The selection of machine-tools is largely a matter of judgment, based on the consideration of many variable factors. No fixed rules can be laid down, but the uses to which the machines are put are divided roughly into three classes which govern to a large extent the types of machine that should be purchased, whether they should be machines of a wide range of usefulness, standardized machines equipped with special tooling or special-purpose machines of special design capable of very large and continuous production. To determine into which of these classes the requirement for new machines falls, an analysis should be made of the following factors: (a) quantity of production required and its duration, (b) method of machining and tolerances and finish required, (c) possibility of a change in design of the product, (d) cost of production, (e) when delivery of machine is required, and (f) money available for the purchase.
When the type of equipment needed has been decided, an inquiry to elicit offers of machine-tools and quotations of prices may be broadcast through trade journals or addressed to a selected list of machine-tool builders. The former method is not recommended as it usually brings representatives of firms that build other types of equipment as well as the particular type desired. Information given in the inquiry should be as complete as possible and in the case of a special machine should be accompanied by a rough sketch and a statement of the method of handling the work into and out of the machine, surfaces to be machined, work that has been performed in previous operations, hardness of the material and data on feeds and speeds, and, if possible, a sample of the work to be performed. The builder should be given as much latitude as possible in making his recommendations, because the buyer is dependent largely upon the manufacturer for information regarding new methods and processes that have been developed among machine-tool users.
Some machine-tool builders, when making quotations, give only a general description of their machine and the price and, if any production estimate is given, merely state the number of pieces per hour that may be expected without accompanying data to support the statement. Other builders send very complete information and guarantee a definite production under certain standard conditions at a definite cost. It is a simple matter for the buyer to check the information given in the latter kind of quotation and compare it with other quotations.
Production capacity, cost of the equipment and its ability to give satisfactory service are major factors to be considered when making a selection of machinetools. Other factors that may be of much or minor importance according to circumstances include proximity and reputation of the builder, service that may be expected, completeness of the builder's stock of repair parts, whether or not the builder furnishes repair-parts lists, floor space occupied by the machine, and convenience of its operation. Another factor of considerable importance is the question of standardization of equipment in a production plant. If a number of machines of one make, type and size are in use in the plant, it is often advisable to continue to buy the same make and size of machine, so that attachments and fixtures will be interchangeable and the stock of repair parts will not need to be increased.
The author concludes his paper with brief references to some common causes of trouble with machine-tools, such as inadequate countershafts; electric motor mountings that are apparently added as an afterthought; insufficient width of belt pulleys; inadequate oil-pumps; cast-iron heads, feed mechanisms, earns and levers; lack of accessibility for making repairs; and multiplicity of oil-cups and oil-holes instead of a system for lubricating all parts from a central reservoir.