On Fri, 22 Aug 2003 10:28:26 +0100, Jordan wrote:

I read that scraping results in better accuracy when making true planes,
than does surface grinding.
I find that hard to understand.
Can someone explain why that would be so?

Jordan

PRO:-
the table on a surface grinder follows the ways to some extent and will also
vary in height if the oiler has been used.
Grinding also produces heat which will distort the object being ground,
Scraping three plates against each other is a primary method rather than a
secondary method i.e. It is not dependant on the accuracy of any other piece
of equipment.
Scraping isn't limited by the size of your surface grinder.

CON:-
If the piece will fit the grinder at work then I can get better results with
an hour of surface grinding than I can with a week of evenings spent scraping.

I burnt my hand on a carbide scraper blade that I was using (hard) for some
rough scraping last week :-(




For true planes you have to go to the primary method of generation which is
the theory that the only surface which can be congruent to an identical
surface in all orientations is the plane surface and the corollary that the
only surface which can be congruent to its reflection in all orientations is a
plane surface. This means scraping three surfaces against each other. Anything
else is an approximation..


Mark Rand
RTFM