Blade Offset

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Blade Offset

Post by dheis on Thu Jun 27, 2013 4:05 pm

I am cutting corrugated plastic (4mm) with the #2 Exacto blade using a D2. I've shortened the blade and broken the tip as it shows in the video. You are suppose to set the offset a 0.140 but it seems kind of arbitrary given that you could set the tip to almost any location. Is there a method to determine the offset more accurately? Like swing the holder and see how big the diameter is?

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Re: Blade Offset

Post by DonekTools on Mon Jul 01, 2013 11:34 pm

There is a pretty easy test.  On a shopbot, if you run a CC command it will execute a circle that starts moving in the +x direction.  It's important to orient the blade at 90 deg to the starting movement when performing this test.  So on a shopbot you would orient the blade as if it was cutting in the +y direction.  If you have the machine move through a circle with the same radius as the tools offset (with the tip just touching the material surface), it will complete the circle pointed in exactly the same direction it started.  If it over rotates, the radius is bigger than the offset.  If it under rotates, the radius is smaller than the offset.  Adjust the radius of your circle until it completes in the correct orientation and you have the correct offset.

You can also look at your corner actions for similar behavior.  Turning too far at a corner, means your offset is too big.  Not turning far enough, and your offset is too small.

https://youtu.be/63u9idcyVhA?t=7m55s

I hope that helps.
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Re: Blade Offset

Post by AnthonyW on Tue Nov 03, 2015 12:53 pm

For anyone messing around with a drag knife on a ShopBot, keep in mind that the input for a CC command is the diameter. The blade offset is the radius. You will need to multiply the blade offset by 2 and put that into the fill-in sheet when you type in CC.

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Re: Blade Offset

Post by pjtezza on Tue Mar 20, 2018 6:33 pm

I don't have a ShopBot and I wasn't sure the GCode I've seen elsewhere would work on my CNC, so I did this with Vcarve by drawing a series of 3" squares with varying cut depths and offsets and cutting them into some thick white paper. It took me 8 squares (I ran two jobs with 4 square each) to find the minimum cut depth and sharp, square corners.

I can't see an easy way in Vcarve to add a lead-in for each square to get the blade oriented prior to joining the part cut path, however.

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