CNC retrofit guide

Five Axis CNC Technology Transformation on a Liaison Machining Center

Old machining centers still have good iron. The control, drives and feedback are usually what limit them. This guide walks through a five axis CNC technology transformation in the order a shop actually does it, with the parameters and checks that decide whether the machine holds tolerance afterward.

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Five axis liaison machining center cutting custom auto spare parts

On this page

1
What decides success before you buy anything
2
What a five axis CNC technology transformation actually replaces
3
Choosing the control and axis configuration
4
Servo sizing and the A1 / A2 drive train
5
Mechanical interface, alignment and feedback
6
Control commissioning and RTCP setup
7
Step by step: the retrofit order that works
8
Retrofit or replace: match the case to the machine

Key takeaways

What decides success before you buy anything

Geometry firstMeasure squareness and backlash before ordering drives. Bad iron cannot be tuned out by software.

Count the axesX, Y, Z plus A and B need five-axis liaison control. A1, A2 and S are separate servo loops.

Match the loadSize servo torque from the heaviest cut, not from the old motor nameplate.

Prove it with a test partA ballbar run and a known test part tell you more than a spec sheet.

Scope

What a five axis CNC technology transformation actually replaces

A transformation is not a rebuild of the whole machine. On most older machining centers the bed, column, spindle housing and linear guides are still within spec after 20 years of service. What ages badly is the electrical and control side: DC drives, resolvers, relay logic, and a control that cannot read modern CAM output.

So the work splits into four packages. The CNC control and operator panel. The servo drives and motors. The feedback system. And the mechanical interfaces that connect new motors to old ball screws. Each package has its own acceptance criteria, and skipping one usually shows up as a taper or a step in the finished part.

The reason shops retrofit instead of buying new is cost per unit of travel. A machine with a 1,000 mm × 1,000 mm table and three-station pallet changer carries a lot of value in its castings. Replacing the control keeps that value and moves the machine from three-axis positioning to simultaneous five-axis contouring.

The limit is the machine itself. If squareness between X and Y is already out by 0.05 mm over 500 mm, no control will fix it. Measure first, then decide.

1
Control and drivesNew CNC, new servo amplifiers, new motors where the old ones are DC.
2
FeedbackLinear scales on X, Y, Z; encoders or scales on the rotary axes.
3
Mechanical interfaceMotor mounts, couplings, pulley ratios for the A1 and A2 axes.
4
Software and postRTCP setup, tool table, and a post processor matched to the new control.

Control choice

Choosing the control and axis configuration

The control has to close five-axis liaison at the same time, not just position five axes. That means the interpolator solves X, Y, Z, A and B together so the tool tip stays on path while the table tilts. If the control only does 3+2 positioning, you get indexed work, not contoured work.

Axis count is where people get confused. A typical retrofit needs seven servo loops: X, Y, Z, A1, A2 and S, plus the spindle orientation loop. A1 and A2 are often the two ends of a tandem drive on the same axis, or the tilt and rotary pair on a trunnion. The spindle loop handles orientation for tool change and rigid tapping.

For the linear axes, direct connection between motor and ball screw removes a belt stage and the backlash that comes with it. It also raises the reflected inertia, so the drive must be sized for it. On a 4,000 mm travel machine, a direct drive with a 40 mm pitch screw and a 2 kW servo is a normal starting point.

Check what the control can store. Tool tables, work offsets, and compensation data all take memory. A retrofit that adds 60-tool random exchange and a three-station pallet changer needs space for those tables plus the PLC logic that sequences them.

1
Simultaneous, not indexedConfirm the control does RTCP with five interpolated axes.
2
Seven loopsX, Y, Z, A1, A2, S plus spindle orientation.
3
Direct drive trade-offLess backlash, higher inertia. Size the servo accordingly.
4
Table capacityTool data, offsets and PLC logic all consume control memory.

