Method 1: Using A Height Gauge (Pre-Setter) Off The Machine
This method uses a dedicated precision instrument called a tool pre-setter.
How it works: You measure each tool in the pre-setter, which gives you an exact length from the tool holder's gauge line (the tapered section that seats in the spindle). You then manually enter these values into the tool offset table in the machine control.
Pros:
Very accurate and repeatable.
Done offline; doesn't tie up the machine.
Safer for beginners (no risk of crashing tools into the part).
Cons:
Requires expensive additional equipment.
You must also set the workpiece Z-zero separately.
Method 2: The Manual "Touch-Off" Method On The Machine
This is the most common method for hobbyists, job shops, and one-off parts. You use a known object (like a gauge block or feeler gauge) on top of your workpiece or a fixed datumn (like the machine table).
Tools needed: A 1-2-3 block, gauge block, or feeler gauge (e.g., 0.100" or 1.0mm).
Pros:
Inexpensive, only requires simple tools.
Very accurate if done carefully.
Directly references the part surface.
Cons:
Takes more time per tool.
Higher risk of human error or crashing if not careful.
Method 3: Using A Tool Touch Off Probe (TTT) On The Machine
Common on production and higher-end machines. A probe is permanently mounted on the machine table.
How it works: You run a macro program (e.g., G65 P9001 T01) which automatically moves the tool down until it makes electrical contact with the probe. The control then automatically calculates and records the TLO value.
Pros:
Extremely fast and efficient.
Highly repeatable and accurate.
Reduces operator skill required.
Cons:
Requires an expensive probe and compatible control.
The probe must be calibrated.
Step-By-Step: Manual Touch-Off Method (Using A Gauge Block)
This is the method you'll likely use most often. Let's assume your workpiece Z-zero is set on the top surface.
1. Secure Workpiece & Install Tool: Clamp your workpiece and install Tool #1 (e.g., a face mill) into the spindle. Jog the machine to a safe position over the part.
2. Jog the Tool Down: Using the handwheel, carefully jog the rotating tool down until it is close to the top of your part. Go very slow!
3. Use a Gauge Block: Select a gauge block of a known size (e.g., 1.000"). Hold it between the tip of the tool and the top of the part.
4. "Feel" the Slip: Slowly jog the tool down in very small increments (0.0001" or 0.01mm). The goal is to lower the tool until the gauge block just slips between the tool and the part with a slight drag. You are not crushing the block, just removing the slack.
● Pro Tip: A better method is to use a non-rotating tool and a feeler gauge. You try to push/pull the feeler gauge. When the drag is consistent, you are at the exact height of the feeler gauge.
5. Set the Offset:
● On the control, navigate to your offset page (usually labeled OFFSET or TOOL OFFSETS).
●Find the row for the tool number you are setting (e.g., Offset #1 for Tool #1).
●With the cursor in the Z-offset field for that tool, you will either:
◇Press a button called "Measure", "Set Tool", or "Part Zero Set". This is the modern way-the control does the math for you.
◇OR manually calculate and type the value. Look at your current machine Z-position (e.g., Z-1.523). If you are using a 1.000" block, your offset value is: -1.523 - 1.000 = -2.523. You would enter -2.523 into the offset field. (The math will vary slightly by control; always consult your manual).
6. Repeat for All Tools: Move to a safe location, change to Tool #2 (e.g., a drill), and repeat steps 2-5. The gauge block height (1.000") remains your constant reference.
7. Verify: After setting all tools, it's excellent practice to verify.
◇Call each tool in MDI mode (e.g., T2 M6; G43 H2;).
◇Command it to go to Z-zero (e.g., G0 Z0;).
◇The tip of the tool should stop exactly at the top of your part. Do this with the spindle OFF and using the handwheel for the first time to be safe.
By understanding these methods, you will be able to accurately and confidently set up any machining job on your CNC mill.
