Lathe speed should be set on the basis of linear velocity of the
surface weighed up against balance of the blank and integrity of the
wood.

There are many references that discuss what this speed (inches/second,
cm/second, feet/second) should be. As diameter, and hence
circumference increases, RPM must be decreased to give the same linear
speed of the surface past the tool.

No one that understands the above will ever simply talk in terms of
RPM nor follow a table of diameter and RPM without considering other
factors.

If the tool is used correctly, the cutting edge has support, and
should not catch. The cutting support will increase as speed
increases if the turner has presented the tool correctly. From this
will follow a better surface finish than if a slow speed was used for
fear of a catch (eg Raffan doesn't use 6 grades of paper starting at
80 grit because he has good technique and takes advantage of using an
optimum speed. He has also learned that not all timbers are alike and
some are better scraped than planed but that is another topic!). If
you don't get catches at modest speeds why do you get catches at
optimal spreeds? The reason will usually be simple: technique can be
improved or poor sharpening of tool leading to less than suitable
bevel (=can't obtain required cutting edge support so you get
catches).

This isn't rocket science and there are enough references for anyone
who is interested (yes, one must actually read, watch and learn) to
improve their technique.

Regarding linear velocity calculations...
Circumference = 3.1416 x diameter of blank
If you multiply circumference by RPM you will have a linear surface
speed.
eg 1000cm x 24 RPM = 24000cm/min (400cm/second)

If you want to try a recommended linear surface speed, convert it to
"distance units" per minute (eg ###cm/minute) and divide it by the
circumference (measured in the same distance units).

Example purely for explanation (NOTE; just for explanation):
Linear speed = 400cm/second = 24000cm/min
Circumference of job1=1000cm : speed = 24 RPM
Circumference of job2=10cm : speed = 2400RPM
The linear velocity in each case of the timber surfaces is the same.

For safety, approach the calculated speed from the slow side, consider
the balance of the blank and the integrity of the wood!

Safe turning,
Alan

On Thu, 08 Sep 2005 09:59:21 -0400, WillR wrote:

Stephen M wrote:
Energy available to propel a loose chunk, shake the entire piece from the
chuck or knock the gouge out of your hand increases with the square of
velocity. Simple truth. What's pitiful as that you think it a joke.




Huh? Wouldn't that be more like Mass times velocity? (as in a proportional
relationship) Maybe the square of the RPM but not the linear velocity.

Steve




1/2 mv^2

..or f=ma

and you cannot push a rope...