All Springer/NP/PCP Air Gun Discussion General > Engineering- Research & Development

Whittlin’ out a GRT-III Trigger

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  by CharlieDaTuna

The GRT-III Trigger, though in appearance seems to be quite simple, is a very complex and precisely machined part. Below is the manufacturing process in a nutshell so that you may see what is fundamentally involved and just why it is so effective, efficient and such a success. Some of the manufacturing processes, procedures and measurements are vague with intent so as not to divulge too much information.

Make a Trigger Prototype:
Of course you need a prototype design to start out with. This provides the basic design and configuration.
Make a basic Dimensional Drawing:From that prototype, an initial dimensional drawing needs to be made and a measurement point of reference is determined. This point of reference is used to determine any and all of the other points of measurements used in and for the CNC machine programming.

The triggers are cut from Aluminum bar stock ¼ inch by 2 inch by 8ft. The stock is cut down to the desired lengths to fit the jig. The prototype jig, used for testing and design, is designed to hold and cut a single trigger while the Master jig is designed to hold and cut 8 triggers at a time.
Building a basic CNC program to cut out trigger (the beginning of the Master program).
The Master program consists of many phases or sub-programs. As each phase of the milling process is set up and tested and approved, it is then added to the final master program in the proper sequence of the manufacturing process.

Making the jigs is an extremely precise, complex and time consuming process as is developing the program for the CNC machine and very expensive to say the least. It takes the sales of a lot of parts just to pay for just that aspect of the manufacturing costs. And…if any error are made, it is very costly, and usually a total loss. 

Make a 1st stage prototype mounting hole setup jig and test:
Build the program for the 8 place 1st stage mounting hole setup
Make permanent 8 place 1st stage mounting hole setup jig:

In order to be able to mill something out it has to be able to be held securely and precisely  in place each and every time, time after time. There is no room for error here or anywhere else in the milling operations.

Preparing the plate for cutting:
This jig is used to perform the operation of drilling out 16 precisely locating holes, 2 in each trigger of two different and precise diameters in the raw material. This is done in four passes on the CNC machine, one for each size hole per trigger and then each hole is reamed with a precision reamer to an exact diameter within one 10,000th of an inch.

The part is then removed from the jig and all 16 holes per side, both sides, for a total of 32 holes, are slightly chamfered by hand to remove sharp edges. It can now be mounted in the Master jig and held precisely in place.

Make a Prototype 2nd stage cutout Jig and test
Make two permanent 8 place 2nd stage Jig (Master Jigs)
 Build program for 2nd stage setup
Build chamfer pass program side “A”, and incorporate sub program into Master CNC program

(Two jigs are used alternating from one to the other as one is being machined and the other being changed over)

This jig is used to mount the plates that we drilled the mounting holes into for cutting out the trigger itself. Once mounted using 16 screws, the CNC machine cuts the 8 triggers out using three passes per trigger. The first pass is the cutout, the second pass is the rough cut and the final pass is the finish cut. That is a total of 24 cuts per jig. Then, the 8 triggers are partially chamfered on side A where the finger sets.   That is a total of 32 passes on the jig for this phase of the operation.

Every time the finished stage of the mounted material is removed from the jig, the jig is thoroughly cleaned prior to mounting new material as is the new plate going in. One tiny piece of trash on the surface of either piece can easily ruin that piece.

Build chamfer pass program side “B” add sub program to Master CNC program
Build finger pad cuts program, four individual operations, side “B” and incorporate sub-program into Master CNC program

The cut out triggers are complete on side A. The jig is removed from the CNC machine, the 16 mounting screws are removed from the triggers and the triggers removed from the jig. They are then inverted and turned 180 degrees and re-mounted with the screws to the jig and reinstalled in the CNC machine to finish side B.

Side B of the trigger is now chamfered and the four individually cut lines for the finger pad are now cut into the trigger. That makes 5 cuts in 8 triggers for 40 more cuts.

The triggers are now removed from the jigs and the basic roughed in trigger is now complete but far from finished. From this point, it goes into several semi-manual phases of manufacturing.

