First let’s talk about the key item on everyone’s list, and that is accuracy. First is understand what your part tolerance is, typically you want something that is at least twice the accuracy that you intend to measure for a DCC machine and three times the accuracy for a manual system, due to increased operator influence on the measurement. IE: if your tolerance is 0.001 you need a machine that is at least 0.0005. Accuracy is typically given in a formula so do the math and work out the formula, also insure that the accuracy stated against a standard such as ISO 10360, and also insure that the accuracy statement is at least 2 sigma to insure reliable results
Determining machine size sometimes is sometimes difficult, because measurement range and part size are not always true correlation. Features can create a need for extended machine sizes. An feature such as a cylinder perpendicular to a plane with some depth can create a need for a longer axis. IE 14-inch-long part with a 8 inch deep cylinder could generate the need for machine with a minimum 22 inch measuring length in order to meet the requirement for measuring that feature. Supplying part prints to a potential supplier and have them quote to your need is always recommended, but the quote should be explained in detail for each item included with the reason why either in writing or in a meeting.
How you plan on using the machine the most is important as this will help determine, things to be considered are number of parts that will be inspected, will it be small lots, one off parts or quantity inspection. If you are measuring small lots or one off parts software simplicity should be high priority, it might be important to have software that would allow you to bring a part to the machine and start measuring immediately without having to create a program, it should also have the ability to store this program for future use. It would also be ideal that the same software would allow you to program from a CAD file if needed. IF the software does not offer this type of flexibility or it is difficult or clumsy to use and not intuitive, you might rule this out as a possibility, but if you are running large runs of the same parts, then the capability to program off line and create programs from CAD may be more important even if it is more difficult to use.
Other considerations that are to be looked at should include, what are the parts like, are they basic prismatic measurements, such as blocks, cases and others or do we do free form surfaces or non-prismatic parts, such as airfoils, blades etc. do I want to do form type measurements, will I be trying to reverse engineer items, each of these need to be reviewed as they can affect the price that you will pay, by increasing the complexity of the software, creating a demand for specialized or dedicated probe systems. Will I need to measure flexible parts such as gaskets, rubber or plastics that are thin walled and can create reproducibility issues. Most of all be prioritize what you must have and what you would like to have and set a budget that you can live within.