# Re: Sean's "methodology"

On 5 Jul, 19:27, Seanpit <seanpitnos...@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
wrote:
On Jul 5, 10:47 am, richardalanforr...@xxxxxxxxxxxxxx wrote:

Oh, and just in case anyone missed it:
The tolerance on the 200mm cube is 2.8 microns.
That's 0.0014%.
It doesn't meetSean'sspecification.
The other cubes are made to slightly lower tolerances.

I don't really know why I have to do this because I know producing an
example for you will make absolutely no difference in your thinking,
but here we go anyway . . . just to hit my head against "the brick
wall" one more time ; )

Here is an example of a granite cube with tolerances less than 0.001%
with respect to flatness and parallelism:

Let's try this in as simple terms as possible, Sean.

We were not talking about tolerance with respect of flatness or
parallelism.
We were talking about tolerance in respect of "reflective symmetry".

If we are talking about tolerance in respect of "reflective symmetry",
you need to take the dimensions of the block into account.

"Undersigned is guide to a batch of three students of Mechanical
Engineering for Masters Program I-STAR who have undertaken a project
of assessing geometrical accuracy of 3 D - CMM. During the course of
conducting project we have come across a situation which requires
certain clarifications as under. A mechanical artifact of Granite Cube
of size 500 X 500 x 500 mm (Hollow design having total mass of 140 Kg)
while being checked using First Principal for checking
perpendicularity has shown value within 5 microns for all the faces
where as flatness values for each of the six faces have been observed
within the range of 3 to 4 microns and parallelism of opposite faces
while checking with digital electronic probe ( gauge head ) are within
maximum 5 microns."

Do you understand the difference between tolerance for flatness,
parallelism and "reflective symmetry", Sean?

You used the term "reflective symmetry".
You told us how to measure it.
You measure it by taking a plane through the centre of the object, and
measuring the distance of opposing points from that plane.

If we have a 500x500mm block, the distance of the faces of that block
from a plane running through the middle is 250mm.
If the tolerance on the flatness of the block is 3 of 4 microns,
that's 0.0012% to 0.0016%, which exceeds your parameters.
If the tolerance on the parallelism of the block is 5 microns, you
just meet your criterion of 0.001% for parallelism only.

However, unless there is *NO* tolerance on flatness, your criterion
cannot be met, as deviations in flatness will reduce the "reflective
symmetry".

So these blocks do *NOT* meet your criteria.

Try again.

RF

.

## Relevant Pages

• Re: Seans "methodology"
... The tolerance on the 200mm cube is 2.8 microns. ... 0.001% requiring a tolerance of 2.5um and the maximum difference in ...
(talk.origins)
• Re: Seans "methodology"
... within the range of 3 to 4 microns and parallelism of opposite faces ... measuring the distance of opposing points from that plane. ... If the tolerance on the parallelism of the block is 5 microns, ... recognize what is commonly referred to as a "carrier wave", ...
(talk.origins)
• Re: Seans "methodology"
... within the range of 3 to 4 microns and parallelism of opposite faces ... Do you understand the difference between tolerance for flatness, ... If we have a 500x500mm block, the distance of the faces of that block ... The tolerance % of one surface point compared with the opposing point ...
(talk.origins)
• Re: Seans "methodology"
... within the range of 3 to 4 microns and parallelism of opposite faces ... Do you understand the difference between tolerance for flatness, ... If we have a 500x500mm block, the distance of the faces of that block ... The tolerance % of one surface point compared with the opposing point ...
(talk.origins)
• Re: ON topic- Flycutting ALuminium
... If it were me (and with that flatness tolerance), I'd not touch the flat surfaces, but I'd rough out your perimeter, all your features & large holes, then send it out to be blanchard ground - then when you get it back, finish your perimeter, features, and holes. ... When you relieve that stress by removing material on one side, the part bows. ...
(alt.machines.cnc)