Automatic Part Fabrication
Part fabrication by automatic machine requires little difference in design from part fabrication by human-operated machine except for the features required for automatic handling and feeding described below. However, burrs are poison! They jam. A human assembler will deburr, discard bad parts, and re-form bent parts; an assembly machine will not unless you provide extra stations to do so.
Automatic Assembly
Assembly by automatic machine requires substantial modification of a design as made for manual assembly, or the assembly machine will be needlessly expensive and unreliable.
Ten design rules for automatic assembly:
- Make the parts robust enough to withstand handling by ma-chine. Fragile parts can be handled without breakage by careful humans but not by machines.
- Avoid near symmetry. A machine may have to orient the part, and it is easier to make a machine discriminate among different orientations if the part is very different in its different orientations.
- Eliminate shapes which can tangle. Humans can untangle springs, pigtails, and hooks, but machines lack human vision and dexterity. Springs are the worst offenders. Helical springs should have closed-end turns. If possible, they should have non-uniform pitch.
- Design parts which will come out uniform from their fabricating process. Non-uniform parts jam in feeders and handlers.
- Visualize the motions the parts go through during assembly, and design them to go together with simple motions.
- Avoid screw, washer, and nut combinations. Washers and nuts have to be held in place by part of the machine while the screw is driven by another part of the machine. Use self-tapping screws or screws with captive washers. Best of all, eliminate fasteners and make the parts snap together or be held together with toy tabs or the equivalent.
- Minimize part count. For each part there must be a separate increment of assembly machine. A special case occurs when there is more than one of the same part and the machine can assemble all the identical parts at the same station with an indexing portion to move the assembling operation from place to place. Printed-circuit-board assembly is an extreme case of this practice.
- Where male and female parts engage, provide tapers on one or both parts to guide them together despite small alignment errors in feeding.
- Consult your manufacturing engineers and part vendors to see if parts can be oriented and loaded into magazines which will then be loaded into the assembly machine. This practice is now widespread in the electronics industry but not spread widely enough. Replacing vibratory feeders with magazine feeders is the best single thing you can do to make assembly machines simpler and more reliable. Furthermore, some of the other rules can be ignored with magazine feeding.
- Design parts which can be fabricated right at the assembly ma chine, as they are used. No magazining or vibratory feeding is needed. Small stampings made from strip stock and helical springs which do not need heat treatment or plating are good examples.
Despite its glamour, full automation is not the most cost-efficient solution to every manufacturing problem. It requires the greatest investment of capital, it takes the most time to design and set up, it requires product design modifications to make it possible, and therefore it pays off only for very large quantities. With a computer-controlled machine which can be quickly reprogrammed (including NC machining) individual batches may be small, even one-piece, but total quantity per year must be large to justify the investment in the machine. Furthermore, there are severe limits to what you can do by just reprogramming control computers: reprogramming does not change fixtures or dies, conveyors, feeders, or many other elements necessary to make a product.
Fully Automatic Machines
There are several types of fully automatic machines:
- Robots, jointed-arm and Cartesian, for machine loading and unloading, for tool handling (welding, deburring, painting) and for some assembling
- Automatic storage and retrieval machines (a class of Cartesian robot)
- Automatic guided vehicles
- Automatic-assembly machines
- Machine tools (NC or template- or cam-controlled)
- Looms, spinning machines, chain makers, headers, and innumerable other specific-product machines
Human-Controlled Powered Machines
Such machines include the following:
- Fork trucks. These do the work of automatic guided vehicles and au-tomatic storage and retrieval machines but trade off capital cost for a human driver.
- Load balancers and cranes. These enable a human to manipulate heavy loads without the cost of a robot.
- Machine tools, human-controlled and -loaded or -unloaded.
- Human-loaded and -unloaded processing machines of many kinds.
Power tools in human hands require more skill but cost much less than automatic or semiautomatic machinery. Powered hand tools use electric, pneumatic, or hydraulic power. Some have internal part feeders, such as riveters.
Human Work with Special Hand Tools
Surgeons, dentists, and optometrists all have developed specially formed, well-made hand tools to match specific tasks. The variety of hand tools used in manufacturing is small, and rarely does one see a truly clever special tool in use. A special case is automobile maintenance, for which many special tools are made in quantity and sold to garages.
