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4DThinker

Joinery fit precision

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Most of what I'm using a CNC for is to cut joinery for furniture projects.  This almost always involves two parts and two separate cut files. The positioning of the cuts and fit between the two parts is very important. 

 

I use several strategies for where to zero out the bit(s) used and they vary with each unique cut.  I consider how I'll be clamping the parts down.  I consider the orientation of the parts on the CNC bed for best access and clamp avoidance.  

 

I  usually cut one half of the joint on a final part, but cut the mating half on a scrap to verify fit.  Not all router bits will be the diameter they claim to be. A bit that is larger than you've told the software will leave tenons smaller than expected and mortises larger than expected for a very loose joint.  A bit that is smaller than it advertises will leave a joint tight.  Often too tight to go together with room for glue.  If my test fit is flawed in any way I'll revise the vectors and recalculate before cutting a second test sample. Only when I am absolutely certain of a good fit will I cut the second half of the joint on the final part. 

 

Once I realize a particular bit is not the size it claims I'll cut a simple slot with it in some scrap then measure the width of that slot with digital calipers.  In Aspire I can edit the tool database and change the default diameter to a more accurate measured value.   It becomes important to keep track of any odd-sized bits.  They aren't useless, but need to be accounted for in any project they are used for. 

 

4D

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I had an issue with some chinesium 1/8" endmills that were always giving me oversized cuts. even after measuring the bit with a digital caliper. So, I just kept telling v carve pro the bit was an increment smaller until it worked out.  The .125" bit needed a setting of .11" to get the results I was going for. Not sure how this works, but if it works I dont futz with it! 

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Your experience doesn't surprise me, Joe.  Often with bits that come from countries where the metric system is used I'll find the real dimensions of an imperial bit is closer to a metric dimension. 1.1 inches is very close to 28mm. 

 

4D

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