Thread: Plane (plain?) Geometry

Originally Posted by IanW

...I converted the mm to a ratio of toe/rear sole. A Clifton #4 comes in at 0.35. my new infill (pretty close to a Norris A5) at 0.34 and the little woodie 0.4. The hybrid came in at 0.57, i.e. the toe was actually longer than the part behind the blade.

Okay, I took that to mean the toe was 0.57 of the length of the "part behind the blade", not of the total length of the sole .

Sorry, that doesn't answer your query...

Cheers, Vann.

I have just one thing to say:

Originally Posted by Vann

Okay, I took that to mean the toe was 0.57 of the length of the "part behind the blade", not of the total length of the sole .

Vann - the ratio is toe length/remaining sole. I guess I didn't explain that clearly.

Cheers,

Originally Posted by NZStu

I have just one thing to say:

Hmmm, yes Stu. I had forgotton that Oriental planes are practically the reverse of Western planes in this regard.

It must be something to do with the way I use planes. The long-toed variety is supposed to be pulled rather than pushed, which might make a difference, but HNT is happy to have his planes pushed or pulled....

So now my changed performance is a bigger mystery. Perhaps its all in my head.

(A thought that only just occurred to me - given the construction of this plane, it's possible there was a bit of body flex in the longer incarnation, which has now been reduced or eliminated, and that is what I'm benefitting from..)

Cheers,

Interesting observation Ian. I will give this some thought today, but the only immediate thing that comes to mind if you have ruled out variable of sole not flat is that the wood is resting on a slightly concave surface (see note further down reply). The closer the blade is to the centre of the chord (sole) the deeper is the sagitta to the curve of the wood. Once you've planed to the natural curve of the plane you can plane no more. Cutting off a bit of the toe moves the iron closer to the end of the chord and the sagitta diminishes, in your case to where you have an ability to plane even with a finer set on the blade. (note)In fact it probably doesn't even have to be because it is sitting on a concave surface. If you manage to introduce the slightest concave whilst planing, the effect will be the same (we're talking tiny tiny amounts here). It's all in my blog.
Also your a,b,c diagram, did you know that the transition from a to b changes the effective depth of cut of 0.1mm initial depth of cut by 0.0000000009mm (for a #7), so that is the reason planes don't need to look like c in your diagram. Even for a #4, the value is only 0.000000004mm!
Cheers
Michael

Beat me to the answer I was going to give Mic, although mine wouldn't have been so technical. Not sure whether I would have been right or not though.

Regards,

Denim.

Ian, to state the idea another way... What you are bumping up against is the maximum error of the plane. For any given blade extension and sole length, a plane with the blade positioned closer to the centre is going to have a smaller maximum error (the planed surface's deviation from flat) than one where the blade is closer to the end. Once you get to that error, the plane stops planing unless you set a greater blade extension. For the given work you were doing, that cut down plane never reached the maximum error for the finer blade setting and you were able to keep planing (if you were to set an even finer blade extension and get reasonable shavings, you might find the limit of the truncated plane too). The other factor which assisted, in addition to moving the blade closer to the end, is the shorter sole length. This too increases the maximum error for a given blade extension and so allows you to keep planing. (technically, it's why we tend to use shorter planes for smoothing tasks). Clear as mud?
Cheers
Michael

Originally Posted by mic-d

....... (technically, it's why we tend to use shorter planes for smoothing tasks). Clear as mud?

Thankyou for the erudite explanation, Michael. Now that you point it out, it makes good sense, and what's more, explains why a sole needs to be very straight & true for very fine shavings. I had realised that the 'upset' when the sole is fullly engaged would be vanishingly small, and had also reasoned that the toe length should not have been a factor (which Stu acknoweledged in a rather good way ). What it boils down to is that either my sole is not quite as flat as I would like to think and/or I was getting some flexing of the sides, which would certainly enhance the propensity to plane a small concavity. As you say, it only takes an imperceptible amount to stop a very finely-set blade from cutting. My shorter plane has a reduced error (the shorter a line, the closer it is to straight) and probably reduced tendency to flex, so the improvement in performance probably stems from both.

