On 09:52 AM 2/15/01 -0800, Hamid A. Wasti said:
>Andrew J Jenkins wrote:
> > I'd still like to know some more specifics regarding optimizing this new
> > rule. My 31's DO live a much shorter life than the 62's, on average, 
> and I'm
> > sure everyone else would ultimately benefit from the information too. A 
> rule
> > is only useful if its substance can be decoded...
>I am not sure what exactly you are asking me to elaborate on.

The nitty-gritty details...

I wasn't being clear, and it seems from you last reply  to Geoff, that what 
you're talking about is high-speed, mass manufacturing, not one-offs, or 
even small runs.

For instance, my tooling runs at  24Krpm, (no, not suitable for tef., 
ceramic, or flex materials..) with a (62mil tool) contour-routing speed 
(PCB cutting tool) of about 30-35 inches per minute, for 62 mil thick FR-4 
material, faster for 31mil material, and significantl;y slower when using a 
31mil  dia contour-router bit, but I have never seen any obvious evidence 
of the chatter problems you're talking about. The tool (or more precisely, 
its software,  and stepper motor controller module) uses ramp-up and 
ramp-down machine code instructions to control the various steppers 
start/stop behaviors involved in the translation process, so perhaps you're 
also talking about a more primitive translation system, I don't know...

What I meant by my question is that
a) I accept what you're saying wrt to the problem, even though I don't 
necessarily see it, because it makes _sense_ to me, from my knowledge of 
b) I accept that it _could_ result in a change to mfg costs, especially in 
large runs (again, if a then b)
c) Because I run a milling machine to produce a great number of my designs, 
I want to understand how to __apply__ the rule, not just in a qualitative 
manner, but in a quantitative manner, ie,

If tool radius R1, then min_path_radius1 = ???
If tool radius R2, then min_path_radius2 = ???'
If tool radius R3, then min_path_radius3 = ???''
result: mathematically, a polynomial (perhaps too much to ask) or 
experientially another _rule-of-thumb_  for specification of min 
milling-path radii to one's design(s). I figured that if you knew enough to 
make the warning, you could also answer that question.)

Perhaps the answer is a simple as "If you use a 62mil dia  tool, use a 
minimum path radius of 63/2 mils)", and perhaps that's a naive assumption o 
my part, but it's a logical outcome of the simple statement "Make the path 
radius larger than the tool", though perhaps no better in the real world 
than making it equal. On the other hand, I could take your suggestion to 
the other extreme, saying instead that then all radii should be measured in 
multiples of the tool radius, like twice, three times, ten times, etc. But 
then I might just cost myself by adding unnecessary post-process hand work 
by doing so...(generally speaking, I don't give a hoot about a 31mil 
offset, and so don't generally worry about that small a radius)  Obviously, 
a radius of curvature of 31.0001 mils isn't going to change the dynamics of 
the situation very much, in fact virtually nil...it's pretty obvious that 
the stresses exerted upon tooling will be almost exactly the same as in the 
case that it was dead-ended.

So, where specifically do we find that there's a benefit to designing this 
way, which outweighs the reduction in the "rectangularity" of the hole?

Does this make a little more sense than my first question?

As you say, mfgs are responsible for determining the best routing speeds to 
maximize their profit margins. Routing speed, material thickness, material 
type, copper weight, tooling flute #s and type all play a role in 
maximizing mill speed and performance. Generally, if one wants a square 
corner, one should simply specify a square corner, and let the pros worry 
about it. Unless I'm not mistaken, the costs only begin to appear in large 

As a designer, I'm just not sure how much one should worry about it. As a 
mfg, one should already know that there are a great number of variables 
that come into play in making the equipment work properly to one's 
advantage, as well as to that of their customer.  I'd like to be corrected, 
or confirmed in that assumption.

I hope that I've made myself clearer.

Sorry for the long-winded-ness of my response...


Andrew J Jenkins. NCMR @ NASA-GRC

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