushed it across the work, meaning the blade would not make consistent contact with the wood and resulting in a unsatisfactory finish.

If it is concave – that is, it has a hollow – then the blade and mouth can be held above the surface of the wood and, in order to make a cut, you will have to extend the blade beyond the mouth than would be necessary otherwise.  The result is that the blade is unsupported by the frog for a relatively long distance and therefore more prone to “chatter” 1)this is caused where the blade vibrates back and forth such that the blade does not make even contact with the wood and, because the wood is not compressed by the leading edge of the mouth, tear-out is more likely 2)where fibres are torn from the wood before they are cut by the edge of the blade, see the previous post for an explanation.

So, hypothetically, having anything other than a completely flat sole is bad news.  But what about in practice?  Exactly how flat is necessary?

In true British fashion we have a rule telling us whether a plane sole is flat or not – the relevant British Standard is BS 3623:1981 Specification for Woodworkers’ Metal-Bodied Planes 3)this is a metric version of the now withdrawn BS 3623:1963.   There is also an ISO standard – 2726:1973 :

the departure of the highest and lowest points on the sole when measured relative to the mean true plane shall not exceed ±0.04mm 4)Section 8.1 Flatness of the sole (excluding corrugated soles)

I have reproduced the diagram from the standard that illustrates the point:

standard plane flatness
error must not exceed ±0.04mm

For now let us assume that the BS standard describes a level of flatness that is good enough for most woodwork . How can we test our planes against it?

First of all you will need a straight edge or a flat surface that has better tolerances than the standard.    I have a 1ft steel rule made by Starrett and these are  apparently guaranteed to be straight and parallel within 0.001″ per foot.

None of my planes are longer than 1ft and since 0.04mm is just over 0.0015”,  it must also be true that if I can’t see any light between the straight edge and the sole, then the sole must be flatter than the standard.

The other way for testing flatness is to place the plane on a known flat surface and use a 0.04mm (0.0015”) feeler gauge to try to get underneath the plane – if you can’t it is flatter than the standard.  In the picture below I am using a piece of float glass:

feeler gauge

Obviously these tests are only as good as your reference surfaces, but you get the idea.

Is flatness of .0015” good enough?

… as you will read all over the internet, Lie Neilsen make the creme-de-la-creme of modern bench planes and according to universal consensus these planes will work “out of the box” without any fettling. Lie Neilsen says:

The soles of our planes are machine ground flat and square to .0015″ or better, regardless of length.

Lie Neilsen

This is exactly the tolerances specified by the the engineers who came up with the BS standard all those years ago, so I guess they knew what they were on about.

Does it really matter?

quite possibly not, however, it is not difficult to get  Lie Neilsen levels of flatness with your el-cheapo ebay plane, and you can read the next thrilling instalment to find out how.

If you are in a hurry to get your old plane working you are just as well to skip this stage and go straight to sharpening your blade. The sharpening step obviously can’t be skipped and may well be sufficient on its own to get the plane working well enough.  You can always come back to the flattening step if you are not happy with the results.

You may also be interested to know that Japanese plane makers deliberately make their planes concave ensuring only that the areas at the toe, heel and immediately in front of the mouth are co-planer.

When you think about it, the fact that the three flat reference points bridge the hollows will mean that the plane will not rock around in use and that the spot immediately in front of the mouth still fulfils its purpose in compressing the wood fibres prior to cutting.    Thus they avoid all the hassle of creating a completely flat sole and have the benefit of reduced friction, since there is a smaller surface area in contact with the wood.

If you are undeterred by these details and want to flatten your plane in any case, read on….


1 this is caused where the blade vibrates back and forth such that the blade does not make even contact with the wood
2 where fibres are torn from the wood before they are cut by the edge of the blade, see the previous post for an explanation
3 this is a metric version of the now withdrawn BS 3623:1963.   There is also an ISO standard – 2726:1973
4 Section 8.1 Flatness of the sole (excluding corrugated soles)

2 thoughts on “bench planes – how flat does the sole need to be?”

  1. Dear sir
    I read your series on flattening planes with great interest and I agree that planes are being more or less mystified, one aspect of mystification being the degree of flatness necessary for adequate planing performance and the equipment required to achieve adequate flatness. Like you I also have flattened my planes on float glass and sandpaper with very satisfactory results, both for finishing boards as well as for jointing boards.
    I only have a single comment: in the text above you say “Does it really matter? quite possibly not….”. That suggests that flatness does.not really matter. But I think it does in all cases but it depends on the definition of flat and how out of flat the plane is. With a convex (belly) plane the planing peformance might not even be too bad depending on where the “pivot point” of the belly is. If it is at the mouth area the effect may be minute. But with a concave ( hollow) plane, that’s different and in my experience most planes that are out of flat are concave.
    What actually happens with a concave plane sole is that at the beginning of the stroke, the toe of the plane rests on the wood. At some point the cutting edge of the plane iron engages with the wood and starts cutting. When the stroke progresses, the heel of a concave plane comes into contact with the woodsurface and effectively lifts the plane a little bit. The result is that the cutting edge will be lifted and no longer engages with the wood, hence no shaving is cut. At the end of the board, the toe of the plane “falls” of the board, the cutting edge is lowered and starts cutting again. The final result will be that you get shavings at the beginning and at the end of the board and not in the middle. With such a plane it is quite impossible to get a good finish over the entire board; let alone a decent jointing of two boards.
    Hence the requirement for flattness should not be exagerated but it is certainly important for accurate work, especially jointing.
    I use a 500 mm straight edge (DIN874/1) and a feeler gauge of 0,002″ (0,05 mm). My personal, very practical requirement is that the beginning of the toe, the part in front of the mouth and the end of the heel of the plane should be coplanar wtihin this 0,05 mm meaning the feeler gauge can not enter between the sole at these point and the straight edge. Other points on the sole may be slightly lower (not higher though!). Given the tolerance of a 500 mm long DIN874/1 straight edge (flat wihtin 12 micrometer=0.012 mm) that brings the maximum difference between high and low spots at 0,062 mm which is well within British standard ( 2 * 0,04=0,08 mm) which sounds good enough to me. With such planes I readily achieve shavings of less than 0,03 mm thick on soft wood and even thinner on hard woods.

    I am curious for your thoughts on this
    With kind regards

    • thanks for the comment

      My conclusion is similar to yours, in as far as it must be possible for some degree of out-of-flatness to impact the effectiveness of the tool, but I do not have any first-hand experience to indicate exactly how much imperfection can be tolerated before there is a noticable impact on performance.

      With hindsight I missed a golden opportunity to learn a bit more about this topic since (like many others, I suspect) I set to flattening my planes before I really learned how to use them properly, and thus was not able to compare performance before and after flattening them.

      Hopefully the British Standards folks were more diligent than me and did some proper tests before arriving at their acceptable tolerances!

      I do have one hunch, which is that a pronounced hump (convex) would be particularly problematic since the plane would tip back and forward in use causing the height of the cut to vary. However, as I say this is just theory.

      Having now learned to use my planes, and made a few bits of serviceable furniture, if I were still buying used planes then I would be sure to given them a proper test before messing about with the sole – this would be informative and might avoid a lot of unnecessary work!

      a bit more about this topic is here :


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