Next, the triggers are each individually milled one at a time in four passes on a computer controlled milling machine. The trigger is mounted into a jig and the lever slot is now be machined in to a specified depth and width. Based on one master jig of triggers, that’s 40 cut/passes. This cannot be done on the CNC machine as the depth of the slot conflicts with the mounting holes and would cut the mounting screws but the excess material in the lever slot area needs to be removed before going any further.

Milling and drilling in the adjustment stages:
Now it’s back to the CNC machine again. A slight flat surface is milled in the slightly curved area of the trigger where the 1st stage adjustment will be drilled later in the process. This flat surface keeps the drill/tap bit from canting due to the drilling angle and breaking later.

 The recessed hole for the 2nd stage adjustment screw is drilled to a specified depth. The hole for the adjustment screw itself is drill/tapped and must be drilled accurately within a very close tolerance.

Next the 1st stage hole is drilled for tapping. The final tapping for the 1st stage adjustment screw is done later after the trigger is anodized.

For all intent and purposes, the machining of the GRT-III trigger is now complete.

Holy smokes….
What this boils down to is that there are over 210 cutting, milling, drilling and tapping operations per each master jig of triggers or more than 26 operations per trigger to this point. That does not include the installation and removal of the 16 mounting screws per jig each time during the loading and unloading of the jigs for the different stages of operations as they progress.

Now a number of the triggers at a time are placed into a tumbler along with thousands of small stainless steel ball bearings and tumbled for about twenty minutes to smooth out any remaining sharp edges, washed and prepared to be sent out to be anodized. The GRT-III trigger blades are now anodized for color, as well as resistance to wear and corrosion.

After returning from being anodized, the final step is mounting each of the triggers individually into a jig for the tapping of the 1st stage adjustment hole to a specific tapered depth. The manufacturing process is now finally complete.

It is now prepared for shipment. The tension spring for the 2nd stage adjustment is cut from a length of stock and the 2nd stage stainless adjustment screw and tension spring are then installed.  The Allen head 1st stage adjustment screw is installed and seated with care to its approximate proper depth.

And finally, it’s tested and prepared to ship.
Each and every GRT-III trigger is now placed into a Gamo or Theoben trigger adjustment jig where it is tested and both the 1st and 2nd stages are adjusted to a point that would be acceptable to almost all shooters making it a true drop in modification. It is then removed from the adjustment jig, placed in a poly bag along with the .050 Allen wrench, packaged up in a 4x7 bubble wrap envelope, addressed and return addressed, sealed and stamped and ready to be sent to its proud new owner.

There you are…as simple as that… and all for $33.00 shipped…lol…

Thank you

And as an afterthought... keep in mind that for many many years before a lot of people attempted to improve the trigger  but no body did and everyone said that it couldn't be done. We eventually took on the challenge. It took us about 6 months of research and development and trial and error for us to make it happen but we finally did it. The first MK-II trigger was born and then as the trigger generations passed, they were greatly improved over the years to what the GRT-III (and GRT-4G) trigger are today.

Bob   aka: CharlieDaTune

just got mine today :D installing it tomorrow.  I cannot wait

I had already read the whole thing on your website (complete with pictures) and I'm still impressed!  I still think they're undervalued, but for that level of air guns, for any more money they'd probably be over priced.  As it is you do a fantastic job of making a silk purse out of a sow's ear.  Thanks from an appreciative customer.  (No, this is not a paid endorsement - I paid for the privilege.)

I've seen several post lately claiming the GRT is over priced.  It is not,  @33$ it i's quite a value ( I own 2). If one was to have a trigger made by a job shop from a print or example they'd not be complaining about the price! It's quite an accomplishment to design, manufacture and sell such an item for $33.  The large number of GRTs in the field tell you all you need to know about its value.

Edit:  This post was NOT aimed at Roadworthy, his mention of value reminded me of some recent posts where questionable practices were thought to be of higher value than a properly designed unit like the GRT.


Hey CDT,
Can the GRT trigger be made to fit the Hatsan 95 or 135?

PS:I'm refinishing, tuning and re bluing the above airguns. I was a little surprised how much internal work the Hatsan's need.  >>Thank you for the guidance in your many detailed articles ;)>>.



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