Rexroth MTX Micro CNC System in Horizontal Grinding Applications

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  Key takeaways
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  What the Rexroth MTX Micro CNC system is built to drive
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  Drive and motor sizing for a horizontal grinding axis
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  Interpolation, look-ahead, and what they buy you on a grinder
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  Tool data and compensation limits
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  When the Rexroth MTX Micro CNC system is the wrong choice
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  Matching the control to the grinding machine

Horizontal grinding machines are a narrow class. The wheel spins in a fixed plane, the table moves along one long axis, and the infeed axis pushes the wheel into the work. There is no tool changer in the usual sense, no five-sided contouring, and rarely more than four controlled axes on the small and mid-size machines. This is exactly the envelope the Rexroth MTX Micro CNC system was designed around.

The compact class covers roughly a triple-axis layout: X, Y and Z linear motors or ball screws, plus one spindle. Where a machine has no separate grinding head, the spindle itself carries the wheel and the control has to keep the spindle loop stable while it interpolates the two feed axes underneath it. That combination is what makes link interpolation useful on a grinder.

The practical limit is not the number of axes. It is the dynamic response you need from each one. A control that is comfortable on a 500 mm table at moderate feed rates will behave differently on a 4,000 mm bed where a 0.5 m/min rapid reversal has to settle without overshoot. Sizing the motor and drive to the moving mass matters more than the control badge on the door.

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  Good fitSingle-spindle horizontal grinders with 3 to 4 controlled axes and table travels up to about 1,000 mm.
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  WorkableMachines with a fixed grinding head where interpolation replaces a dressing axis.
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  Poor fitMulti-wheel cells, five-axis creep feed grinders, or beds past roughly 2,000 mm with high reversal rates.

The motor choice is where most grinding cells succeed or fail. On the reference layout, the X and Y axes use 3.5 Nm Rexroth feed motors, and the spindle runs a larger motor sized to the wheel load. On a grinder, the feed axes are not pushing a cutter through metal at 8,000 rpm. They are making small, precise moves at low feed, and then holding position while the wheel sparks against the work.

That changes the sizing rule. Peak torque matters less than continuous torque and loop stiffness. A 3.5 Nm motor on a light table gives you fast settling on moves of 0.01 mm to 0.5 mm, which is the range that decides the surface finish. If you oversize the motor for a heavy table, you gain acceleration but you may lose the fine resolution the servo needs at the low end.

The drive side has to match. The IndraDrive HCQ021 pairs with these motors and carries the PLC function inside the same unit, so NC and PLC logic share one data path instead of talking across a fieldbus. That reduces the wiring in the cabinet and keeps the I/O response short: 32 input channels and 16 output channels on the drive itself.

For a horizontal grinder, that I/O count is usually enough for the wheel guard interlocks, coolant valves, chuck clamp, and a couple of gauging signals. If your machine needs 60 or 80 I/O points, you are outside the compact class and should look at a larger control with remote I/O.

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  Check continuous torque firstGrinding spends most of its time at low feed, so the continuous rating and the resolver resolution drive the finish.
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  Match drive to motorPair the HCQ021 with the 3.5 Nm feed motors so the current loop and the PLC share one unit.
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  Count your I/O honestly32 in / 16 out fits a single-spindle grinder with basic interlocks; more than that means a different class.

Link interpolation across the feed axes is the feature that changes how you build the machine. On a conventional grinder, dressing the wheel in the correct profile needs either a formed wheel or a separate dressing axis. With linked interpolation and no fixed grinding head, the control can move the wheel along a path that generates the profile instead.

The look-ahead buffer holds up to 999 blocks. On a grinding cycle, blocks are short: an approach move, a feed to size, a dwell, a spark-out, a retract. 999 blocks is plenty for a single part cycle, but it is not a license to run a 50,000-line surfacing program. If your grinding path is generated by CAM and runs long, check the block count before you commit.

The TCP/IP Ethernet port supports high-speed DNC transfer. That matters if you drip-feed programs from a server, or if you want to pull part programs and tool offsets off the machine at the end of a shift. On a compact control, the network interface is often the only way to back up parameters without a laptop at the cabinet.

The NC fixed cycles are the other half of the story. The control ships with sub-programs for center hole drilling, drilling, boring, and tapping. On a grinder that also drills or taps mounting holes before grinding, those cycles save you from writing macro code by hand. They are not a substitute for a proper CAM post, but they cut setup time on simple features.

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  Link interpolationGenerates a wheel profile without a dedicated dressing axis.
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  999-block look-aheadEnough for a grinding cycle, not for long CAM-generated surfacing paths.
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  Fixed cyclesCenter drilling, drilling, boring, tapping built in as NC sub-programs.

The control holds 24 tool data records. On a grinding machine, a tool record is usually the wheel, plus any probe or dressing tool in the cell. 24 is enough for a single-spindle grinder with a few backup wheels, but it is not a turning center's turret. If your process needs 40 offsets live at once, the compact system will force you to swap records mid-cycle.

Each record carries geometric compensation, wear compensation, and radius compensation. On a grinder, wear compensation is the one you live with. The wheel loses diameter as it dresses, and the control has to shift the infeed axis to keep the finished size on target. Radius compensation handles the contact geometry between the wheel edge and the work.

The scaling works both ways. You can add axes or tool records up to the license limit, or strip the configuration back to a bare three-axis grinder. That flexibility is useful when a shop buys one control platform for several machine builds, but it also means the same part program may behave differently on two machines if the license level differs.

One practical note: keep a written record of which license options are enabled on each machine. When a machine is rebuilt or a drive is replaced, the option set is the first thing to check if a cycle that used to run suddenly stops.

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  24 recordsFits a single-wheel grinder; tight for multi-wheel or multi-probe cells.
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  Wear compensationThe key compensation on a grinder, because the wheel diameter changes as it dresses.
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  ScalableSame platform can be scaled up or down, so document the license set per machine.

The compact class has a ceiling, and it is worth naming before you buy. If the machine has more than one grinding wheel running at once, or a wheel changer, the tool and axis count will push past what the control comfortably manages. Multi-wheel cells usually want a full-size CNC with a larger tool table and more interpolation groups.

The second limit is travel and reversal rate together. A 4,000 mm table is not a problem by itself. A 4,000 mm table that reverses direction several times a second at 0.5 m/min is a problem, because the moving mass and the settling time fight the loop. Long machines with slow, single-direction passes are fine; long machines with fast oscillation are not.

The third limit is integration. If the grinder is part of a line and needs to talk to a cell controller over Profinet, EtherCAT, or a custom protocol, check the fieldbus options before you specify the compact control. The built-in Ethernet is good for DNC and file transfer, but it is not a substitute for a real-time fieldbus.

None of these limits are defects. They are the boundaries of a compact platform. The mistake is buying it for a job that belongs to a larger class, then discovering the gap during commissioning.

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  Multiple wheelsWheel changers and multi-wheel cells need a larger tool table.
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  Fast oscillation on long bedsTravel and reversal rate together, not travel alone, set the limit.
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  Real-time fieldbusBuilt-in Ethernet handles DNC; line integration may need more.