Continuous Development of Combined Machine Tools

• 1
  What a combined machine tool actually is
• 2
  How the modular idea took hold
• 3
  The five module groups that matter
• 4
  When a combined setup stops paying off
• 5
  Where combined development stands now
• 6
  Five stages of combined machine tool development
• 7
  Combined line vs flexible machining center

A combined machine tool is a machining system built from standard modules instead of one monolithic casting. The bed, columns, spindle heads, slide units, and fixtures come from a common component family. You bolt them together in different arrangements to suit a part. That is the whole idea: reuse the module, reconfigure the layout.

This is different from a machining center, where one spindle travels and a tool changer swaps cutters. In a combined setup, several heads or workstations can cut at the same time. Cycle time drops because operations run in parallel, not in sequence. The trade-off is that the layout is locked to a part family. Change the part, and you may need to rebuild the fixture plate.

The continuous development of combined machine tools has run along one line for seventy years: replace custom iron with standard modules, then replace hard cams and limit switches with servo control. Each step widened the range of parts a single line could handle.

The concept traces to early 20th century automobile production, when each plant kept its own standards. Parts from one factory would not fit another. In 1953, Ford and General Motors worked with American machine tool builders to push toward common standards for transfer-line components. The goal was interchangeability, not elegance.

Once base units, slide ways, and spindle heads were standardized, a machine builder could quote a line from a catalog. Lead time for the machine itself fell. That mattered more than any single speed gain. A plant could retool a line for a new engine block without scrapping the whole foundation.

By the 1970s, hydraulic and cam-driven heads gave way to servo axes. The module stayed, the control changed. That is the pattern to watch: the mechanical interface is conservative, the control layer keeps moving.

Most combined systems are assembled from five groups. Get these right and the rest is plumbing.

Base and bed units carry the load path. They set stiffness, which sets the achievable tolerance. On a combined line, a weak base shows up as chatter at the second or third station, not the first.

Spindle heads do the cutting. A fixed head is rigid and cheap. A programmable head costs more but survives a part change. Multi-spindle heads cut several holes at once, at the cost of fixed center distances.

Slide and feed units move the head or the work. Ball screws and linear guides have largely replaced box ways on new builds because they hold position repeatability with less maintenance.

Fixture and transfer units index the part between stations. This is where most cycle-time loss hides. A transfer mechanism that takes 2 seconds per index eats 20 percent of a 10-second cycle.

• 1
  Base and bedSets stiffness and the tolerance floor
• 2
  Spindle headsFixed, programmable, or multi-spindle
• 3
  Slide and feedBall screw and linear guide on new builds
• 4
  Fixture and transferIndex time is the hidden cycle cost

Parallel cutting wins when the part has several operations that can run at the same time on different faces. It loses when the part needs one long continuous contour, because that path cannot be split.

Volume is the second boundary. A dedicated transfer line needs enough annual volume to amortize the fixture plates and the setup. Below that, a 5-axis machining center with a pallet pool is usually cheaper per part.

Geometry change is the third. If the part family changes every six months, modular heads help but custom fixtures do not. This is where flexible machining cells, built on the same module families, took over much of the mid-volume work.

At GreatLight we run 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers and 16 mill-turn centers. For most customer parts we machine on flexible centers rather than dedicated lines, because lot sizes run from one prototype to 10,000+ pieces.

Current builds lean on speed-regulated spindles, ball screws, and digital control. A combined cell can now hold ±0.005 mm on a well-fixtured part, provided the thermal load is managed. Spindle growth over a long run is the usual error source, not the servo loop.

Automatic spindle and head replacement is the active frontier. If a head can be swapped by the machine, the same base serves two part families. That is what turns a transfer line into a flexible manufacturing system without a full rebuild.

Small and medium batch production is where this pays. A line that can be reconfigured in a shift absorbs model changes that would previously have idled the plant for weeks.