Anodising

Have built Centering Mic - my threads were to good a fit and siezed on screwing together.

The anodised finish was extremely hard and could not be easily machined. I had to make some parts again!

Tool

My thoughts were to use some kind of tape, grease or even paint, but having not yet done any anodising I was unsure of the reation/contamination with the acid.

Martin

It's worth looking at this video clip that highlights the problem that occurs when anodising screwthreads.

http://www.youtube.com/watch?v=40I8jH6Zs0g

Len. P.

The fits I have achieved will not allow for any further interference to be introduced, further more it appears that there is no fast and hard rule as to controlling the thickness of anodising when using a home setup.

Therefor I think I may have to settle for a good quality primer/etcher followed buy a number of hard coats of paint. At leased with this method I can mask out the areas that I need to keep natural.

To all that contributed to my post. Thanks you!!!!

Martin

Hi Martin,

Anodising does minutely change the dimensions - but depending on your items, you may well not notice them. Anodising is a surface-conversion process - you are converting the surface of the metal into a honeycomb matrix which you can then, if desired, fill with dye and then seal the surface to keep the colour in. There are some notes here which give some background on the process.

Using type II anodising, for a light colour (blue, red, green) you typically need about 0.5 thou total layer thickness; to pack in enough black you need nearer 1.0 thou total thickness. When you're building the ano layer, the very surface is both consumed & converted - its about 50/50 (its actually nearer 60:40 but I can't remember which way!). So, if you're going to colour anodise with a 0.5 thou layer, the item will grow by 0.25 thou for each layer - i.e. 0.5 thou on the diameter.

There is no easy way of masking that doesn't carry risks; and you need to be able to flush out nooks & crannies such as threads between process tanks, so normally threads are left exposed. You will see multi-coloured anodising, or items with some coloured and some left natural - but in these case usually the whole item has been anodised and it has been masked or bleached when it is dyed.

You also asked about the predictability of the coating thickness. Like many engineering processes, if you keep the environment and process constant you can get remarkably consistent results. Using the Constant Current / low current density method, with temperature controlled tanks, we get pretty consistent results (within 0.1 thou or better). Spot checks with our Elcometer show, within the meter's resolution, less than 10% variation in thickness and often better - and the colours match between runs which is our other major criteria.

Depending on how you've cut your threads, you may also find the crests shed their coating and revert to near as small as before coating. Anodising builds a matrix at right angles to the surface, and doesn't take kindly to absolutely acute edges (imagine trying to bend a honeycomb from a beehive around a 90 degree corner). For successful anodising edges need to be 'broken' - radius can be as little as, say, 0.2mm. If you've cut your threads with a full-profile tool, it should be OK; but if the crests have been left sharp they may shed the ano.

Dave

Edited By Dave Martin on 11/12/2012 13:38:51

Edited By Dave Martin on 11/12/2012 13:41:09

I would be worried if threads need to be cut so tightly you can't fit in a thou of anodising. There is a chance they will fret together. It could be be that you can't get them apart again in a few months.

I don't know the Hemingway design, but the design should not rely on threads to achieve accurate positional relationships. Typically either concentric registers and flat surfaces or coned joints to get alignment both axially and laterally.

I would make a test piece and measure the enlargement you get with your process, then make threads that much undersize.

Neil

I must say here I have never had a problem with threads and anodising, cut to standard dimensions and then anodised they have always worked and fitted fine.

The only proviso was that the thread was lubricated with a kilo poise type lube. or a very tenacious Silicon typelube. These were used on all the eyepiece threads and focus thread drives and lasted for years. Some of the Lubes were usable to -50C as the instruments were used in arctic conditions and left out 24/7.

Also that I have seen Kilopoise grease used that was too thick and when cold you could not turn the spindle and in fact once had one snap off.

Is it feasible to anodise the assembled unit and disassemble it after to fit the lens etc?

For those who are not familiar with the microscope see below picture.

Martin

Martin, every part should be anodised seperately at the same time in the bath. If its anodised as one item the internal threads and inserts will not get anodised.

