Every precision part in this project (the dome foot, the friction coupons, the flexures to come) starts on an Elegoo Mars 5 Ultra, an MSLA (masked stereolithography) resin printer with a 7-inch 9K mono LCD and an 18 µm pixel pitch (8520 × 4320) on a 153.36 × 77.76 mm build plate [14]. Out of the box, a resin printer is not a precision instrument, but it becomes one after calibration, which in resin printing mostly means finding the right exposure time for the specific resin being used.
The exposure time decides dimensional accuracy. Each layer of an MSLA print is made by shining UV through the LCD into liquid resin for a fixed number of seconds. If the exposure is too short, the layer does not fully cure, so fine features collapse and parts detach mid-print. If it is too long, light bleeds sideways past the pixel mask, so holes shrink, gaps close up, and every dimension grows. Both failure directions are fatal for a project that will eventually print flexure hinges a few hundred micrometers thin.
The test
To calibrate the exposure, I used the Cones of Calibration V3 by TableFlip Foundry [13]. One small print checks over/under exposure, XY accuracy, fit tolerance, and Z accuracy at the same time, because the model is a plate of paired features that give a pass/fail result:
- Success and failure cones. In TableFlip Foundry's words, "The success cones should print fully, while the failure cones are designed to collapse" [13]. If both survive, you're over-exposing, and if both fail, under-exposing.
- The Sword of Truth and the Monster Skull, a fit test. "The sword should fit perfectly into the Monster Skull but not into the fail holes of the Attunement Block" [13]. If the sword slides into the fail holes, light bleed has opened them up too far.
- The Ale of Accuracy and its Mug, a tolerance check in both directions. In TableFlip Foundry's words, "If the Ale is too large to fit, your exposure time is too high, leading to over-curing and oversized prints. Conversely, if the Ale is too loose in the Mug, your exposure time is too low" [13].
- Thin walls, gaps, and an Attunement Block for Z-height checking (6 mm from raft bottom to block top [13]).
The method
The Mars 5 Ultra has a built-in feature Elegoo calls Section-Based Exposure [14], which allows a different exposure time for different zones of a single print, up to eight of them. In practice the cones model is big enough that only six copies fit on the build plate at once, so each calibration run was one plate with six cones models at six different exposure times, stepped 0.1 s apart.
It took three full plates, eighteen exposure values in total, to converge. The first plate found the approximate range, and the next two worked outward from it and confirmed the best value. Before the number converged, overexposed zones came off with tips and gaps fused shut, and underexposed zones failed the opposite way, with the fine tips collapsed [builder, 2026-08-06].
The result was 2.6 s per normal layer for my resin, judged against the full V3 checklist. Two more settings go with that 2.6 s:
- Bottom layers, 30 s. The first handful of layers gets a much longer exposure so the print welds to the build plate. I stopped optimizing this number after realizing it doesn't affect part quality in my workflow. Nearly every real part prints on supports, lifted off the plate, so the over-cured bottom layers all end up in the support raft that gets cut away. (How I learned that, including the one kind of part where plate-direct printing is actually a feature, is §M4b's story.) This is standard practice, and one less variable to tune.
- Layer height. This is the setting the 2.6 s is tied to, and changing the layer height means redoing the calibration from scratch. I ran one profile for everything and never wrote the layer height into my notes. (The value is still sitting in the slicer profile, and this line stays blank until I read it off there instead of reconstructing it from memory.)
Exposure belongs to the resin
The resin behind that 2.6 s number is Anycubic ABS-Like Resin Pro 2.0, gray. I also have a bottle of Sunlu ABS-Like resin in white, and running it through the same cones test on the same printer gave about 2.8 s per normal layer against the gray's 2.6 s, with 30-second bottom layers still plenty [builder, 2026-08-06]. The two tenths of a second between them put the two resins two full calibration steps apart. So the correct exposure setting belongs to each resin, on that printer, at that layer height, rather than to the printer.
I cannot say why the two resins landed where they did. The bottles differ in both chemistry and pigment, and one comparison run cannot separate the two. Pigment is the usual suspect, since white scatters UV differently than gray does, but that is an explanation I am borrowing rather than one I tested.
Unlike the gray, fresh white prints held onto moisture and stayed damp to the touch for a long time, so Sunlu parts get a real drying stage before they are handled or trusted [builder, 2026-08-06]. Every new bottle of a new resin type now gets a cones plate before it is used for a real part.
Settings box
| Setting | Value | Basis |
|---|---|---|
| Printer | Elegoo Mars 5 Ultra (7" 9K mono LCD, 18 µm XY, 153.36 × 77.76 × 165 mm) | [14] |
| Resin | Anycubic ABS-Like Resin Pro 2.0, gray | product used (name as written on the bottle, full SKU not transcribed) |
| Normal layer exposure | 2.6 s | Measured, COC V3 checklist across 3 plates × 6 models, 0.1 s steps |
| Bottom layer exposure | 30 s | chosen. Irrelevant to part quality when printing on supports (see above) |
| Sunlu ABS-Like, white (comparison) | ≈2.8 s normal layer, 30 s bottoms. Extended drying needed post-wash | builder, 2026-08-06 |
| Layer height | not recorded in my notes (see above) | (none yet) |
| Calibration model | Cones of Calibration V3, TableFlip Foundry | [13] |
| Exposure zoning | Mars 5 Ultra Section-Based Exposure, 6 zones used of 8 available | [14]; builder |
The model, in 3D
The web version of this chapter has an interactive 3D viewer (stl_viewer.html, three.js) for inspecting any STL dragged into it. It ships with an illustrative cone-array demo, not the real COC model. The actual Cones of Calibration V3 STL is TableFlip Foundry's, so download it from their page [13] rather than from us. Their redistribution terms are not stated on the page, so this project links instead of re-hosting, the same policy as the drive-board Gerbers in §M6.