Re: Basic questions

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Gordon Anderson <GAA@...> wrote:

>
> Hello, I have been lurking on this list for sometime and I have a couple of dumb questions that I hope someone can answer for me or point me in the right direction. I am very close to buying a DIY CNC router kit like the blueChick. I have read a lot about the machines and control software but my questions are pretty basic and most likely dumb, here goes:
> 1.) How do I hold material on the table? I will be cutting thin wood, liteply and balsa wood. I will be cutting through the wood to cut out parts. My question is how do people hold the material in place and what do you put under the part you are cutting to make sure you do not cut into the bed of the machine.

Cutting thin materiel with a rotary tool is a challenge. You have to:

1, Sandwich it in a more rigid material (sacrificial material)
2. OR have a pressure foot (spring loaded) on the Z to hold the material down (creates problems if pieces don't have "tabs" to hold them in place after the cut because it will drags things around as they break loose.
3. OR have a vacuum hold down fixture that will pull the material down and even hold the cut pieces in place.

Use a "down-cut" spiral cutting bit so it won't tend to pick the material up as it cuts (on any of the methods)

Of the three I would go for #3 if you intend doing more than one or two of the patterns.

The good news is if you have a CNC router table you can build your own vacuum hold down fixture

A lot of that type cutting is done with a laser so there are no up and down or lateral forces to speak of but they still use the vacuum hold down.

> 2.) I have old drawings with patterns for formers (mostly from model airplanes) that I would like to convert to gcode files to enable cutting the parts. Is this possible? If so where can I learn more about the process. I hope I can scan them in and then convert them to gcodes.

To get old (printed) drawings into a CAD file you can generate G-Code from you must "digitize" the work. That means scanning it (which turns it into a bitmap (collection of dots or clusters of dots) and then tracing that with a vector editing program. Vectors are mathematical "lines and Arcs". You derive the G-code from a vector file.

You hear about "auto trace" programs that will automatically trace a bitmap for you. They work best on clean artwork scanned at high resolution and with distinct density or color changes. Obviously the best is a B & W scan. It still has to compromise because even the best scan is a series of dots.

If your subject matter is too big to scan with normal desktop scanners it gets harder. You either have to have a commercial graphics company do it or you have to do a poor man's scan and use a digital camera. That method requires you to do sizing after the trace (easy to do with a vector object) but may leave you with distortions and parallax errors you will have to deal with.

Since precision for the fit of parts will be important your best approach is to take control of the tracing yourself. It takes longer to get proficient with the tools, but if you learn how to scan, import and hand trace a file you will get better results. You can concentrate on the areas that interlink where tight tolerances are required and you can build files that cut correctly.


Here are some suggestions:

Download a copy of the *freeware* vector drawing tool INKSCAPE. Get some scanned files and import them and try their auto-trace. You may need to futz with settings to get the best results. Learn the drawing tools especially how to use the node edit tool. Your goalis to get smooth lines and arcs and to hold tolerances in mating parts INKSCAPE saves and uses a file format called SVG (Scalable Vector Graphics) The accuracy is as high as you want to make it. There are powerful tools like the weld and trim tools. Being able to hand trace a bitmap you import is a level you would like to get to. BTW Inkscape will import all kinds of formats including CorelDraw CDR formats.

Get a copy of SheetCAM TNG. A demo will let you do simple files up to about 150 lines of code. Full license is $175.00 It will DIRECTLY import the SVG file for you to define HOW you want it to cut.

SheetCAM will do several things for you it lets you define the tool you are using (diameter and flute length). It will let you cut with a an offset based on the tool diameter (a must for accurate sized parts). It will let you define cut direction, feedrate and even let you put in "tabs" in a piece. You can also build a cut file of multiple parts and nest them to cut from one sheet of material.

Once you have the cuts defined you POST (process) the file to match a specific machine or control software (like MACH3) because G-code is a "flexible" language that can vary across different controllers. Done right you should be able to do the file in SheetCAM in a few minutes and punch one button and get a perfect file. Like all things, there is a learning curve and the length is proportional to your experience with computers and drawing with a mouse or tablet.

This is a good place to start and your questions are exactly what is appropriate for this list.

Find out a lot more about the software listed at:

www.sheetCAM.com
www.MachSupport.com
http://inkscape.org


Welocme and keep the questions coming!

TOMcaudle

>
> I hope these questions are appropriate for this list and appreciate any input.