The tubular parts will be put on the jig with internal clips so marks will not show on the outside.

The anodising figure for what you want doing should be AA5 - AA10, this will be perfectly adequate for the use it will be put to.

Clive

Martin,

you might just be able to - but I would strongly advise against it.

Firstly, as well as the lens, you would need to remove any ferrous or other non-aluminium components (I suspect the arbor is steel, plus any ferrous fasteners etc.).

Secondly, with the assembled structure there will be significantly higher risk of carry-over. The anodising process traverses a number of process tanks, and it is absolutely essential that the item is purged off one chemical before it enters the next tank. This is often done by a combination of dipping in / spraying with pure water (reverse osmosis / de-ionised or distilled etc.). Carry-over of chemical from one tank to the next can, in extreme cases, "poison" the following tank, but far more likely is an inferior finish. A little acid carried over into the bulk of the dye tank and affecting the bulk pH will have a slight impact on the dye's performance; far more worrying are small localised amounts. If the workpiece isn't thoughly purged of acid, and a little seeps-out during the dying, you can get symptoms like white whisps or streaks or spots.

Thirdly, it would also be harder to guarantee the readiness and that all the wetted surface area would pass the water break test.

Fourthly, if there are, say, any small aluminium retaining rings or collars, there is a possibility that during the anodising run, if they aren't individually racked then they may loose contact and that component wont anodise properly.

For all the above reasons, when we have camera housings, composite lens mounts etc. to anodise they're always racked as individual items.

Anodising isn't really anything to be scared of if you take your time and common sense precautions.

However, if you take it fast & loose then you can spoil things - e.g. inadequate cleaning resulting in un-anodised patches; or excessive etching beforehand giving a matt surface; or anodising too cold and build up a hard layer that won't take dye; or anodising too warm so you get dissolution and a chalky finish; or touching the item part-way through and getting fingerprints; etc. etc.

I think you're right to be scared of, say, the battery-charger/"let her rip" approach which may give variable/dis-appointing results.

BUT if the item is well made, properly prepared and anodised carefully the impact on the dimensions will be minimal and I really doubt you'll have significant issues.

Dave

Edited By Dave Martin on 12/12/2012 14:43:13

When I had to work for a living, one of the products was a gunsight. The metal parts were all 6061 T6 al. alloy, and they had to be anodised a uniform satin black. There were also many male and female threads that were all 0.5mm pitch, the diameters varied between about 12mm and about 20mm. The customer required that the assembly operations be completed in a clean room, to prevent the ingress of foreign objects such as dust and stuff. The customer also required that the assembly be achieved without the use of grease, to ensure that there could be no contamination of the optics. All this required the threads to be pretty close to "cock on".

In a post above Dave talks about dimensional change. This is real, we had to initially develop a repeatable process which would provide an acceptable "satin black" anodising. The dimensional growth was then established, and the NC programs adjusted as required. From memory it was 2/3 "in" and 1/3 "out", so that for instance if the required spec. of anodising was found to be 0.015mm thick per side, then the dimensional growth would be 0.005mm per side. Before anyone jumps up and down, the 0.15mm and 0.005mm were arbitary numbers.

There were many thousands of sights produced so we must have got something right!!

cheers

Bill

Well, you have convinced me to "give it a try". I have made a shopping list and I'm off to buy the bits.

I also considered powder coating, but it is a bit messy and the fumes from the oven are a bit dodgy, positive side is one can coat many different materials.

I will cut a few threads of similar dimensions and anodise them as a trial first.

One other question, if the threads are found to be too tight after anodising, would it be possible to lap them in, or is the anodising even too hard for this?

Thanks for your input.

Martin

Gents,

Just a couple of question.

I have had problems obtaining suphuric acid (Perth Western Australia is the most remote city in the world). However I managed to find some today, but it is only 34% suphuric acid, is this normal and if not what ratio should I mix?

