Sharpening Made Easy

Knife Sharpening Information and Equipment

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SCISSOR SHARPENING

To understand scissor sharpening you have to think of a pair of scissors as a machine whose purpose is to make one cutting edge ride exactly on the other edge.  They cut with a shearing action and can cut things like cloth and hair that a knife cannot cut.  The first thing we want to do is make sure the edges will meet.  The scissors should open and close smoothly.  Look at the inside of each blade.  You should see a shiny line right along the edge.  This is called the ride line.  If it is badly damaged, worn away or ground away by an amateur sharpener, the scissors may not be sharpenable.

Any work on the inside surface must be done with factory precision and should be avoided until you have the proper equipment and training.  Bad sharpening on the inside surface can render scissors useless - just two sharp edges passing by each other with no shearing action.  Do not touch the inside surfaces until you know what you are doing.

Next, do the scissors open 90 degrees?  If they do not open over 90 degrees they need to be taken apart for sharpening.

Most scissors are sharpened with a bevel.  (The exception is convex salon shears for cutting hair.)  You can see this bevel when the scissors are closed.  We grind only this bevel, not the inside. 

Some scissors, like children's safety scissors or pinking shears, will be ground square.  Scissor angles are measured from square, which is called 0 degrees.  Paper cutting and older cast scissors will be ground at between 5 and 15 degrees, leaving an acute angle at the edge.  Grinding the other way and leaving an obtuse edge, called a negative angle, is bad.  You may see this on low quality scissors or after poor sharpening jobs.  In newer scissors this angle increases up to 25 - 35 degrees or so, but some scissors are deceptive.  Always look carefully for the steepest cutting bevel and duplicate it.  The highest angle found in household scissors will be 40 or 45 degrees.  This is found on the finger blade of knife edge dressmaking shears, although the thumb blade may be at a lower angle, usually 15 degrees.

For standard scissors with both blades the same, grind the bevel until it is right up to the ride line and a slight burr is developed on the inside of the edge.  Sharpen both blades, then before you close the scissors push the blades apart and carefully close them without letting the blades touch.  Now pinch the blades together and open the scissors.  Repeat a few times.  This will move the burr outside the ride and burnish the ride line of the scissors so they operate properly and the blades will not cut into each other.  An alternative method is to run a VERY fine hone (like the Wolff pink hone) absolutely flat on the inside surfaces.

For knife edge scissors sharpen the thumb blade first, then the finger blade.  Remove the burr as described above.  Put the finger blade back in your equipment at the same angle and polish the finger blade or knife edge.  This sharper edge will cut with less effort.  The resulting burr will be small and can be cut off by cutting a piece of paper towel once or twice.  The second cut should be much smoother.

Scissor blades have a slight curve or bow to them to provide tension when cutting near the tips. This tension is adjusted by tightening or loosening the pivot screw.  For most scissors a good adjustment is so they close freely for half of the blade length, but have tension in the final half.  This can vary from 1/3 to 2/3. Cheap scissors have a rivet instead of a screw.  This type may not be worth sharpening.  The rivet can be carefully peened to tighten them.  Be careful with this type not to peen the rivet too much, it is difficult to loosen once you have peened it.

Test your newly sharpened scissors by cutting the following materials, arranged by degree of difficulty:
1. paper
2. Paper towel
3. Wet paper towel
4. Wet facial tissue (Kleenex)
5. Plastic grocery bag material, very close to silk
6. Silk
7. Latex or nitrile gloves
8. Surgical or silicon rubber

Household scissors should be at least a 5. dressmaker shears at least 6.

Another test is to see how many layers of cloth the scissors will cut.  This is a combined test of sharpness, alignment and strength.


Updated September 3, 2011

e-mail steve at bottorff dot com

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