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My client is aksing me if I can manufacture a small but long strip-shaped aluminum-alloy (7075 T651) part. The sectional dimension is about 15mm by 12 mm and the length is about 1000mm. Ideally, this part shall be made as extrusions. However, the client just needs one piece of the product, so he asks me if I can manufacture this product by milling. The main problem is that there's a through slot in the lengthwise direction, and the wall is very thin, about 1mm. The process I come up with is that I mill the slot first when much stock is left around it and hence the wall is thick enough, and then fill something into the slot so as to maintain/increase the rigidity of the part for the next milling process.

So my question is: which material shall I use?

Ideally, this material should adhere/stick to the metal part, and more importantly, it is easy to be removed from the part after I finish all processes. Maybe paraffin wax is a candidcate, but I suspect if it's strong enough. Someone suggested I use PVC soldering rod, but I tried and failed. It didn't stick to the part. I also considered expoxy resin, but it's not easy to be removed and cleared from the part.In thi case, I may need to polish the slot remove the resin. That would be annoying. Can someone here shed light on the material selection? Thank you very much.

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  • $\begingroup$ PVA (white wood glue, Elmers glue) is said to be a good release agent for epoxy. Maybe worth a try. Alternatively, fill slot with another part (Al or acetal or something made to size) and sandwich the lot (mechanically clamped) between rigid Al bars. $\endgroup$ – Brian Drummond Dec 28 '16 at 16:10
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You can get machinable waxes which have a bit more mechanical strength than plain paraffin wax you can actually make it yourself by melting paraffin wax and mixing in shredded polythene at about 15-25% by mass.

The problem with epoxies or similar resins is that it will either adhere and be hard to remove or id you use a release agent it won't adhere at all unless it is held in by other means.

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  • $\begingroup$ Thanks for your help. The material you recommended seems easy to prepare. I'll have a try. $\endgroup$ – Huang Dec 28 '16 at 23:27
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If for some reason the machinable waxes mentioned in previous answer don't work, you might also consider use of Wood's metal as a filler while machining. It's a low-melting point metal alloy. As noted in the Wikipedia article about it,

Wood's metal is commonly used as a filler when bending thin-walled metal tubes. For this use the tubing is filled with molten Wood's metal. After this filler solidifies the tubing is bent. The filler prevents the tube collapsing. The Wood's metal is then removed by heating, often by immersion in boiling water.

It would be less likely (than wax) to shift or squeeze out during machining, but has some important drawbacks that make it not a panacea:
• 10% cadmium content. Cadmium is listed in the European Restriction of Hazardous Substances bull.
• Expensive. For example, about $50 a pound at flexbar.com.

Note, the Wikipedia article lists several variants that may cost less, or be less hazardous, mostly with higher melting points, but some, like Cerrolow, with lower MPs.

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  • $\begingroup$ Thanks for your answer. It's interesting to know these special materials and the wiki page provides useful info of the metal alloys with low melting temperature. $\endgroup$ – Huang Jan 2 '17 at 23:39

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