Grades of aluminium alloy?

[Quandary]

Further to my fitting a boomstrut problem I propose to buy a bit of 25mm alloy plate from ebay and fabricate a swivelling bracket to go on the existing mast fitting, about 10cm sq. the front corners will be rounded and have a 12.5mm. vertical hole drilled top to bottom for the existing pivot pin, the top and bottom back corners will be cut away to about 10 mm. thick to form lugs to take the boomstrut at the top and the kicker shackle below it. Other than the drilling and polishing I hope to do most of the shaping by hand with a hacksaw and file, perhaps an angle grinder.
Ebay offers 25 mm plate offcuts, the more expensive is a good deal bigger than I need and described as 6082 grade, the cheaper stuff is ungraded, or the grade is unknown, the bigger graded piece will involve more cutting but perhaps might polish and stay bright for longer?
Any recommendations from those who know metals?

Any tips for getting the stuck roll pin out of the pivot pin other than a hammer and punch which only seem to mushroom it?

 

[Neeves]

There is a huge range of aluminium alloys. The most common marine alloys are the 5xxx series, and 5083 is the alloy commonly used in the marine environment, because it is the most corrosion resistant and is 'relatively' cheap.. Having said that Fortress use the 6xxx series (6062 or 6063?) - but I think because it might extrude more easily. If you need strength then the 7xxx series, specifically 7075 is very strong (high tensile strength), in comparison to most of the other alloys. There is, or are, stronger alloys with Lithium as one of the alloying elements (but these lithium based alloys are expensive). 7075 cannot be bent nor welded (and I think this is the case with the new lithium/aluminium alloys (primary use - aerospace - think Boeing and Airbus).

Whatever you use you will need to anodise if you want your device to stay shiny, or shinyish, you can do this at home, lots of information on home anodising on the internet and there was an article some years ago in PBO.

I have been using 7075, I cannot source the lithium based alloys. It works easily, I have been using an angle grinder with the thin blades you use to cut stainless and abrasive discs (on the grinder). I have been working 7075 from thicknesses of 10mm to about 35mm. For holes I use normal drill bits, for bigger holes I have some high tensile drill bits (a bit like hole saws). You need lubrication for bigger holes. I also use sparingly a hack saw and files. I finish off with a rotary sander, with various coarseness of paper discs.

7075 is tempered at very low temperatures and industrial working would be water cutting - I take a long time cutting with an angle grinder to keep the metal as cool as possible, because I need the strength.

I also test, for strength, when I have made whatever as I am using the devices under tension.

Most of the aluminium suppliers have copious detail on how to work most of the alloys and also some detail on the best alloys for whatever task or environment.

Be careful of references to corrosion - corrosion tests are comparative and 7075 (and the 6xxx series) is considered poor compared to 5083 - I've been using an anchor with a 7075 shank - and corrosion is almost non existent after almost 10 years - and can be reduced by anodising. Most or all rock climbing gear is based on the 7075 alloy (and anodised - with pretty colours).

Most of the aluminium suppliers here will cut to size the piece you want and you then pay by the kg + a cutting charge. So here I would find out what thickness they stock in the alloy I think best suited - I would then specify the size, in your case the square I need - as this would simply leave me to shape and drill. Much of what I am making is not rectangular and I do need to cut and grind.

I don't quite understand the application - I can be a bit slow

Hope this helps

Jonathan

 

[Quandary]

Thank you for taking the time to make such a comprehensive response.
The application - at the back of my Kemp/Selden mast there is a riveted bracket with a vertical pin which carries a triangular swivelling alloy casting to which the kicker is attached. Almost identical to a gooseneck swivel. I propose to take out the pin and remove this and substitute a square(ish) swivel with two lugs at top and bottom with the flexible strut going to the top one and the kicker tackle to the bottom.
I will investigate home anodising.

 

[rszemeti]

For marine use, you want 5083.

7075 and 7082 ... great alloys but suffer from corrosion. 7082 can be OK, if hard anodised.

 

[Neeves]

For marine use, you want 5083.

7075 and 7082 ... great alloys but suffer from corrosion. 7082 can be OK, if hard anodised.

Any evidence of the corrosion of 7075 - considering I have been using it without issue for 10 years for the shank of an anchor sitting on the bow roller.

