Dimension markers and lofting

A dimension marker pins a channel’s width — and optionally its height — at a point along a segment. Between two markers the channel lofts: it changes size smoothly rather than stepping.

This is how you build a flow-focusing throat, a gradual expansion, or a channel that narrows through a mixing section, without drawing any of them.

Tapering a segment

The quickest way, and the one most people want: select a segment and set Channel size in the inspector. There is a width for each end — start and end, following the direction the segment was drawn.

Give the two ends different widths and the segment tapers between them. That is all a taper is: a marker at each end with different values.

Each field pins its own end and nothing else:

• Set one end only and the whole run holds that width, because size never extrapolates past the outermost marker.
• Set both and the segment tapers between them.
• Leave an end unpinned and it follows the rest of the path — which is what makes lofting across a whole chain work.

An unpinned end still shows a number, because it shows what is resolved there rather than a blank. The pin beside each end tells the two apart: lit means this segment sets it, dim means it is inherited. Click a lit pin to release it.

Sizing every channel at once

To give every channel the same width, click Select all channels. The button is in the inspector when nothing is selected, and in the segment section’s header when only some segments are. Then type a width into Channel size. Rectangular and trapezoidal segments also get a height field.

The new size applies along each segment’s whole length. Each segment gets a pinned marker at both ends, and any other markers on it are removed. The same field works for any selection of two or more segments.

When the selected segments aren’t all the same width, the field reads Mixed and a note gives the narrowest and widest. Setting a size makes them all that size and removes any tapers. One undo puts them back.

Placing a marker anywhere else

For a size somewhere in the middle of a segment — a throat, a step — press D for the dimension tool and click a channel. A marker appears where you clicked, and the inspector shows its fields:

Field	Notes
Width	Millimetres. Required.
Height	Millimetres. Optional — falls back to the chip default.
Position	0–1 along the segment. Editable, so you can nudge it precisely.

To move a marker, drag it along its channel with the dimension tool. It stops just short of the markers either side, so it can’t pass or land on a neighbour.

Delete a marker with the bin icon beside it in the inspector.

What happens with no markers

The segment is the chip’s defaults — 0.8 × 0.5 mm out of the box — for its whole length. The inspector shows this as a greyed “default 0.8 × 0.5 mm” line under the marker list.

That is a sensible size and a large proportion of chips never need anything else.

What happens with one marker

The whole segment takes that width. One marker is a way of saying “this segment is 1.2 mm”, not a way of making it 1.2 mm at one point.

What happens with two or more

The channel interpolates between consecutive markers. Before the first and after the last, it holds that marker’s value.

So a segment with markers at t = 0.2, width 0.8 and t = 0.8, width 1.4 is:

• 0.8 mm for the first fifth,
• widening smoothly from 0.8 to 1.4 across the middle three-fifths,
• 1.4 mm for the last fifth.

Turn on Width in the toolbar (T) to see the ribbon at its true width on the canvas. That toggle only changes how the channel is drawn — it draws each one at its real width instead of as a bare centerline. It does not set any size itself.

Lofting across a whole path

A loft does not stop at a segment boundary. When segments form a connected chain — each joined end to end, no branching — the interpolation runs along the whole chain. A marker at the start of the first segment and another at the end of the fifth gives you a taper that runs across all five.

That is what makes it practical to dimension a long serpentine: two markers, not thirty-two.

The chain breaks at a junction. Where three or more segments meet, each branch is its own chain from there on.

How finely a path is followed

A swept channel is built from a series of cross-sections strung along the path, so a curve is really a many-sided polygon. How many sides it gets is decided per segment, from how long it is and how sharply it bends — a hairpin earns more stations than a straight run of the same length, because that is where cutting the corner would show.

The budget is angular: no two consecutive stations differ in direction by more than about 6°, halved for an export build. On a 6 mm bend radius that holds the facet to roughly 8 µm, which is finer than the xyResolutionMicrons of any printer profile — so the flat spot is smaller than the machine can reproduce rather than merely small enough to look acceptable on screen.

You do not set this. It is worth knowing only because it explains why a tight serpentine takes longer to rebuild than a straight one of the same total length.

Height

Optional, and usually you leave it alone.

A rectangular channel has both dimensions, and the minimum-feature check measures both — for a rect or trapezoid cross-section, height can be the limiting dimension just as easily as width. For a round channel, width is the diameter and height is ignored entirely.

Two practical reasons to set height explicitly:

• A two-piece chip whose split height you want the channel ceiling to sit at.
• A high-aspect-ratio channel — deep and narrow, or wide and shallow — where the aspect ratio is doing something for you. It changes the Poiseuille number and therefore the pressure drop; see Pressure drop.

What the checks say

Two of them are specifically about markers.

abrupt-taper fires when the width between consecutive markers changes by more than your taper ratio — 2× by default. It reports the ratio, the distance over which it happens, and the half-angle, because a 3× change over 20 mm is a gentle taper and the same change over 0.5 mm is a step.

A sharp contraction is not automatically wrong. A flow-focusing droplet generator needs one, which is why this reports as info, rising to caution only once the change reaches twice your taper ratio.

channel-below-min-feature measures the narrowest point along the segment, not an average. A channel that tapers to 0.05 mm at one end fails there, however generous the rest of it is.

When several segments share a marker they are reported together — “3 segments are only 0.05 mm across” — because widening that one marker fixes all of them.

A worked example: a flow-focusing throat

The droplet generator template is built this way. Its throat is 0.35 mm against 0.8 mm channels either side:

1. Draw the cross — a continuous channel with two side channels meeting it.
2. Place a marker just before the junction at 0.8 mm.
3. Place a marker at the junction at 0.35 mm.
4. Place a marker just after at 0.8 mm.

The contraction and the expansion are both sharp, deliberately: the shear gradient at the throat is what pinches the droplets off.

Note that 0.35 mm is near the limit of a standard 4K printer, which is exactly what the template’s caveat says and what the checks will tell you on a coarse profile.

Next

• Chip parameters and split types — the defaults markers fall back to.
• The 3D preview — seeing the loft as a solid.