Drives

Servo sizing and the A1 / A2 drive train

The old DC motors are usually the first failure point. Brushes wear, tachometers drift, and spares are scarce. Moving to AC servo solves the maintenance problem but changes the torque curve. Size the new motor from the heaviest cut the machine will see, plus a margin for friction and acceleration.

A practical way to size: take the maximum cutting force, multiply by the screw radius, divide by the gear ratio. That gives required torque. Then check the acceleration torque for the rapid rate you want. On a 750 mm × 1,150 mm × 550 mm machine, rapids of 24 m/min with a 0.15 g acceleration are realistic.

The A1 and A2 axes on a tandem or trunnion arrangement need a symmetrical gear transmission. Two servo motors drive through matched gearing so the load is shared and the tilt axis does not skew under cutting force. If the gearing is not matched, one motor takes most of the load and overheats.

Keep the original spindle if it is sound. A spindle rebuild with new bearings is far cheaper than a new spindle, and the taper and drawbar interface stay compatible with the existing tooling.

1
Size from the cutRequired torque = cutting force × screw radius ÷ gear ratio.
2
Check accelerationRapids of 24 m/min need acceleration torque, not just continuous torque.
3
Symmetrical gearingMatched A1 / A2 gearing shares load and prevents skew.
4
Keep the spindleRebuild bearings rather than replacing a sound spindle.

Alignment

Mechanical interface, alignment and feedback

New motor mounts are where retrofit accuracy is won or lost. The mount must hold the motor concentric with the screw within 0.02 mm and square to the mounting face. A flexible coupling hides small misalignment but adds torsional wind-up, which shows up as reversal error.

For the rotary axes, the encoder or scale has to be referenced to the actual table position, not to the motor shaft. If the gear train has any backlash, a motor-mounted encoder cannot see it. A table-mounted scale closes the loop at the real axis and removes that error from the part.

Linear scales on X, Y and Z are worth the cost on any machine expected to hold ±0.005 mm. They compensate for screw pitch error and thermal growth. Mount the scale so the read head stays within its specified gap across full travel, usually 0.1–0.15 mm.

After assembly, align the machine geometry. Check squareness between X and Y, then between Z and the table. Check parallelism of the two rotary axes on a trunnion. These are shim-and-scrape jobs, and they must be finished before the control is tuned.

1
Motor mount toleranceConcentric within 0.02 mm, square to the mounting face.
2
Close the loop at the tableTable-mounted scales see gear backlash; motor encoders do not.
3
Scale gapKeep the read head at 0.1–0.15 mm across full travel.
4
Geometry before tuningSquareness and parallelism are mechanical, not software.

Commissioning

Control commissioning and RTCP setup

Commissioning starts with the linear axes, one at a time. Set the direction, then the travel limits, then the reference position. Run each axis at low speed and watch following error. A well-tuned axis holds following error under 0.01 mm during a 5 m/min move.

Then bring up the rotary axes. Set the A and B zero positions with a dial indicator on a known surface. The pivot distance, the distance from the table face to the center of rotation, is the number that makes RTCP work. Measure it, do not trust the drawing.

RTCP, sometimes called TCPM, keeps the tool tip on path while the rotary axes move. To verify it, program a simple circle with the table tilted and measure the result. If the pivot distance is wrong, the circle becomes an ellipse or a spiral.

Finally, load the tool table and the pallet logic. Test random tool exchange through the full 60-tool magazine and cycle the three-station pallet changer several times. Sequencing faults usually show up here, not during cutting.

1
One axis at a timeDirection, limits, reference, then speed and following error.
2
Measure the pivotTable face to center of rotation drives all RTCP accuracy.
3
Verify with a tilted circleWrong pivot distance turns a circle into an ellipse.
4
Cycle the automationRun the full tool magazine and pallet changer before cutting.

Retrofit sequence

Step by step: the retrofit order that works

Follow this order. Doing the control before the geometry wastes tuning time.

