ShopBot PRSalpha 144-60Prototyping Lab · University of Iceland
04 Zeroing
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04Zeroing

Home, job zero and Z zero

Three different zeros. Mixing them up is the most common first-week crash.

C3Machine homeWhere the prox switches say 0,0. Fixed. Everything else is measured from here. Z2Job zero (X, Y)The point your CAM file calls X0 Y0. This example uses the centre of the material. Z2 zeros X and Y, not Z. C2Z zeroMaterial top or table surface — whichever &PWZorigin in the file says. Found with the plate.
Figure 1 · plan

Home and job zero, from above

stock +X +Y 123456
  1. 1Gantry (X bridge): runs along the table on two X motors.
  2. 2Spindle on the Y car: runs across the gantry.
  3. 3X proximity switch · input 2.
  4. 4Y proximity switch · input 3.
  5. 5Machine home 0,0. C3 finds it on the prox switches. Fixed.
  6. 6XY job zero — Z2 at the material centre. Match the centre origin in CAM. A corner origin is also possible, but only when the file uses it. Z2 does not set Z height; use C2 for Z.
Figure 2 · side

Z zero with the plate

spoilboard stock carriage plate inputs1234SB3 screen 123456
  1. 1Spindle and collet nut on the aluminium carriage plate.
  2. 2Bit: C2 lowers it until it touches the plate, twice, then lifts.
  3. 3Ground clip: it stays clamped on the aluminium carriage plate, on the edge. Leave it there; never on the bit or nut.
  4. 4Z-zero plate on the surface the file expects (stock top here).
  5. 5Input 1 on the SB3 screen lights on contact, goes out when lifted. The green dashes are the circuit: plate → bit → spindle → carriage plate → clip → lead → plate.
  6. 6One coiled lead, plate to clip. Nothing else to connect.
Figure 3 · the classic crash

Same file, wrong surface

CAM assumed the stock top; you touched the plate on the bed. Every rapid and retract now runs inside the material and every cut goes a sheet thickness into the spoilboard: clamps hit, bit snaps. The reverse mismatch cuts air. Read &PWZorigin in the file header before C2, every time.[4,54]

stockspoilboard Z0 you touched the stock top cuts as planned stockspoilboard Z0 you touched the bed every move a sheet too deep
MatchFile and plate both on the stock top: the rapid clears the sheet, the cut ends just below it.
MismatchFile on the stock top, plate on the bed: the rapid runs inside the sheet, the cut goes a sheet thickness into the spoilboard.
Homing (C3) in detail
  • C3 homes X and Y. The machine touches off each proximity switch, giving a repeatable start point [QS 15]. It also sets the table base coordinates correctly [UG 20].
  • Input 2 is the X proximity switch and input 3 is the Y proximity switch. Both are configured as limit switches [CR 39; UG 46]. By default, when either one triggers, motion stops and a "table limit" message asks whether to continue [UG 46–47]. VN configures what these inputs do [UG 47; CR 38].
  • Students never change VN or disable limits; report a limit fault to staff [CR 39].
  • If the machine stops with a limit message and nothing is at the end of the table, tell staff.
  • "Move off prox" prompt: C3 asks you to move the spindle carriage off a switch if it starts out sitting on one. Repeated prompts while the carriage is clear mean an input or target fault (polarity, target alignment, wiring). Report it rather than defeating the limits.
  • A proximity target must be met before the hard stop.
  • Where C3 lands (its offset from the switches) is set in TS (Tools → ShopBot Setup) [QS 15; XY 3–6; CR 30].

When to rehome (C3)

  • At the start of every session [MS 3].
  • Whenever the power has been off [UG 20].
  • After the gantry was pushed by hand or moved with the motors unpowered [UG 20].
  • After an E-stop on an alpha. Re-home XY and re-zero Z before cutting again [CR 39; QS 17].
  • After a driver fault or "location no longer accurate".
  • After you have moved the zero point and want the standard one back [UG 20].
Z zero in detail (C2, plate thickness)

Choose the reference to match your CAM file

  • Material top: best when depth must be precise, for example an inlay pocket [QS 14].
  • Table (spoilboard) surface: best for through-cutting wood, because sheet thickness varies [QS 14].
  • The CAM setting and what you do at the table must match. A mismatch ruins material and breaks bits [QS 14].
  • Z must be zeroed every time the machine is turned on and every time the bit is changed [QS 14].

C2 with the Z-zero plate

  1. Work in Move/Cut mode [QS 14].
  2. Check that the ground clip remains clamped to bare aluminium on the carriage-plate edge. The circuit runs through plate → spindle → collet → bit. Never clip onto the bit or nut.
  3. Place the plate directly under the bit, on the surface you chose [QS 14].
  4. Test the circuit. Touch the plate to the stationary bit. Input 1 must light on screen and go out as soon as contact breaks [QS 14]. If input 1 does not light and clear, do not run C2; call staff. ShopBot’s FAQ warns that a failed contact circuit lets the bit keep descending [FAQ]. If the bit reaches the plate during C2 and does not reverse, press the spacebar; if Z still moves, use the E-stop. Do not rely on C2 to detect a failed contact circuit.
  5. Run C2 or click the Z-zero button. Z touches the plate twice, then pulls up to a safe height [QS 14].
  6. Leave the clip on the carriage plate, coil the lead, and put the plate back in its holder on the YZ car — the carriage that carries the spindle across the gantry [QS 14].
  7. If the zero looks wrong, stop and call staff. Staff can check MZ,0 over an open area and verify the plate thickness in TS (default 3.073 mm) [TM 11–12; MS 3]. Keep hands clear during every commanded move. Before a hands-on check: spindle off and stopped, key OFF and removed, keypad closed, no file loaded or paused [UG 5; QS 5].

Manual Z zero (ZZ): not taught

  • Use C2 for Z zero. Manual ZZ with a paper feeler is not part of the student procedure; ZZ is listed in the glossary for reference only [CR 45].
XY zero in detail (Z2, ZX/ZY, fixed offsets)

Home vs job zero

  • Machine home (C3): a repeatable XY origin taken from the proximity switches [QS 15]. C2 and C3 also set the table base coordinates, which software limit checking (SF/VL) uses [UG 20; CR 26].
  • Job (working) zero: wherever you zero X and Y for this job. Z2 zeros X and Y [CR 43]. ZX zeros X only and ZY zeros Y only [CR 44–45]. On the keypad, the blue "Zero Axes" button does the same [XY 5]. VA sets the axes to any chosen value. VA 0,0 is equivalent to Z2 [UG 18].
  • Re-zeroing the working coordinates does not lose the base coordinates. ZT sets the base zero [CR 44]. ST restores the working display to base coordinates [CR 28; UG 19].
  • After setting a temporary 0,0, write down the coordinates so you can find it again [QS 15].
  • 2D offset in FP: the current XY becomes the file's 0,0 for that run only, and Z is not shifted. Use it to place or nest parts [UG 18–19; CR 12].

Fixed C3 offsets

  • Where C3 lands relative to the switches is a staff setting (TS). Students never change it.