On Sat, 30 Jul 2011 17:31:25 -0700, wrote:

On Fri, 29 Jul 2011 13:50:15 -0400, "Existential Angst"
wrote:

"The Dougster" wrote in message
...
A 2- or 4- flute end mill makes a fine trepanning tool on the lathe.

Trepanning of a square blank held on an expaning arbor through a
smallish center hole leaves a square with a round hole, and a disc
with the smallish hole. If what one wants is the disc, trepanning
saves the wear, tear, and irritation of the interrupted cut, and save
turning all those square points into chips.

Assuming the blank is 1/2" thick, idnat why God invented hole saws?

I am struggling to find a good way of cutting out 2.5" disks out of
3/16" steel plate. I have not tried trepanning but it is on the list.

Would a hole saw be useful for this application? How many disks would
one be able to get out of one saw bit?

BTW if I farmed this out to a local shop for plasma cutting it would
cost me $6 per disc. Cutting the same disc with a jeweler's saw costs
1 hour, 2 blades and a sore shoulder.

Michael Koblic,
Campbell River, BC
Greetings Michael,
Trepanning the steel would work well if you have a rigid setup and a
slow enough speed. Hole saws don't have much room for chip evacuation,
so you need to keep pulling the saw out of the hole to clear chips.
3/16 isn't that thick so maybe not too much trouble. A high speed
steel trepanning tool would need to be spun pretty slow but if ground
properly can take a pretty good chip load. A carbide tool needs more
rigidity but will cut much faster. If you can tolerate the noise it's
possible to run the carbide fast with a light chip load if you use the
right grade of carbide. You might be able to unsolder a tooth from a
circular saw blade and use that. Anyway, with a light chip load and
spinning fast the carbide will chatter and the chips will come out as
flakes. Using spraymist with plenty of air pressure will blow the
chips out of the groove so you will be able to make a continuous cut
clean through the the steel. Grind the end of any tool you use at an
angle so that the tool will break through first at the inside diameter
of the groove if it's the discs that you want. Stop feeding the tool
when it is just shy of breaking through. Then use a hammer to knock
the disc out of the plate. There will be a slight burr that is easily
removed with a file or sander. If you cut all the way through there is
a good chance that the disc will shift and pinch the tool which will
probably break the tool. I must reiterate that using carbide in the
above manner will be VERY noisy. Use hearing and eye protection.
Cheers,
Eric