Resources · Reading a spec
Every interrupted-spiral hypotube in the catalog carries three numbers under Cut pattern: pitch, degrees cut and uncut, and cuts per revolution. Between them they fix how much wall is open, how much is bridge, and where the bridges fall. This is what each one does, shown on fifteen parts of a single 1F tube.
Pitch is the axial distance the helix advances in one revolution, 0.005 in to 0.025 in across the catalog. It sets how many turns of pattern fit in an inch: 200 at 0.005 in, 40 at 0.025 in. More turns per inch means more slot per inch and a tube that bends on a smaller radius. It also means more bridges per inch, and at the shortest pitches the bridge count is what carries push and keeps the tube from stretching.
Around one revolution the laser cuts an arc, lifts, leaves a bridge, and cuts the next arc. 120° cut and 24° uncut means each slot spans a third of the circumference and each bridge a fifteenth. The cut share is the flex: 120° of every 144° is open. The uncut share is the torque and the hoop strength: those 24° bridges are the only metal joining the turns. 65° cut and 15° uncut opens less than half of each 80°, so the tube is stiffer in bending and stronger in torque with the same pitch.
CPR is how many cut-and-bridge pairs fit in one turn: 2.5, 3.5 or 4.5 in the catalog. Multiply out and the rhythm closes: 2.5 × (120° + 24°) is 360°. The fractional CPR is deliberate. With 2.5 pairs per turn the bridges land half a pair further round on every revolution, so no two bridges stack on the same clock position and the tube has no preferred bending plane. A whole-number CPR would put every bridge in a line and give the shaft a stiff side.
The 1F family below is one tube, 0.014 in OD × 0.0105 in ID, 6 in long, cut fifteen ways. Three rhythms across, five pitches down. The bend radius is the per-joint contact model for 304 stainless, the figure each product page marks experimental.
| Pitch | 120° / 24° · 2.5 CPR | 80° / 22.85° · 3.5 CPR | 65° / 15° · 4.5 CPR |
|---|---|---|---|
| 0.005 in | 0.51 in | 1.36 in | 1.94 in |
| 0.010 in | 0.93 in | 2.46 in | 3.47 in |
| 0.015 in | 1.19 in | 3.12 in | 3.76 in |
| 0.020 in | 1.15 in | 2.92 in | 2.81 in |
| 0.025 in | 0.89 in | 2.22 in | 1.79 in |
Modelled minimum bend radius. Each cell is a catalog part; click through for the print and the 3D model. All fifteen are $200.
Across a row the rhythm does the work. At 0.005 in pitch the 120° cut bends on 0.51 in and the 65° cut on 1.94 in, nearly four times the radius from the same tube and the same pitch, because the bridges went from a fifteenth of the turn to nearly a fifth of it. Down the first three rows the pitch does the work: doubling pitch from 0.005 in to 0.010 in halves the slots per inch and the 120° radius goes from 0.51 in to 0.93 in.
The last two rows are the interesting ones. Past 0.015 in the modelled radius stops growing and turns back down. At long pitch the slots are far apart and each joint must take a larger share of the total bend; the contact model finds the slot faces touching sooner, so the minimum radius the tube reaches before the slots close is smaller again, while the tube between joints is now stiffer. Two parts with the same minimum radius can feel very different in the hand, which is why the radius is guidance and the bench is the decision.
For a shaft that must steer, start at 120° / 24° and pick the pitch for the radius you need. For a support section that must push, move to 65° / 15° and lengthen the pitch. For a tip that must track, shorten the pitch first, then open the cut. Order the three rhythms at one pitch, bend them, and the next pitch is obvious.
