My machining experiences between 7075 Aluminum and Gun metal bronze to the Juntor watch

Gun metal juntor machining experiences

Gun metal juntor machining experiences

Gun metal old cannon in Spain


What a beginner most afraid when starts to machining are three things, the sound, the smoke and the sparks of the contact of the tool with the work material. With respect to sparks, when using titanium or magnesium alloys there is a great danger of ignition, which means that apprehension to them must be watchful. In any case, that is no problem in the aluminum and bronze alloys that we currently use in the Juntor. Although in the old Juntor we use alloys of steel, titanium or bronze to aluminum, we are now opting for aluminum and bronze alloys of tin called Gun metal, since the current Juntor has a marked tactical component.

Loud sound in machining appears when cutter has a large diameter. This is because cutting surface increases. So cutting deeper, faster and more feed surface of the workpiece on each step sound increases. If the beginner is not get used, it will seem that the machine bursts. For veterans this sound is a music to their ears. At the beginning I recognize that I was very afraid to machining. Now the sounds helps me to know that I have to decrease the cutting speed or change the milling cutter.

Smoke is exactly the same. It frightens when you do not know what the matter is. It happened a couple of times because I measured wrong tool length. With excessive friction of the tool to the metal in the area without cutting. That is a big problem when machined brass because it can change the color of the piece and darker. If you work with tin bronze it is a problem as the surface acquires an almost black patina in a few days. That may be that the content of the alloy particles such as the small percentage of nickel, zinc or lead having these alloys blacken them.  Gun metal juntor machining experiences are great.

If cutting speed and feed are correct and the cutting surface tool , bronze machining sounds great. With the aluminum alloy happens the same. If the cutting surface tool is sharpened and variable helix, it is not hear loud sound at the cut. Moreover it produces quiet the smoothness of the sound. Gun metal juntor machining experiences are pleaseant with 7075 alloy too.

What has pleasantly surprised me has been the color that has emerged from the machining of bronze. It is a rather reddish gold and not the classic gray of the so-called Gun metal. I think it’s due to the low content of lead and the predominance of copper. It is possible that in the machining, the sliding of the planes also places the majority of copper atoms in a homogeneous way and in the reflection of light, the color of copper predominates. They are speculations from the theoretical notes that I write more ahead. So machinability of both is very good


A few theoretical notes.



Tensile strength, which indicates the strength of the material at the mill cut, it is similar in both materials. This is the ease in which the different planes of the crystalline structure are dislocated or slid in the machining. Both alloys 7075 aluminum and Bronze Gun Metal have a cubic crystal structure with a centered face (Face-centered cubic FCC). Then when there is a shear stress on one of its faces, the Burger vector, slip or stress required will depend on the interplanar sliding planes. In the case of these two alloys the relationship between sliding planes and direction the Burgers vector is very good. Thus tensile strength is in each alloy,


7075 Al 489 MPa o N/mm2

Cu 90, Sn 5-8  Zn 2 (Gun metal) 340 MPa o N/mm2


With these data ,we are not afraid  to start machining alloys with cubic crystalline structures to face centered. Metals of this cubic crystal structure with the centered faces would be Au, Pt, Fe, Ag, Pb, Ni, in addition to Al and Cu. So, Gun metal juntor machining experiences can be very  enjoyable.

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