Scale X-1 Problem

[Mike_F]

Hi,

I am sure that sometime in the distant past I have read of this problem before but I had never had the opportunity to try myself. I needed to machine a mirror image of a component and being lazy, did not want to resort to running the piece through the CAD/Cam process again so I decided to try using the Scale feature in Mach. I set the X axis scale to -1 and machined the part. All went extremely well and it was only on close inspection that I noticed in each corner, the machined arcs were the wrong way round. I have attached a photo that shows the error. The correct one is on the right. I had to highlight the edge with a pencil as it did not show up too well.

Everything else is perfect but because the arcs are reversed, the tool has taken a very small cut into the wanted part.

Is there a solution to this or should I just not be so lazy? It seemed such a simple way to do the mirror image 

Mike

[Graham Waterworth]

I have seen this with g-code that uses R instead of I & J it is to do with the 180 degree rule and the shortest path.  Could be something totally new as well 

Graham

[BR549]

I think Graham hit the nail on the head. It is the R method that causes the inversion of the arc useing scale to mirror. I just tested both methods. IJ works ok.

(;-) TP

[Mike_F]

Thanks for the replies, I think this is probably the reason. Now the biggie - is there a simple solution to this problem? Is it a function of the post processor to use either IJ or R modes for arcs and is it something that can be easily changed within the post processor? I don't fancy trawling through the whole of the code, looking for these instances.

I am fortunate enough to be sponsored by EdgeCam but have been unable to get onto the training scheme and I desperately need to know how their post processing wizard works. I always thought that wizards were pretty fool proof but the EdgeCam one has certainly got this fool beaten

Thanks again but I still need some help,

Mike

[BR549]

I am afraid you will have to get with edgecam for that help. I was suprised to see they still used the Rmode. Things like this are the reason most Cams moved away from it long ago.

(;-) TP

[Mike_F]

I have just sent an email to EdgeCam with a link to this thread.

Something is puzzling me though. In the same program that the photos were taken of, all arcs are done using the R word and there are thousands of them yet, only three of them appear to be reversed. This just does not seem consistent.

Cheers,

Mike

[BR549]

That is the problem with Rmode the problems only show up in certain conditions(;-) That was the reason IJ mode came about.

Rmode is VERY simple in nature and very easy to understand the basics but the math does not always work out in actual useage.

(;-) TP

[Mike_F]

Just a thought - does Mach change all the G2s to G3s and vice versa when it scales at -1 or does the fact that all X coordinates are negative give the same effect of reversing the direction of the arc. Just trying to get my head around what is happening.

Mike

[BR549]

Don't have a clue as to how it works I only know that it does.  You may want to readup on the Rmode it may explain to you why it does not always work as expected.

(;-) TP

[Mike_F]

I received a very quick reply from Jim Swales at EdgeCam and it is a post processor issue. The fix was very easy and within EdgeCam there is an option to use IJK mode only. Once I had selected that, and re-ran the code generator, there was not an 'R' word to be seen. Now to see if that solves my problem - I'm sure it will.

Incidentally, while exploring EdgeCam a little more, I discovered that if I use 'splines' instead of 'line arcs' for the nc smoothing, I get very few arcs at all in the code but the splines are all made from very small segments. It will be interesting to see what the difference is, if any is perceptible.

Thanks very much for the pointers.

Mike