1.   Do you make and sell these hammers?
I do not.  This is a hobby for me, not a commercial enterprise.  
If you can can weld, have access to metal cutting tools, a belt grinder, drill press, various hand tools, are good at scrounging materials and have basic fabrication skills, you can make a Cricket for yourself with information in this document and watching the videos that are up on YouTube.  Although I used a mill and a commercial vertical band saw, it is not necessary to have them to build a Cricket.  Everything I cut on the vertical band saw could be cut with a Port-A-Band or zip discs.  A guidance system can be made without using a mill….  The videos show an alternate method to the original Cricket.
I tinker, I learn…. then I teach..  
2.  Can this Air Circuit configuration and component set be used on an air hammer which has the Cylinder mounted above the Ram?
Yes, it certainly can.  The only modification to this document would be the reversal of the hoses to the Cylinder ports.
3.   I’d like to have a longer stroke….  How can I get that?
A longer Cylinder could be used (it will use more CFM of air).  
Stroke length with a single Roller Valve Air Circuit is limited by numerous variables such as Cylinder length and diameter, air pressure and the volume of air you allow to flow through the Air Circuit.  If you want a stroke longer than can be managed with a single Roller Valve, you will have to implement a stroke control environment using two Roller Valves (one at each end of the stroke to signal the 4-Way Valve to switch the direction the Ram is moving).  
A second Roller Valve would also require a second Ramp and Dock.  Finally, it would require using a 4-Way Valve that has an Air-Assist on both ends of the 4-Way Valve to move the spool.  
4/18/2022...  One of my cardinal rules has always been to keep hoses in the air circuit as short as possible...  Shorter hoses facilitate less air to move and compress within the air circuit and thus snappier ram movement... and shorter strokes.   
If you really feel you would benefit from longer strokes, you could lengthen the hose lengths between the  4-way valve and the cylinder ports...  Keep in mind though, this will slow the cycle rate...
4.  Why is it important that the Air Circuit Components be installed in as tight a cluster as possible…  
It takes a finite amount of air to cycle the ram once.  It takes 200 times that amount of air to cycle the ram 200 times a minute.  The less air in the circuit, the less time it takes to compress it and move it through the circuit.  
5.  Why did you use a Roller Valve for the Air Gate?
Because I already had the valve.… and it could be plumbed to do the job.   The plumbing is unusual for a Roller Valve, but it works.
Any three-way valve can be used for the Air Gate.  
6.  I’ve seen a feeding frenzy when people talk about ram to anvil weight ratio.  It’s been stated that it takes a 20-1 ratio to achieve great forging….  It appears the Cricket has a low ratio. What’s the story there?
There has always been a lot of discussion about this.  I don’t disagree that more anvil mass is better….  But the question is… How much?
There was a study done that identifies relative efficiencies compared to a “1 to 100” ratio.  That is…  The highest efficiency would be achieved if your anvil were 100 times the weight of your ram.  The study identifies that 90% of the 100-1 ratio efficiency can be achieved using a 4-1 ratio. Even a 3-1 ratio gives you about 87% of the 100-1 efficiency.
If this information has any validity, I doubt the Cricket is going to have a problem.  I have a Fairbanks 150# mechanical at home that I doubt very much has more than a 5-1 ratio, and it’s considered an industrial grade machine.
7.  I’ve heard that Alignment Couplers are not reliable for this application.  What gives?  
I’ve heard the same thing.  If that proves to be too true, I’ll modify the interface between the Cylinder rod and the Ram assembly.
Personally, I believe the benefits of the Alignment Coupler outweigh the negative aspects.  I recommend buying a heavy-duty coupler.  I have been told that the most likely part of the coupler that will fail is a “conical washer” inside the unit and.…  I’ve also been told if this is replaced with high density plastic, the coupler will last much longer.  This is something I will try if I need to.  
8.  Are any operational features available with this Air Circuit dependent upon these specific Air Circuit components?    
Yes…The Single-Hit and Clamp implementation on this Air Circuit depends upon a design characteristic of the Norgren 4-Way Valves. 
Management of the spool in their valves is completely independent of the air used to drive the Cylinder.…. This guarantees the spool will be in its normal position if pressurized air is absent from the Spool Control Port.  
Other 4-Way Valves may not work the same way… Parker for one.
9.  How does keeping Butterfly Valve 2 close to the 4-Way Valve improve performance?
Installing Butterfly Valve 2 close to the 4-Way Valve keeps the volume of air in the Air Circuit to a minimum.  The more air in the Air Circuit, the longer it takes to move or compress it, slowing the actions (and performance) of the hammer.
10.   I’ve heard about solenoid activated valves.  Have you considered using them?
I have not.  I’m not sure introducing electricity into this environment would be a good idea.
11.   Is the downward movement of the Ram just gravity fed?
No….  The Ram is driven down by air pressure (force = pressure times area) plus the weight of the falling mass.  The velocity of the Ram at impact plays a huge part in the total impact also.
12.   How much air (CFM) does the Cricket use?
I don’t have any way to measure exactly how much air is being used.
I believe the original Cricket could easily be run continuously with a single-stage 5 HP air compressor.  
One of the great advantages of this specific Air Circuit configuration (including components) is that the hammer can be run with a small air compressor.  At about a 50% duty cycle, operating at 80 PSI, it only requires about 4 CFM of air.  I have run, with great success, the Cricket using an old Craftsman 2 HP compressor (Rated at 7.8 CFM at 40 PSI) that has a 20 gallon air tank.
