Machine tool 101: Part 6, Machine servers

Manufacturing Engineering, Jun 1994 by Aronson, Robert B

Workpiece transportation, chip removal, and coolant supply are three systems critical to the basic machine tool cutting operation. Transport of the workpiece involves both the pallets that carry the workpiece and the various means of getting the pallet to the machine tool.

PALLET PARADE

Before automation, a machinist simply carried or wheeled a workpiece to the machine and attached it to the worktable, helped by a crane if the part was large. This is still the method if speed isn't a problem or the job is a one-of-a-kind part.

Operators of today's high-volume automated production equipment attach the workpiece to a pallet, a metal slab with slots or holes in its base that mate with pins and tabs in the machine tool work area. Pallets allow workpieces to be easily positioned in one or more machine tools without taking time for fixturing. Pallet configurations are not totally standardized, although an ISO pallet standard exists, and design variations impede pallet interchanges among machines of different manufacturers.

In a low-volume operation, an operator manually feeds the pallets to the machine. For higher volume, a two-pallet worktable allows the machine to work on one part while the next is being prepared. A horizontally or vertically mounted rotary table can become the fourth axis of a machine tool. Two or more tables, mounted at 90deg to each other, give a machine tool additional axes.

A "pallet pool" ensures constant work flow. It's an area holding one or more pallets until needed. In an automated system, the pallet pool can be the basis for lights-out operation. The machine tool or cell will operate untended as a loading system feeds in the pallet-mounted parts as needed.

A parts magazine is another way to simplify loading to a single machine, especially smaller, stackable parts. Operators manually load the magazines which feed parts to the tool's work area.

Lathes and turning centers working with tubular stock use magazines linked to the headstock that feed directly to the work area. Stock is automatically indexed forward, positioned, then machined.

GIVE THE PART A HAND

In high-volume repetitive operations, or where handling is a simple pick-and-place task, fixed automation may be best for feeding parts. A programmable robot is effective with parts where:

* Positioning takes advantage of the robot's multiple axes (up to six in most cases); for example, if the part is fixed to a carrier or dunnage, lifting it out can be a complex maneuver;

* Weight is from 3 to 250 kg;

* Variety shifts frequently; or

* Parts are too heavy for workers to move or too hazardous for them to be near.

Stationary robots serve from one to four machines, often a cell configuration. Rail-mounted robots can serve additional machines. Part cycle time largely determines how many robots to use. If cycle time is long enough, one robot handles two or more machines. With a short cycle time, one robot can serve only one machine .

Robot manufacturers usually design end effectors, or grippers, for a specific part or part family. When feeding machine tools, the robot often has a two-gripper end effector so it can pick up and deliver a part in one operation.

If the robot has to distinguish among parts, the sequence can be programmed, and the robot will respond accordingly. End-effector sensors can also identify which part it has and what program to use. The robot may also have a "search program" so it will look in more than one location for a part. All robot programs are linked to the machine tool's program for efficient operation, so that, for example, the robot knows the machine has secured a part before it releases its gripper.

Often a robot can be reused on another job by simply reprogramming and changing the end effector. This can be a major benefit over hard automation.

Like machine tool programming, robot programming has been simplified. Instead of establishing a path by jogging the arm from place to place with a teach pendant, better systems allow the operator to establish a workpath on-screen with a prompt program to help. Lock outs on some programs prevent the robot from carrying out commands that will cause a problem, like asking the arm to move a load faster than is practical.

The machine controller and robot, as well as other part-handling mechanisms, exchange data two ways. With highly integrated systems, the machine's control gives all necessary commands to the load/unload equipment. More frequently, the control passes a message, called an M function, to the feeding system to do its job. This is the control code category for all auxiliary functions.

PART TROLLEYS

When more than one machine is involved and a large number of like parts are being made, larger tables can present work to several tools positioned around the periphery. When serving two or more machines beyond the reach of a robot, RGVs, AGVs, or SGVs work well.

A rail-guided vehicle (RGV) has a pallet carrier that rides a straight set of tracks, usually from 20 feet to several hundred feet long. Operators can push parts from machine to machine, but more often, the carts are motorized and computer controlled. In larger systems, the RGV serves machines on both sides of the track.