This topic keeps coming up with novices, so I thought it worth writing.
Whether it is a new or old chisel or plane blade, before it can be sharpened the back must first be flattened.
This is more critical with chisel blades than plane blades. Chisel blades must be flat at the back of the bevel since this area acts as a fence when paring. With plane blades one might take a short cut and use a micro backbevel (such as David Charlesworth’s “ Ruler Trick ”). There is no short cut with chisels.
One of the problems facing some new owners of shiny new chisels is that the polishing process dubs the side edges of the blade. The effect is to cause a slight curve at the edge that runs all the full length of the blade, and ends at the bevel edge. This effectively reduces the cutting width of the blade. The diagram below will illustrate this effect.
In Figure 1 the red area represents the rounded edges looking down at the back of the blade. In Figure 2 this area can be seen again in the cross section of the blade. Figure 3 represents the area that remains unsharpened since a flat honed bevel does not reach into the lower areas.
The target area for a polished, mirror finish is the 25mm wide section immediately behind the bevel. The remainder of the blade only need be flat.
The bottom line is that some chisel blades represent more work than others – this could be the price of a pretty, polished blade. Moral of the story: purchase the cheaper, unpolished version if you have this choice.
Tools of the trade
There are a number of ways one might flatten the back of a blade. There are mechanical methods, such as using a disk sander or a belt sander. There are hand powered methods, such as Japanese waterstones and sandpaper. Of all these I believe that sandpaper on a flat substrate is the safest method. It is the method I have chosen to demonstrate. Note that this is the method I use – others may prefer their own methods.
For ease of illustration I will demonstrate the flattening of a few plane blades.
My lapping surface consists of a 1m long by 200mm wide and 10mm thick sheet of float glass. This is attached to three layers of 19mm thick MDF. Contact your local glazier for an off cut. This is the cheapest option.
There are different types of sandpaper. I am no expert here. Norton has recently introduced a new aluminum oxide based paper, the 3X. Reports are very promising. Up till now the most durable I know of is Zirconium Oxide belt sander belts, but this is only available in 80 and 120 grits. Generally I use emery paper. This is the black wet-and-dry paper. It does not last as long as Zirconium Oxide but it is easier to obtain, cheaper, and durable enough for most applications. One sheet is usually sufficient for at least three plane blades.
This is cut into thirds and contact glued to the glass surface. I use inexpensive poster spray contact glue. It sets quickly, is easy to pull free, and can be re-applied.
The sanding progression
As with honing the bevel, there is no point in jumping grits. Begin with the lowest that will create a flat surface. Move to the next level that will remove the scratches of the previous level. Continue along this path until the finished surface is both flat and mirror-like. I usually begin with 80 grit, and then move through 120, 240, 360, 600, and 1200 grit. At this point I move to polishing compounds.
Technique
The one absolutely essential criterion is that the blade is kept FLAT on the sandpaper. It must not be rocked as this will dub the side of the blade.
To make sure of keeping the blade flat, one’s weight must be centered over the blade, pushing downward as you slide the blade sideways.
You can do this with fingertips on the back of the blade, or you can use a large magnet as a grip.
View Image
magnet
View Image
pressing down
To keep track of progress it is advisable that one marks the area with a permanent marker. This will help one see where are the high and low spots.
Make sure you keep the sandpaper free of swarf, the filings that are removed by the sandpaper. Do this frequently. It is the messy part of the process.
Progressive results
This is what a dubbed blade looks like. This needs to be removed completely before moving to the next grit. It may be necessary to drop back a grit is the one you are using is working too slowly. There is a trade off, however, in this regard since very coarse grits may leave deep scratches, which in turn take more time to remove.
These scratches looks like 120 grit. I didn’t have anything coarser to hand.
As one moves through the grits, so there should be an even replacement of the previous grit level. The following picture illustrates a blade at 240 grit.
After 1200 grit I move to a honing compound. I have tried “Autosol”, a metal polish, but found that this does not produce the shine that one gets with Veritas green rouge (in “crayon” form). The Veritas rouge has a rating of .5 microns.
One can scribble this directly on the 1200 grit. It works, but there is the possibility of grit contamination, which results in a slightly foggy surface.
Better to either rub the crayon on a sheet of MDF or a hard leather strop (the one in the picture is the horse butt strop from Tools for Working Wood .
Some results
This is a blade at .5 microns.
and another just for fun …
Derek Cohen
Perth, Australia
January 2007
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Tools & Reviews at http://www.wkfinetools.com/contrib/dCohen/index.asp
Replies
Derek, thanks for all the work you put into that demo, it's great and will be bookmarked! The tip for using a magnet to hold the blade flat is a good one (another use for the GripTites, LOL).
forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
>>The tip for using a magnet to hold the blade flat is a good oneI think this is simpler and better. Gives a lower "centre of gravity" to the grip on the blade.http://www.geocities.com/plybench/grip.jpg BugBear
Nice site, BB. I saw the spot on shaprening nickers, which I'll print out for when the rabbett plane gets tuned up soon.
Re: the blade holder, couldn't really see any details. If I follow the links in the planes section, will I find more info? Gotta go right now, finishing up a project.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
Here's the descriptive text:
To hold and move the blade I use a small piece of scrap, with a shallow slot to fit the blade, and rounded corners for comfort.
