Scary sharp hollow backed chisels...

[woodbloke]

I wasn't really happy with the 'Scary sharp' method of lapping a chisel back that I was playing around with the other day with the Blue chip...there was an undesirable element of 'drubbing' at the front as I was holding by the handle and using a full width piece of w/d. This afternoon I bought a much smaller piece of 6mm float glass about the width of a bit of w/d paper but only 60mm wide, which is about 2/3rds the length of a chisel blade:

In this first pic above you can see the three chisels I was messing around with. The middle skew and the Blue Chip have been flattened on the first paper, 150g which was stuck to the glass using some spray on adhesive. The skew nearest the camera is as I was using it...a pretty patchy, sort of mottled back and not really flat. The next pic shows the finished backs having been polished to 1200g, which is the finest paper I could find in town this afternoon:

What is absolutely essential is that the handle is never touched in case it's raised, even once will be disaster! The technique, which is borrowed from MrC (David...many thanks ) is to move the chisel back and forth on the stone so that the edge hangs off the far edge for about 50% of the time, and pressure is always applied directly onto the blade back. The result is a really excellent polish on the backs but even better, if this technique is strictly adhered to:

.....you will achieve a hollow back to the blade. The chisel is being held by the lightest of pressure using the little clamp (freebie from F&C) so that there are two points of contact on the back of the square and as you can see, a feeler gauge can be inserted . Total cost to do this was about £4, so not bad for some lapped chisel backs...LN's next - Rob

 

[Anonymous]

4, so not bad for some lapped chisel backs...LN's next - Rob

But Rob, suely dead flat is even better than slightly concave. and the LNs come supplied dead flat.

nice tip thoguh

 

[Waka]

Little LN gloat in the background Rob, who could you have learnt that from?

 

[woodbloke]

Tony wrote:

But Rob, suely dead flat is even better than slightly concave. and the LNs come supplied dead flat.

Dead flat is obviously highly desirable, no question. A slightly concave chisel back however will always register against any flat surface as two points will be in contact at any one time, the cutting edge and the point on the blade behind it where it touches the surface it's resting on, either a piece of wood or a Spyderco 10000g. With a dead flat stone, repeated back polishings on a stone (as in removing the wire edge) may cause the blade to go slightly convex if the handle is even minutely raised in the process. The critical thing is that the vertical pressure needs to be applied to the back of the blade at all times...I've been guilty of holding a chisel by the handle when polishing off a wire edge That's the theory anyway

Waka wrote:

Little LN gloat in the background Rob, who could you have learnt that from?

Waka - if it's the last picture you're looking at, ie the Veritas LA Jack....should have gone to Specsavers :lol: :lol: - Rob

 

[bugbear]

It is a truth universally acknowledged that a man in posession of an engineer's square being used to check flatness is in need of a surface plate.

BugBear (with apologies to Jane Austen)

 

[David C]

BB,

very good!

Rob,
One huge advantage of the very slightly hollow chisel back, is that there is a much higher probability that the wire edge will be correctly polished away on a flat polishing stone.

If using water stones this also helps to cut down the stiction a little, though the former point is much the most important, for the best sharpening.

We can't get perfect flatness ever, and a minute hollow is almost always hugely preferable to a minute bump

David C

 

[bugbear]

We can't get perfect flatness ever, and a minute hollow is almost always hugely preferable to a minute bump

Amen brother!

BugBear

 

[Anonymous]

We can't get perfect flatness ever, and a minute hollow is almost always hugely preferable to a minute bump

David C

David
'Perfect flatness' is not required. It is irrelevant that we cannot attain it. We do not look for or need it. These are woodworking tools :wink: The 'level of flatness required' is down to the use one has in mind for the item.

Flat to less than a thou deviation is perfectly adequate in a chisel (surface grinders can go much better than that (when I test LV/LN planes on our CMM (accurate to a few nanometres) they are often flat to less than 1/2 a thou) and better than anything other than very slight concavity as this may encourage the cutting edge to dig in.

Clearly a convex chisel is useless and much worse than a concave one, but visibly flat when held against a reference edge works best in my experience.


One thing I have found is that thick plane blades (LV/LN LA planes) make far better paring chisels than many chisels i have tried :? as they are very flat right up to the cutting edge

 

[David C]

Tony,

Not clear if we are in agreement or disagreeing?

The observation about perfect flatness was purely to illustrate an important truth.

Many of my chisels have considerably more than one thou" hollow in the full length of back.

My reasons, as stated are to do with sharpening and I find no disadvantage in use from some hollowness. The old long paring chisel, (photograph on my blog) has considerable hollow forged into the length of its back and I understand from Ray Iles that this was considered correct and desirable.