If you have a small air tank on your compressor and its motor runs continuously to fill it, you might try using a gulp tank (at least 30 gallons) with the compressor.
13.  Why are butterfly valves used (as opposed to ball valves)
It is common for home-built air hammers to have ball valves in use because they are available locally, are inexpensive and the hammer will run with them.  However, they can negatively affect the controllability of the hammer.   I have found that butterfly valves work better.  
The best valve would be one that allows air to flow through the valve  proportionally to the movement of the lever.  For example…  If the lever is moved half way, half the potential air movement (volume) through the valve would occur.  
Ball Valves are designed such that, after the lever is moved just a short distance, most of the (potential) air volume can come through, so the lever swing for controllability is very abbreviated.  Using a Ball Valve results in difficulty in designing a linkage from the treadle that gives you Ram speed proportional to the treadle movement.
A butterfly valve has a “wafer” inside (rather than a ball).  When the butterfly valve is closed, the wafer is 90 degrees to the air line (cutting off air flow).  As the valve is opened (by linkage and treadle movement), the wafer moves in unison with the lever and provides a much more proportional airflow mapped against the movement of the lever.
There ARE true proportional valves available, but they are very expensive (Hundreds of $).  
A butterfly valve is not truly proportional (and costs more than a Ball Valve), but is much better than a ball valve (for this application) and is a great price/performance compromise.
14.   What are “cushions”?
A definition is provided in the Glossary.
15.   How can I tell if a Cylinder I’m looking at has cushions?
Most model numbers of Cylinders will include an indication that the Cylinder was ordered with cushions.  If you look on the manufacturers website for a product catalog, it will generally have a breakdown of the model numbers.  
I’ve heard most Cylinders are custom made these days, using a model number the customer has defined by specifying the features needed.
In lieu of that……
Look at the end caps and see if there are small screws in addition to the Ports.  If the cushion (rate of deceleration) is adjustable, there will normally be small screws on the end caps of the Cylinder.  If you do not see small screws, move the rod in and out to each end quickly.  If there is a cushion, the internal piston will not collide violently with the end caps.  You will feel the piston slow down significantly before it hits the end cap.    Some Cylinders will have a single cushion on either end to support specific applications.
Although not specifically required, I recommend using Cylinders with cushions.  It’s cheap insurance against damaging your cylinder by mistake.  If you don’t have cushions, you will need to have Ram travel limits that will prevent the piston from hitting the Cylinder end caps.  You must consider also, that each cushion effectively reduces the effective stroke by around 3/4 of an inch, so if you want/need a full (unencumbered) stroke of (say) 8 inches, you need to buy a Cylinder with a 10 inch stroke.
16. Could  I use a hydraulic Cylinder?
I would expect you could.   In fact, it may seem that it’s a great idea to use a hydraulic cylinder because the rods are generally thicker.
However, there are several issues that need to be considered.  All are associated with that heavier rod.  
One problem is the force differential between the front and rear of the cylinder.  For example….  If you use a 2” bore cylinder with a 1 & 1/8” rod (a common combination) the surface area (on the internal piston) that pressurized air has to move the piston is significantly different between the two sides of the cylinder piston.  Pressurized air from the rear of the cylinder would have 3.1416 square inches of area to push on.  Pressurized air from the front of the cylinder would have only 1.37 square inches of area to push on.  This may cause operational problems if the same air pressure is used on both sides of the piston.  The force differential may drive you to need to use two regulators to balance the force.
Although the same components may be used, this issue may require you to use different plumbing on the 4-Way Valve.  Ports 3 and 5 may need to be used for input air (allowing two regulators).  Port 1 would become the only exhaust port.
Another issue is alignment.  If you us a solid connection to the Ram, alignment is even more critical than when you use a cylinder with a smaller diameter rod.  Smaller diameter rods can flex a bit if the alignment is not perfect.  A hydraulic cylinder rod may not flex at all.  If the alignment is not perfect, lateral stress on the bushings will cause early failure of the cylinder.
The third issue I’m aware of is the amount of force required to lift the Ram (while cycling).  It must be strong enough to quickly raise the Ram to allow rapid cycling.  It takes at least 3.5 times the Ram’s weight in force to accomplish this (some have said as much as 5 times).  This, depending upon your total falling weight, may require very high pressure on the rod side of the cylinder piston to achieve satisfactory performance.  Be sure you know the pressure limits recommended for your Air Circuit components.  Routing of that higher pressure should only be where it is required.
17.  I just want to make a simple hammer.  How would I do that?
You can simplify the Cricket by eliminating features.   Most people get along just fine without single-hit and clamp features.  This would eliminate the need for the Air Gate, simplifying construction and linkage to the treadle.
It is not altogether necessary to be able to move the stroke…  if you are not into using tooling, a single stroke position may just be fine.  This would eliminate the need to make the Stroke Adjustment Assembly.
The roller valve could be installed above the table, in a static or movable position... eliminating the need for the Ramp Dock Assembly.  This would, however, bring hoses above the table.
04/18/2022...  An original design criteria was to keep hoses from being above the table...  In retrospect, the hoses associated with the roller valve are not a burden as long as they are routed so they are not near the ram action.