DMG Five Axis Tool Change: Point Adjustment Explained

• 1
  Why DMG five axis tool change starts with geometry
• 2
  Machine zero, work offset, and the tool offset chain
• 3
  Spindle orientation and key alignment at the change point
• 4
  Arm, cam, and gripper: where a swing error really comes from
• 5
  Making the adjustment without losing the reference
• 6
  When you should stop adjusting and call the builder
• 7
  Symptom to cause: a quick diagnostic table

A tool change on a five-axis machine happens at one fixed point in machine coordinates. The spindle travels there, the arm or chain presents the tool, and the two must meet inside a window that is often narrower than 1 mm in each axis. Everything downstream of that window is compensation. Everything upstream of it is steel.

So the first question is never which parameter to edit. It is whether the machine zero that the controller uses for the change position still matches the physical reference the builder set at installation. On most DMG five axis tool change cycles, the change position is stored as an absolute machine coordinate, not as a work offset. If that absolute value drifts, no amount of tool offset work will fix the symptom.

The second question is which body moves. On a gantry-style five-axis center the Z ram travels to the ATC; on a trunnion machine the table often swings clear while the spindle stays put. The correction axis is different in each case. Reading the ladder or the change macro tells you which axis the controller is commanded to move, and that tells you where a correction can legally live.

A change that fails only after a long warm-up, or only on the second shift, points at thermal growth rather than at a wrong number. Cast iron and ball screws do not expand uniformly, and a change window sized for a cold machine can close once the spindle has run for three hours. That is an engineering boundary, not a bug.

• 1
  Absolute, not relativeChange position is stored in machine coordinates on most DMG cycles.
• 2
  One moving bodyIdentify whether the ram or the table reaches the ATC.
• 3
  Thermal driftA window that closes after warm-up is a growth problem.

Machine zero is the builder's reference. Work offsets shift the part relative to that reference. Tool offsets shift the tip relative to the spindle gauge line. A tool change point sits in machine zero space, so it is largely independent of G54 through G59. If a change fails only on one program and not on another, the change point is probably fine and the problem is elsewhere.

The chain that matters at the ATC is spindle gauge line to tool taper to tool tip. Any error in that chain shows up as a change that is correct in X and Y but wrong in Z, or the reverse. Measure the tool in the presetter, then measure it again in the spindle with a height gauge on a known surface. The difference is the chain error, and it is usually a chip or a worn taper, not a parameter.

Drawbar force belongs in this chain too. A drawbar that releases late leaves the taper engaged while the arm pulls, which produces a Z-direction error that looks like a change point problem. Check release timing before you touch any offset. On a healthy machine the tool should be free before the arm starts its stroke, with a visible clearance gap.

Keep a written record of the change position values and the date they were verified. A machine that has been moved, leveled, or had a ball screw replaced needs the change point re-verified even if the controller value was never edited. The number in the controller is only meaningful against the machine it was taught on.

• 1
  Work offset is irrelevantChange point lives in machine coordinates, not in G54.
• 2
  Measure twiceCompare presetter length against in-spindle length.
• 3
  Drawbar timingLate release mimics a Z change point error.

Most five-axis tool change cycles begin with spindle orientation. The controller drives the spindle to a fixed angular position so the drive keys or the stud slots line up with the arm. If orientation drifts by a few tenths of a degree, the arm can still engage, but it will bind. The change sounds different before it fails.

Orientation error is easy to misread as a change point error because the symptom appears at the same moment in the cycle. Watch the orientation move on its own, with the ATC inhibited, and mark the spindle face. Repeat it ten times. A repeatable position within a few hundredths of a degree is normal. A wandering position points at the orientation encoder or its belt.

Key and slot wear matters on high-hour machines. If the drive keys are worn, the tool seats at a slightly different angle each time, and the arm sees a different target every cycle. That is an intermittent failure that no single parameter value will cure. Replace the keys and re-check before adjusting anything in the controller.