Jonathan

Edit,

Low Friction Rings, see the current thread, - Allen's rings are made from 6082 and anodised (though the 5xxx series might be the alloy of choice for marine application both Fortress and Allen (and I assume others) use different grades - suggesting corrosion is not the only issue determining choice. I think 5083 is the choice to build vessels - and the application and environment for 'components' have different demands. If corrosion was a major issue - then carabiners and other items of climbing equipment would soon succumb and my anchor shank - that we use as our primary anchor would have failed long ago. I do admit our anchor shank is not bright any shiny - it has a matt patina - suggesting some superficial corrosion.

If you need strength and lightweight and are neither welding nor bending - my choice is 7075. There is a lithium based alloy called 'Weldite' (I think that is the name) which is strong, light and presumably can be welded. It was developed in America - I think it might be quite expensive. Airbus wings used to be machined, may still be, from single billets of 7075, and then I assume, the top and bottom surfaces, riveted together. I stand to be corrected but aircraft on carriers would have been very dependent on 7075 - though the move now is from 7075 to the newer lithium based alloys (making lithium a very critical strategic metal, exacerbated by use in batteries). Most of the aluminium companies make a lithium based alloy (China is a big producer).

I did try to source a lithium alloy - but it proved impossible in Oz, we have no aerospace industry so no major usage. 7075 is very easy to source here in a variety of plate thicknesses.


I think you mentioned the plate thickness to be 25mm - you will find the thin cutting discs wear quite quickly. You either need a grinder that takes big discs, or you need a lot of discs or you need to cut from both sides. My grinder is small, takes the smallest discs, and as the maximum plate I worked with was 35mm I cut from both sides - I then ground down - but you need to cut slightly oversize to allow for subsequent grinding.

Please use goggles and either the guard or gloves (or both). Angle grinders are lethal. Keep anyone (and pets) well away - you do not need to be interrupted (a lack of concentration, or interruption, and the blades can lock up). Secure the item being worked on, engineers vice, lots of 'G' clamps. Its very (very) noisy! (my workshop is largely subterranean). Do not be discouraged - its actually easy and satisfying - just needs planning and caution.

An alternative to home anodising would be to find someone who makes jewellery (from aluminium) the pretty colours come from the anodising. I am sure you might find someone who can anodise small components (as you can galvanise small steel components) - it just takes patience to find them (and I have not found them in Sydney ). However subcontract anodising will have a cost.

 

[rszemeti]

I have too much experience of corrosion of 7075, mostly in planes. You don't need much to corrosion to seriously weaken the structure, google "crevice corrosion" to learn more.

You could also try google, the Wikipedia entry for 7075 says: "It has lower resistance to corrosion than many other aluminium alloys"

I appreciate you have used one piece of it in one application, but that is just one isolated instance. I've seen enough 7075 rotting in planes to tell me your particular use case was not typical.

For marine use, I would stick to 5083 for larger items, and consider 7075 for smaller, machined parts, IFF I could anodise them,

 

[Neeves]

Thanks rszemeti,

Your experience with 7075 in aircraft has strong validity - aircraft parts and the hull of vessels are a bit big to anodise!

I am aware of the comments on resistance to corrosion but Wiki say, of salt spray testing (from which you and others quote results)

"The salt spray test has little application in predicting how materials or surface coatings will resist corrosion in the real-world, because it does not create, replicate or accelerate real-world corrosive conditions. Cyclic corrosion testing is better suited to this."

I too would stick to 5083 if I were having a vessel built (and I think economics would point that way as well), no doubt - but for items where high strength is needed and in an anchoring application 7075 currently seems appropriate.

Of our anchor - only the shank is 7075 the fluke of the anchor is welded 5083. The shank is removable. The components I make are part of the rode but are not critical - and show no adverse effect, yet, to the marine environment.

Just out of interest and as you are here - where do the more recent lithium based aluminium alloys fit with regard to corrosion. I understand they are increasingly finding application in aeronautics (hence the huge increase in production). And secondly - if these alloys cannot be welded - how do they join them altogether? bolts from the same alloys?

Quandary - apologies for the thread drift - I thought I might hi-jack the thread, temporarily.

Jonathan

 

[Neeves]

Quandary,

I'd stick to known alloys. I would try to anodise.

If I understand correctly you are going to drill a 12.5mm hole through a 25mm plate the full length of the piece, 100mm (for the swivel pin).

You will need a decent heavy duty bench drill, a very good vice to hold the piece, a long drill bit and a lot of cutting oil. It can be difficult to drill, steadily and continuously and apply cutting oil simultaneously (I am a typical husband and cannot multitask) - it needs a good spray bottle and everything held rock steady.

