jointer sole length vs. stock length? ratio?

[Johnny Kleso]

Tub of water is what they use to test levels..

[Will Blick]

Interesting post Russell....

So, getting back to the original issue, and applying what you have learned...what would be your estimate of hand plane length vs. board length. Assuming all the obvious things are done, such as lowering the high spots, at some point, you can not stop a shorter plane from concaving a longer board.... I am interested in your thoughts...

[Casey Gooding]

I believe the general rule is a plane can accurately joint a board approximately 2.5 times it's length. I think the important thing is not so much that a board is perfectly straight, but that two edges match up.

[Roderick Gentry]

There is an inherant relationship that will create a straight edge without measurement, automatically. It has been referred to, there is a relationship between blade projection and sole length that will give a certain degree of concavity for a given sole length and blade projection, and for a given shaving size. The length of the board does not affect this. But while it might be within an accuracy/length limit of a thou over 22" it is a lot more than that as the 22" segments are connected, but in my experience it yields sufficent performance to joint edges over say 7' table lengths. One can tighten the specification if one is willing to reduce shaving thickness but it eventually gets impractical. So another option would be to lengthen or shorten the required sole length to get back in the sweet spot.

If one is jointing, rather than shooting an edge, the width of the board or it's aspect ratio also comes into it. As boards get wider their stiffness increases, so the joint will tolerate less spring. So a plane that might be fine for a board with an aspect ration of 1:14, say for a table plank, would not be fine for a guitar plate that was nearer 1:2.

So the relationship has a few factors in it, but it could be said to exist, and if one prefered to express it in the form of sole to board length with the other variables assumed, it could be done that way.

[Roderick Gentry]

So imagine you have a board whose edge has a 30 foot longitudinal concave radius to it, or call it 3 foot if that helps. Run a jointer plane along that edge and it can't cut. Now take a piece with the same radius, but convex. The plane will be able to cut into that. Somewhere between those two extremes the plane goes from taking a cut to not taking a cut, and that is the point of automatic "straightness". It is a function of blade pojection relative to sole length.

[Will Blick]

Fully agreed, great contribution....

I like your point about board thickness as well, or more importantly, board flex... we often forget about the downward pressure applied when hand planing....the shorter the plane, less area this force is spread over, the more flex. A power jointer has a big benefit here due to much longer beds.

and agreed, thickness of shaving plays a very significant role also.... so it make sense to finish joint planing with thinnest shavings... I will start implementing this.... thx

[Andrew Pitonyak]

Let B = the bed length of the plane. Let B=24"


Where is the cutting edge of the blade? I will assume that it is 25% from the front, so 75% from the back of the plane. For our example, this means 6" and 18".


Let D = the blade depth = distance from the bottom of the sole to the cutting edge = shaving width along a flat board. Let D be a very thick shaving of say 0.01"


Assume a perfectly flat board and place the front of the plane firmly on the board and take a shaving. I expect the shaving to have thickness D while the front of the plane is held firmly in place.


Take a shaving for a bit until the entire plane is over the wood. If the front is held down firmly, then the back 18" of the plane is of height D above the board. Now, let the back end drop to touch the board. The plane is now at an angle. I assume that the uncut portion directly in front of the blade is touching the uncut portion wood and the back end is now a depth of D (0.01") lower. So, what is the angle? The angle is given by asin(D/18). This yields the tiny angle 0.0318 degrees, which tilts the plane and changes the cutting depth to sin(90 – 0.00318) * D. The difference in height is about 1.54E-09 = 0.0000000054"


The main advantage of the long sole is that the plane rides of the hills and valleys of a board that is not straight.


Sometimes the length is a problem, not because of the length, but because of the weight. I have some twisted and cupped boards. If I use my #8, the weight of the plane flattens the board so the cup and twist is gone. To some extent I can avoid this by putting weight down on the rear of the plane so that the cutting edge reaches the hill before the weight pushes the board flat. I can also use a lighter plane, which means not as long.


How the plane behaves with respect to a hill or valley depends on where pressure is applied to the plane. Pressure applied to the back will PROBABLY (no testing done) allow a 24" plane to ignore valleys that are easily 12" wide... until the back of the plane enters the valley.


If you consider what occurs when the front of the plane enters a valley and pressure is applied to the front of the plane, the equations change... Also, the front is not as long, so the angle will be greater for the same depth.

[Roderick Gentry]

"The main advantage of the long sole is that the plane rides of the hills and valleys of a board that is not straight."


I don't believe that, though it is the dominant view. The reality is you can't tell what length of plane will interface with what wave train of bumps most effectively (just running the boards, I guess the next part of this post is an analysis of fit, but who does that and picks up a specific plane). So imagine a 10' boards. Having bumps even 1" A block plane would get 'er done pretty much just as well as a jointer. But there would be a board where the longer sole would work better, maybe 10" spaced bumps. And because of that the longer sole plane is the better choice as you say. But the reality is that the jointer when used properly would straigten any board just as well regardless of what the bump pater was so long as it is being used for it's default cut. And as far as that is concenred a plane half the length with half the balde projection would work just as well as the longer jointer, but would take a long time to get the result because of the finer shaving.

The constant is that for a given effective cut, a given blade projection and sole length will produce a given "strainess" of board regardless of what board you start with. (That is an approximation if you want to get super picky about it, but the principle is correct). And this principle remains true regardless of what board you start with. I didn't make this up, I learned it in the Krenov plane video.

I don't want to overstate my point. I guess the length of plane sole, and it's effects, is default function also, but it varies depending on the board you start which while the default function relative to blade projection is a constant.

[Roderick Gentry]

Fully agreed, great contribution....

