Making your own concave cutter mandrels

[Faceting Frank]

Searched old subjects on the forum and found a couple on making concave cutter attachments for your faceting machine, but could not find one on making mandrels. Have made all my mandrels for my concave cutter and nearly finished a mold for coating a copper mandrel with an old BATT lap metal.
So will start posting shortly on how I made it and other info on making mandrels.

[RoughCreations]

Should make for interesting reading, look forward to it. I wonder whether similar processes could be used for making gem-carving bits that you could use in a hand-held rotary handpiece?

[Faceting Frank]

Finished making the mandrel mold. Made it out of old faceting machine parts, so not concerned on how it looks, as long as it works.
This is to make BATT lap metal coated copper mandrels. The whole thing is made of ally.
The main body consists of two pieces of flat bar bolted together and bored through; the base is also flat bar with a piece of 1.2mm with a ¼” hole in it to accommodate the mandrel shaft. The mandrel shaft protrudes through the end of mandrel body and fits in the hole, this ensured the mandrel is central to the main hole at the end. The top part of the mold keeps the other end central. A recess to one side of the hole allows the molten metal to be poured in the gap between the mandrel and the mold.
I will line the inside of the mold with alfoil, this should make it easy to take apart. Have tapped one side of the mould 6mm, should I need to force it apart.

[MakkyBrown]

Best of luck Frank hopefully it works nicely.

[Faceting Frank]

All ready to try it out tomorrow. The plan is to produce two 12mm diameter mandrels, one for 3k and one for 60k. The hole in the mold is bored 12.5mm and the copper mandrel turned 11.5mm. That’s not much of a gap, but at the moment I don’t see any reason to turn the copper any smaller. The alfoil liner worked well and was easy to keep in place on assembly by keeping the drill bit butt in the hole.

[MakkyBrown]

Worth keeping some heat on the mold before and a bit after pouring to make sure it flows over all the copper. Could be worth working out the melting point of batt alloy (which is going to vary depending on how old the donor lap was) and maybe heat to about 50 degrees above that.
I wonder if tinning the copper first might also help.
Looking forward to hearing how you go.

[Faceting Frank]

Your right about tinning the copper first, didn't do that and there were some hollows in the tin down to the copper. Other problem was no hole to let the air out. So drilled a hole, turned the copper down to 10.5mm and cut the tin up into little bits. Dipping each bit in bakers fluid and putting then one by one in the recess at the top and letting them melt in. So will find out tomorrow how it went.

[Faceting Frank]

Wish I had done some research on making molds for metal casting before making my ally mold. Never realised that silicon molds can withstand the heat from low melting point metals.
https://www.youtube.com/watch?v=D_BGMBmc7Pg

My ally mold didn’t work with the heating setup I had because it was getting too hot at the bottom and the poured metal sort of pitted at the bottom. So, getting myself a silicon mold making kit and giving that a go, confident that it will work ok with the copper pre tinned.

[Faceting Frank]

The silicon mold stuff comes as a two-part liquid or two-part putty. Since there is no detail in the mandrel, opted for the putty. Found a YouTube vid on how to make a two-part mold with it.
The putty is mixed together by hand and will stay workable for a few minutes. Made a plastic oversize mandrel and pressed that into what will be one half of the mold, as well as a bit of wire to create an air escape. When the putty has fully cured you coat it with Vaseline and press the other half of the mold onto it. With the plastic mandrel and wire still in place, the putty will fill the recesses and they allow the two halves to be accurately aligned.
Looks a bit bodgy, but they fit together perfectly.

[MakkyBrown]

I'd wonder if you to stick with an aluminum molds and go about 2mm(or more) oversize and machine back to size. Use your blow torch to heat the sides of the mold. You are probably never going to get it perfect with out machining afterwards.
MB

[Faceting Frank]

I know I would have to machine it to size. The three mandrels I would be using 1200, 3k, 60k would have to be identical in diameter. There are a lot more vids on YouTube about silicon molds for pewter than the one I posted, which convinced me it would work and it did on my second attempt.
Key was to pre heat the copper core of the mandrel before putting it in the mold, hot enough so the molten metal would stay molten as it sank to the bottom.

[Faceting Frank]

I didn’t make the mandrel out of just the BATT metal with a shaft through it because I didn’t think it would be rigid enough. I use polyethylene mandrels for my 1200 grit work, as it does a much finer finish than copper. But its soft and bends if you put much pressure on it. So, I’ve changed all my shafts from brass to stainless steel, though have not tried any out yet.
The finish on the mandrel is with auto feed on the lathe, did a brilliant job. To good in fact and will have give  it a 600 sandpaper finish.

[MakkyBrown]

Well done Frank, looks good. I might give the silicone mold a go sometime.

[Faceting Frank]

Made three mandrels in total, ended up having to machine them to 11.5mm instead of the intended 12mm.
Found I could hold the two halves of the mold together with rubber bands, never had a problem with them melting. The aluminium bar is to rest the little flat bar onto, it is held by hand to keep the shaft central in the mold at the top. The protruding shaft at the bottom fits in the mold.

[Faceting Frank]

Started using my new mandrels today. Haven’t cut many concave stones in the past because of the length of time it takes to set my machine up to use the concave attachment I made for it. The plan was to cut flat facets to the 1200 stage, then concaves on a copper 1200 grit, 3k BATT and 60k BATT, all 11.5mm diameter.
These concaves are much deeper than the ones I’ve cut in the past and found after cutting the 1200 grit the 3k wasn’t reaching the bottom of the concave. So turned the 3k mandrel to 11.4mm diameter, which worked perfectly.
The runout I’m getting on the dial gauge on my faceting machine with these mandrels is 0.04mm, don’t know if that’s contributing to the problem. But dropping the diameter by 0.1mm fixed it.