Offsets CAD, CAM related

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Offsets CAD, CAM related

Post by CVH » Mon Aug 12, 2019 6:39 am

Win7pro 32bit Wininstaller Qcadpro 3.23.0

All, Cad's & CadCams,
As engraver I usually use conical shaped engraving cutters.
Cutting edge diameters down to 0.10mm and in special occasions even smaller.
I still get many errors while offsetting polys or entitiy strings.
I am very intrigued how Qcad can works around that for offsetting to cnc paths.

Prefering a 3rd party CAM included in my driver soft I don't need to create the paths offsets.
My CAM has it's own peculiarities, nevertheless it produces perfect workable paths and pockets for me.

Where I used all these offsets initially for was merely for presentation.
We usually get a drawing with splines-lines these days.
Everyone can draw sharp paths but we can't cut them with a round tool.
(see file, pockets at Eagle's neck)

I first solved presenting this with an approx lineweight.
These use a round pen but are limited in choise.
Doing so, the pockets are represented rounded but fatter too.
It was allready better to use an offset with 1/2 the lineweight.
Later it showed this isn't allways correct for inward and/or outward scharp/round edges.
Not correct or rather not preferable or unnecessary detailled.

Later I constructed a sceme with offsets and counter offsets.
Depending the result wanted chosing rounding or hard for the first operation and rounded for the second.

Super fine engraving points don't like to cut in a narrowing corner, semi-death stop and reverse.
Engraving is one, that takes time, re-dressing an engraver point is an whole other issue.
By chosing the offsetting distance a bit larger then the actual cutter radius in that depth we can try to keep as much scharp details as possible and have a continuous tangentially connected cutter path.
At present I use these offsets to export pockets to my CAM.

We learned not to set the spline approximation too high as in the example in this report: ... sk_id=1765
Ever since I am cleaning up offset paths.

On my wishlist are:
- Round Poly
- Select self-crossing

I recently set aside a drawing item in the file include as example and tried some offsets.
As I think this isn't as demanding as in the earlier report the results strike me.
These are not too tiny to detect.
Maybe this is a good example to digout where Qcad fails

Eagle offset faults.dxf
(431.81 KiB) Downloaded 276 times

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Re: Offsets CAD, CAM related

Post by CVH » Mon Aug 12, 2019 10:02 am

Additinally I use offsets to check for uncut areas of my cutter sequence.
Pocketing with a very fine cutter point and about 1/3 stepover will take to much time.
Even worser, such a cutter will not last that long.

In the picture the cleared areas at a depth of 0.05 by my actual used sequence.
I am emulating how my CAM is exporting cutterpaths.
The other way would be to import the actual G-code as a dxf.

The design was allready adapted as prior noticed with a minimal rounding of 0,06 so the 0,10 cutter would not get stuck in the corners.

Every cutter leaving at least something to cut for the next cutter in sequence.
(Not cutting and rather touching/rubbing will worn a cutter much faster.)
from the inside out:
0,50mm pocketting, 0,30 offset, 0,20 offset, 0,10 offset. A good rule of thumb is times sqrt(2).

The gain of superseding and replacing the pocketting job with a 0,70mm cutter did not justify an additional cutter swap.
And mostly because other parts of this design could not be cut by a 0,70 cutter.
At a different place in the design I even needed additional 0,30 pocketting because the 0,50 would not enter.

By offsetting I can inspect the cutted areas at the bottem giving the cutter tip diameter.
Again, more offsets.
When needed I can add an additional wider offset with the same cutter as pre-cutting avoiding pocketing with a very fine cutter.

Just to sketch my workflow.
Did not find any affordable CAM that keeps my cutters whole like my approach.
Did not try un-affordable ones.

Eagle Cutter sequence.png
Eagle Cutter sequence.png (29.61 KiB) Viewed 3843 times

John Hyslop
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Location: Melbourne - Australia

Re: Offsets CAD, CAM related

Post by John Hyslop » Tue Jun 22, 2021 2:35 am


Not sure what most of your comments mean "soft & hard" ? :?:
Anyway here is the Eagle with offsets 0.3 & 0.04 for comparison ..
This is from AutoCad 2020 I'm stuck on this perpetual version as I gave up
subscription to updates 2 years ago and now using Qcad which I'm very happy with
Subscription to Product Design Suite Did Cost Me AU$3600 a year e_ugeek
So I'm very forgiving on the minimal amount of rework I sometimes need to do in Qcad..

PS This was the deleted post that was meant to be a PM , I've reinstated it so people don't get confused when reading the reply..
Eagle offset-Acad.dxf
(580.78 KiB) Downloaded 4 times
Last edited by John Hyslop on Tue Jun 22, 2021 9:09 am, edited 3 times in total.
IF IT IS TO BE IT IS UP TO ME [ Ten most powerful 2 letter words ]

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Re: Offsets CAD, CAM related

Post by CVH » Tue Jun 22, 2021 4:27 am

John, thanks for the input ...

'Soft' is short for 'Round Joint', inside corners are round like a cutters does.
'Hard' is 'Mitter Joint' like the ones you added so kindly from ACAD.
(They are perfect in global and also in very minute detail)

What I first do when a customer gives me a design is a representation of what is feasible with a round cutter.
In this case a sub 0.01mm engraving point.
For engraving points: The deeper, the larger the diameter at the surface.
The top finishing pass only a few mills deep having about a 0.08mm diameter at the surface.
  • Offset inside, D/2=0.04mm, 'Round Joint' (best of Clip/noClip/Misc Alternative) => Cutter path
    Osset outside of path, 0.04mm, 'Round Joint' (best of Clip/noClip/Misc Alternative) => Actually cutted shape

Many customers initially don't realize a cutter can't cut hard inside corners. :wink:


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