If a 25mm cube prints as 25.04 it may not be because the scale is off. You would only know that if you printed a second 50mm cube and it was 50.08 My guess is that on your printer the 50mm cube would be 50.04
The printer should have both a scale factor and a bias. you would hope the scale is 1.000 and the bias is 0.000 Let's assume your printer has a scale of 1.0 (it is perfect) and a measured bias of 0.04 that is added to all surfaces. If so them it would print a 25mm hole as a 24.92mm Actually it is worse. The plastic shrinks when it cools and there are bumps. And when a hole shrinks there is nothing but air to hold back the plastic that is shrinking like a rubber band. On the outside the other plastic holds it from shrinking to much. So inside and outside diameters shrink differently. The other effect that makes hole under sized is that, let's say you are making a left turn with the print head trying to follow a curve. There is going to be more plastic on the inside of the curve then the outside. Inside diameters will have a little to much plastic. Then add in those surface imperfactions and shrinkage. My conclusion is that if you care about exact precision, put the part in a lathe and use a boring bar to remove the 0.2mm of extra plastic. PLA is very easy to machine if you go very slow so it stays cool. If you don't care about being exact use sand paper over a round form to enlarge the hole. Boring is an easy operation if your lathe still has hand cranks. Or change the hole size and reprint the part. Most printed parts require some post processing if you care about precision and appearance. The other solution is to change the hole size in the CAD file. On Tue, Aug 4, 2020 at 1:49 AM Gene Heskett <[email protected]> wrote: > On Tuesday 04 August 2020 03:42:59 andy pugh wrote: > > > On Tue, 4 Aug 2020 at 01:45, Gene Heskett <[email protected]> wrote: > > > While I was out, getting a legal thing underway, the 6807 bearings > > > showed up, but they do not fit, the pieces that go inside need a BIG > > > hammer, and where they fit in the main body shell is too small by > > > half a mm. > > > > What is the hole size in the CAD? Did you ever print that calibration > > cube? > > yes to the latter, and its average is about 5.02mm per 5mm cube but is > not linear. 25mm measures 25.04, and if anything, the recess should be > big, not so tight it can't even be started with a small hammer. And the > hole size isn't shown in freecad, or I don't know how to display it. > I'll have to chuck and center up the main body in a 4 jaw and run a > boreing bar thru the hole, probably an HSS one, or for threading as a > normal carbide chip isn't sharp enough. I'll try that later today. It > not quite 5am local time ATM. > > Cheers, Gene Heskett > -- > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author) > If we desire respect for the law, we must first make the law respectable. > - Louis D. Brandeis > Genes Web page <http://geneslinuxbox.net:6309/gene> > > > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