I have not noticed any tendency to dig in, though theoretically this might appear to be an issue.

LV have introduced a lapping machine for some of their blades, and these are indeed the flattest that I have ever seen.

best wishes,
David

 

[Anonymous]

Hi David

I agree with you that slightly concave is preferred to convex

As an engineer, I have issues with this phrase you use in posts and in your DVD though that 'We can't get perfect flatness ever' as if that was some mystical state that we cannot approach but wish we could.

How flat is perfect? Are we talking to within a mm? a micrometre? nanometre? picometre? femtometre? are we talking about one atom 'out of position' along a length? (sorry if this sounds facecious, I didn't mean it to). By the way, physists can arrange atoms in a single layer using electron microscopes - pretty near perfectly flat :wink:

Would a woodworker not think that an engineers 'straight edge' is in actual fact straight, and flat, when it might well deviate by a 1 thou or so over 24"?

I find the phrase 'We can't get perfect flatness ever' completely meaningless and say that flat is preferred to either concave or convex (though often harder to achieve).
I would say that for a chisel, a thou or two convex over its length would not be noticeable in use (unless it is a bump near the cutting edge) and do not bother flattening my LN chisels when they arrive, flat, from the factory.

Having said all that, i agree that convex is generally the worst evil for cutting tools

 

[David C]

Dear Tony,

I suppose I am guilty of extreme pedantry, and A level Physics definitions. i.e. that the philosophical concepts of flat and straight are absolutely unattainable in the real world.

I have posted many times about cheap straightedges which have no stated tolerance supplied.

My reasons for preferring slightly concave chisel backs are entirely to do with polishing off the wire edge on a waterstone which itself is unlikely to be entirely flat.

Hope this is making some sense,
best wishes,
David

 

[Joe]

To me (a non-engineer) the statement about the unattainability of "perfect flatness" simply makes the point that "flatness" has to be understood within the context of defined tolerances. It's about deciding what constitutes an acceptable margin of error rather than searching for the holy grail. This is useful, I think.

Joel

 

[bugbear]

To me (a non-engineer) the statement about the unattainability of "perfect flatness" simply makes the point that "flatness" has to be understood within the context of defined tolerances. It's about deciding what constitutes an acceptable margin of error rather than searching for the holy grail. This is useful, I think.

Joel

Yes - one tires slightly of woodworkers claiming that their straight edges, or squares are "dead accurate" .

Americans (bless their patriotic cotton socks) are particularly prone to claiming that workshop grade Starrett equipment is "dead on", a claim certainly not made by Starrett...

BugBear

 

[Harbo]

Sorry Bugbear but I am puzzled by your last comments - what is the point of using a square if it is not accurate - dead or otherwise?
My test of accuracy is to scribe a line from a flat, straight base, roll the square over and check against the original line. If they match then the square is "accurate" and should give consistent 90 degree results? Which after all is what the square is required to do.
I have an old rosewood 6" Marples square which is OK up to 2" widths but over that is no good. Generally I use to my ("accurate") Axminster Engineers square set and for larger widths use my recently acquired 12" Engineers square - Ally Pally Model Engineers Show £12.50.

Rod

 

[Paul Kierstead]

It's about deciding what constitutes an acceptable margin of error rather than searching for the holy grail. This is useful, I think.

Well said. And, to add to it, the real point is understanding there is always an error and ensuring the error falls on the side that does the least harm (or,possibly, even some benefit).

 

[David C]

BB's point is that none of your engineers squares are likely to be perfectly square. If you were to scribe very fine lines on a piece of metal with a straight edge, when testing them, and examine with magnification, the errors should become visible.

They are probably 'bench grade' and should be square within a certain British Standard, defined tolerance.

i.e. something like within a thou or two per 4 " of blade length, or perhaps over the entire blade length.

There also exist higher spec squares, which will have less error.

Test squares are about £40, whilst Reference grade which are even better might cost £70-140, for a 4 to 6 inch square.

These figures are very approximate and for illustration only.

best wishes,
David

 

[bugbear]

whilst Reference grade which are even better might cost £70-140, for a 4 to 6 inch square.

chuckle. Of course, when you're using equipment up at that level, various "normal" procedures generate errors far greater than the accuracy of the tool.

It's surprisingly easy to be measuring accurately enough for the temperture (and associated thermal expansion) to be "an issue".

Ditto surface cleanliness. If your tool is accurate to (say) 1/10 thou, pieces of dust of that size become "an issue".

It's all about tolerance.

BugBear

 

[David C]

To return to the original post, I am delighted to hear that the objective is being achieved.

I thought the 'movement one and two' strategy might work with abrasive paper, but had not tried it.

best wishes,
David