In the event that you design and build a master key system for a customer, there are a few methods and rules to follow to make system planning easier and more efficient. First let’s look at some of the rules.
Rule 1. In a very large system, no cut in any change key should match the cut in a master key in the position on the key. For example, the master and change keys we just worked with are listed here.
MK 342442 CK 455654 CK 201023
Note how the master key and the change keys do not have a similar cut in any one position. Now, let’s look at another
example. Let’s say we have the following master and change key cuts.
MK (3)4244(4) CK (3)5565(4) CK 20102(4)
The numbers in parentheses are illegal numbers for change keys since they have the same number as a master key in the same position. Change keys may duplicate two or more numbers without a problem.
Rule 2. The depths between the successive cuts on a key should be no greater than seven for a 0.015-inch master pin system and five on a 0.023-inch system. This rule prevents a
very deep cut next to a shallow cut that can cause problems during keymaking. For example, a key with a code number 447019
would have an excessive depth change between the third and fourth cut and the fifth and sixth cut.
Rule 3. On a 0.015-inch system, there should be at least two cut sizes between successive cuts. (A 0.023-inch system is de- signed to allow single number changes.)
Rule 4. There should be no locks in the system set up with the same number pattern.
Of all the rules presented, Rule 3 may be the most confusing. In a system that uses master pins in steps of 0.015, it’s very easy to key pick a lock with a change key that has similar cuts. If the system is set up with at least two pin size differences between change keys, the overall pin difference expands to 0.030—a difference that can prevent key picking. This double spacing of the key cuts is often termed double step progression. Locks that use master pins of 0.023 inch difference in dimen- sion can have keys with cut numbers just one value in
difference. This method of keying a system is called single step progression.
In developing a master key chart for a customer, you will often use a pin chart to define the chamber pinning of the locks and the cutting of the master keys. Let’s use a two- chamber lock to look at a simple example of a pinning chart. Looking back atFigure 49,the master key had a cut of five and two. We will use these numbers as the base numbers in the chart shown here.
In this chart, we have listed the even bittings on the left with the odd bit- tings on the right. Now we can begin to list the change keys that are possi- ble in the two- chamber system. Chamber 1 Chamber 2 MK 2 5 4 7 6 9 8 1 0 3
Let’s first take the four in the left-hand column and combine it with the seven in the right-hand column to give us a
change key of 4, 7. Next, let’s take the four of the first column and combine it with the nine of the second column giving us a change key with the cuts 4, 9. If we continue using this method, your change keys with a four first cut would give us 4, 7
4, 9 4, 1 4, 3
Now we can begin combining the six cut with the numbers in the second column to give us
6, 7 6, 9 6, 1 6, 3
Next, using the eight with the numbers in the right column we get
8, 7 8, 9 8, 1 8, 3
And finally, combining the zero with the numbers in the right column we get
0, 7 0, 9 0, 1 0, 3
Putting all these tables together gives us
4, 7 6, 7 8, 7 0, 7
4, 9 6, 9 8, 9 0, 9
4, 1 6, 1 8, 1 0, 1
4, 3 6, 3 8, 3 0, 3
But are all of these keys legal keys? According to our rules, especially Rule 2, there can be no more than seven cut depths between successive key cuts. Therefore, the key 0, 9 is an ille- gal key and should be eliminated from the system.
Now you can see how a simple two-chamber pin tumbler lock can give us 15 change keys and one master key. A six-chamber pin tumbler lock can give us many more combinations. In fact, using a six-chamber lock with a two-step system can give you 4096 possible change keys and a master key. A single-step sys- tem can yield 15,625 keys! Of course, inside these locks would be stacks of small master pins making the locks easier to pick than a standard level one lock. For this reason, one general rule locksmiths use on master key systems is to only place one master pin per chamber into each lock.
To show how one master pin per chamber would work, let’s look at a lock coded 4, 5, 3, 1, 4, 2. By adding a number one master pin to the first chamber, we get a master key and change key codes 4, 5, 3, 1, 4, 2 and 5, 5, 3, 1, 4, 2. Now let’s add a number two master pin to the second chamber of same lock. Our list of possible keys becomes
4, 5, 3, 1, 4, 2 5, 5, 3, 1, 4, 2 4, 7, 3, 1, 4, 2 5, 7, 3, 1, 4, 2
Now let’s add a third master pin to the third chamber. If it is a number three pin, you would get the following key combinations: 4, 5, 3, 1, 4, 2 5, 5, 3, 1, 4, 2 4, 7, 3, 1, 4, 2 5, 7, 3, 1, 4, 2 4, 5, 6, 1, 4, 2 4, 7, 6, 1, 4, 2 5, 5, 6, 1, 4, 2 5, 7, 6, 1, 4, 2
This gives us eight possible combinations for three simple master pins. Note how the number of keys is doubling each time we add a new master pin. If we were to continue to the sixth chamber, we would have 64 keys and 63 different locks. However, our system with 63 locks and 64 keys has two problems. First, some of the change keys will have a cut the same as the master key in at least one position. Often, this becomes an unavoidable situation when using a single master pin in each chamber. Second, if a number one master pin is used, it becomes very possible to have change keys with cuts only one digit apart. The solution to this second problem, when dealing
with a 0.015-inch master pin system, is to not use a number one master pin in any chamber of a small master key system. In order to reduce the chance of key picking, there is one “trick” locksmiths or lock manufacturers may use. For this ex- ample, let’s use a three story hotel with 60 rooms on each level. Now, you can set up the 60 rooms on the first floor with 60 of the best pin combinations and key codes and a master key. You can then reuse these 60 pin combinations and key codes on the second and third floors. But wouldn’t this system al- low a first floor key to open a second and third floor lock? It would unless you change the keying of the locks from floor to floor. If the keyway changes, then the keys will not enter the locks, rendering them useless except on the floor and for the room they are intended.
The master key for all rooms would need to enter all of these different keyway grooves. Special master blanks are available for this purpose. In fact, it can be possible to create a level three or four system using locks with different keyways. In these systems, a master key can be created to fit only the key- way on one floor. A series of grand master keys can then be created to open the locks on the first and second or second and third floors. A great grand master key can be made to open all locks in the entire hotel. All these keys can be made using the proper master blanks.