Size problems

[tlempicke@rhtc.net]

Just got my R&P machine together and it seems to work just fine. One problem I seem to have is dimensions. When I run the calibrate routine it is right on, however when I cul a piece 12 inches in diameter it is about 1/4 inch off. (Too small)

Here is what I am trying to do-
Generate drawing in Bobcad.
Convert to SBP using converter I got from Shopbot folks
Cut part

I am inclined to think the problem is during the conversion, but I have a hard time seeing how this could be. Any ideas out there?

Tom Lempicke
tlempicke@rhtc.net (mailto:tlempicke@rhtc.net)

[llarson@softdisk.com]

Tom,

Just a thought...are you using a 1/4" bit that is set to follow the centerline of the tool path? If so this could account for the 1/4".

[birdsofplay]

Larry is on the right track IMHO

The CC comand, et al, has an argument I or O or T
which places the bit Inside, Outside or TRUE
(in the middle of the bit). Of course you must
tell the SB what your current bit size is first.

In CAD you can probably make the correction also.
That way the bit would always be where "you" want it regardless of cut direction or bit size.
It just makes the CAD job a bit messy.

What we all need is a "Big Book" of Insights,
Tricks, Techniques and examples.

[bwclark@centuryinter.net]

Tom,

If you are having problems with the converter, I would
be VERY interested in finding out why. With that said, I have been
using Bobcad since October of last year and have not had a problem with
the conversion data.

Are you including any toolbit offsets in your drawing. If you cut a
12" X 12" peice of material with a 1/4" bit, you "part" will only be
11.75" X 11.75" when finished. What you need to do in Bobcad is this:

1. Draw your original geometry.
2. Under the "Other" menu, select "Offset" and then "Chain"
3. A dialog box will ask you Distance. Enter 1/2 of your toolbit size.
1/4=.125 3/16=.09375 1/8=.0625 ect.
4. Now select a side of your geometry. The crosshair will change into
an arrow. By moving the arrow up/down or left/right, will
determine the direction of the offset and the side (inside or
outside) the offset will be generated. The program will then
ask you "Selection OK"? Select Yes if ok, No if the selection
is not right. If you did not select the right geometry, try
again until you feel comfortable with the offset routine.
5. Lastly, now Bobcad will ask you if you want to continue the offset
routine. Select "NO" and you now have an offset of your part.
6. Now you need to generate G-code from your offset, not the original
geometry. So either blank the original geometry out (explained
in the Bobcad manual) or be careful on what you select in the
NC-Cam menu.

Good luck and hope this helps.

Bruce Clark
bwclark@centuryinter.net (mailto:bwclark@centuryinter.net)

[giggalo]

If you want to make a true cut to the size you want " Say cut a 12 inch circle" and have it a true 12 inch, always add half the bit size you will be using, I found this to work in most cad programs, Bruce said it best useing Bobcad.

Ron

[wecarve353@aol.com]

We are new shopbotters. To experiment, we inserted a felt tip pen in the router, and tried to draw an elipse. We programmed a diameter of 20", with a 1.5-to-1 ratio for the elipse. We also chose the true cut. Instead of a 20" x 30" oval, we got 22" x 32".What did we overlook?

[davenadeau]

Bridget,

Did you check your "unit values" with the VU command? Unit values are the number of steps per inch (or a multiple thereof) and they only need to be set once when you set up your ShopBot (or after you install new ShopBot software). It is unit values that allow the tool to move the correct distance when you issue a command, so that may be what caused your ovals to be cut the wrong size. The exact unit values required for your tool are a function of the gear ratio of the motors, the size of the gear on the motors, and the stepping setup of your control box. Fortunately, you don't have to guess these values... the unit values for your tool are on a piece of paper in a manilla envelope shipped with your tool. Contact us if you need these values.

Dave

David Nadeau
Technical Support
ShopBot Tools