Machining 9 carat gold

[Richard1]

A friend has talked me into making a set of darts for him out of annealed 9 carat gold.

This involves taking a piece of 6mm bar 55mm long, turning a dome on one end and drilling it 0.087" turning it round and drilling and tapping the other end 2BA then turning a series of grooves 1/32" wide at 1/8" spacing most of the way down the outside.

Never having turned gold before can anyone give me any guidance on speeds, feeds, clearance angles etc.

As there is about £800 worth of material to make 3 darts I don't want to get it wrong.

Richard

 

[albertorc19]

Well, that's new and quite unusual. I worked at a jeweler's shop many years ago and I remember gold is a soft material that can be cut, hammered to shape with soft tools, it is a material very easy to work with. The engraving tools at the shop were not sharpened oftenly because of gold's softness.
The 9 carat figure implies that you have an alloy 9 parts gold and 15 parts of another metals, the most common are silver and copper which make pure gold harder but not by much. I think that you could start with tools, feeds and rpm for brass. Being such a soft material you'll have to be extra careful when fixing your stock to the lathe chuck, perhaps you'll have to use leather or wood packing because steel jaws will leave deep marks. Finish is quite easy with gold, fine grit sandpaper and polish compounds will be enough for good looks.
One important thing to remember is that you'll have to collect ALL the swarf produced by the machining process because it will be certainly melted again therefore it is very important that your machines are squeaky clean prior to machining in order to avoid even the tiniest swarf of steel or alumminum to mix with the gold swarf. A good solution is to put a piece of paper where most of the swarf is going to fall, at the shop we had to use special aprons to prevent the loss of small pieces and we also had a paper linen drawer in our lap where all gold's dust fell.
Please post a picture of your gold darts when you finish them.
Regards

 

[GailInNM]

I have never turned gold but I will make two possible suggestions.

Get some copper rod, 6mm or 1/4 inch diameter, and practice turning it. Most copper rod comes in either 1/2 or full hard state, so heat it to red and let cool to anneal it and it will probably be similar to the gold.

To tap the 2BA hole, try to get a thread form tap that size. The soft gold will be difficullt to tap with a cutting tap as the chips will not break. Remember that a form tap takes a larger tap drill than a cutting tap. When drilling the holes peck drill with very shallow pecks to keep the flutes clear.
Gail in NM

 

[jwcnc1911]

I'd say put something down to catch the chips for sure! Deliver the good darts then melt and sell the chips.

Gold is really soft and I would imagine it would tend to push off like machining lead alloys or copper.

Main thing, save the chips just in case you have to melt it and start over.

 

[enfieldbullet]

if you screw up you can always gather the bits and start again, just takes a few minutes with a torch.

i have worked with silver in the past, and can say it should be a pretty easy material to work with. probably less rake angle for the tools would work best.

and lastly, they aren't called noble metals because of their price, but because they machine, form, hot and cold work, cast and polish beautifully.

 

[Lakc]

If he has the money, who are you to say no.

Seriously, consider building a die to cast a single dart and knurl it once cast. Then sell the die to Bottleson. I would really hate to break a tap off machining it.

 

[Henry]

I am goldsmith... 9 karats ( 10K gold is the minimum karat designation that can still be called gold in the US) can have a lot of different properties depending in the other metals mixed with the gold, could be almost as hard as steel if mixed with nickel and quenched or as soft as tin is mixed with silver. You can find mostly copper and silver normally, if it is white gold it can be palladium, nickel or silver, sometimes it has zinc, cadmium, platinum, palladium, aluminum,etc,
But I think that is a good idea to do a test with copper or brass, both are used to learn jewelry because their properties.
To anneal the metal will change their properties a lot, for 9 carats you will need boric acid to protect it against oxidation or other gases in the atmosphere, after it you will need pickle the piece. Could be really easy to pour melted metal in an improvised mold ( could be a piece of wood coal carved ) to get as close to possible to the final piece.

