martin54 wrote:You really need to be experimenting yourself with something like this, lots of different variables to consider. What works well for John with his machine could be completely wrong for you with your machine. Plywood also varies a lot with grade so again hard to say how well what you are trying to do will work.
Maybe start with trying to cut letters bigger that you want to using different speed & feed settings, small text generally you need to be cutting fairly slowly. A coat of sanding sealer or something similar that will soak in & can help bind the fibres together & prevent tearout often works.
So i'm trying similar speeds and feeds (30 ipm feed 15 ipm plunge) with the same bit he's got, and my main problem isnt tear-out (i've been experimenting using stained and/or sealed wood, and that seems to help, as you suggest). My issue tends to be with the fuzzies left behind in the groove, even after 2x slow passes (second pass helps, but doesn't really leave it clean). Particularly with "deeper" cuts like 0.025 (vs my best so far at 0.011"), the fuzzies seem to be worse.
I've tried samples of Oak, Birch, cherry and walnut, as well as birch plywood (as I mentioned before), cured with various stains , danish oil and/or shellac.
My best settings are 2x slow passes (30 ipm feed or less at ~18000 rpm) using a 0.011 depth instead of 0.02 in finished birch wood (not ply, gave up on that for now for tear-out reasons), which leaves no almost no surface tearout and reduces the fuzzies considerably but is still lacking. I'd be happy to sand the finished milling work by hand, but getting in the little grooves that a 30 degree bit leaves is somewhat problematic, to say nothing of accidentally sanding away the stain and/or sealer and having to do that over, too!
P.S. Why does a slower feed rate (vs faster spindle speed) give good results on this sort of thing ? I know they arn't exactly interchangeable in a linear fashion, but in terms of pure simplistic chipload, I would expect a faster speed at a given feed to give a finer finish ? I'm running a dewalt DWP611 as a spindle, so my SLOW speed is 16000 rpm, with a high of 26900, based on research. I've tried using GWizard as a calculator based on depth of cut and tool geometry and it seems to recommend I max out my spindle speeds for a V-tipped 2-flute bit in the woods i'm using to get the best finish. I ignored it and used a slower spindle speed based on what I've read here.. but i'm wondering why it would recommend the higher speed? I've heard as a rule of thumb that smaller radius bits (which, a V-bit is effectively at low plunge depths) benefit from non-intuitive positive effects from higher than normal spindle speeds. Anyone care to tell me if this is true/why it's true ?