Thread Tools
This thread is privately moderated by BMatthews, who may elect to delete unwanted replies.
Mar 06, 2019, 03:58 PM
B for Bruce
BMatthews's Avatar
Thread OP
Discussion

Sharpening carving knives for balsa or harder woods.


I posted this in a thread but it's something worth putting here for posterity too. Hope it helps some of you.

ut in a flat line and wanted badly to actually dive into the work. It SEEMED like a good idea but was a total disaster.

On the other hand by far the most typical factory sharpening is the hard beveled shape shown in "B". I tried to show why this isn't a good detail carving shape in the second part of the sketch below showing how the hard heel will lever up the actual cutting edge. This makes it tough to maintain a nice flowing cut and doubly difficult to make a neatly curved hollowed cut.

The sweet spot is the apple seed shape where the part of the bevel that we use to guide the cut is immediately behind the cutting edge and the rest flows smoothly up and out of the way. This makes our lives easier when it comes to carving shapes and nice details.

Did some reading and learned about the better and more durable "apple seed" shape. But even there we walk a fine line between too blunt and edge to work well in soft balsa and too sharp which tries to act too much like "A" and wedge itself deeper too easily. Then there's the compromise of a finer angle which proves to be less durable and going too blunt and having a durable edge but one that needs too extreme an angle to the surface before the cutting edge ingages and a shaving comes up. Basically Papa Bear's knife, Momma Bear's knife and Baby Bear's knife....

I went down and held an angle indicator against the blade of one of my knives and tried to mimic the angles that I hold the blades at for the initial shaping of the edge for both my balsa and very soft wood carving knives and my more durable edge knives I use in the wood working side of things. Shown below.

I cannot emphasize enough the gains to be had from a leather strop that is charged with a good polishing compound. It makes ALL the difference. Not only for getting a "shaving sharp" edge but also for polishing the sides of the apple seed so it glides smoothly through the wood.

We don't need fancy stones to do all this... though it's a nice long term option. We can do all the sharpening we need with just some of the black or dark grey silicon carbide "WetorDry" sandpaper in the right sort of grits and some light oil or soapy water to lubricate the paper. 320 does a good job on coarse shaping. Then some 400 to get the scratches out and polish the sides of the apple seed nose and sides of the bevel. Some 800 gives us a pretty nice polish and starts a bit of a shine. From there flip the 400 over and do some initial stropping on the back of the paper. Yep, the back. seems like there's enough Silicon carbide dust from the manufacturing or the other paper in the pack to make the back side mildly abrasive too. And it puts a darn nice finish on the edge and sides. Finally go for the mirror edge and hair shaving finish from the leather strop charged with some fine polishing compound. What you want is not the reddish "rubbing compound" but instead the white super fine "polishing compound" found in automotive places. Another source of stuff to charge the leather with is mag wheel polish. Mother's being a good option. I also used to have some stuff that came in cream form in a tube. Don't recall the name. Anyway if it's a white material it'll likely work. You'll know it's right when the first few strops of the blade over the leather gives you a nice mirror like finish and the leather is rapidly turning black from the metal removed.

Hopefully needless to say but if using sandpaper like this to sharpen everything is done using a draw stroke instead of a push stroke which is used when using sharpening stones.
Sign up now
to remove ads between posts
Mar 07, 2019, 01:16 PM
Registered User

thanks for posting that


a while back i was looking at adjustable jigs for sharpening
and came across this explanation. I like the apple seed shape.
Mar 24, 2019, 09:12 PM
What could possibly go wrong?
I'd also been posting recently about what a revelation it is to use well-sharpened tools and I totally agree re. the leather strop. I've actually just had one of these arrive over the weekend - https://www.carbatec.com.au/sharpeni...op-single-face and a tube of Pfeil polishing cream. I don't have it handy now but it gives an equivelent grit rating for the cream, it was very fine, on the order of 1000 or so. I've got a couple of water stones I bought years ago and could use a lapping but I've done very well with an affordable hardware store oil stone. The coarse side is a little too coarse IMO, useful only for restoring old tools that have been rusting away for years, but the fine side seems to be about 400 to 600. It's capable of a pretty good edge. Currently I prefer surgical scalpel blade, the standard but thicker type (I forget, number 25?) stropped as soon as they lose their edge. They can easily be sharper than new after the first time on the strop.
There's nothing like using truly sharp blades for working with, once you experience it you never go back. You make a very good point about polishing the bevel though, most people learn to hone the cutting edge but neglect to maintain the bevel. If you're carving rather than just trimming it makes a huge difference.
Apr 02, 2019, 05:47 PM
B for Bruce
BMatthews's Avatar
Thread OP
If it leaves a more or less mirror shine on the steel then it's a lot finer than 1000. I suspect the whole grit system starts to fall on it's nose at those sizes but more likely it's equivalent to more like 10K or even finer.

I've found that automotive Chrome and mag wheel polish makes a pretty good dressing abrasive for leather strops. But if you're doing OK finding the proper stuff then great.

