Joinery fit precision

[4DThinker]

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

[joe the gas man]

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! 

[4DThinker]

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

[4DThinker]

This coming week is the last week of classes for the Fall semester 2017.   As is typical, I'm getting a rush of requests for CNC work from desperate students.  One of the more challenging requests is shown below.  The legs are for a small table.  Already cut as 2-side 3D files on our CNCs, the notch in the top couldn't be cut with the part lying flat.  The only workable approach is from the top.    I've carefully clamped the leg onto my variable angle clamping fixture.  I used a digital angle gauge to tilt the fixture 1.45 degrees, which is the side taper of the legs.  Then I had to clamp the leg to the fixture in the vertical position it will be in when assembled.   I added blocking around the shape so once I've got the first leg cut I can simply swap in the next 3 which need the same pocket cut.  The notch they need is a simple 90 degree corner so the legs can be bolted to the corners of her table frame.  Repeatable clamping and toolpath alignment are the critical requirements here. 

 

I have the toolpath ready, but haven't yet gotten up the nerve to cut the first one.  It is hard to tell where the center from the fixture is.  I'm just not yet sure I've picked the best origin point to set for the toolpath.   I'll post a results photo once I've finished cutting one good one. 

 

4D