Shape is one size in Designer and another when cut.

I am making some holders for some tools and want to cut recesses in MDF the shape of the tool. I'm using the 1/8 straight carving bit and when I make a rectangle in designer that is 2.5x0.75 it comes out 2.64x0.88. A hole that is set to .782 dia comes out at .92. Each of the shapes is off but it isn't consistent. They're off by .138, .122, .140, .130, .137, .139. If it was off by .125 I would understand it's the diameter of the bit but the inconsistency is driving me crazy. The couple of rectangles I have set up as regions so I don't see where to set an offset or move the bit to the inside or the outside. Do I have to do the math every time I want something to come out exact? I've attached the MPC. I did switch the circles to DRILL to see if that makes a difference but I won't get to carve that until tomorrow.

Thanks!Attachment 88447

Years ago I made a Slot Board to quickly take the guess work out of slots.... You could do the same.... Remember the wood will get bigger and smaller with Humidity.

The Resolution of this HOBBY machine might be messing you up.... Do you put tape on the bottom of the board to let the brass roller have something to bite into?

When you define a carve region it is where the CENTER of the bit will pass, so the carve region will be larger than the defined area equal to the diameter of the bit.

The inconsistency could be due to runout, possibly due to the tightness of your y and z bearings,

I design and build wood clocks where the center holes on the wheels and pinions (gears) must be precise, because I press brass tubing into them and it must be a tight fit. I found that the Carvewright does not drill precise size holes. In fact, as a test, I drilled a hole with a drill press using a 1/8" drill bit and then with the drill press a 1/8" hole using the Carvewright 1/8" cutting bit. The drill bit hole was a tight fit to 1/8" brass tube, but the Carvewright bit fit was loose. So the 1/8" cutting bit is a bit larger than 1/8". Check the actual diameter of your 1/8" carving bit.

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Originally Posted by Zap

I am making some holders for some tools and want to cut recesses in MDF the shape of the tool. I'm using the 1/8 straight carving bit and when I make a rectangle in designer that is 2.5x0.75 it comes out 2.64x0.88. A hole that is set to .782 dia comes out at .92. Each of the shapes is off but it isn't consistent. They're off by .138, .122, .140, .130, .137, .139. If it was off by .125 I would understand it's the diameter of the bit but the inconsistency is driving me crazy. The couple of rectangles I have set up as regions so I don't see where to set an offset or move the bit to the inside or the outside. Do I have to do the math every time I want something to come out exact? I've attached the MPC. I did switch the circles to DRILL to see if that makes a difference but I won't get to carve that until tomorrow.

Thanks!Attachment 88447

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There is no "1/8 straight carving bit" all carving bits are tapered and have a rounded/pointed tip. The cutting bits are straight and have a flat bottom and are used for doing cuts.

The 1/8" carving bit has a rounded tip but otherwise is straight.

Attachment 88449

The 1/8" carving bit is ONLY for soft material such as Balsa, Basswood or Foam. It is a straight edge bit with rounded tip. It breaks very easily when used on harder materials than suggested.

ah yes, it's a cutting bit. just typed the wrong word. :)

This is MDF so I expect minimal amount a wood movement. But yep, I use blue tape every time. My regions are off in both X and Y so I don't think this is the roller skipping. I'm thinking the variation could just be a bit of random skew when using the caliper. I didn't have time to get out and try it again today but I'll shrink the regions by 1/8 and try again.

Thanks!

Are you using the 1/8" cutting bit to carve a carve region? If so you are trying to fool the machine and it is not working out for you.