What is a time basis?

The time basis states how a time was arrived at. Four bases are common in a machining estimate. Estimated: derived in advance from geometry, material, machine and experience, without a program. From CAM: calculated from the toolpath lengths and the programmed feeds. Simulated: the NC program run through a model of the machine and its control. Measured: the time a real run took, from the control, an MES or time booking. The basis says where a time comes from, and so something about its uncertainty, not whether it is right in a given case.

Why do times on different bases differ?

Each basis leaves something out. An estimate rests on reference figures and comparable work and misses the detail of the program. A CAM time often assumes the programmed feed, leaves out loading and unloading, and takes little or no account of axis acceleration and deceleration, the control's look-ahead, tool changes and probing cycles. With short segments and frequent changes of direction it can therefore come out lower than the real time. A simulation gets closer to the machine the better its model of kinematics and control. A measured time holds for that one run, operator overrides and interruptions included, and varies between batches. Hours booked in time recording often include more than machine time, such as waiting, measuring and reclamping, and are only as reliable as the booking discipline.

How does the time basis work in practice?

An estimated 14 minutes and a measured 14 minutes are the same number with a different uncertainty. Keep the basis per line and you can see which times are least certain and which to check first when the margin is thin, for example with a CAM program or a trial part. In post-calculation you compare estimated with measured times, which only makes sense when the basis of each time is known. When an experienced colleague corrects a time, the number changes but the basis does not: without a CAM program, a simulation or a measurement it is still an estimate.

What is the time basis often confused with?

The time basis is not accuracy: an estimate can sit close to reality and a CAM time far from it. Nor is it a kind of time: setup time, cycle time and machining time are kinds of time, and most can sit on more than one basis. Setup time, for example, does not come from CAM, but it can come from an estimate or a measurement. A labelled estimate is one basis, not the whole idea.

How do you see the time basis in Blake?

In Blake every calculated time is an estimate and is labelled as one, with time, rate and quantity as the inputs on the line. An adjusted time requires a reason, and who, when and why stay with the line. More on estimating.