I read a number of threads related to circles not coming out round but none of them seemed to explain exactly what I am experiencing so I thought why not add one more…
I am working on cutting out the bottom of an ammo box for 9mm rounds. This picture is of my first test cut. It came out pretty good and the holes were accurate to my model but they were a bit small.
After doing a couple more tests in pine I found that the idea hole size is 10.2mm. With that all figured out I moved on to cutting it out of cherry. Just to be safe and to improve surface finish I dropped the feed on all of the operations by 100 or 200 mm/min (depending on what the operation was).
I cut the part out of cherry and the surface finish was acceptable and everything seemed to go well. I did some minor cleanup sanding and then starting inspecting the part.
First issue… The thing was not square. It was about 1mm out of square over a distance of ~200mm.
Second issue… The holes were not round. Cutting a 10.2mm hole was previously giving me about .5mm on all sides of the bullet. Now in the y direction I have zero clearance (it is actually an interferance fit) and in the x direction I have about .75 - 1mm on either side.
I adjusted the machine to square by snugging of the x/y tensioner bolts on the center section and manually setting the steppers to be square before powering the steppers on. Now the outside of the box is square.
I did a bunch of tests boring just the holes in cherry… slowing the operation wayyyyyyy down (from 600mm/min 100mm/min)… Doing a repeat pass to account for tool deflection… moving the part to the far back corner of the machine (that area rarely sees any action)… rotating the piece of cherry so the grain is running along the opposite axis.
The only thing that seemed to make any difference at all was rotating the wood. Things were slightly improved but not enough to say it wasn’t just a fluke. After all of this, I noticed that the wood was burning and the bit was heat stained so I ran to HD and picked up a new freud spiral up cut 1/4" bit.
I ran a test with the brand new bit in pine running real slow after making sure the gantry was square and the result was a box with square sides but holes that are not round.
You can find pictures of my build here New MPCNC build in Detroit.
Parts were all printed by a friend of mine. I’m running a RAMPS 1.4 with the standard (non-endstop) firmware and some no-name A4988’s I got on amazon in a printed case with two 40mm fans.
The part was designed in fusion 360 and CAM was done in fusion as well using guffy’s post processor.
When I get home from work I can upload some pics of the holes I am getting now.
I am running out of ideas so any suggestions would be much appreciated.
