Tag: how to strop

Posted on

Derek Cohen’s Stropping Compound Comparisons and Using a Strop

The following is a reposting of Derek Cohen’s “Stropping with green compound verses diamond paste”, which originally appeared on his website, In The Woodshop in January 2009. I thought it was quite well done (and not just because he likes my horsebutt strop) and since I often get questions about how to apply the compound, or what compound to use, he kindly granted permission for me to repost.  I collected some of my own thoughts on stropping here. Although he discusses a woodworkers chisel, the basic principals are the same for paring knives, spokeshave blades, etc.  He has a number of other tutorials about sharpening on his website. At the bottom of this post is an update after using the horsebutt strop for five years.

COMPARISON OF GREEN COMPOUND AND DIAMOND PASTES

I have been using a horsebutt strop with green buffing compound (.5 microns) for about two years. The strop is excellent with a hard and resilient surface, but I would recommend glueing it to hardwood for certain flatness. Used on their own they have a tendency to curl slightly, and this can lead to dubbing if one is not careful.

With the green compound I add a dribble of baby oil (which is just scented mineral oil). This turns the wax into a soft paste and allows the leather to soak it up. Used as a “crayon” alone it can become thick and collect on the surface (although it must be noted that the act of dragging a blade across this will remove – scrape away – the excess wax). Lee Valley also notes, “Frequent, light applications are better than less frequent, heavy applications”.

This is my strop with green compound applied.

For the purposes of comparison I set up another (unused) horse butt strop. There were two points of departure, but I did not think that these would interfere with the results. The first was that I used the rough (flesh) side of the strop since I wanted to save the smooth (hair) side for future use if this experiment failed. And second, I did not glue the strop to hardwood (but instead held it flat on ply) for the same reason.

To prepare the strop for the diamond paste I first moistened the surface with the baby oil.

The diamond paste was part of a batch I bought on eBay about 2 years ago (.5, 1, 10 and 40 microns). This is oil-based.

LV (Lee Valley) and TFWW (Tools for Working Wood) both sell water-based diamond paste. I generally use waterstones and, that this is oil-based, is not a concern since the two mediums are not used together.

So, dribble a little diamond paste onto the strop…

… and massage it in …

One observation about paste is that the excess sits on the surface of the strop in exactly the same way as the green paste. I would say that the criticism levied at the green rouge should equally apply to other pastes. Still, as I pointed out earlier, the excess is scraped off as one drags the blade across the leather.

I used 1/2″ and 3/4″ Blue Spruce chisels on Radiata Pine end grain. The chisels could cut but really needed work.

The chisel was stropped both back and bevel. Note that this involved drawing the blade towards oneself, with the sharp end trailing. Otherwise the blade will slice up the leather.

The result – the chisel was now several orders of magnitude sharper and had no difficulty taking fine shavings in this horrible wood:

Below is the result with the diamond paste – the chisel was now capable of excellent work:

Having gone back-and-forth between the two pastes, I concluded that they appeared to produce a cut that was almost identical. With he green compound, however, the chisel did appear to cut a tad more easily and leave a slightly smoother surface. When stropping it felt smooth, while the diamond paste felt “gritty” – but this may have been due to the rougher leather (still, it was not a “roughness” that I experienced).

Bottom line – I think that either system would work well. I shall continue using the diamond paste strop as I think that this should improve as the leather builds up more diamond within it.

USING A STROP

For those new to this, I have a few images of the stropping technique I use. Others may do this differently and, if so, I hope that you will post here.

Stropping the back of a chisel (or substitute a BD [Bevel Down] plane blade here):

Note that the blade is angled at 45 degrees, held flat, and pulled towards oneself (i.e. sharp end trailing). Do not push the blade bevel first – it will slice up the leather.

 

This chisel, like my BD blades, is hollow ground. Find the point of contact, then drag the blade back (as outline above).

Stropping a BU blade:

My BU [Bevel Up] blades typically have a 25 degree primary bevel and a high secondary/micro bevel (e.g. 50 degrees, to create a 62 degree included angle). The microbevel makes it difficult, if not impossible, to freehand strop accurately (that is, maintaining the secondary bevel angle). Consequently I only strop the back of the blade. This works well enough to give the blade a second lease on life.

