Is there a spcific reason why plane irons are 25 degrees?

[J_SAMa]

Even in a bevel down plane, where the bevel angle doesn't affect the angle of attack at all... Does it have to do with the balance between sharpness and edge retention?

Sam

 

[G S Haydon]

I can't wait for the responses on this one, and I hope mine's not the worst . Because It works! If say you went to steep, say 45 degrees or more in a bailey type the bevel would ride up on the timber, especially on end grain . To shallow, say 5 degrees, it would be a really weak edge with no support, would be really hard to hone that shallow too. 25 > 35 is fine (based only on my experience with bailey type planes) I don't stress to much on angle, I free hand it to what feels a natural angle, I don't micro bevel either (that's not to say I'm right, far from it, there is no one way of doing it, just works for me).
On edge retention I have always been taught a shallow angle is sharper but will loose it's edge quicker, a steeper angle is not so sharp but will hold it's edge longer.

 

[Jacob]

30º is generally preferred for honing as it works for most things and is easy to judge (half of corner of equilateral triangle, third of a right angle, gradient 1/2).
25º just happens to be an angle less than 30º (rounded off) but 29, 28 etc would do just as well. So there's nothing specific or magic about 25º as a grinding angle, except convenience and easy to remember.
So it's hone at 30º grind at less than 30º, say 25.
Most new tools are ground at 25 on the assumption that the user will hone a steeper angle usually 30º

 

[Jelly]

Erm, ish... It's not as important in hand tools, but if you look up "relief angle" with reference to wood machining there should be plenty of info... (usually as engineering papers/theses, all the useful information is)

Basically if the iron was honed to 45° and bedded at 45° then the flat of the honed Iron would ride on the workpiece, so the iron needs at least a little relief to cut down into the timber... In hand planing that rarely matters beyond there being some to make the iron cut, but it can make a significant difference to chip formation/extraction in machine tools where each knife might be moving at 12000RPM and making it's entire cut in six thousandths of a second.

 

[TobyC]

From IanW, http://www.woodworkforums.com/f152/question-about-infill-plane-159762/#post1558072.

To work properly, the cutting edge needs "relief", or clearance, immediately behind the edge. Because wood is a variably compressible material, the relief angle needs to be at least 10 degrees to cope with a wide variety of types. It's impractical to sharpen a metal blade to less than about 20 degrees (& even that is too acute for many) so the compromise is usually 25 degrees (usually in the form of a secondary bevel). Add 10 degrees for relief, and this sets the minimum bedding angle for a bevel-down blade at 35 degrees. That would be a minimum, and would be ok for some woods, but could cause problems in many others, which is one reason you just don't see planes with bedding angles between 30 & 40 degrees, they are either >40 or <30. Once you go below 30 degrees, the practical solution to the relief problem is to flip the blade over, in which case the bedding angle is now the relief angle, and the bevel angle becomes the 'cutting angle'. A 'standard' block plane has a bed angle of 22.5 degrees, which, with a bevel angle of 25 degres gives you a cutting angle of 47.5 degrees. Almost the same as the 'standard' 45 degrees of the most familiar bevel-down bench planes.

Of course there is a little more to the function of planes than cutting & relief angles, but these do set some important limits to design.

Toby

 

[woodbrains]

Hello,

There are no hard and fast rules as such, as in if there is a moderate variation, nothing disastrous will happen. However, there are lots of variables that need to be considered, and this has led to the classic consensus that a grinding angle of 25 deg, honing at 30 deg has come about.

Relief angle in hand tools isactually more important then has been stated previously. Wear bevels that are created on the blade from use will increase the honed bevel, so if this is too close to the bedded angle to start with, the relief angle will be reduced to too small a value and cause problems. 30 degree honeing bevel will give a good margin of 15 degrees of relief angle, so the iron will get dull and be sharpened before the relief angle is breached. In traditional oil hardened and water hardened steels that have been most widely used, 30 deg is a good angle to maintain edge retention and 'thin' enough to be sharpest. 25 is sharper and good for softwoods, but will break down quicker in harder woods. Newer steels such as A2 might work better with a honing angle of 35 which still gives adequate leeway for relief angle. Since these steels edges hold up longer than carbon steel, the wear bevel should not get so troublesome that a 10 deg leeway will cause any problem before resharpening. I would not, however, go any higher that 35 deg in any steel, as the wear bevel can change the honing angle quite rapidly. Think about it, a wear bevel is essentially a radical rounding over of the edge right at the cutting edge, and it will not take much to increase the honing angle to near 45 deg, which will breach the relief angle and cause problems. Draw it out large scale and you will see the effect.

Another advantage of a 25 deg grinding angle is, since most of use use secondary bevels, more honings are possible between grindings than with a steeper bevel. Again, draw it out, a 28 deg bevel will not allow as many 30 deg honing a in the space left, compared to a 25 deg, when you take into account removal of enough steel to remove the wear bevel and get to sharp steel again. Of course a 20 deg primary bevel will allow more honing a still, but will take longer to grind initially ( by hand) or be more likely to burn with a dry grinder. It is all about happy mediums. If you have a wet wheel grinder, it may be good to grind at 20 and get more hones between grinds. I still stick to the classic 25/30 for most thing because it works. I have a Ron Hock A2 iron which doesn't seem to suffer like other A2 steel from crumbling edges, so don't bother changing the formula here, either. The other A2steel iron I have is in a bevel up plane, so higher angles will not breach the relief angle, so I go higher to retain the edge and also to deal with tricky grain.

Mike.

 

[Dangermouse]

And of course it all depends on what method you use to sharpen the blade or chisel. :wink: :-"