I like your point about board thickness as well, or more importantly, board flex... we often forget about the downward pressure applied when hand planing....the shorter the plane, less area this force is spread over, the more flex. A power jointer has a big benefit here due to much longer beds.

and agreed, thickness of shaving plays a very significant role also.... so it make sense to finish joint planing with thinnest shavings... I will start implementing this.... thx

On board flex I was thinking of something different than your equally valid point. My point is that in setting up a plane:

- The default cut is concave, length of the board, until they start to make very subly convex plane soles.

- The default cut will create a sprung joint, unless you back it out.

- While you can close together a sprung joint, or as you point out regulate it with blade projection aspect ratio, this needs to be regulated not only in terms of board length but width to length aspect ratio.

So take my original example. except let's consider the guitar plate only. 20 inches long, 10 inches wide, and 1/8" thick at glue up along the 20 inch side. aspect ratio 1-2. Now cut out a strip that is aspect ratio of 1-14, otherwise similar. That would be a strip 20 inches long and 1.42 inches wide. The guitar plate 10x20 is 345 times stiffer than the 1.42x20 strip as far as closing the edge joint. So when working with the plane's defaul setting one has to account for that.

The take away is that the crossgrain stiffness of boards rises exponentially with width, and you may have to alter your approach to deal with it. You don't need math except to realize how significant this factor can get. It isn't practical to use planes and their default settings on guitar tops, which is why we have shooting boards and sanding boards, etc... But there are many other situations where the default planing ability of planes is very useful.

What this has to do with the original question is that you can meaningfully set plane length in terms of board length, but width ratio also has to be considered, and you can set that in terms of felt flex, or reasonable clamping pressure.

[Will Blick]

again, great post.....

I was mostly thinking of edge jointing, whereas the edge was thinner than the blade width...otherwise, a power jointer is the ideal solution... or as you mention, sandpaper on FLAT surface.... assuming you have a flat surface large enough

valid point regarding material strength, which is a function of the wood type and dimensions.... again, a power jointer solves this, as you push the wood on the large flat beds, so the wood is always supported, vs. a hand plane where some of the wood is being flexed at different amounts based on hand pressure, and position... obviously center has potential for most sag, so clamping strategies also come into play...

What this thread has made me appreciate is.... reference surfaces are the key, as we know 1:1 will assure flatness equal to the tools tolerance, i.e. 100% of the wood touches the beds, throughout the entire cut. The further we stray from this ideal, the more risk the board not being straight. A well tuned and FLAT power jointer truly can earn its wings in this regard. Of course, its the wood length that is the significant factor here... 5ft board you must deal with differently than a 1ft board, assuming your goal is max. flatness. Although I never thought about this much in the past, I have always instinctively used my neander tools on smaller length boards, and my power tools on larger boards. Of course the marriage of the two is ideal... a single pass of hand plane after power edge jointing gives you a glass like surface, vs. power jointer.... although when I put new blades on my jointer, it can match my hand plane surface, but it doesn't last long....

[Johnny Kleso]

I have made high precision parts for over 30 years and high precisionmachines..
No matter how big or expensive the machine you always haveto check the part for accuracy, there is never a given in a machine shop..

When you'er figuring formulas where your accounting for the wood, badly tunedplane as well as user experience?
I know where must be some relationship and ratio that longer is better but I think its futile tothink you don’t have to check yourwork..
There is no such thing as FOOL PROOF as fool are so Ingenious

[Will Blick]

I doubt you will get an argument from anyone with comments like, "check you work", or "check your tools", etc....

I have a 5ft starett straight edge, its straight to .0002" per ft... this was the best investment i made in my shop....
It enables me to get my long jointer beds, as close to dead flat as one could expect for ww. The finished wood is equally as flat....
As I mentioned, the key is proper starting reference, otherwise you will tune tools that will be poorly aligned, but with no knowledge of such...

As for user experience.... a thread like this gives people info to think about, hence the value of forums...

[Darius Ferlas]

I read somewhere, possibly on this forum, that a reasonable ratio is 1:1.5 (combined table length to the length of material) and this is what I had no choice but test with my 6" jointer - 54" long table. The longest piece I ran thrugh it was 80" long. It worked but it was a serious chalenge and I think the fact that the pieces were bed rails helped me in deciding the quality of the cut was good.

The longest pieces that I was able to verify as to their straightness were 60" long boards for a table top glue-up.

[Brad Pearce]

I have both a 6" machine jointer [46" bed] and a 22" hand jointer plane [Lie-Nielsen]. The hand jointer is absolutely true as far as I can discern. The machine jointer is as true as I can make it, which is within 4 thousands of an inch infeed and outfeed. This is measuring with a Veritas 50" straight edge. With wood pieces shorter than 25 inches the machine jointer works fine and is very fast, with longer pieces it fails, just a bit, but it magnifies with additional cuts. Then I need to take it into my own hands with the hand jointer, where it all goes square. I could not live in woodworker's world without both tools... Also, the hand jointer is indispensable with larger stock [i.e. My shop can’t accommodate an 8” jointer.].

[Eddie Darby]

I was curious about the relationship between bed length and assured flatness, i.e. no thinking, the tool would be fool proof...
As an example... my power jointer has 40" beds on each side... when I joint a board 40" or less, I am ASSURED straightness as the reference surface is as long as the stock...

Even with your 40" board on 40" beds you need to 'know' the general state of your board, since how you hold the board, and where you put pressure on the board as you feed it, will affect the outcome. Knowing if a piece is cupped or bowed, twisted all must be factored in so you can run the board correctly through the jointer.
Sorry you will just have to accept that in woodworking you have to use your brain.