1. Out of Spec Precious Metal
The first sheet of 12 Gauge 925 Sterling Silver I used happened to be the only one of three sheets to measure incorrectly at 1.22” x 6” versus the specified 1.25” x 6”. In order to use the silver sheet (purchased from Rio Grande) that did not meet our specification, I redesigned the double-sided coin to be smaller.

• Lesson: Measure all precious metal items received from vendors and be sure to contact the vendor upon finding problems like the under-sized 925 sterling silver sheet.

2. Poor Mounting of Material
Discovered after breaking several bits (0.8 mm and 0.3 mm) that the outside edges of the silver square (1.25” x 1.22”) were 0.05 -0.1 mm higher than at the middle of the sheet. The single wood screw that I employed to hold the silver sheet down into the mount put unequal forces in the middle of the coin versus supporting the whole square.  Added L-shaped metal brackets to better hold the sheet down uniformly flat.

• Lesson: Ensure the silver is held down flat and take Z-zero measurements at several spots for verification before carving.

3. Too Fast and Deep CNC Machining
Broke a 0.8 mm ball nose bit during a Roughing operation. This may have been due to my attempt to carve un-annealed silver with increased feeds & speeds. For success, I lowered the depth of cut from .15mm to .10 mm. and annealed the silver sheet.

• Lesson: Always anneal the silver sheet before carving each major toolpath (such as roughing and finishing).

4. Un-Optimized CNC Toolpaths
Broke several bits (0.3 mm ball nose) during Finishing toolpath operations due to a 3D design issue where the offset of the Finishing toolpath was too small (0.15 mm should be okay in normal circumstances). This caused the small bit to scrape the ragged outside walls leftover from the roughing toolpath. Changed the offset to 0.3-0.5 mm to ensure the bit cleared the walls.

• Lesson: Verify the toolpaths using the Preview Toolpaths feature in Aspire. Unfortunately, on jagged designs such as the Rose Coin, trial and error might be necessary.

5. Mounting and Software Situational Awareness
Discovered late in the double-sided operation that one or more toolpaths did not carve centered in the silver square. This problem is caused by several factors. For one thing, I’ve verified several times during the design stage that some part of the 3D design was not centered in Aspire. This is a huge problem that must be resolved by better situational awareness. If you get the design under control, there is still the physical mounting of the silver sheet that must be centered. Maybe more significantly, I found that the wood mount was not exactly equal to the size of the silver square. The mount was actually 1/32” larger than the silver square in each direction (x and y). Lastly, if using a wood screw to help hold down the silver square to the mount, it must be screwed into the wood straight. If it is screwed into the wood crookedly, it can possibly shift the silver square in both the x and y directions; especially, if the mount is larger than the silver square in the first place.

• Lesson (1): Frequently check that the 3D design is centered in Aspire before saving and using the toolpaths to carve precious metal.
• Lesson (2) Ensure the wood mount is perfectly matched to the silver sheet. If not, start a new mount. 

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