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Did you know there are three ways you can touch off your tools?

Because of how Machine Offsets add up, there are several ways CNC machinists can set their Tool and Work Offsets.

This is especially true for Tool Length Offsets.

Tool Offsets can be either Positive or Negative.
Depending on your Machine Shop equipment you should use one or the other.

Regardless of how you set your tool length offset, you apply it the same way.
Right after the tool change and after turning on your spindle and moving to your X Y position above the part.
The very first absolute Z movement should be the line where you apply the tool length offset.


Positive Tool Offsets (gage line tool length offsets)

In the case of Positive Tool Offsets, the offset represents the Length of the tool measured as a distance from the Gauge Line of the spindle (typically spindle nose) to the tip of the tool. The longer the tool, the larger your Tool Length offset will be.

In such a case, Part Z Work Offset will represent the distance between the same Gage Line to the top of the part.

Pro's of Gage Line (Positive) offsets:

  1. Tool Length Offset remains the same between many machines.
    You can just pull the tool out of one machine, Put it into the other one, Type in its Tool Offset and off you go. There is no need to touch off the tool again.
  2. Positive offsets are the easiest to wrap your head around.
    Each definition represents exactly what it does.
  3. Positive Tool Offsets can be measured offline on a pre-setter and then tools can be quickly loaded into the machine without the need to tough off each tool on the machine individually.
  4. In addition to offline pre-setters, most tool tough-off probes also operate in the Gage Line system

Cons of Gage Line offsets:

  1. Can be less practical on machines without tool pre-setters
  2. You are forced to operate with large negative Z Work Offset values
  3. If your tool pre-setter goes down (who never had a dead battery?) you will have a hard time trying to touch off your tools the alternative way.

In summary. Positive or Gage Line Tool Offsets are practical on modestly to highly standardized/automated machining environment.

Negative Tool Offsets

Negative Tool Offsets are dead-simple.
In its easiest form, it represents the (negative) distance between the tip of the tool to the top of the part.
In such case Z Work Offset will equal zero:

This Tool Offset setting style is often used on older machinery in non-automated or non-standardized machining environments.

The initial Top Of Part flat surface is often faced and filler gage is used to set the tool of the top of it.

Pros of Top of Part Negative Tool Offsets:

  1. Dead-simple. Tool Offset represents the distance between the top of the part and the tip of the tool.
  2. This Tool Offset setting style is supported by default on most machines. Just jog the tool to the Top of Part and press "Write Offset"

Con's of this Offset Type:

  1. Tools need to be re-touched for each and every job whenever the height of the part changes
  2. Tool offsets are not interchangeable between several machines
  3. The part needs to be faced in order to set the tool offsets
  4. Can not set tool offsets off a curved or irregular surface: Imagine you need to rework an already machined part. How will you set the tool heights?

Top of Gage Block Negative Tool Offsets:

You can, however, have a little bit of both worlds.

If your machine is not equipped with an automatic tool pre-setter you have to use your machine's manual tool offsetting routine.
You can Touch Off all your tools of a pre-defined gage block off the top of your table or ground vice surface.

In this case, Z Work Offset will equal the distance between the top of the touch-off gauge and the top of your part.

It is extremely handy to touch off your tools of the solid vice jaw using a 1x2x3 block or a special analog Tool Probe with a dial scale.
Just make sure to use the same height (for example the 2-inch side) all the time.

Because when machining in a vise the part stick-out is the most important value, it is easy to calculate the proper Z Work Offset height.
All you have to do is set your Z Work Offset to -2.0 (in the case when you tough your tools 2" above the top of the jaw) and then add the height of your part above the vice.

Pros of Top of Gage Block Negative Tool Offsets:

  1. The same tools can be used without the need to re-set their Tool Offsets between jobs. Only your Z Work Offset changes.
  2. This Tool Offsetting Routine is supported by most CNC machines.
  3. No need to face the top of your part to set your tools or work offset. Just calculate the distance between the top of the gauge and the top of your part.
    A dial indicator can be used to set the Z Work Offset
  4. A tool pre-setter can be used to set the tool offsets. You would need to supply the Machine Gage Height value to the pre-setter software to calculate the proper Tool Offset value.

 Con's of Top of Gage Block Negative Tool Offsets:

  1. The math involved can be a bit challenging at first. Thus not many beginning machinists use it.
    Eventually, however, most figure out and "invent" this method for themselves
  2. Tool Offsets will be different between different machines. So they need to be touched off again when used on a different machine.

Which Tool Offset Setting Type to Use in Your Shop?

It depends on your equipment and shop practices.
You should, however, stick to one particular Tool Setting style and use it across the shop to avoid machine crashes and mishaps.

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