 

Update: It is now 5 years on, and I continue to use the same strop with the green compound. It looks the same – the horse butt leather has lasted extremely well. What of the diamond paste? Well I did not like the feel – the subjective sense of grittiness remained. I learned that others experienced something similar.  The strop with green compound is less used since there is a sharpening bench alongside the work bench, and it is just as easy to use a 13000 Sigma ceramic waterstone, and in some cases this is better since there is no chance of dubbing the edge. I have continued to use the strop with green compound to “refresh” mortice chisels since these are honed with a rounded bevel.

I glued the undressed strop was to hardwood with the smooth (hair) side facing up. I have kept it that way, and it is used after honing blades to ensure that the wire edge on plane and chisel blades has been removed.

April 2014

Posted on

How to Strop a Knife

Stropping is a motion which pulls the cutting edge away from a substrate—leather, paper, wood, etc.—perpendicular to the cutting edge, with or without additional compounds. Stropping not only produces a very sharp final edge after sharpening, but it is an easy way to renew a slightly dull edge without having to go through the entire resharpening process. I tend to strop my knives whenever they feel a bit dull, or I have to apply excess pressure when using it, or when edge paring very thin leather.  I find stropping the quickest, easiest way to keep the knives used for leather paring sharp.

.

TECHNIQUE

The above video illustrates the technique I use in stropping, using the materials I will discuss below. I strop all the knives I sell using this method, and use it to keep my own knives in shape. There are two key aspects. First, always draw the blade away from the cutting edge to avoid digging into the leather, which is sometimes called a “trailing stroke”.  Second, it is paramount to hold the knife at the exact bevel angle it was made, and keep this angle consistant throughout the stroke. If you raise the angle, even a few degrees, the cutting angle will rapidly become too obtuse and you will have to resharpen or possibly even regrind it. Similarly, the back needs to be kept flat.

I find a strop that is 12-15 inch long and 2 or 3 inches wide ideal. If it is shorter you will have to reposition the knife an inordinate number of times, which slows the process and may introduce more errors.  If the strop is too long it is difficult to maintain a consistent angle on the blade throughout the length of the stroke. If the knife is slightly wider than your strop, just angle it a bit so it fits.

I count the number of strokes I do on each side to keep them even, 12-15 times on each side is a reasonable starting place.  If the knife is still not sharp, strop some more. If it still is not cutting well, it may need to be resharpened or reground. Although you are *just* rubbing a knife on a piece of leather, don’t be fooled that you are not doing anything: all the black marks are metal that have come off the blade.

Eventually, however, even careful stropping will gradually create an obtuse cutting edge. It may look sharp and have a mirror shine, but it will need to be resharpened using your preferred sharpening system.

MATERIAL FOR THE STROP

I prefer a two stage stropping. First I strop on the flesh side of horsebutt, which is dressed with a .5 micron green honing compound.  Horsebutt strops available here. Then I do a secondary, final, stropping on undressed flesh side of calf. This is why I flip the strop over in the video. I find it gives an excellent final “bite” when paring leather, though some people prefer just the hair side of the horse butt, others skip this step completly.  Other substrates for strops are wood, MDF, binders board, cowhide, mat board, etc. Anything firm and  flat can work, although a material that compresses too much will round over the cutting edge more quickly.  I prefer horsebutt over cowhide because the surface lasts longer, it is firmer, and it is a traditional material for high quality strops.

I generally use the strop on a hard flat surface but some people mount them to wood or other flat material.  Since I use mine two sided I find it easer to just flip it over.  The speed that you strop at does not seem to make much of a difference, as long as a consistent angle is maintained. There are also a variety of leather belts and discs to attach to power machinery, but I find it is too easy to round an edge using these, and it is not really much of a time savings since stropping does not take much time by hand.

COMPOUNDS FOR THE STROP

My preferred stropping compound is a .5 micron green chromium oxide buffing compound. I now sell a convenient 1 oz. bars of them, and my sharpening system also now comes with them. I like the edge this compound gives to the knife, and it does remove metal fairly quickly. Chromoglanz is another popular option among bookbinders, though I don’t know how precisely the abrasive is sized, and I personally don’t like the way it feels when you are stropping—it is very slippery. It seems to be better at polishing than establishing a cutting edge. There are other types of powders and honing compounds available as well, jewelry suppliers often have a wide variety. Quarter and half micron diamond paste is an expensive, but addictively fast cutting strop dressing and a real joy to use.

Careful stropping can keep an edge tool cutting well for a long time.