Tag Archives: 7075 alloy

Anodizing 7075 Alloy: Micro-Tel handles

Quite a few good tutorials exist for anodizing of aluminum, and pretty decent results can be achieved in any home shop equipped with a a sink and a few chemicals. For good results, with all the basic items mastered, the most critical item is the aluminum alloy. Generally speaking, any type of pure aluminum, and Al-Mg alloys are very much suitable for the anodization process. Zn, Si, Mn (and to some extend, Cu containing) alloys don’t work well.

The handles for the Micro-Tel MSR-904A receiver were machined from 7075 alloy, because of its strenght, and availability. 7075 has about 6% Zn, and 1.5% Cu, both of these elements are known to cause trouble when anodized. However, one can still try.

A quick, step-wise description of the process:

Step (1) – throughly clean/degrease the workpiece: first, I use methylated spirits, then, hot water and detergent, then rinse with water. Wear clean gloves.

Step (2) – etch with about 10% caustic soda. Room temperature.

eloxal naoh bath

As you can see, the part will turn black. This is typical for certain alloys.

Step (3) – use about 5% nitric acid to remove the black layer. Dip for a few minutes only. There will be some faint grey residue which needs to be brushed of mechanically (use a very clean brush – otherwise, it will contaminate the surface). Had to repeat the caustic etch twice to get a uniform and shiny surface.

eloxal hno3 bath

Step (4) – anodize. Mount the piece with heavy aluminum wire. For 7075 alloy, pure aluminum wire works. Alternatively, use thick titanium wire. Current needed is about 1.5-2.0 Amps per 100 cm2. I used 2 A, for the handle. As cathode, use a sheet of aluminum, lead, or titanium. I use just plain aluminum and it is working just fine. For the liquid, about 15-20% sulfuric acid (dilute 37% battery acid with destilled water 1:1 ratio). Keep at room temperature, cool with some ice (applied to the outside) if it heats up too much. Typical time needed is 30-60 minutes, depending on the temperature and layer thickness. Don’t let the acid heat up too much – the layer will stop growing.

eloxal oxidizing

eloxal pwr supply

Step (5) – densify by boiling in distilled water. Needs to be really boiling, not just hot!

eloxal boiling

After the first attempt – everything looked fine after step 4, but the handles turned pretty dark after densification.

eloxal handle too dark

Pretty much, a full failure.

So, etched off the oxide layer with 10% caustic soda, and repeated the process, with two modifications:

(1) Keeping the acid rather warm, about 30°C, and reduced anodizing time to 20 minutes. This will give a thinner layer.

(2) Added a bit of ammonium acetate to the water used for densification. You may also add a very small amount of acetic acid. Keeping the bath slightly acidic prevents darkening during the densification process for Zn/Cu containing alloys.

The final result:

eloxal handles final

It’s not perfectly silvery color, but a slight yellow-orange color (like lightly colored wood). And the layer is certainly not very thick. But good enough to protect the 7075 alloy from forming corrosion spots over time.

Micro-Tel MSR-904A Microwave Receiver: a set of new handles

For a while I have been looking for a set of of spare handles for the MSR-904A, but to no avail; the unit was missing the handles – had some rough rack-mounting fixtures. Fortunately, most of the Micro-Tel equipment uses the same kind of handle, so at least I know how they should look like.

handle micro-tel

These handles are actually of a very nice design, and don’t look too complicated to fabricate. So I thought I would give it a try and machine a set of spares.

For the material I selected a piece of 7075 alloy T651 temper (fully hardened, stress relieved). This is a quite strong alloy, and it needs to be because the MSR-904A is heavy, and the full weight rests on handles if it is set down on the floor/carried around, etc.

How to machine such handles – well, it is best done using a CNC mill, and luckily, I have one in the basement, even if it is just a small machine.
fkm349vl mill

It is a FKM349VL table-top mill, not a very heavy machine, but pretty capable if used correctly. This is not for heavy loads and throughput-optimized toolpaths, but it can yield very usable accuracy, and the surface finish is very nice, provided that good tools are used. Typically, I get parts that are reproducible to within 0.01 mm, and absolute accuracy typically better than 0.03 mm, depending on how the machining goes.

handle intermediate pieces

Starting from a plate, first, the opening of the handle was machines, and chamfered. The original handle uses a radius chamfer, but well, I only have tooling for 45° chamfers around, fair enough.

Then, the holes were drilled, and countersunk, followed by machining of the left and right (short) edge.

A bit more tricky, the piece where the handle is mounted to the case. This is fairly thin, and some excess metal was left to allow easy clamping of the piece. Sure, there are other ways of achieving the same result, but it saves a lot of time if the vice doesn’t need to be re-adjusted, and if everything can be done in two or three clamp positions.

As one of the last steps, the excess metal is taken off.

handle removing excess

All in all, considerably more effort than I thought:

(1) Cut the plate to approximate size; machine one long side flat

(2) Machine handle cut-out

(3) Machine chamfer (upper side)

(4) Machine left and right edge (short sides)

(5) Spot drill, drill, and countersink holes

(6) Turn around – align

(7) Machine other chamfer

(8) Machine mounting piece

(9) Mount set of 4 pcs upside in vice

(10) Cut-off excess

(11) Cut recess of mount piece, left and right

(12) Machine front chamfer, 2x

(13) Deburr, finish

handles final

These were the tools used –

handle tools

And, not to forget, EMC2 with the Axis interface. Thank You EMC2 (LinuxCNC) team for providing such great software, free of charge!

handle axis

I might still anodize the handles – but first need to do some tests with 7075 alloy (never anodized this alloy before, and don’t want to waste the handles!).