On machines with a dual-contact or Big Plus taper, face contact and taper contact must both be present. A tool that only contacts on the taper sits deeper than the offset assumes, which shifts the effective change point in Z. Blue-check the taper on a sample tool to confirm contact pattern before chasing numbers.

• 1
  Orient firstThe cycle always starts with a fixed spindle angle.
• 2
  Ten repeatsWandering orientation means encoder or belt, not offset.
• 3
  Blue-check the taperConfirm face and taper contact before editing values.

The ATC arm is a cam-driven mechanism with a fixed stroke. It does not know where the spindle is. If the arm overshoots or undershoots by a millimeter, the cam, the roller, or the arm stop has moved. Adjusting the change point to hide a cam problem moves the error into the next cycle and usually into the tool taper.

Check the arm stop and the cam follower first. A worn roller changes the effective stroke length by a small but repeatable amount. A loose arm stop lets the arm settle differently under load. Both produce a swing error that appears only in the last few degrees of travel, which is exactly where the change point is least able to compensate.

Gripper spring force and jaw wear matter more than most operators expect. A weak gripper holds the tool slightly off-center, and the arm presents it to the spindle with a lateral error. The spindle then fails to seat cleanly. The symptom looks like a change point that is out in X or Y, but the real fix is a gripper rebuild.

After any arm work, re-teach the change point from scratch rather than nudging the old value. Start from a safe clearance position, jog in small increments, and confirm the tool seats with hand pressure before running an automatic cycle. Two hours of careful jogging is cheaper than one crashed spindle.

• 1
  Cam before codeA fixed-stroke arm cannot compensate for a worn cam.
• 2
  Gripper centeringWeak springs present the tool off-center.
• 3
  Re-teach, do not nudgeAfter arm work, build the value again from clearance.

When the diagnosis points at the controller value itself, change it in single-axis steps and record the old number first. A typical correction window on a five-axis center is a few hundredths of a millimeter per step. Anything larger than 0.1 mm in one edit is a sign that you are correcting a mechanical fault with software, and it will come back.

Move the axis to the change position in handwheel mode with the ATC inhibited. Confirm the spindle nose and the arm gripper are concentric within the builder's tolerance, usually a few hundredths. Then enable the ATC in single-step mode and watch one complete cycle with the feed override low. Never test a change point correction in automatic mode on the first cycle.

Record the date, the old value, the new value, and the reason. On a machine that runs two or three shifts, that log is the only way to tell a one-time correction from a slow drift. If the same axis needs correction every few months, the problem is mechanical and the parameter is only masking it.

Confirm the result with a real part. A change point can be perfect at the ATC and still produce a taper error if the tool seats differently under cutting load. Run a test cut, measure the feature that depends on the tool, and compare it against the drawing before you release the machine back to production.

• 1
  Small stepsA few hundredths per edit, never 0.1 mm at once.
• 2
  Single-step firstWatch one full cycle before automatic mode.
• 3
  Log every changeDate, old value, new value, reason.

There is a point where more adjustment makes the machine worse. If two axes need correction in opposite directions, or if the required value changes after every cycle, the geometry is no longer consistent and the controller cannot model it. Stop and measure the machine, not the parameter.

A change point that only works with a warm spindle is a known limitation on some five-axis designs. If production requires a tight change window from a cold start, the honest answer is to add a warm-up cycle rather than to push the cold value. That is a process decision, not a repair.

After a crash, the change point is not the first thing to check. Squareness, spindle runout, and taper condition come first, because a bent spindle nose changes the reference the change point is measured against. Correcting the point before the spindle is a waste of a shift.

If the machine has been re-leveled, moved to a new foundation, or had a linear guide replaced, treat the change point as unknown. Re-teach it from clearance. The old value was valid for the old geometry and has no meaning on the new one.

• 1
  Opposite correctionsInconsistent geometry, not a parameter problem.
• 2
  Cold vs. warmAdd a warm-up cycle instead of chasing the value.
• 3
  After a crashCheck squareness and runout before the change point.