Program Verification & First Article
Writing the program is only half the job. This page covers the verification workflow that separates a production programmer from someone who crashes machines: dry run, single block, reduced feed, and first article inspection.
Concept
Program verification is the process of confirming that the program will produce the intended part before committing to a full production run. The sequence is: syntax review → coordinate review → offset review → CAM simulation → dry run on the machine → single block → reduced feed → first part cut → inspection. Each step catches a different class of error.
Why It Matters
Every crash started as a "should be fine" program. Mental simulation is unreliable: you read over the same wrong coordinate three times and still miss it. A 5-minute dry run at reduced feed catches errors that cost 4 hours of machine repair, a broken spindle, and a scrapped part. The cost of verification is tiny compared to the cost of a crash. The engineering habit of verification is what lets you run unattended programs with confidence.
How It Works
Before Hitting Cycle Start — Checklist
- Units — G21 (metric) or G20 (inch)? Confirm both in the program and on the controller.
- Work offset — G54 set to the correct datum on the part? Check X, Y, and Z origin.
- Tool length offsets — H01, H02... measured and loaded? Not zero?
- Tool radius offsets — D values match the actual tool radii?
- Spindle direction and speed — M03 forward, S in the correct range?
- Z clearance — is the tool at a safe Z above the stock, not at cut depth?
- Tool number — does T01 in the program match what is physically in the spindle?
- Startup block — does the program begin with G21 G17 G90 G40 G49 G80?
Verification Sequence
| Step | What You Do | Catches |
|---|---|---|
| 1. Syntax review | Read the code top to bottom on paper | Missing %, wrong O number, typos |
| 2. Coordinate review | Sketch the tool path on paper | Wrong coordinates, wrong direction |
| 3. Offset review | Check G54, H, D on controller | Missing or wrong offsets |
| 4. CAM simulation | Watch material removal in CAM | Obvious collisions, wrong tool path |
| 5. Dry run | Tool at safe Z, run at rapid | Gross coordinate errors, limit switches |
| 6. Single block | Block-by-block at reduced feed | Per-motion errors you would miss at speed |
| 7. Reduced feed cut | First cut at 25–50% feed override | Tool deflection, chatter, chip issues |
| 8. First article | Measure every dimension | Offset errors, tool wear, dimensional drift |
First Article Inspection
After the first part runs through at reduced feed, stop and measure. Check every dimension the drawing calls for: hole positions, depths, diameters, and surface finish. If dimensions are off, the cause is almost always an offset issue, not a program issue. Adjust the G54 or H offset by the measured error and run a second part. Only after the first article passes inspection do you increase feed to production rate.
Example
Before first cut checklist for a drilling program:
- G21 G90 G17 active in the startup block
- G54 origin at the correct corner of the stock
- H01 tool length offset measured and loaded (not 0)
- Spindle S2000 M03 forward rotation
- G00 Z50 above the stock (not hovering at Z−5)
- T01 in the spindle is the Ø6 drill the program expects
- G80 at the end of the hole pattern
Run dry run with the tool at Z50 (no cutting), watching the XY positions. Then single-block the first hole down to Z-15. Confirm the tool is at the right X/Y before allowing it to feed down.
Common Mistakes
- Running full feed on first part — use 25–50% feed override until the first part passes inspection.
- Skipping dry run — dry run at rapid with tool above stock catches gross coordinate errors before any cutting.
- Trusting CAM simulation — CAM does not know if your G54 is wrong or if H01 is unloaded.
- Not measuring the first part — "it looks right" is not inspection. Measure every critical dimension.
- Adjusting the program to compensate for offset errors — if the hole is 0.2 mm off in X, fix G54 X, not the X coordinates in the program.
Practice
1. Why use reduced feed on the first article?
Ans
So you can hit feed hold immediately if something looks wrong, before the tool cuts into the part. At full feed, a 0.5-second delay can scrap the part.
2. What three offsets must be verified before running?
Ans
Work offset (G54 X/Y/Z), tool length offset (H), and tool radius offset (D).
3. CAM simulation looks perfect. What might still be wrong?
Ans
Work offset, tool length, tool number, units, spindle direction — anything in the physical setup that the CAM model does not know about. Only the machine dry run reveals setup errors.
4. The first part comes out 0.3 mm too large in X. Do you change the program X coordinates?
Ans
No. Fix G54 X offset by the measured error (add or subtract 0.3 mm). Changing the program makes it wrong for the next setup.