Are Machining Centres Taking Over Jig Bore Work?

The escalating challenges of producing tight tolerance parts in a production environment and the growing need for cutting components with complex free-form surfaces are driving the demand for jig borer-like machines with high productivity. By William Malanche, Mitsui Seiki USA Inc.

In rare instances, machining centres can accomplish what would be considered jig borer applications, however that success revolves around the machine design and fitment, coupled with the requirements of the part and the tolerances to be achieved. For example, if the part is flat with little depth and it requires jig bore type positioning accuracies (typically 0.0002” and under) then most modern day controls have the capability to apply enough pitch error, inclination and/or straightness compensation to “laser in” a pretty good line at a specific height. But this assumes that engineers will use a machine that has a certain mass, structure, and design capable of repeating accuracy. Without that fundamental trait of repeatability, it is impossible to get consistent results using electronic compensation.

A good analogy surrounding machine tool accuracy and high precision manufacturing is the sport of target shooting with a rifle or handgun. If you hit the bullseye, is that accurate? Absolutely! But if you can’t keep all of the shots in a consistent grouping, you are far from precise. You can have a fancy scope with all kinds of adjustments for distance, elevation, and windage (like CNC control level compensations), but unless you have a solid stance or bench to shoot from (a good solid machine tool foundation), steady hands, or consistent trigger pull, among others, you can’t reliably expect to hit the target. There are the fundamental mechanics that need to be in place, so you can apply the technology effectively. The same applies to machines.

Legacy Jig Borers

In the early days, jig borers were designed to be built from the table surface. Builders began with a stable base material like cast iron, with a low CTE (Coefficient of Thermal Expansion) put it on a solid three-point footing, and hand scraped the table surface – which was essentially the gauge line of the machine. Every measurement was taken in relationship to that line and the machine built out from there. It took a lot of adjustments to make sure that everything stacked above and under the table was square and perpendicular to that gauge line surface. Typically, those adjustments were done by hand scraping, fitting, measuring the results with traceable gauges and squares, and then repeating the process to the required level.

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