This doesn't have to apply only to Haas but that is what we use so the description is written for a Haas control.
There have occasionally been questions about setting tool offsets. Here is the technique we use with a Toolsetter; it is slightly different to what I have seen described and I think it has some advantages.
The first picture shows the tool being brought down to the Toolsetter until the dial is zero; a close-up of the dial is inset. The Z coordinate for this tool position is entered using TOOL OFSET MESUR. A very important thing to note here is that the top of the Toolsetter must be above the location on the part that is Z zero and must be below the toolchange position. But the exact height does not need to be known.
Offsets are entered for all the tools this way.
The second picture shows a cap with a hole placed on the Toolsetter, this cap brings the dial to zero, a dial gauge is in the spindle and the Z axis is brought down until the spindle dial is at zero. The Z coordinate for this position is recorded and called Zs.
The third picture shows the dial in the spindle moved over to the part with the Z axis brought down until the dial reads zero. In this example Z zero for the part is at the finished surface so a previously machined part is in the fixture. The Z coordinate for this position is recorded and called Zw.
Now the difference between Zs and Zw is obtained; when the Toolsetter has been placed so it is higher than the part Z zero, this difference will be a negative value and is called Zd. Zd must be a negative value for error prevention.
The Zd value is entered into the Z column for the Work Zero(es).
Advantages:
It is quick and accurate. The accuracy depends on the machine and dial gauge accuracy; provided the neither Toolsetter nor the cap are not sitting on a film of coolant which can introduce an error of 1 to 2 thou.
Using a Toolsetter has the inherent advantage that the tools are being brought down to a spring loaded plunger not against a solid block. If the operator overshoots a few thou nothing serious happens. Although if the operator puts the Toolsetter above the toolchange position and hits NEXT TOOL rather serious things happen.
When multiple parts are held in fixtures that have been whipped up quickly and are not as precise as they could be, it is an easy operation to get Zw for each part position and have a specific Zd for each part work zero thus compensating for the fixture error.
When the Toolsetter is at the correct height and the Zd is a negative value it is difficult to get a wrong tool offset. The most likely error, at least in my experience, when entering values is putting in the wrong sign. When the value should be negative and a positive is entered all that happens is the tool goes well above the part and cuts air.