I think you have cleared up the mystery for me - thanks!

Cheers,

Hmm ... there is one other variable that might explain it - you swapped the blades and the result improved. Perhaps the difference is in the blades? How does the little smoother perform with the blade taken from the hybrid?
Just covering all the bases.
SG

Originally Posted by Scribbly Gum

Hmm ... there is one other variable that might explain it - you swapped the blades and the result improved. Perhaps the difference is in the blades? How does the little smoother perform with the blade taken from the hybrid?
Just covering all the bases.
SG

Good point, SG, but I did cover that base. The problem occurred with with either blade, so I don't think that is a factor in this case. I know that swapping blades can sometimes make a huge difference - the first time I put a thicker blade in my old #5 workhorse was an eye-popping revelation. However, I would have been surprised if I had noticed any difference between the blades in this case, because they are ground to precise thickness & width, so they seat equally well, & the sharpening bevel is the same. The only variable was length. I carefully sharpened each to the best of my ability, so that should also have been pretty consistent. I also tried both blades in the woodie, and decided it went better with the shorter blade, because the longer blade made it a bit less comfortable to hold. The 'pillow' on the hybrid is lower and a touch longer, giving more room.

So I'll go with Michael's explantion as my working hypothesis for now. In any case, I'm very happy, because the problem is cured...

Cheers,

Don't you love it when a plan comes together .........
Cheers
SG

Can only add this: when Terry Gordon built and tested his A55 prototype he then added 8mm to the toe for the production version.

Originally Posted by IanW

Thankyou for the erudite explanation, Michael. Now that you point it out, it makes good sense, and what's more, explains why a sole needs to be very straight & true for very fine shavings. I had realised that the 'upset' when the sole is fullly engaged would be vanishingly small, and had also reasoned that the toe length should not have been a factor (which Stu acknoweledged in a rather good way ). What it boils down to is that either my sole is not quite as flat as I would like to think and/or I was getting some flexing of the sides, which would certainly enhance the propensity to plane a small concavity. As you say, it only takes an imperceptible amount to stop a very finely-set blade from cutting. My shorter plane has a reduced error (the shorter a line, the closer it is to straight) and probably reduced tendency to flex, so the improvement in performance probably stems from both.

I think you have cleared up the mystery for me - thanks!

Cheers,

It might be that your sole is absolutely flat and the plane is perfectly rigid. You will still see this problem. Even a perfectly flat plane will plane a very slight curve at its maximum error because the toe-end, heel-end and edge of blade define an arc of a circle.
You grow the chord (sole) and the arc (this is what I call the natural curve of the plane or the maximum error) gets flatter (it's why we choose larger or smaller planes for jointing or smoothing resp.). You decrease the blade extension and the arc gets flatter (which is one reason you should have the sharpest possible edge) and you move the blade position towards the centre of the sole and the arc gets flatter. In other words, moving the blade position towards the centre effectively makes the plane perform like a longer plane with a 'standard' blade position. If your plane was the size of a #4 it may have in fact been performing more like a #5...

Cheers
Michael

Originally Posted by rsser

Can only add this: when Terry Gordon built and tested his A55 prototype he then added 8mm to the toe for the production version.

Be interesting to me to know why, Ern - 8mm seems like a very fine adjustment, but p'raps it made a big difference to balance or something like that?

I think a toe can definitely be too short, in which case it won't give a firm register as you begin the cut. The actual length needed depends on the overall length & balance of the plane. On one of my early attempts at making a small smoother, I made the toe far too short, which made it awkward to start smoothly. It was a dog of a thing for several other reasons, and almost put me off planemaking for good, but I guess it did teach me some good lessons, once I was able to diagnose its major flaws. Another mistake was to use yellow glue to laminate the sides & add-on sole, and it de-laminated soon after I moved back to Brisbane from a much drier climate. I wasn't the least upset!

So, all good fun, and still learning....

Cheers,