Back bevel sharpening

[Karl]

Ok, so this may be a thick question. But i'll ask it anyway :lol: :lol:

When honing a blade with a back bevel, is it better to keep the primary grind angle low (say 20 degrees), with a honing angle a couple of degrees higher (giving a 22 degree honed edge). If a back bevel is introduced of 15 degrees, this would then give an overall sharpening angle of 37 degrees, and an effective pitch of 60 degrees.

I only ask as I am going to prepare a blade specifically for planing some difficult timber I am working with. My normal routine gives a 35 degree honed edge. If I am then to add a 15 degree back bevel, this would give an overall angle of 50 degrees.

Surely a sharper edge can be obtained at the lower angle?

I have had similair thoughts regarding the preparation of high angle BU blades - is it possible to achieve a "sharp" 50 degree blade?

All thoughts are welcome.

Cheers

Karl

 

[woodbloke]

karl wrote:

I have had similair thoughts regarding the preparation of high angle BU blades - is it possible to achieve a "sharp" 50 degree blade

I haven't used a back bevel much on my normal BD planes and when I have used a steeper effective pitch at about 60deg it hasn't worked very well on trickier timber. However all my BD blades are honed so that the effective pitch, including the 12deg bed angle, is 50deg and they are very sharp. Final micro bevel is at 38deg on a Spyderco 10000g stone ceramic stone with Mr C's ruler trick used on the back - Rob

 

[Karl]

However all my BD blades are honed so that the effective pitch, including the 12deg bed angle, is 50deg and they are very sharp. Final micro bevel is at 38deg on a Spyderco 10000g stone ceramic stone with Mr C's ruler trick used on the back - Rob

Rob - I presume you mean the BU blades? 12 deg bed.

Cheers

Karl

 

[Chris Knight]

Karl,
I think you'd need to consider the clearance angle under the blade with your suggested example. At a bed angle of 15 degrees, your clearance angle is zero, so your bed angle is going to need to be a good bit more to give you an effective angle.

 

[woodbloke]

However all my BD blades are honed so that the effective pitch, including the 12deg bed angle, is 50deg and they are very sharp. Final micro bevel is at 38deg on a Spyderco 10000g stone ceramic stone with Mr C's ruler trick used on the back - Rob

Rob - I presume you mean the BU blades? 12 deg bed.

Cheers

Karl

Karl - correct, typing too late at night - Rob

 

[PaulO]

Karl,
I think you'd need to consider the clearance angle under the blade with your suggested example. At a bed angle of 15 degrees, your clearance angle is zero, so your bed angle is going to need to be a good bit more to give you an effective angle.

I think the original post was regarding a BD plane, with a 45 degree bed, the 15 degree back bevel giving an EP of 60 degrees. So by grinding and honing the front bevel at 20/22 degrees, he is actually increasing the clearance angle, to 45-22= 23 degrees.

 

[Chris Knight]

Paul,
Yes, you are right if Karl did indeed mean BD - I wasn't sure.

 

[Jeff Gorman]

If you start the stroke partway along the length of a piece, the greater angle will make it more difficult for the edge to get under the surface.

 

[David C]

Karl,

Which difficult timber are you working with?

David

 

[Karl]

Thanks for the replies folks. Sorry for the delay in replying - just got in from work.

I am working with a BD plane - Clifton 7. I mentioned the point about BU blades as it seemed equally relevant.

David - the timber is Oak, but in long lengths (7'). The grain changes direction, and I am getting tearout with a standard 45deg blade, even when set for a fine cut. I was therefore planning on trying a 60deg EP, introducing more of a scraping cut.

This is my first foray into back bevels, and i'm feeling my way a bit.

Cheers

Karl

 

[David C]

Karl,

I would have thought that 60 degree Effective Pitch should do it. If this does not then increase EP 5 degrees at a time, till tearout ceases.

Steeper EPs only work with fine shavings, (Hoadley).

I find I only need a very narrow BB, 0.2 to 0.3mm is plenty. This is simple to hone at 15degrees, with a few strokes on a 1200 waterstone, followed by polishing at the same angle.

