How to bore a graphite furnace
Note that web-based summaries of standard operating
procedures are not exhaustive.
Not all of the relevant safety information is included in this document.
Further training is required for safe operation and will be provided
in person.
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The straight walled graphite furnaces used in the 12-TEL multianvil
assembly are machined from graphite rod. Periodically, when it's time to replenish
the supply of heaters, we find that we have forgotten the procedure for this.
So, here is the "last known good" way that produced good quality
furnaces. The issue is that the furnace bore must be straight sided so that
the wall thickness
is constant throughout the length of the tube, in order for the pressure medium
to fit perfectly.
- Cut the rod into 14 mm lengths using a cutting jig in a low speed diamond
cut off saw. A steel saw blade (rather than diamond) is adequate for this,
but watch your fingers!
- Use the real lathe. The hobby lathe in the lab isn't up to the task. Use
580 rpm spindle speed.
- Graphite dust is messy, so lay some paper across the bed of the lathe beneath
the collet to catch the graphite dust. Wear a dust mask and eye protection.
- There is an oversized 5/32 collet that works well for holding the 4 mm
OD rod. An 11/64" collet would probably work too. The 4 mm collet is too
small for holding the delicate furnaces. Make sure the collet is only just
tight enough to grip the graphite. You'll have to keep an eye on the collet
closer while you're working when it's so loose.
- Pre-drill an undersized hole. A 7/64 HSS bit works fine for this, and you
can drill the entire furnace length in a single pass. Be careful to center
the drill well and don't drill too quickly.
- Use a short-flute ("stub-length") solid carbide drill bit for
maximum rigidity. We last used a diamond coated 1/8 inch carbide bit with
a 1/2" drill depth.
Drill the length of the tube in several (~3) very slow passes, brushing graphite
powder from the bit between each. Drill all the way to the end of the flutes.
- Check the fit with a length of MgO extrusion. Make sure that both ends
of the hole have an equally tight around the MgO.
- Grind the furnaces to size using the furnace grinding jig. Each end should
be ground a little, with the final grinding reducing the furnace length to
13 mm (+/- 0.01).
ACW 2012
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