Because I didn’t want to leave any unturned stone, I decided to make a “computer assisted manufacturing” version of my hammer by placing my SolidWorks model in GibbsCAM in order to produce the necessary toolpaths that will be read and executed by the CNC mill.
Again I chose to work with brass because machining steel is a lot slower.
The problem with machining is to solidly hold the part so the cutter can shave the material. Machining small parts is always difficult due to the relative proportions of the part compared to the clamps.
This time the hammer would be made in a single block with the pins (hammer pins and sear pins) being integrated instead of holes as on the original. This would add a bit of machining time but since a 1/16″ end mill had to be used for the details of the teeth there is enough room to detail the pin close to the hammer head.
The justification would be that if the built-in pins break they could be shaved off and drilled to accommodate the steel pins that exist on the original model. This would extend the life of the hammer a bit more.
Since the hammer has different thicknesses, it must be machined from both sides. This would require two setups in perfect relation. This could be achieved by using a jig that is also prepared with the CNC mill.
As it could be appreciated, often to machine some parts it is required to make other parts to hold them. This means more planning, more material, more machining time and is hence more expensive.
Furthermore, how the part would be machined has its own importance. In this case, since my stock is a length of brass bar of 1 inch by .5 inch it made sense to have the head at the top and the teeth region still attached to the main bar at the bottom. That way, when the bar is flipped for the second setup, there are still supports at the head and the main bar. By working the sequence carefully for the second setup it is possible to leave only two small tabs and completely machine the part undisturbed.
The CAM software allows simulation of the toolpaths. The following pictures show the machining steps for the jig as well as the sequences for the first and second setups.
For the jig I found a piece of quarter inch aluminum scrap having several extras holes already in it: they are not shown as I am using the ideal plate as my model for machining but none of them interfered with the function of the part. Our CNC mill has a large block with a pattern of threaded holes. My first jig setup was temporarily held with clamps and drilled with holes that will keep it in position. I also added two holes corresponding to the pivot hole of the hammer.
With a ½” endmill the first slot was roughed out.
Because of some tight spots in the second region the roughing was done with a 1/4” endmill…
…but it still required a 1/8” endmill to finish up the tip of the hammer spur. Both slot contours have an offset of .0025” all around when compared with the stock bar and the finished hammer. This gap is just enough to keep the piece located for both setups. There is a boss with a hole to accept the tip of the hammer pin. This with the hammer spur and the balance of the stock bar will be enough to precisely position the part.
Once the jig is prepared, it is time to install the stock and start machining. Shaping the hammer involves a lot of toolpaths so, to minimize the number of images, I show the progression in stages. Here the first one covers the drilling operations. The 3/16” hole for the hammer pivot, the small 1/16 relief holes for the teeth and a spotface to reduce the height of the pin that will push against the secondary sear.
After that, this picture shows the result for the regions that could be machined with the ¼” endmill.
The progression continues with the cutout full depth of the hammer head and all the regions that a 1/8” tool can have access to. I also have a deeper cut close to the teeth so when it is turned around for the second setup it will be completely open.
The last stage for the first setup is to finish, with a 1/16” endmill, the detail cut of the teeth and of the hammer spring pin close to the head.
The partially machined bar is placed in the second notch of the jig and roughed out with the ¼” tool.
This is followed again by the 1/8” and…
…the 1/16 endmills.
To complete the second setup, the 1/8 endmill would cut through the thin 1/32” skin that was preserved and leave only 2 tabs.
Once satisfied with the virtual machining, it is time to generate the G-Code and transfer it in the memory of the CNC mill.