I leave the bevel configuration as normal. There may be some advantage in the edge being rather obtuse as steep EPs create more heat and blunt quicker than a normal blade.

please report on results
David

 

[ivan]

Back bevels (on a bevel down blade) look like a spiffing wheeze, but I wonder if they are just the result of armchair day dreams. If you think you need a back bevel, then you also certainly need a close set capiron.
By close set, the best eveidence appears to be just a few thou (about 4) from the blade edge. Once you've put on a 15 deg backbevel, how do you get the capiron 4 thou close?

 

[Philly]

Ivan
When you start raising the pitch above a certain point (say 60 degrees) the blade takes a different type of shaving where the shaving breaks at the actual cutting edge, making chipbreakers un-necessary. So you can set the chipbreaker (or back irons as I prefer to call them :wink: ) safely back away from the cutting edge.
Hope this helps
Philly

 

[David C]

Ivan,

I can assure you that steep back bevels are nothing whatever to do with "armchair dreams".

This technique allows me to plane any 'unplaneable' timber.

Cocobolo, rosewoods, interlocked Santos, rippled Tasmanian Blackwood, the list is endless.

This is one of the topics on my next dvd from L-N.

David Charlesworth

 

[ivan]

I use a back bevel on the LN No9, but confess I have not tried it on a bevel down plane. A 50 deg frog + very close set capiron is as far as I usually get; looks like I'll have to add another blade to my shopping list and give it a try!

 

[Karl]

Apologies for the delay in responding - been tied up with the day job.

I left the blade configuration as normal (25 deg primary grind, 35deg hone), then honed a narrow 15 deg back bevel. Was surprised to see the sharpness achieved - I thought such a significant range (50 deg) wouldn't produce a sharp edge, but it did.

Anyway, planing ensued, and I am v. pleased with the results. Tearout significantly reduced, but at the expense of an increased work rate! That back bevel does make the plane more difficult to push - even with a fine (1thou) shaving and a nicely waxed sole. Also, taking such fine shavings meant that it took quite some time to remove the deep tearout which had been caused by my thicknesser.

David - I will be very interested to see the next DVD - any idea on when it is likely to be out?

Cheers

Karl

 

[Jeff Gorman]

Karl wrote:

I left the blade configuration as normal (25 deg primary grind, 35deg hone), then honed a narrow 15 deg back bevel. ........................

Anyway, planing ensued, and I am v. pleased with the results. ..........................

I think Karl would also benefit from an extremely fine shaving aperture. When working with a back bevel, the cap iron setting is immaterial.

 

[bugbear]

Back bevels (on a bevel down blade) look like a spiffing wheeze, but I wonder if they are just the result of armchair day dreams. If you think you need a back bevel, then you also certainly need a close set capiron.

I don't quite follow that; high effective pitch and a close cap-iron are separate concepts. Further, since high EP blades remove a different sort of shaving, the effect of close cap-irons is massively reduced.

By close set, the best eveidence appears to be just a few thou (about 4) from the blade edge. Once you've put on a 15 deg backbevel, how do you get the capiron 4 thou close?

I assume you're referring to the famous (and only AFAIK) japanese reasearch into cap-iron effectiveness, done in the context of a "super surfacer". IMHO all that reserearch proved is that cap-irons on hand-planes (almost) never perform as chip-breakers.

This is not to say they don't do anything; in a Bailey plane the capiron ensures that the blade is pressed firmly down onto the bedding.

(as an aside, the other purpose they they often serve is to jam shavings, but I don't think that's intentional )

BugBear

 

[Philly]

So just to confirm - a "back bevel" is applied to the flat, non-bevelled side of the blade.
A back-bevel applied to the bevelled side is actually a "micro-bevel", a very different thing.
Philly

 

[Jeff Gorman]

Philly wrote:

>So just to confirm - a "back bevel" is applied to the flat, non-bevelled side of the blade.
A back-bevel applied to the bevelled side is actually a "micro-bevel", a very different thing.<

With respect, and to split hairs, as far as I recall, in the quarters (either rec.woodworking or the OldTools group) where the term 'micro-bevel' appears to have first been used, it was used to indicate a third very narrow, usually highly polished bevel applied to a honing bevel, making three bevels on one side of the blade ie a 'grinding bevel' (conventionally 25deg), 'honing bevel' (usually 30deg) and 'microbevel' (32 deg or a bit greater).[