Bit and program accuracy

[richard_saylor]

This is a two part problem.
First off, I'm having troubles finding upcut spiral bits that are actually .5 inch. Most range from .49 to .495 inch. This includes Bosch, CMT, Whiteside, MCLS and various other brands. Yes, my caliper seems to be accurate because it measures the shaft size on all my .5 inch bits to be exactly .5 inch. The bits are bought from reputable dealers and are not marked as seconds.
Secondly, what kind of accuracy should I expect when cutting a pocket toolpath. Knowing that a bit is .495 inch, I can either designate a new tool with this cutting diameter, or selecting a virtual .5 inch tool and alter the drawing to obtain the proper size. Using the second scenario, if I want a circular pocket toolpath of 2.2 inches with the .495 inch tool, wouldn't I give the pocket a 2.21 inch diameter? With experimentation, I must give it a 2.226 inch diameter toolpath to obtain the 2.2 inch hole.
To test the accuracy of my inside vs. outside caliper, I've measured the tip of the tool and then cut a .25 inch deep path through hardwood and measured the width of the cut with the inside caliper. The widths are exactly the same.
Any ideas?

[ssflyer]

Unfortunately, I've been finding this to be more prevalent, lately. My Onsrud bits seem to be a bit better, in this regard. So I simply add my tools to my database for the tool, itself, with the correct diameter. Kind of a pain, when you've got several, with differing diameters...

[richard_saylor]

A follow up on part two of my problem.
I created an End Mill tool of .495" to model exactly the diameter of my physical End Mill. I created a 2" circle. Using the .495" tool I created a Pocket Toolpath inside the 2" circle.
After routing the circle, I measured and the pocket is 1.98". Am I expecting greater accuracy than the ShopBot can accomplish, or is it something else?

[MogulTx]

You have to also know how the material is going to react to your bit. If you tell the software that it is a .495 bit, there may be the issue of not measuring the bit to the most extreme diameter of the bit. ( In other words, it may be cutting a larger path than what you are expecting, and so it may, in a cutting-outside-the-line application, leave you with a slightly smaller diameter. If cutting inside the line, it may leave you with a slightly larger diameter.

Sometimes the material will encroach /swell into the cut path due to swelling out of the grain of the material being cut (foams sometimes do this, as do woods- especially those that are still green or have a higher than expected mositure content.) Also: many things will temporarily expand from the heating /trauma of the material from the cutting process...

You may need to investigate a little bit to figure out what issue you are having and to pin it down a little further!

I measure bits all the time that measure out at .116, but that cut like .125. I am trying to figure out how it does it. I figure it is becasue the spiral is slightly off center and I am not measuring the true diameter of the bit, but rather the offset bulk of the cutting faces, which are not necessarily symmetrically laid out on both sides of the centerline of the bit...

[richard_saylor]

As stated in my first post: "To test the accuracy of my inside vs. outside caliper, I've measured the tip of the tool and then cut a .25 inch deep path through hardwood and measured the width of the cut with the inside caliper. The widths are exactly the same." From this, I would assume that my measurement of the bit is right on.
As far as type of wood and any resulting swelling, I get the same results from pine, bone, plastic and Ipe.
My need for accuracy comes from doing inlay with various materials and other interference fit parts for my guitar business. The tendency for the machine to cut pockets too small and profiles too large, compounds the error. I'm constantly having to experiment with my drawings to get things to fit.

[gc3]

spindle runout, bit deflection, machine backlash could also be factors

[richard_saylor]

"spindle runout, bit deflection, machine backlash could also be factors "
Doesn't it seem that all of these would create a larger cut rather than a smaller one?

[bob_reda]

If it is consistent it could be that your unit values may need tweaking. Also, I don't know what software you are using. I use aspire and for inlays I use the same bit for both the male and female.

Bob