Tool Length Offsets on the Mill - 4: Storing the Length in Memory
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
From the previous page you discovered that the controller can retrieve tool offset data from memory and that the operator or set up person was the person who placed that data into memory. For newer machines the controller keeps the offset values in the same location as the tool number and compensation is made automatically. However for somewhat older machines (the FANUC 6Mb shown in these videos) the offset data must be associated with the tool in the block of code. This is accomplished by combining the location of the data (the address) with the G43. The "H" word is used for this. For example:

G43H1 tells the controller to offset the currently selected tool with the value found in the H1 memory location. It is customary in machine shops - if not absolutely law - to have the H number correspond to the T number. In other words if the tool number used was 3 then the offset for that tool would be found at H3. Be careful here. Unless there is an obvious reason not to, follow this rule. A look at a few blocks of code using G43.

  • N1000T1M6 - changes the tool (M6) in the spindle to tool #1 (T1)
  • N1005 - empty here but there may be a specific “safety” string in this space. Check your company's procedure.
  • N1010G43Z.1H1 - This block tells the controller to compensate for the length of the tool (G43) by looking in memory at location #1 (H1) and adjusting the Z length according to the value it finds there. Next to go immediately to Z.1 on the part.

With tool length compensation the programmers never needs to concern themselves with the length of tools. The length of each tool is taken care of at the machine. The only thing a programmer needs to remember is to write a block of code for each tool change that adjusts for length. That code word is G43 and the associated word, H. However, as mentioned, newer machines simply store the offset data with the tool number and do not require a separate H word.

Tool offsets on most modern CNC machines are accomplished with touch probs as show above
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label: CNC mill,. Offsets, G43 H43,