If you have any question of any problem with my English I will try to explain myself better.
Throw away the chips and tell where, when I listen the word recycle, I think about gold and platinum, absolutely everything that touch the metals is processed that include the water for wash my hands.

 

[aonemarine]

Sounds like a good project for lost wax casting....

 

[Arcane]

To add a bit to what Henry said ( excellent info): it has been my experience that gold that has been remelted and cast more than once or maybe twice will change color depending on what it has been alloyed with, usually taking on a pinkish hue. This can be improved by bursting (a process using sodium cyanide) or by plating after the part is finished. Rather than turn the piece from gold rod, it might be better to cast it using a lost wax process and turn it just to touch it up. I cast a lot of gold and silver in my younger days but have never turned either. BEST of LUCK!

 

[Richard1]

I will be seeing the guy later today so will find out what the gold is mixed with (I hope). As he had already bought the gold bar before asking me to make the darts it seems that they will have to be turned rather than lost wax.

I have already made one out of brass to work out how to hold it and the order of machining. It is also to show him what it will look like in 6mm as his current darts are 1/4 solid tungsten.

Will today see if I have any scrap copper to practice machining as assuming this is a gold, silver, copper alloy I hope it will be similar.

Will keep you updated and post photos.

Richard

 

[Richard1]

No photos yet I have dropped my camera. Yesterday I made 2 practice darts one out of copper and one one out of sterling silver. I found drilling to be the worst part, followed by threading. If gold machines like this I think it will be OK.

The gold in question is a silver, copper, gold alloy.

The dart tips are 0.090" diameter, TiN coated hardened steel, force fitted 7mm deep. Any suggestions on what size the hole should be to force fit these? Given how soft the gold will be I am not at all sure that the tips will stay in if they are going to see a lot of use.

Richard

 

[Lakc]

When you said thread 2ba I was assuming you meant both ends for a hammerhead tip.
http://www.hammerheaddarts.com/
You may need some type of an insert to keep the point from hitting the gold directly.

 

[Henry]

Use this for cut the gold it will work really nice with tiny amounts and will made drill in gold a lot easier. Like I said the fit will depend mostly at the 9 K gold that you use. Dip the part to be inserted in a little amount of acid that attack that steel that you are using, at the same moment that you are inserting it, that will oxidate the metal and will made it thicker inside of the fit, I think that this will made that fit for ever ;-).

 

[Richard1]

Thanks for all the good advice.

I think I have some Methyl salicylate if I can find it for drilling, never would have thought of using it though. Thanks Henry.

I have never heard of Hammerhead darts until Jeff mentioned them good information there which looks like it might come in very useful in the future. Looks like Hammer head and ACE are interchangeable and ACE are listed on Ebay.

This was supposed to be a one off for a friend! So far I have made 6 sterling silver darts, currently away for hall marking, the gold is arriving on Wednesday to make 3 gold darts and he is talking about some platinum darts.

 

[Lakc]

Years ago I made a soft tip version of my Bottelson 3092's in plain steel, then had them gold plated by a friend.

 

[jwcnc1911]

Man this forum is filled with knowledgeable people! I was following this as I've cut everything from CGI to brass to inconel but never gold.

Any one ever cut unicorn antler?

 

[Richard1]

Finally the pictures I promised all those months ago.

9 carat gold

I think they turned out rather well.
The gold was a little nerve racking considering that each dart has over £200 of metal in it and the only way to find out how it cuts is to cut it and see.

Richard

 

[Henry]

Nice work!!! the next one in platinum

 

[Lakc]

What did the weight come out too? Looks good.

 

[Richard1]

Next one in platinum but only if my friend is paying for them.

I don't know how heavy they are I didn't have anything the weigh them with when I made but I will ask him tomorrow.

He just gave me 6 lengths of 5/16" sterling silver today and told me he has registered the name PMD (precious metal darts). If he can sell them it looks like by accident I might have a job.

Richard