From Drawing to Process Plan

Good programs start before the machine: they start on paper. Reading the drawing, choosing the datum, and sequencing operations determines whether the part works or scrapes. This page is the bridge between engineering drawing and G-code.

Concept

A process plan translates a drawing into ordered machine operations. Step 1: read the drawing and identify critical dimensions, datums, and surface finish requirements. Step 2: choose work zero (datum). Step 3: sequence operations — roughing first, finishing last, holes after profile. Step 4: select tools and cutting parameters. Only then write G-code.

Why It Matters

Writing code before planning produces a program that machines the wrong features in the wrong order. A part that needs a Ø10 H7 hole should be roughed to Ø9.8 then bored or reamed — not drilled to size directly. The process plan determines the tool list, cycle time, and whether the part holds tolerance. Skipping planning is the #1 cause of scrapped first-article parts.

The Process Plan Template

1. Material and stock — what are you starting from? Raw bar, plate, forging? Size and condition?
2. Datum — which feature does the drawing reference? Set work zero there. The datum should be a finished surface, not raw stock.
3. Operations in order — face, rough, finish, drill, tap, inspect. Always face first to establish Z0.
4. Tools — which tool for each operation? What H/D offset numbers? How many tools total?
5. Cutting parameters — RPM and feed from tooling data, not guesswork.
6. Inspection — which dimensions to check after each setup? What gauge?

Drawing Review Checklist

• Dimensions — identify all dimensions and their tolerances. Which are critical? Which are nominal?
• Datums — where do the dimensions reference from? That is where G54 should be.
• Surface finish — Ra requirements determine whether you need a finish pass.
• Holes — what size, tolerance, and depth? Drill directly vs drill-ream vs drill-bore?
• Threads — standard or custom? Internal or external? Tapping vs thread milling?
• Chamfers/fillets — what size? Which tool can produce them?

Example: Plate 100×80×12

A rectangular plate with a pocket and 4 holes. Process plan:

1. Face top surface with Ø63 face mill (establishes Z0)
2. Rough pocket with Ø10 end mill (leave 0.2 mm finish stock)
3. Finish pocket to size (light cut, good surface)
4. Spot drill 4×Ø6 holes (center drill)
5. Drill 4×Ø6 holes
6. Deburr edges

Work zero at lower-left corner on the top surface. Tool list: T01 face mill, T02 roughing end mill, T03 finishing end mill, T04 spot drill, T05 drill. Five tools, five operations, one setup.

Common Mistakes

• Drilling before milling the flat surface — holes start on uneven stock and wander.
• Choosing work zero at a corner that is not the drawing datum — dimensions accumulate from the wrong origin.
• Skipping roughing and trying to finish in one cut — too much stock causes tool deflection and poor finish.
• Not planning inspection points — you must know what to measure before you measure it.
• Underestimating tool changes — each tool change takes time; consolidate operations where possible.

Practice

1. Why rough before finish?

2. What is the first operation on a raw plate?

3. Why spot drill before drilling?

4. What is a datum, and why does it matter?