Scissor Sharpening

[stevebot]

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 does not cut well.  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 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
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.  Fold a heavy piece of cloth 3 times and you have 8 layers.  This is a combined test of sharpness, alignment and strength.

[Ken S]

Interesting posts, Steve. I am glad we now have a separate subforum for scissors.

Ken

[Rob]

Agreed. Content, not rhetoric.  Excellent :-)

[Rafael]

As I said in a previous post I just opened a sharpening shop. I am kind of newbe so I have many questions as new tasks are coming in from clients.

So ... how do you sharpen with Tormek T7 scissors like the one in the picture, especialy the right side (thumb) blade.

http://www.gradinamea.ro/_files/Image/articole/original/TAIS012960.jpg

[Herman Trivilino]

No easy way, Rafael. Curved blades are hard to do well on the Tormek, but at least this one is curved the easier way. Take it apart at the pivot, obviously. From there you could try the knife jig, or free hand it. It might be possible to do it with a homemade jig where you attach a platform to the base of the scissors jig. Attaching a second smaller platform to the top of this platform might be necessary.

http://forum.tormek.com/index.php?topic=1592.msg6634#msg6634

[Rafael]

Thanks. I'll see what I can do today.

[grepper]

I've sharpened a bunch of hand pruners (secateurs).  I can't remember if I used the knife jig or the scissors jig, but it was easy, worked well and got them nice and sharp with an even bevel.  Try both of those jigs... you may prefer one over the other.  Or as Herman suggests, practice free handing them or build something like his knife rest design.  That would work well too.

If you are just starting out, especially if you are already taking in business, you might find it really helpful to get some junky knives, pruners, scissors and anything else you may get to sharpen and spend a few hours practicing.  Just a few hours experimenting goes a long way with the Tormek.

Get an old knife with a really concave edge (or grind away just at the middle of the blade) and see what it takes to make it flat again so you don't see light under the blade when held against a flat surface.  Put some good chips in an edge and see what it takes to remove them.  Bust the tip off a knife and practice grinding a new tip.  Practice grinding down a bolster.  Grind off the edge on a pair of scissors and get them sharp again.  Learn how much metal you can remove before you have issues closing at the tips, etc.  That kind of stuff.

[Ken S]

Excellent advice, Grepper. One of the benefits of learning on your own knives, etc. is that if you do a bad job while learning, you don't have to worry about the customer.

Herman, I believe your HK-50 is one of the most beneficial things to come from the forum. I think it is actually far more versatile than just small knives. (You do, too, hence the machete sharpening.) I understand why Tormek has not chosen to manufacture it, as the sharp blade is hand held, rather than being secured in a jig. From a manufacturer's standpoint, this translates into potentially increased liability. From a user's standpoint, it translates into a very versatile accessory. As I have stated earlier, I would recommend placing the threaded holes to secure the platform very accurately, and making a jig to be able to repeat them. This will allow the user to make additional platforms for other tools, such as very short chisels.

Follow Grepper's sage advice and practice much.

Ken

[Rafael]

Thanks guys. I did it free hand. The client is happy. Even if I know I do need some more practice. Thanks again. Your tips will be very useful in the future.