Dimensioning and Tolerancing

I have my Rev B Folger Delta. Prints great.

I just noticed while printing a clutch alignment tool. My OD in the STL is 0.82" but its coming out to 0.79" after the PLA print. My MendelMax doing the same print gives me 0.81" diameter.

Any thoughts?

deltas are known from having poor dimensional accuracy

Well, perhaps because people calibrate them ad hoc by e.g. fiddling with diagonal rod length grossly out of its true length instead of calibrating it properly using either dc42's or my approach. If your delta does not have play and has towers (almost perfectly) perpendicular to the bed then you can calibrate it very well and in few hours. Just use the approach which works an you will get part accuracy of 0.1 mm per surface rather easily. And if you play a lot with extruder calibration and print slowly then you can get accuracy of about 0.05 mm per surface. But that extra step is probably not worth the additional effort.

Quote
hercek
instead of calibrating it properly using either dc42's

It depends on how many factors you are calibrating (endstops only or including delta radius).

For example, David's 6-factor calibration using his IR sensor gives different results for me:

G32 Calibrated 6 factors using 16 points, deviation before 0.027 after 0.025
M665 Diagonal 214.35, delta radius 105.22, homed height 236.18, bed radius 90.0, X -0.20°, Y -0.77°, Z 0.00°
M666 Endstop adjustments X-1.33 Y0.37 Z0.96

G32 Calibrated 6 factors using 16 points, deviation before 0.035 after 0.030
M665 Diagonal 214.35, delta radius 105.19, homed height 236.17, bed radius 90.0, X -0.19°, Y -0.77°, Z 0.00°
M666 Endstop adjustments X-1.34 Y0.39 Z0.95

G32 Calibrated 6 factors using 16 points, deviation before 0.034 after 0.033
M665 Diagonal 214.35, delta radius 105.17, homed height 236.15, bed radius 90.0, X -0.22°, Y -0.79°, Z 0.00°
M666 Endstop adjustments X-1.33 Y0.38 Z0.95

So, if I using 6-factor calibration, every time I using G32 I getting slightly different dimensions of printed parts.

Edited 1 time(s). Last edit at 03/30/2016 02:46PM by roboprint.

Yes, but notice that the differences seem to be within 0.05 mm. That is quite good. You probably have some play in the printer. Or maybe end stop or z-probe are not repeatable well enough. Or number of other possible reasons. But regardless, the error in your printed parts are hardly bigger than 0.2 mm. If your extruder is correctly calibrated then I would assume them within 0.15 mm maybe even 0.1 mm.
Notice that the thread starter complained about the error of 0.76 mm. Well built delta should not have such a big error.

I'll be doing a new calibration shortly. I just used a 104% scale factor to fix my issue for now. Just needed to get a few tools printed.

Changing the rod length to fix the delta radius seemed hokey to me because i knew it would change the dimensional accuracy

I disagree on the statement that Deltas are not that precise. I have build two Deltas now and the tolerances are phenomenal +/- 0.03 mm easy.
Cpk value is >2.5
I do agree that from what I have seen is that not many do build a Delta precise enough causing bad print results.
- Poles do need to be 100% rectangular to the bed
- Arms (ball to ball) do need to be exact the same
- Design need to be stable
- Arms and Poles do need to be stable. Swinging arms (not stiff enough) are often the cause of big problems. (especially the cheap buys)

Edited 1 time(s). Last edit at 04/10/2016 03:42AM by evandene.