On Tuesday 04 August 2020 05:47:46 Chris Albertson wrote: > 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 > Thats been my impression too. And there is no bias adjustment. only scale, but there is a definite, easily lost in the measurement noise, bias.
> 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 About what its done or worse. > 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. Exactly. > 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. That is a 200 meters of pla part, 42:12 hours:min to print. I'll bore it on the lathe as my linuxcnc driven hand dials can go .0001 in radii per click. The other two parts that fit the ID of this bearing, can be driven in with difficulty, so cleaning that surface up would be beneficial too. The difficulty level there makes me think the assembly screws would break it without exerting enough force to bring the parts together at the center of the bearings bore depth. > 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. freecad can display the .step, but the .step file isn't the src. > 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 designed to brute force > > steel, isn't sharp enough for this except the single tooth thread > > cutters.. 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
