The cell decision.
Recently we visited Kurt Mfg Co, Minneapolis, MN. You may know them for their vises and related tooling products. But they are much more than that. The firm has one of the nation's most fully equipped job shops--doing precision work for aerospace and computer industries.
On the surface, they look like many similar shops: Management is installing new MRP systems, flexible manufacturing systems, fully automated painting systems and much more. They even manufacture a microprocessor control unit of their own--in two models, one a basic programmable controller for robots and machine tools, and the other a CNC unit for OEM or retrofit.
Beneath the surface, however, there is a difference. Much of the work is done on conventional CNC machines programmed manually or with an APT time-sharing service.
What is unique is the arrangement of machine tools, or, more specifically, the re-arrangement of the machines. If the work order is small, the job goes to a CNC machine, but if repeat orders are a big factor, or the run is fairly long, Kurt engineers will take the job off the CNC and create a manufacturing cell for the task, relocating machine tools as required.
There is no magic, no computer diagnosis, CAM, or other supersophisticated planning device--just good logic from the minds of the engineers in charge. It's a grass-roots approach that really works.
As an example, let's look at a cell to show the mixture of simple and complex machining equipment. Many of the workpieces go into computers, and there is a surprising amount of long-run production, even of the very expensive components. The spindle shaft shown in the drawing, for example, could have been machined on a single NC lathe with attachments, but now it's done in a cell for higher production efficiency.
Originally, the part was machined from a forging of 440 stainless steel. Today, it is made from a casting of the same material, processed in the cell, broached, heat treated to 58 Rc, and then ground on five diameters to 0.0001, with runout between journals and between different diameters held to within 0.000 050.
In the cell, the casting is first cut on a Mazak 4L turning machine, followed by additional turning on a second 4L. The workpiece then goes to a Natco drilling machine, where it receives 10 holes. Eight are 1/4-20 tapped and two are 0.406 dia. Next it goes to a special machine with a Burgmaster head for chamfering, and then to a lead-screw tapper with automatic indexing. The last station in the cell mills two slots 0.375 wide.
Carbide tools serve in the turning and milling machines, and HSS tools make the holes and threads.
In a second cell, the operations are (1) Ex-Cell-O boring machine, (2) broach, and (3) combination drilling and milling on a special machine built by Kurt. The machine drills, reams, and taps 12 holes, and then mills two surfaces. The complex machine is directed by a simple timer, working through hydraulic actuators.
After the cell operations, the part goes to heat treating, and then to the grinding department. Originally, this job started on NC machines, but the engineers chose dedicated equipment when the customer requirements called for higher production rates, and the shop load precluded tying up the NC machines for the work.
Kurt engineers figure that a typical maximum NC run will be 10,000 parts per year, possibly augmented by a Zagar head for tapping. Beyond that, the cells come into play. Many runs are for 100,000 parts per year in several batches.
For shorter runs, Bridgeports still serve, especially for one-off parts. The new Cincinnati FMS system, sometimes called a "super cell,' will handle intermediate orders from 30 to 400 pieces per month. Management feels that the FMS is not for extremely precise work, say 0.0002 tolerances, nor for overly complex parts such as those requiring 75 holes each.
The concept of the cell group is similar to group-technology approaches to the logic of manufacture. It is a flow-through method of machining, a work center, where it is possible to avoid stacking and rehandling of workpieces. Not every operation fits into a cell, so the firm still has central areas such as grinding, heat treating, and even some turning. The lathe department, however, serves mainly for secondary operations.
A project leader follows each job through inspection, but all thought isn't left to the engineers. A spokesman for management said, "We want operator involvement, too. Our engineers set feeds and speeds on the high side and let the operator use the override if he thinks it's necessary to slow down.'
In addition to sharing information and operating decisions with the operators, managers feel that in the future there will be more sharing of information with vendors and customers. This will involve marketing decisions as well as pure technical facts. Such communication will be necessary to assure proper design, tolerance, and manufacture of complex parts that must interact with other parts in the elaborate systems encountered with high technology.
Quality assurance demands good communications. One of the new techniques just getting started is called source inspection. Here, gage readouts in the shop QC area feed through remote lines directly to the customer's computers, so the customer doesn't have to use incoming inspection. He knows the parts are good before they leave Kurt's shop. This kind of QA saves time and money.
For more info on Kurt's job-shop capabilities, circle E53.
Photo: In a typical manufacturing cell, one operator handles all tasks. Use of auxiliary manual equipment speeds up overall productivity compared to previous setup using only CNC lathes.
Photo: Operations 3, 4, and 5 are shown from left to right. Multiple-spindle drilling is done on a Natco machine--the chamfering operation and lead-screw tapping on equipment built by Kurt.
Photo: Operation 6 mills two areas on the spindle flange.
Photo: In a second cell, the operator air-gages a part after an Ex-Cell-O boring operation. The inset shows parts after broaching and final machining.
Photo: Drawings show surfaces turned on the two Mazak 4L machines. Some diameters will be ground to tighter tolerances, e.g., the 1.583" dia will be ground to 1.5749, 0.0001, with tir of 0.000 05. Some dimensions were omitted for simplicity.
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|Title Annotation:||Kurt Manufacturing Co. job-shop|
|Author:||Miller, Paul C.|
|Publication:||Tooling & Production|
|Date:||Jan 1, 1984|
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