Cutting petrified wood

Jewelry Discussion
1

While cutting petrified wood with the automatic feed function, the
stone cupped on the inside. Hand feeding I could cut a flat surface,
polishing created the same problem. Cupped and scratched surface. The
stone is obviously harder on the outside than on the inside. Any
expertise or opinions on this would be most appreciated.

2

Hi William.

My first thought is to set the auto feed as slow as possible. Also
check to be sure your blade hasn’t been warped.

When polishing use a hard surface rather than padded so undercutting
is minimized.

The differential hardness will make the job take longer than you
might think it should so patience will be helpful.

HTH Pam In Mesa, AZ where it is (unbelievably) still Spring the second
week of May!

3

Hi, sound like blade wobble. Is the face you are offering to the
blade perpendicular to it? Is the peripheral speed of the blade about
correct? (a smaller blade on the arbour will be too slow) A slow feed
should reduce the cupping but you will see a larger cutting slot. As
for grinding and polishing you will need to increase the time spent
on the finer grinding stagesto reduce the undercut. It would be wise
to add an extra stage as well, ie if you grind say 320 grit then 600
and then 1000 I would say go from 320 to 500/600 grit, then 800/1000
and then 1200/2400. This will again reduce considerably the undercut.
Polish on a harder surface such as perspex or a metal lap (you could
even try a hardwood). Aluminium will do if copper is out of your
price range. The polish can be anything you would normally use but
tin oxide works well with perspex.

Nick Royall

4 
a smaller blade on the arbor will be too slow

Isn’t it the other way around? The larger the blade the slower the
speed of the outside edge, smaller blade outside edge turns faster.

Richard Hart G.G.
Denver, Co.

5 
Isn't it the other way around? The larger the blade the slower the
speed of the outside edge, smaller blade outside edge turns
faster.

Richard, rethink it. A small blade has a smaller circumfrence than a
large blade. If both turn at the same RPM, the smaller blade is
moving a shorter length per revolution, thus the smaller blade, if
the RPM is the same, gives a slower speed.

Perhaps you’re thinking of pulleys. If you have a given drive pulley
on a motor, and a driven pulley on the sawblade arbor, then a smaller
driven pulley on the arbor will give a higher speed to the blade…
But a smaller pulley on the motor, gives a slower blade speed…

Peter

6

Peripheral speed is basically a linear motion so a bigger blade will
turn slower than a smaller one to obtain the same rate. My warning
was about using a small blade in a saw designed for a big blade as
this will give too low a speed to cut efficiently and will have more
wobble because the centrifugal forces are too low. The optimal
peripheral speed will be about the same for just about any blade
diameter of similar construct.

Nick Royall

7

a smaller blade on the arbor will be too slow

Isn't it the other way around? The larger the blade the slower the
speed of the outside edge, smaller blade outside edge turns
faster.

If the shaft on which the blade is mounted is turning at the same
speed, the larger diameter blade will have the fastest speed at it’s
cutting edge.

For sake of description assume a blade with a 10 inch diameter
turning at 100 rpm

10 x pi = 31.4 The circumference is 31.4 inches

31.4 x 100 = 3141 inches per minute speed at the outer edge of the
blade

Now let’s use a 20 inch diameter blade turning at 100 rpm.

20 x pi = 62.8 The circumference is 62.8 inches.

62.8 x 100 = 6280 inches per minute speed at the outside edge of the
blade.

The larger blade in this case is twice as fast as the smaller blade.

8

The larger diameter, the more cutting inches per revolution.

Dave Leininger

9 
Isn't it the other way around? The larger the blade the slower the
speed of the outside edge, smaller blade outside edge turns faster.

Thanks to all who replied on and offline. I knew that as a polishing
buff increases in diameter, the outside edge increased in speed, so
same would be true for lapidary saw blade.

I have been on pain meds for a double whammy of a kidney stone and a
massive sinus infection that I am now recovering from. Surgery was on
the 10th for the sinus issue, I posted on the 11th heavily medicated
Pretty sure it will be a week or two before I can focus or
concentrate enough to be able to use tools or machines and not be a
danger to myself or others…

Richard Hart G.G.
Denver, Co.

10

Richard, take care of yourself old friend. Thats the first rule of
survival. Always enjoy your posts and would like to see more,

Cheers from Don in SOFL.