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CNC advice thread?


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Guys, I'm rebuilding my home build CNC.


One thing I have problems with is this. I make the tool paths to cut the panel, maybe have to turn the panel over to cut recesses, paint it, then replace it to allow the engraving path.


If I cut out a panel I have clamped or used double sided tape to hold the large piece I'm cutting it from, but how do I get the panel in exactly the same place every time so the underside and most important, engraving is perfect??


Thanks for any tips


(not a laser, a router btw).

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I dont know what CNC you got, and or how precise it is.


If its a reliable machine, and you got homing switches, you can home it and it should know its bearings.


If its a china model, the thing will never be precise, not even if you got homing switches.


But there are other ways you can do it. Start by cutting a angle, where you add som numbers on each axis of the cut line, so you dont have the angle all the way out on the edge of the cutting area.


Remember those cordinates, it would be smart to have the same numbers on each axis, fx X-20 Y-20


And remember to cut a cirkle in the corner of the angle, so that you can place Square object all the way up the corner.


Then you paint the thing, and when its dry you engrave a cross at a specific cordinate fx the middle in X-10 Y-10.


Then when you place you object, you put on your engraving bit, and position the bit right in the cross, and then you can read in mach 3 or whatever you use what the machine Thinks the cordinates is.


Then compensate these cordinates in your CAM software to get this position.


And there you have it... It will run precise...

Its abit annoying to to that every time, but it Works.


Hope that helped.






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Mr. Burns, reverse cutting requires the use of registration holes. I always mill a .125" diameter hole somewhere on the workpiece. When cutting the first side you can establish your zero points wherever you want. When you cut the backside you need to first, create your g-code file using that hole as you your zero point, then use a .125 steel pin in your collet and set your zero point on the mill. Doesn't matter where as long as you have room for the milling operation. You then move your work piece into position under that steel pin and when they are lined up perfectly you have your work at zero and you can start cutting.


I tend to push my workpiece into the pin and drop it down to the table as I use double sided tape so it hard to slide the workpiece around.


Another more complicated method is to set 4 steel pins at each corner of your table. Before doing anything else,you cut 4 holes in your workpiece the same diameter of your pins. With the pins fixed to the table you can now drop you workpiece down on to these pins. This allows you to flip the workpiece easily. The caveat is your zero point must be at the centre of the 4 pins and when you work out your g-code file you must centre your drawing at the same point. Think of it this way. The zero point on your drawing should allow you to mirror the drawing using the zero point and it should register perfectly regardless if it's back or front view.


Was that clear enough? I can get into more detail if you need it. Btw, the accuracy of your machine will dictate whether in fact you can do this. You must have excellent repeatability, otherwise you will never get your registration to be accurate enough to do this reverse cutting operation with any degree of success.


John W

aka WarHog.


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