NC drilling: tolerance and surface finish.

In hydraulic block manufacturing, NC drilling tolerance is not a single number. Every dimension has its own function and therefore its own tolerance requirement: a mounting hole and a valve cavity are not machined to the same precision.

What does ±0.02 mm NC drilling tolerance mean?

This value says that the dimension may deviate from the figure given on the drawing by at most two hundredths of a millimetre. That is about a third of the thickness of a human hair. On a cavity diameter this deviation directly determines how tightly the valve seats and how much the o-ring is squeezed.

Tightening the NC drilling tolerance is not always "better"; every tightening increases machining time and cost. The right approach is to keep the critical dimensions tight and leave the non-critical ones free.

Quality on the sealing surface

The o-ring takes the imprint of the surface it sits on. If the surface is too rough, micro channels form and leakage starts in time; if it is too polished, the o-ring slips and is hard to fit. For this reason the sealing areas of the cavities are machined differently from the remaining surfaces and, where necessary, ground.

Deviation in deep holes

The deeper the hole, the more the tool drifts. The two ends of a long passage may in reality be a few hundredths of a millimetre out, even if they look to be on the same axis on the drawing. This deviation can stop two passages that should intersect from meeting properly. The solution lies in planning the drilling sequence and the reference face before machining: if all dimensions are taken from the same reference, the errors do not accumulate.

Burrs: the smallest detail, the biggest risk

A burr always forms where two passages intersect. A piece of burr left inside the block can break off at first start-up, travel to the seating surface of a valve and stop the system. Removing the burr at the intersections mechanically or thermally, and then washing the passages, is for that reason an inseparable part of manufacturing.

Dimensional control and records

When machining is finished the critical dimensions are checked against the NC drilling tolerance: cavity diameters, depths, port positions. Recording this check is what makes the second batch come out the same as the first when the same part has to be produced again.

In short

If the dimensions hold, the part works; if the surface is right, it seals; if there is a record, it can be produced again. We describe the machining process and the NC drilling tolerance we work to on our precision machining page and the control chain on our quality page.