ShopMill Zero Offset and Marker Setup: Fixing Setup Errors

Problem Scope

Two ShopMill features generate most "the part is machined in the wrong place" and "the block runs twice / not at all" complaints when an existing program is being modified rather than written from scratch:

1. Nullpunktverschiebung (zero point offset / work offset) — the part datum does not land where the drawing says it should, or the program moves after a settable offset is edited.
2. Marke setzen (setting a marker / label) — a named start and end marker delimit a program section so it can be repeated, translated, rotated, mirrored or scaled. Mis-scoped markers repeat the wrong blocks or silently repeat nothing.

Both faults look identical at the machine: the tool cuts air, cuts in the wrong quadrant, or crashes into a clamp. The diagnosis path is different for each, so separate them before touching the program.

How the Offset Chain Resolves

The programmed coordinate is not the machine coordinate. On a SINUMERIK/ShopMill control the position the axis actually reaches is the sum of several independent offset stages. If the part is in the wrong place, one stage in this chain is wrong — not necessarily the one you edited.

Rule: a Z error of exactly one tool length points at tool data, not at the work offset. A constant X/Y shift on every tool points at the work offset or base offset. An error that appears only in part of the program points at a programmable translation or a marker-delimited transformation.

Diagnosing a Zero Offset Fault

1. Reference all axes and confirm the machine coordinate display is plausible at a known position (e.g. table centre or a hard reference feature).
2. Open the zero offset list and record every non-zero value, including the base offset and any offsets not used by the current program. Note which offset the program header actually selects — ShopMill program headers select the work offset explicitly.
3. Zero out the programmable translation mentally: search the program for translation, rotation, mirroring and scaling entries and check whether each one is cancelled before the next machining block.
4. Re-probe the datum on the actual clamped part, into the same offset number the program header selects. Use the workpiece measuring cycles (edge, corner, hole/boss centre) rather than an eyeballed handwheel touch-off.
5. Verify by MDA/manual jog: with the offset active, command X0 Y0 (in the workpiece coordinate system) and confirm the spindle centre sits on the intended datum feature. Then command a known point such as X50 Y50 and measure the actual travel.
6. Check Z separately: with the tool that was used for the touch-off, drive to Z0 in the workpiece system and confirm it touches the reference face. Repeat with a second tool to prove the tool length data is consistent.

Symptom-to-cause table

Markers (Marke setzen) and Program Repetition

A marker is a named label placed in the program sequence. Two markers — a start marker and an end marker — bracket the section you want to act on. The repeat/transformation entry that follows then references those marker names.

Typical use cases: repeat a pocket sequence at several positions, machine the same contour again at a deeper Z, or apply a translation, rotation, mirroring or scaling to a group of blocks.

Correct structure

Program header      (blank size, work offset, safety plane)
  Tool call + position pattern
MARKER: START1      <- start marker, name it descriptively
  Contour / pocket / drill sequence   (absolute positioning only)
MARKER: END1        <- end marker
TRANSLATION  X.. Y.. Z..              (or ROTATION / MIRROR / SCALE)
REPEAT  START1 .. END1   count = n    (references the marker pair)
TRANSLATION  cancel                   (return to the base datum)
  Next operation
END OF PROGRAM

Rules that prevent the usual failures

• Unique names. Two markers with the same name make the repeat resolve to the first match. Use names tied to the feature (POCKET1, HOLES_L), not M1, M2.
• Nesting must be strict. Marker pairs may be nested one inside another, but they must not overlap or cross. A crossing pair is the classic cause of "it repeats the wrong blocks".
• Absolute positioning inside the repeat. An incremental move inside a repeated section accumulates on every pass, so pass 3 lands three increments away. Convert to absolute, or make the step-over deliberate and documented.
• Tool changes inside a repeat. Repeating a section that contains a tool call re-runs the change every pass. Move the tool call outside the markers where possible.
• Cancel transformations. A translation, rotation or mirror that is not cancelled remains active for everything that follows, including the next tool. This is the single most common reason a program that ran correctly last month now cuts in the wrong place after an edit.
• Repeat count semantics. Confirm on your control whether the count is the number of additional passes or the total number of passes. An off-by-one here is a full extra cut. Verify with a dry run before committing to material.
• Deleting a marker. Deleting one marker of a pair leaves an orphan reference. Delete the repeat entry first, then both markers.

Editing an Existing Program Safely

When optimising a program that already runs, the risk is not the new code — it is the interaction with state left behind by the old code.

1. Copy the program under a new name before editing. Keep the known-good original untouched until the new version is proven on the machine.
2. Map the state first: list every work offset selection, every transformation, and every marker pair with its line range. Do this on paper before changing anything.
3. Change one thing per verification cycle. Edit the offset, or the markers, or the transformation — not all three, then run.
4. Graphic simulation is the first filter. It catches marker scope errors and uncancelled transformations quickly, but it uses the offsets stored in the control, so it will not catch a datum probed to the wrong face.
5. Dry run above the part. Raise the datum in Z by a safe amount (via the offset, then restore it), run at reduced feed, and watch the X/Y path.
6. First cut in single block with feed override low, hand on feed hold, especially at the entry of each repeated pass.
7. Restore the Z offset and note in the program header what was changed and when.

Verification Checklist

Documentation to Pull Before You Start

The ShopMill operating and programming documentation supplied with the machine covers zero offset handling, the workpiece measuring cycles, and the marker/repeat and transformation entries in detail, including the exact softkey paths and the repeat-count convention for your control and software version. The machine builder's supplement takes precedence where it differs — base offsets, fixture data and probe configuration are builder-specific. Confirm your control's software version before applying any procedure from a generic manual, because menu structure and the availability of individual transformation entries vary between versions.