Camber and some Sketchup models

[BrentB]

Hi

Some time ago, perhaps even a couple of years, there was a discussion about cambering plane blades and the difference between low angle and regular bench planes.

I have been learning Google Sketchup and have finally been able to draw a model of a cambered plane iron. Using these models, I was able to look at various camber situations and even produce a small calculator which lets you specify your blade width, the shaving thickness you want, and the bedding angle and returns the camber needed.

http://www3.telus.net/BrentBeach/Sharpen/camber.html

While working with the calculator and the various models, I worked through the general problem of cambering bevel up plane irons. I am unsure of the resolution of the question last time around, but these models should help the discussion.

For slight to moderate camber, a shop made jig will give you excellent results honing these cambered irons. Even for larger cambers - scrub planes - a slightly modified version of the jig works just fine.

 

[Philly]

Brent
Fantastic! Thanks for putting that together - must of taken a lot of work.
I'm off to read through it again,
Philly

 

[Chris Knight]

Brent,
Thanks for an excellent article - great job!

 

[Paul Chapman]

Great stuff 8) Many thanks, Brent.

Cheers :wink:

Paul

 

[graween]

Hi brent.

Really an extraodrinary site "bravo" !

Thanks a lot for putting together all theses informations.

Thanks.

 

[Derek Cohen (Perth Oz)]

Hi Brent

This is just a quick response. I must read the whole article more carefully. However my eye was caught by the following ..

Looking at the side of each blade, we can see the thickness of metal to be removed. The thickness is greatest for the 50 degree primary, thinnest for the 25 degree primary. However, the length of the new surface is shortest for the 50 degree primary, longest for the 25 degree primary. The area of each of the parallelograms is the same - sketchup computes this area as 0.006 square inches.

What I note is that you offer full bevels, not microbevels, which is what most sensible people would hone on a BU plane blade.

I am also not sure how much you can extrapolate from Sketchup drawings?

Have you read the article I wrote on cambering BU plane blades? I had the help of a friend in compiling the mathematical model (he is a Head of Department and professor of maths at one of the local univesities). I tend to believe his figures ..

Bob wrote:

Below I plot the "angle of primary bevel" used as a function of the mount of metal that needs to be removed relative to the 25 degree bevel. By way of example, a 30 degree bevel requires 25% (or 1.25 times) more metal to be removed relative to the 25 degree bevel. A 45 degree bevel requires 2.15 times more metal to be removed and a 15 degree bevel requires only about half the metal to be removed.

My article on cambering BU blades ..
http://www.inthewoodshop.com/WoodworkTechniques/TheSecretToCamberinBUPlaneBlades.html

I'll read a bit more ..

Regards from Perth

Derek

 

[BrentB]

Hi Brent
What I note is that you offer full bevels, not microbevels, which is what most sensible people would hone on a BU plane blade.

In fact, I show grinding and honing for all three primaries. In an attempt to look all around the problem, I modelled each angle Lee Valley sells.

I am also not sure how much you can extrapolate from Sketchup drawings?

These models are like engineered drawings - they are exactly what you have. All lengths are accurate to fractions of an inch and angles to fractions of a degree.

Since they are exact, they show only what people are intending to get. If you use a jig, you are probably close. If you work freehand, who knows.

Have you read the article I wrote on cambering BU plane blades?

Your article, which I admit I could not understand, was part of my motivation for doing the sketchup models. I think your pal solved the wrong problem.

I don't understand why my models and your experience are at odds. The honing model shows that honing the higher angle primary should require removal of only 1/6th of the amount of metal - should go 6 times as fast. That would justify buying the high angle blade, if you feel you have to use high angle blades with a low angle plane.

I might grind a blade and hone it at these angles to see what happens. Again, does reality reflect the model. If it does not I will have to revisit the models. Hopefully if the models are wrong someone will point it out before I have built the grinding/honing jig and done the grinding and honing of a test blade.

 

[Derek Cohen (Perth Oz)]

Your article, which I admit I could not understand, was part of my motivation for doing the sketchup models. I think your pal solved the wrong problem.

I don't understand why my models and your experience are at odds. The honing model shows that honing the higher angle primary should require removal of only 1/6th of the amount of metal - should go 6 times as fast. That would justify buying the high angle blade, if you feel you have to use high angle blades with a low angle plane.

