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EGLM Grenade Launcher Trigger Mechanism Enhancement


PMO Gordo

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After using my EGLM excessively, I've broken both trigger attachments that came with the purchase. My analysis showed the designs of the trigger rod and cam were the culprits (see picture below). The way the trigger mechanism works is like this: when the user pulls the trigger, the two trigger plates will be drawn toward the user, which in turn moves the trigger rod in the same direction. The trigger rod will then ride in the slot in the trigger cam and cause it to tilt and engage the trigger inside the launcher body. The trigger rod is only 3mm in diameter, and to make matters worse, it has two cuts for the shims near the area which engages the trigger cam. As the slot of the trigger cam is the only place that touches the trigger rod, all the pressure is focused on the weak spots - the cuts on the trigger rod. The trigger rod broke exactly at the cuts as shown in the picture.

EGLM+Broken+Fire+Mech_V2.JPG
EGLM+Trigger+Cam.jpg

 

To prevent the same thing happening again, I decided to create an enhanced trigger mechanism using 3D-printing technology. I began my design by spreading the pressure point to more areas on the trigger rod. To be specific, I added two slots on the trigger cam, so there were three areas on the trigger rod engaging the trigger cam instead of one. I also removed the cuts for shims on the trigger rod to increase its structural integrity. The new trigger cam looks like this.

EGLM+Trigger+Cam+V1.jpg

When I uploaded the design of the trigger cam to Shapeway, they informed me the walls on my design must be above 3mm for printing in steel (the original design has 2mm walls in most areas). As steel is the cheapest metal on Shapeway, I decided to made my design fatter.

EGLM+Trigger+Cam+V2.jpg

And then it occurred to me that there's no need to use a trigger rod anymore. I could use two screws with 3mm thread and nuts to secure the trigger cam to the trigger plates, which would increase the strength of the trigger mechanism and save more materials. So I removed the center slot on my trigger cam design.

EGLM+Trigger+Cam+V3.jpg

The price for my newest design costs US$30, which is expensive for such a little part. Does anyone have any suggestion on where to cut more materials on my design?
 

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I wonder if you aren't looking at the wrong solution? Perhaps a better engineered pin with spacers and without the notches/grooves that lead to the failure might be a better and cheaper approach? The springs on each end will retain the pin and the grooves for them should be ok. If you had some cylindrical spacers either side of the trigger cam and between the trigger plates they would keep everything in place. You could have spacers between the trigger plates and the springs too. Without the grooves the pin shouldn't fail as easily. Pins and cylindrical spacers are easy to knock up on a lathe. Depending on diameters the spacers might be just bits of standard sized tube parted off or carefully sawn to length.

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Thanks for the suggestion, Mike. Before designing the new trigger cam, I also thought about adding a spacer to the rod the goes from one trigger plate to the other, as well as through the slot on the trigger cam. But seeing the slot is only 3.5 mm in width and the rod is already 3 mm, I couldn't find any spacer that fits that size.

 

Manual lathe work is a great idea if I could find someone to do it. I've heard it's cheaper than CNC or 3D printing.

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Ahhhh. I think I see what you mean but that isn't, quite I think, what I was meaning. My solution would have a 3mm pin which runs as shown in your picture. It would have a groove for the spring at either end but no grooves anywhere else. The spacers would be machined to length to fit as follows in ascii art ;):-

             _        __        _
            | |      |  |      | |
            | |      |  |      | |
         __ | | ____ |  | ____ | | __  
    __o_[__]|_|[____]|__|[____]|_|[__]_o__
   [_|_|______________________________|_|_]
    ^ o [__]| |[____]|  |[____]| |[__] o
    | ^  ^  | |  ^   |  |  ^   | |  ^  ^
    | |  |  | |  |   |  |  |   | |  |  |
    | |  |  |_|  |   |__|  |   |_|  |  |Spring
    | |  |   ^   |     ^   |    ^   |Spacer 
    | |  |   |   |     |   |    |Trigger Plate
    | |  |   |   |     |   |Spacer
    | |  |   |   |     |Trigger Cam
    | |  |   |   |Spacer
    | |  |   |Trigger Plate
    | |  |Spacer
    | |Spring
    |Pin

The spacers would be all 3mm ID or as you are in the US and tend to favour Imperial then 1/8" = 3.175mm  and, say 5mm OD or 3/16" 4.7625mm. If you could go to a bigger OD and a better fit on the 3mm rod for the ID it would provide better support to the rod and the trigger plates and trigger cam.

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