Sharpening plane irons and chisels

[Rocker]

I like a tip from David Charlesworth's book "Furniture making techniques". He recommends, instead of polishing the whole of the back of a plane iron, to form a narrow very low-angle micro-bevel on its back face by placing a thin 6" ruler lengthwise on the sharpening stone and resting the plane iron's back on it. Doing this speeds up sharpening considerably. However, he says that this should not be done with chisels. I cannot see any valid reason why not. Can anyone suggest one?

[GeoffS]

Rocker
I'd suggest because you want a truly flat back to a chisel, particularly a paring chisel, maybe not so important on a mortising chisel. However I think I would prefer them to be flat also.
Most plane blades being much wider than chisels the idea would be most useful for them. Not sure that the purists would approve though.
Cheers

[Rocker]

Geoff,
You still have not explained exactly why it is so essential to have a truly flat back to a chisel. If my trigonometry is correct, if we assume that the micro-bevel on the back of the blade is 0.5 mm wide and the angle between the plane of the back of the blade and the micro-bevel is 1 degree, then the edge thus formed will be less than 0.01 mm from the plane of the back of the blade. I doubt very much that this will make any significant difference to the way that the chisel performs.

[GeoffS]

Rocker
Agreed - it does seem a VERY small amount. But just yesterday I was lifting some runs of glue (Araldite) off perspex. Now, I know that ain't quite woodwork, however a sharp paring chisel did the job very nicely with no visible marking on the perspex. I was able to lay the back of the chisel flat on the perspex and knew that the cutting edge was on the perspex. I am not going to try it (perspex is too damned expensive) but I wonder if a chisel sharpened with even the smallest bevel on the back would have done as well.
Maybe somebody else can try - I'd be perfectlly happy to be proved wrong.
Cheers

[derekcohen]

Rocker

I must agree with Geoff. You will lose accuracy when paring with a chisel whose back is not perfectly flat.

By way of an illustration, consider that the back of the blade was concave (rounded outward - have I got it right?). If you attempted to pare this way, what would happen is that the chisel would move along the curve, cutting a scoop (convex). In other words, a rounded blade back will not follow a straight line.

Now this is taking the situation to an extreme, but it IS just an illustration of what I am getting at.

Plane blades, on the other hand, are a different kettle of fish. You are cutting with the front of the bevel. You can manipulate the rear of the blade to alter the cutting angle. For example, adding a 5 degree backbevel with raise the cutting angle to 50 degrees. This will turn a standard 45 degree plane (such as a Stanley) into a plane that is better suited to cutting hard woods such as Jarrah. Keep on hand a selection of blades with different degrees of back bevels for increasingly difficult grains. As you increase the back bevel, so you get closer and closer to a scraping plane. The downside is that these blades are harder to push through the timber.

Regards from Perth

Derek

[LineLefty]

Would you not need to extend the plane iron from the ruler the exact same amount each time you did this though? Makes repeatability hard.

[derekcohen]

Hi Adam

I agree that a backbevel is more work, which is why I tend to avoid using them.

Here is a link to a backbevel jig that can add repeatability:

http://www.estimatortools.us/forum/article_view.php?faq=3&fldAuto=23

Regards from Perth

Derek

[outback]

Does this mean then, in your opinion, I am better off flattening the whole of the back of the blade, rather than using the ruler trick?

I seems like a quick easy way of making curlies rather than wearing out paper that's all.

[beejay1]

By way of an illustration, consider that the back of the blade was concave (rounded outward - have I got it right?). If you attempted to pare this way, what would happen is that the chisel would move along the curve, cutting a scoop (convex). In other words, a rounded blade back will not follow a straight line.
I would repectfully point out that you have it the wrong way round Derek. Concave is inwards and convex is outwards. You did askhttp://www.woodworkforums.ubeaut.com.au/images/icons/icon7.gif
beejay1

http://community.webshots.com/user/eunos9

[LineLefty]

Thanks derek. I might try it on an older blade I have. The blade supplied from HNT gordon was as flat as I could determine straight from the factory so I'm loath to back bevel that.