Daniel's Work Thread: Toward a Hackerspace Scanner

[daniel_reetz]

I will share everything when it is ready. The parts are only off by thousandths of an inch in the files listed in the first post - I corrected them because it annoys me, not because it makes a difference in the final parts.

Are you planning to cut a scanner soon?

[bonesgeorge]

Well, actually, we have been cutting the parts with hand-tools, and after doing some math, we've been toying with the idea of making the cuts for those who would prefer to assemble a scanner rather than going through the whole building process. We're not sure yet if we could come out with figures that worth the working ours, but if it's okay, we may even start to ship scanners!

But first we want to see how much was it to get some parts cut perfectly to be used as some clichés, and to make some other calculations to make this decision.

I will share everything when it is ready. The parts are only off by thousandths of an inch in the files listed in the first post - I corrected them because it annoys me, not because it makes a difference in the final parts.

Are you planning to cut a scanner soon?

[daniel_reetz]

Yeah, it's cool - I'll post a longer post about how I feel about all that later today.

New problem. Was just test-assembling this scanner for Sean of Paper Upgrade, and I noticed that some parts fit more tightly than expected. Inside cuts are smaller than I expect, and outside cuts are larger. By .01" in both cases.

My first guess is that the bit is .005" undersize from the expected size of .25". Anyone agree/disagree with this? It's kinda tough to measure the bit directly... from its widest points it is definitely smaller than .25 but by more than I expect. We saw this when we were cutting scanners at CNCRouterparts in Seattle, but it was different then - we were using cheapy router bits from Bosch and not the nice Onsrud cutter I have now.

Anyway here are some pics. Maybe I won't ship this scanner to Sean after all - I'll get some better birch ply and re-cut with a reduced bit size in Vcarve, and that will probably be better.

[dpc]

What's the measured diameter of a plunged 1/4" hole using that bit?

[rob]

Onsrud cutters are undersized? What is the model number of the bit you used? Is the bit at all discolored? Do you have a micrometer? Are you conventional cutting or climb cutting? Have you measured some other part to see if it's off by proportionally larger, or off by the same 0.01"?

I have two Onsrud 57-280's (1/4" downcut 7/8" length) that I haven't used yet. Tomorrow when I get to NextFab I'll try to remember to put it in a micrometer and see what I get. There's also a used one that NextFab has, I'll measure that, too.

In the end, it could just be a combination of the CNC machine, the cutter, and the wood. Cut some inner and outer squares and measure them. See if they end up having different dimensions in x and y, or different dimensions for outer and inner cuts.

But in the end... does it matter? I mean, you've got a creamy pine filling which can surely compress if you just force the parts together. That thing isn't going to crack, and may in fact end up being a bit more solid when the parts are put together slightly oversized.

[daniel_reetz]

It does matter- the parts are not fitting together they way they should. I do want things to be a tight fit, I don't want them so tight that they crack the plys (happening on these pieces).

[daniel_reetz]

Update on me: I am going to be traveling for the next four days, so no real updates will be forthcoming until after that.

[rob]

OK, the measurements on the bits show up as 0.246 inches, so your bit is probably within tolerance. I still can't believe 0.005 makes any difference.

[rob]

I measured the same parts you did on the run I did today. In my file, the outside cut is 1.4982", and the inside cut is 1.4990". The outside cut measured 1.4990" (difference +0.0008") , and the inside cut measured 1.5075" (difference +0.0085"). Both cuts are in the Y direction. Clearly since inside cut > outside cut, these parts will fit. The real question is, why are the cuts different in the first place? Both cuts are in the Y axis, both are using the same bit, and both measure nearly the same in the file. I say inside and outside cuts, but note that these are both outside profiles. I'm just referring to a concave and a convex cut.

Not only that, but since my bit is 0.004" undersized (so the radius is 0.002" smaller than it should be), the outside cut should have been 0.004" larger than 1.4982" (so, it should be 1.5022") , and the inside cut should have been 0.004" smaller than 1.4990" (so, it should be 1.495").

So. The first thing is, the ShopBotters recommend that you always micrometer your bit to get its exact diameter. Good advice. Some shitty bits have been measured to be 1/64" undersized (nearly 0.008" in radius). Now that's a shitty bit.

Anyway, the 1.5022" outside cut turned out to be 1.4990" (difference -0.0032") and the 1.4950" inside cut turned out to be 1.5075" (difference +0.0125).

Note, a different outside cut measured 1.9885", with a file size of 2.0009". Adjusting for the bit, the cut should have been 2.0049", so a difference of -0.0164". This outside cut was in the X direction.

The only thing I can think of is bit deflection, part deflection, and machine flex. If machine flex is not the problem, the first two problems could be solved by cutting all parts about 0.015" oversized, and then running a final pass at the exact size. This way there's not a lot of lateral force to take off the last 0.015".

Other than that, I'd have to carefully debug the machine to find out what's going on. But since the parts all seem to fit... there's not much point

[daniel_reetz]

Pursuing this inside/outside thing a bit... will put links in this post.

This guy on shopbot forums claims the inside-outside discrepancy is due to climb/conventional.
Here's another post detailing climb vs conventional programming strategies and their effects on part quality.