Good machining practices: Test cuts

This is one of many articles in a series that is meant to reduce setup, program and run time errors by forming good habits as a CNC machine operator. Discussions include properly proving a new program, making test cuts, appropriate inspection techniques and hundreds more procedures that are necessary to master in theory and practice.  

Every machinist, no matter how experienced gets a little nervous when it comes time for a close tolerance finish pass. Typically, the closer the tolerance the more passes one will make, progressively sneaking up on the finish dimension. This can be time consuming but most consider it the cost for a quality part. I do not know of any estimators that factor in the meticulous actions taken by top-notch machinist while figuring part process time for costing.

In reality, it is not necessary to take a test cut but only a test incision into the part geometry. Only cut deep enough to insert the proper gauge into the geometric feature. If you think about it, most close tolerance diameters are a tight fit because another part is going to be pressed inside. Due to this fact, all closely dimensioned features usually have a lead in chamfer. Only cutting to the depth of this chamfer leaves the machinist stock in the case of an error. Even if there is not a chamfer, is it not better to contact your customer and report a out of tolerance dimension only a fraction deep than informing them that the whole feature diameter is in non-conformance.

Machinist should be trained on the proper procedure for test cut / finish cut operations. All machined parts, machine tools and even cutting inserts have a tolerance. The insert tolerance can become a critical factor on very tight fit parts. Inserts should not be changed between a test cut and a finish cut. Many issues can be created by changing the cutting insert from dirt lodged between the insert and the seat to tightening the cam lock screw tighter than the previous operator. In the event a tragedy occurs and an insert has to be changed: blow out the pocket with compressed air and flip the insert over instead of rotating it to expose a new tip. This will minimize the tolerance factor due to indexing.

Another process improvement gained by making a shallow test cut is the fact that more stock is left on the part to be removed during the finish pass, improving the surface finish and creating carbide glaze.

Test cuts can also be employed to inspect the geometric accuracy of machined profiles. Removing a large amount of material for a large cast blank only to find out the programmed profile is wrong or the wrong diameter tool was selected by an operator. Making a light (.01) deep test cut into the material and then inspecting and verifying it was machine correctly is a part saving operation well worth the time and effort. The best way to implement this is to have a separate operation programmed into the part program. Leaving this responsibility to the operator by offsetting the machine to the test cut depth leaves this operation at risk and could be overlooked.

Remember to test cut before you cut.