I've got water stones here that I use for my wood working toools. The medium stone for restoring an edge is 800 grit. And from there I give it a couple of passes on a 4000 grit then a few passes on the polish charged leather.
Apr 03, 2019, 02:53 AM
What could possibly go wrong?
That polishing cream came with the strop, at their replacement price I'll be looking for alternatives. Thanks for the tip on mag wheel polish, now you mention it I think I've heard that before but it;s a good one. I tried it on some hobby blades inc. an MA razor plane and it did a good job though it takes some time to really put the edge on it, and you can actually blunt all that good work pretty easily if you're not careful. But it's still better to have than not.
Apr 05, 2019, 01:53 PM
What could possibly go wrong?
I found my little Eze-lap paddle sharpener that I saw mentioned recently (can't find the post now) and as I was making some balsa sheet pieces with a utility knife I thought I'd give it a shot, just for reference. It's a medium grade. The knife is a retractable type where you can snap segments off the blade as the tip gets blunt. Mine was already ready to replace so I tried it with the paddle and the result was worthwhile. By that I mean I only spent about 1 minute with it just trying to clean up the cutting edge, and sure enough it came out about as sharp as a new blade would be (they're not that super-terrificly sharp when new anyway). I got that knife for one job, shaping/carving thicker blocks of balsa such as nose or windshield blocks, where ideally you want to make a cut up to a few inches wide in one pass. They don't hold an edge for long because you have to apply a lot of pressure and inevitably you end up either sawing or hacking, no cheap thin blade can take a lot of that treatment and stay sharp for long. So the lapping paddle did a fair enough job, someone who just wants to keep working and isn't obsessively into sharpening could get a good edge back on it and go back to work in a couple of minutes with one of these and from memory thye're not expensive.
Later I had a go on the same blade with the strop and polishing compound, and yeah, BIG difference. You can get a real edge on it fairly quickly, though I suspect the polishing compound is the secret. I wonder if in days of yore the old fashioned barber with cut-throat razor, when using his traditional leather strop without polishing compound, relied on a bit of texture in the leather to act as abrasive. The smooth, soft leather on my paddle strop looks to me like it's nothing more than a medium for the polish, which is what does the real work. I'd love to get a genuine old-timey barber's leather strop and use it the way we've seen in old movies, holding one end with the other attached to the bench, back-forth-back-forth a few times, always keeping a literal razor-sharp edge and never letting it get dull. A little and often being the philosophy. That would be a nice knick-knack to pick up but I bet they're hard to get (I just realised I happen to know the last of that type of shop in the area, the very old barber either retired finally or went to his eternal reward, and the shop hasn't even been cleaned out though it's sat there closed for over a year. Still with the same faded posters of the latest gentlemen's hair fashions for 1974 on the wall and chequer-board linoleum on the floor, real traditional, you know the type. What a shame the last of them finally closed... but I must contact the estate and ask about the strop!)
Jun 21, 2019, 01:53 AM
Registered User
For #11 blades, I've found the so called Edjer very useful, but that's probably not the best for carving. At an old job, someone who didn't know any better got a Diamond "stone". This worked ok at first, but not for long. We got a Norton Crystolon stone, which is what I still use at home, with baby oil*. Compared to other stones, the coarse side was very fast and the fine side quite fast. However, I suspect you guys are talking about finer stuff that I might not have the patience for. I have a little stone that looks kind of like marble. It's quite fine, and fast enough for small blades. Recently, I picked up a stone at a hardware store for someone who wanted to sharpen her kitchen knives, which were in horrible shape. It looked like Crystolon, but was very slow.

For the friend with the knives, I tried 400 grit, used, wet or dry. Not hopeless, but slow. Will bave to try fresh stuff, with coarser grit. So many dull kitchen knives.

Can anyone suggest something cheap that works quickly that I can get cor my friend? There seem to be a lot of types. Also, how do you tell the difference between a waterstone and an oilstone?
Jun 21, 2019, 05:08 AM
What could possibly go wrong?
Lincoln, what I've been calling an oil stone may or may not be the right term, I mean the type of 2-sided stone from the hardware store you used with the baby oil. Mine is Bear brand and I just used 3-in-1 oil. A water stone is what the wood tools store calls their stones that look a little like hard clay and you literally use, and store, them in water, I use a plastic lunch box. As you say these are pretty fine grade.

I got one of those diamond sharpeners for kitchen knives, it certainly sharpened them up but you were right again, it didn't last long.
Of the types I've used, for your friend's knives I'd go for the 2-sided hardware store sharpening stone with a light oil. By the way if they get too full of grit and rust from the blades you sharpen, a wash in kerosene should restore it. I add a little oil every time I use it. I'd think you could lay those old knives on it with the same angle as the knife's bevel and get those looking fresh again first, then that should get it ready to sharpen with a regular kitchen steel (that's what we called our sharpener anyway) or maybe a small hand held version of the same type of stone. I've seen those in the hardware store too. There are plenty of alternatives but if you have that type of stone handy that's a good choice. If the flats of the knife look real bad you could give them a rub on the same stone but (fresh) wet and dry will probably be easier, and a buff and polish if you want it to look nice. This is for initial restoration, but for ongoing use a kitchen sharpener should be adequate after that
Jun 23, 2019, 03:23 PM
Registered User
The problem is that I'm spoiled. The Norton stone I have at home is very much faster than the hardware store stone, even though both are nominally silicon carbide. Sigh.
Oct 03, 2019, 01:27 AM
Registered User
FWIW, X-acto used to make leather strops. Some are on ebay:

https://www.ebay.com/itm/Vintage-Rar...r/293220718612


Quick Reply
Message:
Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion wood carving micro_builder Life, The Universe, and Politics 28 Oct 28, 2018 02:52 PM
Discussion Settings for X-Carve & Balsa wood/Ply? Patrik_NN CAD/CAM 9 Jun 06, 2018 06:58 PM
Discussion I Have taken Up Wood Carving Chophop Life, The Universe, and Politics 10 Jun 02, 2018 09:48 PM
Discussion 3,007 Carved Wood Blocks Big Foot 48 Life, The Universe, and Politics 3 Aug 30, 2016 09:26 AM
Discussion carving a balsa prop.... géry 46 Vintage & Old-Timer Designs 0 Jan 17, 2016 09:00 AM