Gloves are another issue, do nitrile gloves provide the same protection as vinyl? I know with my experience dealing with conveyor belt, that nitrile was always recommended where oil or acid was present.

Martin

Martin, I use about 15 or 20% for anodising, so adding your acid to the same volume of water (add acid to water, NOT water to acid) would get you there.

I have had perfectly acceptable results using the acid from dead car batteries given to me by a local garage, added to the same vol of water, but that's difficult now that most car batteries are sealed.

Andy

My second attempt at anodising 12x66x3mm strip unknown grade (I assume it came from aluminium sheet used in small aluminium boat manufacturing).

The first one I did, I had problems with maintaining a suitable connection, I used titanium strips milled from sheeting, I simply bent the strip into a hook and dangled it into the acid bath, in turn clamping the other end of the strip to an aluminium angle which spanned the plastic container. This all proved to be too flexible and failed to provide a suitable connection.

The second attempt I squeezed the hook tight with pliers (see the marks above the hole) this worked immediately and bubbles started to appear as soon as switched the power on.

The colour was "Parker" blue diluted 1:1 with distilled water, immersed for only 5 mins then sealed over simmering distilled water for a further 5mins then left in the water for a further 15mins.

Futher improvements to the fastening will need to be made prior to my next episode, any suggestion to this would be greatly appreciated.

Martin

Congratulations Martin, that looks very good - nothing like some success to lift the spirits - you're certainly on the right road now.

Re the fastening of the part to the hanger - so far all I have done is taper the end of the hangers - a very slow taper that is - and just gently pushed these into suitable holes in the component to give a tightish fit. This has proved more than sufficient on such parts that have a hole. On the cylinder heads I bent the hanger back on itself in a 'U' to a spring fit inside the cylinder head. This does leave very slight noticeable marks at the contact points once coloured but as these are hidden on assembly they are of no concern. By just hanging the part lightly on the hanger the surface will anodise beneath the point of contact reducing the electrical condutivity as it does so.

Writing inks have certainly given me the best results so far and I have since bought some other but as yet untried colours. Tha's certainly a lovely blue.

I still think you could protect the male thread from anodising by wrapping with PTFE tape - now you are set up why not try that on a test piece - I would be reluctant to 'lap' the thread by bringing the two parts together - they may bind irretrievably - but lapping the male with a small piece of hard wood with the paste embedded into the end grain works well if used like a thread chaser - letting the thread gradually form itself into the grain as it runs down the thread. I have used this method on many occasions when a chaser isn't available to polish a thread for that final fit.

Hope this is of use - Ramon

PS - I see the posts are obscurred by the adverts again - nothings changed much there then.

Edited By Ramon Wilson on 26/12/2012 16:08:19

Edited By Ramon Wilson on 26/12/2012 16:09:27

hi all, a new boy here but as regards acid strength, "normal" industrial strenght is 95% or so. To arrive at a consistent and repeatable bath strength the use of a suitable hydrometer may prove handy, depending on strenght or sg an ordiary battery hydrometer may work ?

Noel

Thanks Ramon,

I will let you know how I proceed.

We have high temperatures for the next few days (41C) so I may wait until next week, or I will have to chill the acid bath with frozen water bottles, not the ideal!!!!!!

Martin

Gents,

I have just successfully anodised a small part using Parker Black Ink.

Approximately 20 mins. in acid bath (20%) @ 15v amps @ around 0.4-0.5

10 mins in 1:1 ink/distilled water.

5 mins. in steam and a further 10-15 mins in simmering distilled water.

I also had success on another part using thread tape to protect the thread from anodising.

The following pictures are an indication of the colour, in reality the finish is much better than the photo's show.

Martin

Nice work Martin, very nice Black.

For some reason black ano dye is more expensive than the rest - good to hear the ink works as well as the other colours - all I need to do now is find a nice bright green.

Pleased to hear the thread tape idea worked too.

Your time is much shorter too - so far I've run for 40 mins but will try reducing it next time.

Did you get any leaching of colour after the steam when simmering?

Regards - Ramon