DoN. Nichols wrote:
On 2008-08-02, RoyJ wrote:

Sheesh! Talk about a confusing chart.


[ ... ]


If you want to try it out on the quick and cheap, a standard stove
element runs around 40 ohms and is rated above 1000 watts at 240 volts.
Run two in parallel for 20 ohms. I've been getting these for $1 each at
my appliance recycling place. The resistance is not particularly fussy,
the VFD is just looking for a place to dump the heat. An advantage of
the stove elements is that you will never have to worry about
overheating them.


But -- the hot resistance is higher than the cold resistance.
Which is your 40 ohms? Measured while cold? I calculate about 57 ohms
hot from your 1KW at 240V. This means that a series of stops in quick
succession (like tapping a lot of holes in one workpiece) may reduce the
braking as the elements get hotter.

Don,

What resistance wire are you assuming. For a stove element I would have
assumed a FeCr or FeCrAL wire and my Kanthal data shows a resistance
increase with temperature of 4% to 5% at about 500C depending on
composition, the maximum resistance increase is given as 8% for some
wire but at temperatures around 1300C which isn't likely to be desirable.

What kind of tapping do you intend to do on the CNC machine?
Rigid tapping, which requires the spindle to feed down at precisely the
thread feed rate?

If not that -- why not go for a tapping head which releases when
the feed stops so the tap freewheels, and then reverses the tap when you
start to withdraw -- all while keeping the spindle rotating in the
forward direction. I use one for tapping in a drill press, and it can
be used in a milling machine as well. There are versions designed for
CNC machines which can even be handled by tool-change robots.

Enjoy,
DoN.