You can start out with any lock, but I will suggest starting with an average sized four-pin Master padlock. They are easy to impression, and blanks can be easily obtained at a hardware store. You should get more than one blank for practicing. Five is probably a reasonable number.
Below are some depth and spacing data for the Master padlock, which as you will see later, can be useful (although not necessary). The depths are measured from the bottom of the blade of the key, up to the bottom of the cut where a pin will rest.
Cut # Depth Cut # Depth
0 .280" 4 .220"
1 .265" 5 .205"
2 .250" 6 .190"
3 .235" 7 .175"
The distance from the shoulder of the key to the first pin is .185", and the spacing between pins is .125" (you really don't need these last two numbers, but they may be helpful references as you are first learning to recognize what the marks look like).
Another good approach to using a practice lock is similar to that sometimes recommended for learning picking: Get a lock cylinder and remove all the pin stacks but one. After you have impressioned the one pin lock, add another pin stack and try again. Continue adding pin stacks until you can impression the whole lock.
Six inch, #4 Swiss-cut round or pippin files are normally used for impressioning (the files are called 6", but are actually about 8" long including the tang). Both types of files taper down to a smaller cross- sectional size towards the tip. The round file is usually used in conjunction with a small flat or triangular file which is used to shape the flat sides of the cuts on a key. The pippin file has sort of a teardrop cross section, rounded on one side, and with two flat surfaces meeting at a knife edge on the other side. The flat surfaces are used like the flat file above to shape the sides of cuts.
Chances are that you won't find these kind of files in your local hardware store, just because they have finer teeth that are required for most purposes. Locksmith suppliers carry them, of course. You can also get them through a machinist's or jeweler's supply house.
The particular #4 Swiss-cut pattern is used for impressioning work because it leaves a very fine, slightly dull, and slightly corrugated surface on the blank, which permits visible marks to be made by the pins rubbing on the blank with very little pressure. A few locksmiths use a #2 Swiss cut pattern because it cuts faster, but most authors specify the #4. Having tried both types, I strongly recommend the #4 also.
It is also a good idea to get a handle for the file, as it permits better and more comfortable control of the file. A file card is a special brush made to clean the teeth of a file. The soft brass of the key blanks tends to clog up the teeth on an impressioning file a little bit, which affects the quality of the fine surface you are trying to produce on the blank. Don't be cheap - get a file card too.
A few tips on using files: Files cut only on the forward stroke. So, push the file slowly and evenly forward with gentle cutting pressure, and draw back the file without any cutting pressure. Particularly when impressioning, do not apply pressure when drawing back the file, as it tends to polish the surface of the blank (a dull surface is needed when impressioning). Hold the file with an extended index finger pushing down on the top edge of the file to control cutting pressure. Light cutting pressure will produce the finest finish for producing visible marks. Use heavier pressure to remove material rapidly, followed by lighter strokes to finish the surface for marking.
Soft brass blanks are the best for impressioning. Steel blanks are much harder than is desirable, and aluminum blanks develop fatigue cracks easily when using hard turning tension. If you can only find bright plated brass blanks, you will have to file the plating off the top of the blade with your impressioning file. Only file deep enough to remove the plating, because with some locks a #0 cut requires the full un-cut height of the blade. With the plain brass blanks, you also need to smooth the top of the blade with your impressioning file in order to leave a surface that will show marks - just be careful not to take off too much.
Note: Some lock manufacturers use #0 and others use #1 to indicate the highest depth cut. For consistency, #0 will be used in this manual when referring to the highest depth cut (unless otherwise noted), which is equal to or very close to the full un-cut height of the blade of the key blank. A #1 cut refers to a cut which is one step lower than the un-cut height of the blank.
Some people like to prepare the blanks by either thinning them down in width with a flat file, or knife edging the top of the blade. In both cases the idea is that a very thin piece of metal can more easily be deformed than a thick one. In the case of thinning down the blade, it can also be wiggled around more in the keyway. When thinning a blade, do not thin the area immediately adjacent to the shoulder of the blank where the blade enters the keyway. You will be applying hard turning tension on the blank later and it is important not to weaken it at the point where most of the turning stress is applied.
Knife edging is used more often when the pull-out method (more about this method below in section 5.5.) of obtaining marks is to be used. Knife edging is used to thin only the top of the blade to make the initial marks more visible. To knife edge the blade, file both sides of the top the blade at about a 45 degree angle. The idea is not to make it really sharp like a knife, just to make the edge weak enough to mark more easily on the top surface.
As an example of the utility of knife edging or thinning, I took a new blank for a Master padlock and prepared the flat top surface of the blade with my impressioning file. After some wiggling, I could see one mark at the tip of the blade, which is enough to start with. But, I then knife edged the blade and wiggled some more. This time I could easily see marks from all four pins. With the knife edging, less wiggling was required and the marks were much more visible.
MAKING THE MARKS
There are three commonly used methods for making the marks. They are called wiggling, tapping, and pulling. In each of the methods, the blank is inserted in the keyway, then turned hard to bind the pins. Usually turning pressure is applied in the direction you want the lock to open, but you can try both directions to see which leaves better marks. It is important to make sure that the blank is evenly seated on the bottom of the keyway before applying turning pressure. If you are holding it tilted, some of the pins will already be pushed up and won't leave any marks.
When impressioning, you will need something to hold the blank because of the repeated hard turning tension used (the tension is harder than is used for picking, but not hard enough to break the blank). A small pair of vice-grips (no larger than the 5" size) works well. Attach the vice-grips like a handle, aligned with the long the axis of the key blade (not at a right angle like a turning wrench). There are also some commercially made handles for impressioning. There is at least one with a trigger handle to help pull out the blank uniformly each time, when using the pull-out method.
