Relearning old misteaks

[Gordon]

Sometimes short cuts do not save time but actually add time.

I have been working on Paddleduck two cylinder engine. Plans are free and available as the original sketches or as DXF/DWG. I downloaded the DXF drawings and loaded them into my CAD program. The drawings are metric and I want inch sizes so I rescaled them 1/25.4 and everything looks good. I also have been trying to set up and learn CNC on a small Grizzly mill and this looked like a good project to experiment and learn on. Unfortunately what is close enough for manual work is not necessarily close enough for CNC. 20MM=.7874in which is generally close enough to 3/4" (.75) but it is about 1/32 off and when the mating piece is also 1/32 off in the other direction it means things do not line up by 1/16. When it is loaded into a CNC program the program just does the cut or hole where the drawing placed it without compensating for the slight error in conversion.

I should have just redrawn the whole thing from scratch instead of trying to use the DXF. Lots of little variations which I would have picked up had I been doing manual milling.

The good news is that I am learning a lot about the CNC mill. I can now make a part which would have taken me three hours manually and after experimenting with the program, learning how to run the program and remaking the spoiled parts using the CNC mill I can do it in only three days. Don't laugh, it was five days the first time. Good thing this is a hobby and not my means of earning money.

 

[Hopefuldave]

Ahhh.... 20mm is nowhere near 3/4", 19mm is.close!

I don't think.you can scale between metric and inch sizes like that, setting the inch increments to 1/32", you'll end up with parts that don't fit....
You need to make the conversion with dimensions accurate to the thou", and work to that accuracy, or better work in metric units (like.the majority of the world's engineers) and.use the drawings as they come

 

[Wizard69]

If you are using a CNC machine why would you even bother to convert to English dimensions? Just set the CNC machine to millimeters and let it go. Sure you will need to do some work with your cam program if it isn't too intelligent about inch and metric dimensions but in the end it should be far quicker.

I can understand the conversion if you are doing manual work and all your tools are inch only. Your CNC machine though shouldn't care one bit.

 

[Cogsy]

The drawings are metric and I want inch sizes so I rescaled them 1/25.4 and everything looks good.

This isn't a rescale. If you've done 1 inch = 25.4mm then that's an exact conversion. As long as you stick to that, all holes, mating parts, etc will line up perfectly.

 

[Gordon]

You will get no argument from me that the imperial system is nonsense and the metric system is superior. However I live and work in the part of the world which uses inches instead of millimeters and the stock and the tooling readily available is in inches regardless of what the rest of the world does.

My biggest problem is with stock sizes. For instance the base is 50 MM wide. On something like the base that dimension is not critical. Two inch stock is available and is 1.968 MM so that is close enough and what I will use. However a hole centered at 25 MM from the x=0, y=0 is now .984" and not centered in the piece. When the DXF drawing is loaded in the CAM program the hole is placed off center unless I make a point of moving it to center in the 2" piece. That is why I said that I would have been better off to just redraw the piece at 2" and center the hole at 1". My trying to use the DXF drawing to saved time actually ended up adding time. That follows on many parts. 5 MM thick stock may be readily available elsewhere but I am stuck deciding whether to use 1/8" or 3/16" thick stock or maybe milling it down to 5 MM. If I use inch sizes I have to compensate somewhere for the difference and I would have been better off to have done this right in the first place. Thus my remark that shortcuts do not always save time and some of us have to relearn this from time to time.

As far as the rescale/resize is concerned, if I load the DXF into my CAD program set to inches when I measure a 10 MM piece it tells me that it is 10" so resizing to 1/25.4 now makes it true size. I am not sure if that is a function of my CAD program or the original CAD program but that is the way it works with the CAD program which I have been using for the last 20+ years. I think that it is the difference in the way Autocad handles entities and the way my program (VisualCADD) handles the entities. The fact is that when I tell the program to rescale x=1/25.4 and y=1/25.4 the parts come out true size.