Re: [CAD_CAM_EDM_DRO] Measuring accuracy

Hi Greg,

It sounds like you have been looking exclusively at 3D cam. Many 3D CAM
packages generate a 'Z map' which is like the pixels on your screen,
where the colour of each pixel defines the depth of that point.
Obviously if your stepover (the 'pixel' size) is large then inaccuracies
can creep in.

AFAIK pretty much all 2.1/2D CAM packages use some form of mathematical
equation to offset the drawing contours. With modern computers the
results of these calculations usually have a resolution thousands of
time better than your machine. Some 2.1/2D CAM packages break arcs into
small line segments before processing them(it makes the maths MUCH
easier) but the line segments are kept small enough so that the
inaccuracies are less than your machine's resolution.

The big question is do you need 3D CAM for your work? If you are
generating complex 3D curves then you do but if your part is mainly
pockets, holes and vertical sided cuts then 2.1/2D CAM is better. Get
demo copies of Dolphin, Vector, DeskCNC or my own SheetCam and see if
they will do the job for you.

Les




Gregory Kamysz wrote:

> I just found out that the accuracy of my setup won't be defined my the
> mill but by the software I'm using. I've looked at all of the sub-1k$
> software and they all seem to work the same. There is always some
> rounding in the numbers. Much more tolerance than accuracy in the mill
> would cause. If I write the G-code, it's not a concern but if I use a
> software to generate G-code I'll be lucky to get +/-.05mm before I even
> start the spindle.
>
> The error I'm talking about is when using a .25mm step over and my part
> edge falls somewhere in between a step. Sure, I can cut parts at .01mm
> stepover but who is going to wait weeks for a part to run on the mill?
> If I want something pretty to look at, it's not a concern, but if I want
> to use the part in an assembly, it's unreasonable.
>
> Am I missing something?
>
> Greg
>
>
>
>
> Jon Elson wrote:
>
>
>>>Gregory Kamysz wrote:
>>>
>>>
>>>
>>>
>>>
>>>>What kind of test does one run on a mill to define it's accuracy? Is
>>>>there a standard code or method used to do this? Or do I just run some
>>>>parts and see if I've made scrap or something useful?
>>>>
>>>>
>>>>
>>>>
>>
>>One of the other tests I do, with no special equipment, is checking
>>orthogonality
>>of the spindle to the actual X-Y plane of motion. (My table is banged up
>>enough that there is no way to easily place a flat on the table repeatably.)
>>
>>I wrote a CNC program to move in a complete circle of some diameter.
>>When done, it returns to the center of the circle.
>>I use a 1/2" end mill to cut the circular track in a piece of scrap. I then
>>mount a dial test indicator on a copy of an "Indicol" holder, and sweep
>>the circle. If the height is the same all around, the head is "trammed"
>>correctly. If not, the head needs to be realigned.
>>
>>Because of concave wear on the bottom of my table, I get a saddle shaped
>>curve to the circular track. It is a small, but detectable defect in this
>>65 year old machine.
>>
>>Jon
>
>
>
>
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