Wiggling is accomplished by applying turning tension, then wiggling the blank up and down causing the top of the blank to rub against the tips of the bound lower pins.
Tapping is a variation of wiggling. The blank is inserted into the keyway, then a steel rod is placed in the hole in the bow (handle) of the key to provide turning tension. A small mallet is used to tap on the bow to make the impressions. Tapping on the top of the bow pushes up the tip of the key by lever action, and tapping on the bottom of the bow pushes up the back of the key by direct action.
The pull-out method only works after you have cut down to at least a #1 depth, hence the popularity of knife edging the blank, then using the wiggle method to see if there are any #0 cuts to start with. To use the pull-out method, apply turning tension, then pull out on the blank (don't try this method on disk or wafer locks, because the disks may bend or break). Unlike the wiggle and tapping methods, the marks produced by pulling will not be exactly where the pins are, the distance away being related to how far you pull the blank out (maybe 1/16"). For this reason it is helpful to scribe lines down the side of the blank after the pin locations are found by the wiggle method, to use as a reference when filing. The advantage of the pull-out method is that it can leave more easily visible marks than the previously mentioned methods.
There is more than one way to implement the pull-out method. One technique involves attaching a C-clamp to the bow, then using the C- clamp to provide turning tension on the blank. A screwdriver is placed between the side of the bottom end of the C-clamp and the face of the lock, then the screwdriver is twisted to pry the C-clamp (and therefore the blank) in a direction out from the face of the lock (no more than about 1/16").
An effective hybrid approach is to first put turning pressure on the blank, then add pulling pressure (without actually pulling the blank out enough to start making marks - the pressure is just take up any slack between the blank and the pins and to put more tension on the pins) using your vice-grips or a commercial impressioning handle, then bump or tap the blank up and down to make the marks stand out more than more than they would otherwise. Remember to file where the pins are, as with other pull-out techniques (see section 5.5. above).
There is an optimum amount of turning tension to apply to the blank for any particular lock. It is the rubbing action of the pins against the blank that polishes the surface of the blank to produce the little marks used for impressioning. If too little tension is used, the pins will move too easily and not mark. If too much turning tension is used, the pins will jam and not mark - the pins have to be able to move a little to polish the blank's surface.
You will have better control of the impressioning action if you hold the blank and handle with your hand up near the head of the blank and the face of the lock, rather than having your hand farther away.
Wrist action, rather than action from the elbow is more effective in moving the blank within the keyway to produce marks - the recommended action is more to tilt the key up and down from the wrist with a bit of a snap, verses just lifting and lowering the blank.
SEEING THE MARKS
The mere act of preparing the flat top of a soft brass blank with an impressioning file, inserting the blank in a lock and removing it, without any wiggling or turning, will leave marks on the blank. There will be some streak marks where the pins have dragged across the specially prepared surface. Try it and you will know these marks look like so you will not confuse them later with the useful marks.
The useful marks you get are not really depressions in the surface of the blank (except maybe when a pin is almost at the shear line - if you start seeing deep gouges, the lock is probably about to open). A mark is normally just a subtle change in the reflectivity of the surface of the blank. The impressioning file leaves a slightly dull finish, and marking will slightly polish it. To see the marks turn the blank in the light. When you hold it at the right angle, the marks appear as little tiny shiny dots. They can be hard to see in bright light, so if working outdoors, sun glasses may be helpful. Some people like to use a magnifier to see the tiny dots - even with a magnifier, you still have to turn the blank in the light just right to see the marks. With a little practice, you will locate the marks very quickly.
If impressioning a dirty or weathered lock, you may find little specks of debris on the surface of the blank after marking. If there any doubt as to what you are looking at, wipe off the top of the blank to see if you actually have a mark rather than a tiny speck of dirt.
FILING THE MARKS
The rule for filing marks is simple. If you see a mark, you file there - if not, you don't (except when using the pull-out method - in which case if you see a mark, you file where the pins are; see section 5.5., above). Whatever you do, don't be tempted to guess - if you're not sure if you have a mark or not, don't file there. Work on making and seeing the marks first.
File only 2 or 3 strokes at a time before looking for more marks, because you only have to file a cut a few thousandths of an inch too deep, to pass by the shear line (a shortcut, allowing more filing at one time, follows in section 8.1.).
As the cuts are filed deeper the sides of the cuts will start to become parallel with each other, looking something like the letter U. If you leave them that way the key will get stuck in the lock. Use a flat file, or the flat side of your pippin file to angle the sides the cuts at about a 45 degree angle from vertical, making the sides of the cuts look more like the letter V. The bottoms of the cuts should remain rounded. It can be helpful to look at some other keys, then try to duplicate the shape of the cuts.
Some locks have fat pins and some lock have skinnier pins. There seems to be a natural tendency to use the middle part of the file, leaving fairly wide cuts. The cuts only need to have a radius a little bigger than the radius of the pin tips. For locks with skinny pins, try using the file more towards the tip, where it is narrower.
If you can see more than one mark at a time, it is ok to file them all at once or one at a time.
Sometimes a pin will stop marking before it reaches the shear line. So, don't be surprised when a pin that has stopped marking starts marking again after some of the other pins have been brought to down the shear line. Just keep filing until the pin stops marking again.
Full Credit To Mark Wanlass, Frank Spencer and Paul Vorman