figure 8 pattern ?

[Dave the Nave]

For many years now,I've seen people using a figure 8 motion when lapping something flat on a surface plate.And when I asked those in the shop why the fig. "8" movement,They're reply was "I don't know really,this is the way I was taught".
I can remember a time when my boss chewed me out for lapping a part on a surface plate using a straight linear back and forth motion.I guess that was a "no no", but the part did look good though.And I always use 240,320 &400 grit wet or dry sand paper.
Can anyone explain the physics behind the "figure 8" method?

[spkrman15]

Hey Dave,

A guy came to my work and lapped a part with the figure 8 method. He told me the reason he did it that way was to keep the the part flat. It seems we put different pressures on parts if we go in straight motion or in circles. A figure 8 patters helps balance out these tendencies, keeping the part flat. Hey it saved me about 1200.00 bucks. So he couldn't be that off. Hope that helps Spkr

[Thrud]

Lapping anything in one direction only will not make it flat. A linear stroke leaves microscopic ridges and grooves running along the direction of the stroke. You can never get a flat surface - it will not wring to a gauge block.

Think back to how you were shown to draw file. Alternate directions. You can see the progress of your work. When you scrape something flat, you always alternate direction of the cuts.

If you look at an orbital sander it produces circles on the finish exactly the same size as the motor shaft offset. It cannot produce a flat surface.

A Random Orbit Sander - now we are getting somewhere! Flater surface and it will not wring with a gauge block either! Why? The random orbit is just that - a random orbit. Spiraling in and out to maximum and minimum orbits constantly - in the same direction! The grit will only cut cw or ccw depending on construction of the machine.

The microscopic tearing of the metal in the direction of cut leaves a "surf's up!" curl on the high spots - you have to remove this in the opposite direction to even the surface out. The figure eight pattern is a fair approximation of randomized direction of cut and will produce a flater surface eventually.

Of course, this only applies to metals (grainular in nature). Other materials have different physical properties and are dealt with differently.

If you still want more info I suggest you contact www.asm-international.org for books and papers on specific metals operations and books on failure analysis.