Coring

After the last blog it is a logical step to continue this month by discussing coring – after-all, a core is just a larger hole.  But the technique for coring is quite different.  I should say techniques, since there is more than one way to cut a core.
 
The first one I’ll mention is the ultrasonic corer, used to cut long, straight, small holes in beads, etc.  The “drills” are often hollow and diamond-tipped so they are technically core bits.  I know little about this technique unfortunately though I’m sure I could find a use for it in my workshop!  I don’t think it is used by many jade carvers, but who knows, maybe one of them won the lottery?
 
The second method I have used successfully however.  It employs beautifully engineered core barrels (see Picture 1 below) which have an internal supply of water to cool the work, lubricate the process and clear the rock flour, delivered through an external water supply hose and a slip ring (collar) which fits over the stem/shaft - water flows to the inside of the barrel through a hole in the stem.  Because the water is under pressure from the tap (or overhead reservoir) you can end up with a certain amount of water spray and have to restrain the water feed pipe from turning with the core barrel.  The advantage is that these are well engineered tools and you can use them in cutting longer cores.  With this method you can reduce the rate of “pumping” the core barrel (see last month’s blog by Steve) because the water pressure cleans and cools things for you to an extent, at least with small jobs.

Picture 1 on left shows a well-engineered core barrel (gripped in the chuck by the stem in the top left) with water supplied to the inside of the barrel via a slip ring.  The flexible feed-line needs to be restrained when coring or it wraps around the core and chuck.

The third method is a variation on the second.  I recently bought a couple of C-Cut Diamond Coated Hole Saw’s (See Picture 2 below), though there are many companies which make them.  They are very affordable, and will last a while if you treat them carefully.  They range from 6 - 100+ mm diameter – quite a choice!  I found using them straightforward, simple, clean and easy.  So I’ll describe this method:

Picture 2 on left shows one pack of core barrels I purchased recently.  The hole which allows water into the inside of the barrel can be seen in the left-hand barrel.

• Firstly, I carefully marked the centre of the hole on my workpiece.  In fact, I marked it on both sides as I have always cored & drilled from both sides in the past to avoid blowout on the back face
  
• I nailed the piece to a block of wood to avoid the risk of its turning in my fingers and injuring myself
• I placed the block and work in an old Tupperware container and filled it with water to cover the workpiece (see Picture 3 below)

Picture 3 on left shows the bi clamped in place on the block of wood, water covering it and the action about to begin.

• We’ll assume the core barrel is already tightened in the chuck of the drill press and power has been turned on!
• I brought the core barrel down gently onto the work in the correct position and let it start cutting, whilst holding the block of wood firmly to avoid any movement.  It was running at about 750 rpm but up to 1500 is fine for small work (but larger cores need slower speeds – check with the manufacturer’s recommendations)
• The water cooled the work and barrel inside and out through a small hole in the core barrel below the water level.  I applied pressure to the corer for a couple of seconds and then pulled back to let water in to cool and clean the work, before repeating the pressure
• Once I was about half-way through the work I removed it from its confining nails, turned it over, re-clamped it in place with the nails, refilled the container with clean water so I could see what I was doing, repeated the careful alignment and carried on with the coring procedure
• It was good! (see Picture 4 below)
  

Picture 4 on left shows the core cut and removed from the bi.

• I removed the internal core from the barrel with a piece of wire through the hole (see Picture 5 below)
  

Picture  5  on left shows the core before removal from the barrel.  It is easy to remove it - simply stick a piece of wire into the hole on the left and force the core out.

• But I was thinking to myself that turning over the work was probably unnecessary because the flat-faced wooden block was supporting the piece so I tried coring, very gently from only one side with the second piece, and indeed, it worked.  There was minimal blow out, but would be worse if I had applied a lot of pressure to the barrel, which will also shorten the life of the tool as well
• You can tell immediately when the corer has cut through the stone because it won’t grind into the wood – it’s too soft and spongy for it.  And the “feel” changes
• This worked with all of the remaining pieces
• And no splashed water or mess!

 
If you are planning on using a large diameter core barrel, or need to core through a thick section of rock you would need to check the recommended rpm and maybe buy the second type of core barrel I described above.  And clamping the piece becomes important, rather than just holding the block of wood.  But start small and build up!
 
Happy coring!