1
1. Survey the machineRecord squareness, backlash and screw pitch error before touching anything. Squareness targets are 0.02 mm over 500 mm for X–Y and 0.03 mm over 500 mm for Z–table. Log the numbers; they are your baseline.
2
2. Strip the old controlRemove the control cabinet, DC drives, resolvers and obsolete cabling. Label every wire that stays. Keep the spindle drive interface documentation if the spindle is being reused.
3
3. Fit motor mounts and drivesMachine and align new mounts to 0.02 mm concentricity. Install AC servos sized for the heaviest cut plus acceleration margin. Use rigid couplings on direct-drive axes and matched gearing on the A1 / A2 pair.
4
4. Install feedbackFit linear scales on X, Y and Z with a 0.1–0.15 mm read head gap. Fit table-mounted encoders or scales on the rotary axes. Route cables away from spindle power leads to avoid noise.
5
5. Align the geometryShim and scrape until squareness and parallelism meet the baseline numbers from step 1. Do not tune servos on a machine that is mechanically out of square.
6
6. Commission the controlSet directions, limits and references axis by axis. Tune each loop to hold following error under 0.01 mm at 5 m/min, then set the pivot distance for RTCP.
7
7. Prove with a test partCut a test part with simultaneous five-axis contouring. Measure the result against ±0.005 mm and check surface finish. A ballbar run at 1,000 mm/min shows the contouring error across the working volume.

Decision table

Retrofit or replace: match the case to the machine

Use this to judge whether a transformation is worth doing on a specific machine.

Machine condition	Retrofit	Buy new	Why
Iron sound, control obsolete	Yes	No	Castings outlast electronics by decades
Squareness out by 0.05 mm	No	Yes	Geometry cannot be tuned out
Needs 3+2 indexing only	Yes	No	Cheaper control, no RTCP needed
Needs simultaneous contouring	Yes	Sometimes	Depends on spindle and thermal stability
Spindle worn beyond rebuild	Risky	Yes	New spindle often exceeds retrofit value
Machine over 4,000 mm travel	Yes	Rarely	Replacement cost scales steeply with size
Short remaining production life	No	No	Neither pays back; outsource instead

FAQs

Questions engineers ask before a transformation

Can an old three-axis machine become a five-axis liaison machining center?

Only if the machine has, or can take, two rotary axes. A three-axis bed mill usually needs a trunnion table or a tilting head added, which changes the structural loop and often exceeds the value of the retrofit.

If the machine already has A and B axes that were never interpolated, the transformation is mostly a control and drive job. That case is common and usually pays back.

How accurate can a retrofit machine be?

Accuracy follows the mechanical condition. With sound geometry, linear scales and a proper RTCP setup, a retrofit can hold ±0.005 mm on contoured work and reach Ra 0.8–1.6 μm on aluminum.

If the iron is worn, no control will reach those numbers. The survey in step 1 tells you which case you have.

Do we need linear scales or are motor encoders enough?

Motor encoders close the loop at the motor, so they cannot see screw pitch error or thermal growth. On a machine expected to hold tight tolerance over a long run, scales are the safer choice.

For roughing or short parts, motor encoders are often acceptable. The trade-off is tolerance drift as the machine warms up.

How long does the retrofit take?

It depends on scope. A control and drive swap with existing rotary axes is a shorter job than adding feedback and re-aligning geometry. Plan the commissioning and test-part stage separately from the mechanical work.

Do not compress the alignment stage. Rushing it moves the problem into the tuning stage, where it costs more to find.

What should be in the acceptance test?

A ballbar run, a squareness check, and a test part with simultaneous five-axis contouring. Measure the part against ±0.005 mm and check finish with a surface tester.

Also cycle the tool changer through the full magazine and the pallet changer through every station. Automation faults rarely appear during a light test cut.

Can the retrofit keep the existing tooling and pallets?

Usually yes, if the spindle taper and drawbar are kept. Tool data has to be re-entered in the new control, and the tool table must match the physical magazine positions.

Pallet interfaces are mechanical, so they transfer as long as the pallet changer logic is rewritten for the new PLC.