Hi Brent

Your conclusions run counter to a not only the results reached by a highly experienced mathematician, but also my experience in using the Veritas blades in BU planes for some years.

Perhaps we are talking about different things. What wrong problem do yiou think my pal solved?

I know that you sharpen BD planes, but do you own and use BU planes? My article was written in reaction to the difficulties experienced by myself and others in cambering the HA bevels. My experience in using these planes led me to recognise that a low (25 degree) primary bevel made for less steel to remove when adding a HA secondary cambered bevel. Conversely, if you attempted to hone a camber on a HA primary bevel then you would create more work for yourself than you would want.

I took this intuitive observation to a mathematician, who confirmed it.

Obviously we both cannot be right.

I look forward to the observations from your real life experience.

Regards from Perth

Derek

 

[bugbear]

I had the help of a friend in compiling the mathematical model (he is a Head of Department and professor of maths at one of the local univesities). I tend to believe his figures ..

I think the formulae and graphs he supplied are accurate - for the assumption he's working to.

Having read his stuff several times, he appears to be calculating how much material needs to be removed from a blade with an existing straight primary to bevel to make the end profile of the blade round, as if one were doing an initial grind with the blade perpendicular to the grinding wheel.
This appears to leave the edges of the camber (at the side of the blade) completely blunt - this is explicit in his little picture of a right-triangle at the edge of the blade.

Of course, what we want to know is how much material must be removed to gives a sharp cambered blade, so I think professor Loss has done the wrong math (but done it correctly).

BugBear

 

[bugbear]

I am also not sure how much you can extrapolate from Sketchup drawings?

CAD packages are pretty trust worthy in that regard.

As long as Brent's drawings are accurate, I would expect Sketchup's derived quantities to be accurate too.

BugBear

 

[bugbear]

Hi


http://www3.telus.net/BrentBeach/Sharpen/camber.html

I'm fairly sure the blade shape needs to be an ellipse for the cut to be a circle, so talking about a camber radius is (itself) an approximation.

I'm unsure of your grinding effort predictions (not the volume removed - I'm happy with that).

BugBear

 

[BrentB]

I'm fairly sure the blade shape needs to be an ellipse for the cut to be a circle, so talking about a camber radius is (itself) an approximation.

When you tilt a circle you get an ellipse. The major axis of the ellipse, the horizontal axis, is unchanged. The minor axis, the axis along which we measure the camber, does change.

When grinding/honing an iron to have camber I don't worry too much about whether the resulting shaving has circular cross section. The goal is an smooth curve from thickest in the middle to nothing at the edges. If you really want a circular result you will have to take a lot of time during grinding/honing.

I'm unsure of your grinding effort predictions (not the volume removed - I'm happy with that).

I built the jig in the final sketchup model on the camber page (updated today) and am grinding an iron like the second last model. I am hand grinding on a bench stone and doing some scans as I go. The work is a little slower than I expected because extra pressure is required on the outside of the iron since a lot more metal has to be removed there. I can't believe anyone would be able to do this on water stones. My Silicon Carbide stones are plenty rugged though. A coarse India might be a better bet, but I don't have one.

Interesting that Lee Valley has completely abandoned oil stones in favour of water stones. We finally get a Lee Valley store in town and it doesn't stock any oil stones!

 

[BrentB]

A reader of my camber page, Mark Wells, figured out a simplification of the jig that I have documented

http://www3.telus.net/BrentBeach/Sharpen/camber.html#jig2design

His modification can be used with any shape iron, but it works particularly well with cambered irons.

Other threads on this forum have discussed the benefits of wide or narrow rollers. Mark thinks the smallest possible point of contact is good enough. He may well be right. Certainly for cambered blades, more camber on the jig than on the blade is not a problem.

Here is the sketchup model from that page. I may have fumbled the URL - it does not display in preview.

 

[BrentB]

One final note on this topic - this one covering bevel down scrub irons.

I made up a jig and took an old Stanley #40 1/2 blade through grinding and honing.

http://www3.telus.net/BrentBeach/Sharpen/scrubjig.html

I have also reorganized all the pages dealing with cambered blades. I think one more reorganization is needed, but it is a little better now.