Engraving marble

[steve_g]

This is my first attempt at engraving marble and I can’t say that I’m pleased. The first try was .040 deep at speeds ranging from 2ips to .5ips. Second try was .020 deep at .25ips. I used a 60° V-bit that was too dull for any other purpose. The marble was picked up at a local flooring store.

I’m curious what others are using for bits and speed and if my experience is typical. The samples shown are 6X6. The fracturing around the engraving reminds me of chip carved arrow heads, not the look I was after!

Steve

[billp]

Steve,
I've never done cuts that shallow in marble, so I'm gonna have to guess at this one; I'm wondering if the depth isn't TOO shallow, so that you are really dealing with both the polished face, and the underlying stone at about a 50/50 depth ratio (?).
If so, there is then the issue of the not too sharp bit ( which HAS to also translate as out of balance) you'd need add that extra vibration to the mix as well.
In the samples I showed at the Camp those letters were all close to .25" deep,one was done in multiple passes, and the other was a slow, single pass.
And THEN there is also the issue of the marble itself;

MARBLE (Scientific term) - A metamorphic (recrystallized) limestone composed predominately of crystalline grains of calcite or dolomite or both, having interlocking or mosaic texture.

"Interlocking or mosaic texture" would seem to lend itself to "tear out", so I'd opt for the sharpest bit you have on hand.
If that piece were still registered on my machine I would take a small paint brush and coat the insides of each V cut with some plain tap water. Then I'd place a plastic sheet around the edges of my machine to keep the soon to be created (airborne) marble slurry from sticking to your machine....And then with the sharp bit I would cut the same file, but a little deeper...
Please keep us posted on your findings...

[nailzscott]

Steve,

Like Bill, I have not cut marble that shallow either. Here is one I cut using a 90 deg vbit at .3 inches/sec but the plunge was what caused most of the problems, until I slowed it to .1 inches/sec. I got about 4 or 5 of these out of a bit before it was shot. As it got dull, I would get more chipping around the edges of the lettering. These letters are anywhere from .060 to .140 deep.

[upsman]

Scott, what are you slowing down your Stepdown, feed rate, or feed rate? And what V-90 are you using? I have Onsrud 37-87 a large v-90. Would like to know if a different V-90 would work any better or doesn't matter. Thanks

[billp]

Scott,
Ramping into the cuts may help to solve the plunge issue...

[steve_g]

Scott, Bill:

Work has gotten in the way of play and I haven’t gotten back to trying these suggested parameters. Hopefully tonight!

I’m wondering, what router speeds you’re using?

Steve

[billp]

Steve,
When I ran a PC router I did everything at 10,000 rpm because of the obnoxious sound the router made when running at higher speeds. When I switched over to a spindle I stayed in the 10,000 rpm area for material like marble .

[nailzscott]

Tim, my feed rate is .3 in/sec and the plunge is .1 in/sec, all cut in one pass. I think my router speed is 13k. As far as vbits - a few pieces cut like this picture ends up trashing the bit. I have used good bits and some that are as cheap as I can get - even standard bosch, sears, and several from big box stores. None seem to last more than another.

I have cut 30 or so small hardwood signs, then use the same bit on marble until it dies. I just keep cutting with it until I see the chipping begins on the lettering. I have run out of vbits before and even sharpened the carbide (not recommended) to make it through a couple more cuts.

Most wear variation seems to be from the variation in the marble pieces. Sometimes you can see the crystal type veins in the marble and they definitely wear the bit down. I try to chose my specific marble pieces better now to stay away from these hard sections. Depending upon the hardness of the marble surface, you can watch the tip sometimes and see the glowing red, but as long as they are cutting without chipping out the letters, I just keep on using them.

Bill, I have not tried ramping, but the problem with a faster plunge was that my Z would loose steps. I figured since the surface of the marble is harder, ramping would keep the bit in the hard section longer.

[gc3]

what about a toolpath with a climb/conventional cut?