Use a brand new, scary sharp tap. Use cream (I have used the little coffee creamer packs, not the non-dairy abonations) for tapping fluid. Animal fats are supposed to work, but I have not tried them. Bacon fat was suggested.
If you can get at a tapping machine, so much the better. I found that running the tap fast, helped.
I did a series of 5/16", and 3/8" holes in some buss bars for a large radio installation grounding assembly. The first two holes destroyed a couple taps, and the holes. Once we got the tapping machine set up, the rest of a couple hundred holes, only took a few minutes, and turned out very well.
Try a test hole or two, on a scrap or cut-off.
I'd suggest drilling the holes on the large side of the scale. Not like you are relying on the threads to hold up any structure, so you can afford to be pretty generous with the drill size.
If you just want some "pips" to locate things why not use say 1/8" copper snap head rivits, no need to form proper heads on the reverse, just a shallow CSK hole and a punch in the center to spread the end. Obviously no tapping needed just a hole.
Bacon fat or lard is better. If you use milk then clean everything nearby as soon as you have finished, milk splashes stain and rust steel very quickly.
Use a very sharp (or roll-form) tap, and some arrangement to keep the tap vertical is almost essential - a drill press or the like will do at a pinch. Without some such arrangement it is very difficult to tap copper, even a slight misalignment from vertical will cause the tap to stick.
When (dry-) turning or drilling copper I find high speed helps, but that's a bit impractical when tapping.
As with all tapping, a slightly oversize hole will make life much easier.
As to taps, carbon taps are plenty sharp and hard-wearing enough, but HSS taps are tougher and break less easily - and toughness counts, especially if you are hand tapping and aren't using a tapping machine or drill stand etc. to keep the tap absolutely vertical.
I marked out the position of the holes (sheet of glass, surface gauge, dividers, fat permanent marker as substitute for layout blue)
Following my previous difficulties locating a hole accurately in some soft material
I didn't dot punch; given that I was planning to use 4BA threads (around 1/8", but a fine thread) I was using a 3mm drill, as per Zeus chart. I drilled a 3mm hole in a piece of scrap mild steel, and clamped the piece of scrap in place, this faking a drilling bush.
I then used a hand drill (Stanley #803) to make a substantial start to the hole.
Having "marked" all the holes in this way, I changed to using a drill stand to get the holes straight.
(no drill press, I'm afraid) with my Bosch drill. Typical DIY drill, hammer action, multi speed, reversible jobby.
Having made the thorough holes, it was time to tap. A second hand Presto carbon steel
4BA taper tap (very long taper) was fixed in a Millers Falls hand vise
The jaws in this have a vertical 'v' groove, and the handle can be replaced with a brace adapter.
I held the vise in a very small brace (5 inch sweep).
This arrangement allows very good directional accuracy, since the assembly is around 16" long from tap to handle.
The only problem is that even a tiny brace can generate a lot of torque.
I therefore only used this arrangement to start each thread, around 3-4 turns (in the hallowed 3/4 turn forward,
1/2 turn back method).
The recommended bovine lubrication was used :-)
I then put the tap in a small 'T' holder, and finished each thread.
The tap was regularly extracted (tedious!) and cleaned of chips using a small brass brush (actually a sueded cleaning brush).
Bottom line - I had no trouble, and now have 8 nicely tapped holes, populated by 8 4BA studs, and the copper plate sits on my hob nice in a stable and safe fashion.
On or around Thu, 15 May 2008 12:10:38 +0100, bugbear enlightened us thusly:
how does it work? I might do one for here. Got one of the ones that someone recommended, it's better than the ones I had but still plated steel and will eventually rust just the same, although it seems to work quite well.
I have now; made a soup last night, in a largish (6-8 pint, I guess) aluminium Tefal pan.
The simmering was gloriously even over the entire area of the copper, so I guess it's working as designed.
I was slightly disappointed that the simmering was quite fast; it appears that the cheap diffuser also sheds some heat somewhere other than the pan, since (from memory) the simmering on the aluminium diffuser was slower (but less even).
I was using my hob's smallest burner, at minimum level.
I suspect that the copper plate will perform its anticipated task of avoiding food burning rather well.
When I was an apprentice electrical fitter in the '60's I had to make hundreds of copper contacts and I was told kerosene was the correct cutting fluid when tapping threads in copper . It worked for me. Regards, Russell