To remove the pin (I cannot be of much help), oil, a wooden mallet and wooden pin - at least you should not splay the pin. I wonder if you might have more success if you try to rotate the pin first - soak hot water.

Jonathan

 

[Daydream believer]

a lot of cutting oil.

Paraffin or failing that, WD 40

 

[Quandary]

Paraffin or failing that, WD 40

Kerosene heating oil?
I was hoping to do a lot of the cutting with a hacksaw and just use the angle grinder for finishing?
I have a drill stand with vice under and a Record in the garage loft.
Lots of useful information, all appreciated.
Home anodising kits seem to run out at close to £100 so I might defer that until I find an opportunity to get it done somewhere?

 

[Neeves]

I've used WD40 - its convenient as it comes with a tube allowing you to direct the liquid where you want. I had previously used cutting oil, but ran out.

Kerosene heating oil? what's heating? Its the middle of autumn here, 23 degrees (C).

You are going find it very hard work cutting 25mm plate with a hacksaw. You might have more patience than me - but cutting a decent straight cut is not easy. I have found an angle grinder with the thin blades effective. Just be cautious.

I have a big, heavy duty vertical bench drill stand, bolted down. I also have a vice device that fits to the platten (?) and holds articles vertical. I've just drilled a 4.0mm hole in 9mm 7075 - but only 30mm. The hole you are aiming for is, obviously, possible - but a challenge to get it right.

Home anodising - I used a plastic box, (picnic box) a piece of roofing lead, a battery charger and acid. I tried to keep it simple. Nothing went wrong. The result was 'OK'ish. I gleaned all the instructions from google.

BUT - Its not fantastic, needs a fair amount of tuning - and I'm not strongly motivated toward commercial quality (I'm not selling anything). My interests are in the devices, not what they look like - and if corrosion is an issue I have not seen it. Your conclusion is similar to mine - find someone who has a bit more skill - if I were making lots, I'd persevere.

In your case I might paint it with white epoxy. Any wearing surfaces are going to lose their anodising quickly anyway (so lubricate), as long as you do the epoxying carefully and don't go around your deck with an ice-pick - it should last.

Jonathan

 

[Quandary]

My drill stand has a removable clamp that fits to the base, if I have a perfect flat base to one edge of the plate it should stand upright while I drill it, I can get away with a hole about 75mm deep using spacers and washers above and below which will help with ensuring free rotation. So my plan is first to find a square true edge and take about 25 mm off the opposite front corner, set up the drill and go down through the remaining 75mm. The lump left standing behind the cutaway will be reduced in width to form the top lug, the back corner below will have a triangle taken off both sides to form the bottom lug. Drill 8mm. holes in the lugs then clean up rounding off corners and edges, the angle grinder will definitely be used for this. Smooth the surfaces a bit then apply etch primer followed by 'special metals primer' (purchased for priming a propellor) and spray with a silver or black aerosol. If that does not work I will abrade again and use the epoxy as suggested or perhaps flowcoat? My choice of finishes is influenced by having most of the stuff mentioned to hand already.
I was an architect, starting in the days long before CAD when everything was done by hand on a drawing board, and my grandfather was one of the cabinet makers working on liners including the Titanic so hopefully there is an element of precision in my genes, I can be quite dogged with tasks, my biggest problem is probably over optimism but I will let you know how it works out, if I don't come back you will know the answer was 'not well'.
Again, my appreciation of the considerable effort put in to helping me.

 

[Neeves]

Good luck

A lovely and evocative family history. You certainly have the background.

I'm like you - use what is left over from the previous task - seems a good idea to me.

Actually what you are making is quite simple its just a bit big for most DIY equipment. I was thinking you would need quite a big drill, 100mm min available drill length, 100mm for the piece and then whatever for the vice to hold the piece. But you are cutting it all back to 75mm which will help immensely. The edge, or end, of the piece you buy should be at 90 degrees - it is quite wide and if the platten is at 90 degrees you are well on the way.

When I have finished with the angle grinder I sand/polish using a normal disc sander with 'velcro' discs. I start with a sander that simply rotates, its very fast, and as the papers get finer I move to a Makita orbital sander - which is a bit more gentle.

If you are not anodising you will not need a mirror finish as you will want is rough enough to key.

Jonathan