BugBear
So you must make one for each blade, no? Sorry, I need to take some time and explore your site more. I'll post if I then have questions.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
So you must make one for each blade, no?
Yes, one per blade width, but they're rather simple to make.
BugBear
that's a detailed essay.
but I must take exception with you that chisels "must" have flattened backs... chisels already have flattened backs, no need to flog that horse...what counts is a sharp cutting edge, period... whenever I happen to check my chisels, they already have a flat back, from day one.
JP
You wrote: I must take exception with you that chisels "must" have flattened backs... chisels already have flattened backs, no need to flog that horse...what counts is a sharp cutting edge, period...
Perhaps I was not clear enough in making the point that a flat back for chisel blades - and the approach taken here - is essential if you have a dubbed back (which is quite common among many modern chisels). Of course one should try a tool out before attempting any work. The point I made at the outset was that a dubbed blade provides a reduced working area. Regards from PerthDerek
DC, I have always polished my backs to a mirror finish with great (labor) success but how do we tell the MFG of the tools that they have screwed up.,other than not buy their goods. Paddy
If you chop out your dovetails by hand, like many of us do, I think a flat back is essential on a chisel. If the corners, say 1/16th to 1/8th on each corner, are slightly rounded over, you'll never get that 'crisp' line to register the tails with. Then, when it's all put together, you will definately have gaps.
I've actually proven to myself using an unflattened (from the factory) back vs. the exact mfg. chisel with extensive flattening and polishing from me in the shop, and the outcome was quite noticeable. With the unflattened back, I wasn't able to get a crisp chop across the line, and it was 'rounded' just enough to cause gaps when the pins were finally mated to the tails.
Just my .02 observation.
A sharp chisel is the intersection of two very flat and polished surfaces. Anything else, in my opinion, is a gouge, however slight. Some guys, much more talented than myself, can accomplish anything with a sharpened spoon. I haven't been blessed with that ability.
Jeff
thanks derek, that is very well illustrated. i have flattenend the back on a few blades and it does seem to be the most time consuming and boring activity ever. i have a small wooden plane with a dowel thru it for pulling. that cheap little blade took forever to flatten. once done however, it does work relatively well.
thanks again and i think i'll be starting with some less fine grades of paper next time as you advise.
Derek,
Thank you very much for your post on flattening the backs of chisels. It is a classic.
Have you written a textbook on the care of hand tools?
Are you thinking of writing one? I have read many tutorials and texts. some of them are impossible to follow. Yours is as good as a video. That can only come from a very high degree of skill, and the ability to convey the thoughts that were intended.
The only problem with posts like yours is that it makes the rest of us feel inadequate. :-) After reading yours, I thought about writing a tutorial too. But then I realized that I have to develop some skills first. Darn! My intentions were good.
Please feel free to write more such tutorials whenever you get the urge. I had flattened the backs of my Hirsch chisels, but I hadn't tried to get a mirror finish on them. I believe that I stopped at about 220 grit. I will now take care of that.
Enjoy.
Mel
Measure your output in smiles per board foot.
Goodness Gracious... after reading this I may go back to power tool woodworking...
Lotsa overhead in those processes and procedures
Gee Charles, I think that you forgot to read the first sentence at the intrduction to the tutorial.
Feel free to add your constructive insights.
Regards from Perth
Derek
Ha ha Derek.I guess me and Charles will have to resort to the use of our Surface Grinder, equipped with Special Rubber Wheel for imparting a space age flat mirror finish....Well I assume Charles has a surface grinder.Philip Marcou
I guess me and Charles will have to resort to the use of our Surface Grinder, equipped with Special Rubber Wheel for imparting a space age flat mirror finish....
Gad Philip
I don't know where you guys get this from? I assume you are teasing, but just in case some think you are serious ...
Firstly, the article was to emphasise the importance of a flat and smooth blade back. And especially for novices.
Secondly, we are referring to a small area at the rear of the blade, not the whole back. I said focus on the first 1", but it could just as easily be 1mm. It is not practical to say this, however, since someone will take me literally and try to restrict their honing to that area. Anyway, we all know that the more we lap the back of blades, the further up the blade "grows" the polished area.
Thirdly, the demonstration was conducted here on plane blades - but only because they make it easier to see what is going on (also, I was doing a blade test at the time, and so it was convenient. The blades shown were used as controls). However I stated early on that this particular process was only critical for chisel backs as plane backs could get away with a micro backbevel.
The fourth point is that a definition of a sharp edge is when two surfaces come together at one point. Linked to this is the recognition that both surfaces need to be of equal smoothness. A surface honed with a 800 waterstone is not considered particularly smooth compared to one honed with a 8000 waterstone. If one of the two surfaces was honed with the 8000 and the other with the 800, the edge would reveal the serrations of the 800, not the "smoothness" of the 8000. Consequently, to have a blade - plane or chisel - that has a back that does not compliment the front is to reduce the effectiveness of the sharpening effort and to prevent the possibility of obtaining a sharp edge. In short, the back of the blade must be as smooth as the front. Whether you want to do this for 1mm or 1" is up to you.
Regards from Perth
Derek
Derek,
From the perspective a new woodworker, I would like to thank you for sharing your knowledge in a way that even I can understand.
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