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Archive for the ‘Knife sharpening’ Category

There are fewer knives now in hospitals that need sharpening, if you don’t have any you can use this article to sharpen your kitchen knives.

At one time we used to sharpen scalpels, but now I would be surprised if it was done anymore because disposable blades are now probably universally used.

We also sharpened knives for use in the mortuary.

Much of sharpening, either needles or knives is about practice. With a knife, once you have had a look at the video below you will get the general idea about angles. Having got that it is practice and more practice. Don’t expect to get it right first time, it may take you ages to get a knife sharp but as you get better you will speed up. one point to remember is that you are better doing it little and often, don’t wait till the knife is really blunt, touch it up before it gets anywhere near being blunt. The same applies to needles.

Not cutting well is an obvious indication of being blunt but you should not let a knife get to this point. You can feel with your thumb if it is blunt but better have a look under a bright light.  Hold the knife with the sharp edge upwards towards the light and look along its length twisting the knife from side to side slightly, if it is sharp you will not see any light reflected off the cutting edge, any blunt parts will show a reflection from the light.

The test for a sharp knife is to be able to cut a piece of photocopy paper. Hold the piece of paper using in one hand. Use the other to try to cut downwards through the paper from the top edge. If the knife easily cuts through the paper it is sharp.

As well as getting a sharp edge you should also look at the knife from the side view, hold it up to a light or examine it with a hand lens, you may find that it has a rough-looking edge to it rather than a smooth line,  this is because someone has caught something hard with it and it has put a nick in the edge. You will feel this if you run your fingernail along the edge. If this is the case you will need to run the edge of the knife over a sharpening stone to get rid of the nicks, this will make it blunt.

The photo below shows the knife being held at right angles to the stone. Just run the blade up and down the lubricated stone gently following the line of the blade (This one is slightly curved), until the nicks have gone. Then sharpen it.

DSC_8090-1


Off the subject slightly but an interesting side note. In the pathology laboratories  knife blades are used in a machine called a microtome  (from the Greek ‘small’ and ‘to cut’) to slice samples that have been embedded in wax so that they can be examined under a microscope. In this case the blades are sharpened using an automatic machine. The blade is put into the machine and turned on, a revolving glass plate with grinding compound on it sharpens one side with semi-random movements. After a short time the blade is automatically turned over and the other side is done, and so it goes on for a set time.  Microtomes can also use glass knives and diamond coated knives, the latter are used to cut very thin sections of hard material like bone for examination with an electron microscope. Never seen these last two.


There are a number of different tools for sharpening a knife.

1. Sharpening steel. Made of steel, diamond coated or ceramic. Below is one made of steel.

These are not for sharpening a blunt knife, they are for keeping a sharp knife sharp.

steel

 


2. Sharpening stones, hand. Natural or artificial stones.

A piece about sharpening stones.

Read my article on these under the title ‘Sharpening stones’. There is no point in me writing how to do it when there are many places where you can see it done. Click on the link below to watch the one I like.

An amusing but informative demonstration of knife sharpening.

You can also use fine sandpaper. In this case you will need a surface plate. As I say in the Sharpening stones article you can use a piece of thick plate glass to do the job, or you can buy a surface plate, these are often made of a thick piece of Granite that has been ground and polished to be very flat. With these the wet and dry sandpaper of made wet by running it under a tap. Place it on the granite plate and surface tension will hold it there.

Wet and dry paper will come in many different grades running from about 80 grit to 1200 grit, the higher the number the smoother it is. You will need something from say 400 upwards. Grits between 60 and 200 are more for woodworking.

Jewellers rouge.

This is a very pure red iron oxide powder that is mixed with water or oil and used on a surface plate for a very fine final polish. It can also be mixed with wax to form a bar for use on a power polishing wheel or it can come on paper like wet and dry abrasives, this latter is sometimes called crocus paper.


3. Motorised stones . These will have a large round wheel made of a bonded grit, this wheel will revolve quite slowly with the lower quarter turning through a water bath to keep it lubricated. These will need to be ‘dressed’ every so often. In use the surface will become uneven  so dressing is the process of making sure that the wheel is even all across its width. It is done with a dressing tool which has a diamond tip or a dressing wheel stick that is simply an abrasive stick with a grit harder than that of the machine. With this will be a second wheel of hard rubber or sometimes leather that is used to get the final finish on the blade after it has been on the grinding wheel.

Below is one made by Tormek. The abrasive water wheel on the right.

tormek

The diamond dressing tool

 

stickThe abrasive dressing stick.

stone


4. Pull through sharpeners . Steel or ceramic.

Below is an example of a pull through sharpener, this one is a two stage one, it first does a fairly course sharpen then on the second one it puts on the final edge. It has two ceramic wheels that the knife is pulled through, these are designed to put on the correct angle. You can get three stage ones with the ceramic wheels being of different colours. There are some that have a water bath that keeps the wheels lubricated.

Simple ones can have steel wheels.

knife

To finish off a knife you should run it over a leather strop to get it super sharp. Any old bit of reasonably thick leather will do, a leather belt for example. Glue your piece of leather to a flat piece of wood to make it easier to use.

The angle does not matter much. too flat and you will only polish the side of the blade, too steep does not matter much because as the leather is soft it will find the right angle, it is easy to judge the angle because it will be the same as you used to sharpen it. You can add jeweller rouge paste to the leather to speed up the process but I would not bother. Always pull the blade towards you otherwise you will cut the leather. Run its full length over the leather on both sides about fifteen times each side. You will get an edge like glass.

Below a rather fancy strop, but as I said and old leather belt will do.

strop

 

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