Milling Operations: Facing, Peripheral Milling, and Squaring Stock

Milling Operations: Facing, Peripheral Milling, and Squaring Stock

Squaring a block of mild-steel stock is one of the most fundamental tasks in manual milling. The finished part must have six flat faces with all adjacent faces perpendicular to one another. This lesson walks you through the planning, machining, and inspection steps needed to hold flatness within 0.003 in and perpendicularity within 0.002 in per inch.

Why Squaring Sequence Matters

Every dimension and feature you add later depends on the accuracy of your reference surfaces. If you machine faces in a random order, errors compound: a slightly bowed face becomes your datum, and everything measured from it inherits that error. The correct approach is to establish one reliable reference face first, then use it to create a second face perpendicular to the first, and continue in a logical order so each new face is located from a face already proven accurate.

A standard six-face squaring sequence is:

1. Face 1 (datum) — machine the largest flat surface first; this becomes your primary datum.

2. Face 2 — flip the part so Face 1 sits on parallels; machine the opposite face parallel to Face 1 and to thickness.

3. Face 3 — stand the part on Face 1; machine a side face perpendicular to Face 1.

4. Face 4 — flip and machine the face opposite Face 3.

5. Face 5 and 6 — machine the remaining two faces (ends) last.

Annotate your operation sheet with this sequence before you touch the machine. Your instructor will verify the logic before cutting begins.

Setting Up the Vise and Parallels

A Kurt-style vise with hardened jaws and matched parallels is the standard setup for squaring work. Key setup rules:

• Clean the vise bed, jaws, parallels, and workpiece with a brush and rag. A single chip under a parallel can tilt the part by thousandths.
• Set parallels so the workpiece stands above the jaws enough to allow cutter clearance but is gripped firmly — roughly half to two-thirds of the stock height in the jaws.
• After clamping, tap the top of the part with a soft mallet while tightening the final clamp to seat it fully on the parallels. Verify seating by trying to slide each parallel; it should be snug.
• Re-indicate the vise whenever you reposition the part. A dial indicator swept along the fixed jaw should read within 0.001 in over its full length. If it does not, adjust and re-sweep.

Face Milling and Peripheral Milling

Face milling uses a shell-type face mill whose cutting edges are on the bottom face. It removes material quickly and leaves a flat surface. Use it for the large flat faces (Faces 1 and 2) on mild steel with a step-over of roughly 40–50% of cutter diameter.

Peripheral (slab) milling uses the side cutting edges of an end mill. Use a ¾ in end mill for roughing the sides and a ½ in end mill for finishing passes where dimension control is critical.

Climb vs. Conventional Milling

• Conventional milling (cutter rotation opposes feed direction): chip starts thin and grows. Greater cutting force pulls the part into the cutter, but it is stable and preferred for finishing passes on a manually fed knee mill because backlash is less likely to cause chatter or dig-in.
• Climb milling (cutter rotation matches feed direction): chip starts thick and thins. Smoother surface finish and less heat buildup make it suitable for roughing passes, but only when the vise is rigid and backlash is minimal — check that the knee mill table locks are snug on axes not in use.

As a general rule in this lab: rough with climb, finish with conventional.

Speeds and Feeds for 1018/A36 Mild Steel

For HSS end mills in mild steel, a surface speed in the range of 80–100 SFM is a reliable starting point. Convert to RPM using: RPM = (SFM × 3.82) ÷ Cutter Diameter (in). Feed per tooth for a 4-flute HSS end mill is typically 0.001–0.002 in per tooth in mild steel. Adjust if chatter appears — reduce feed or increase RPM slightly.

Safety Rules for This Lab

• Eye protection (safety glasses with side shields) is required at all times on the shop floor. A face shield is added when using cutting fluid.
• Lockout/Tagout (LOTO) applies whenever you change a cutter, adjust a quill, or reach inside the machine envelope. De-energize the machine, apply the lock, and tag it before your hands enter the work zone.
• Never wear gloves when operating a rotating cutter — gloves can catch and pull your hand in.
• Brush chips away from the work zone with a chip brush, never by hand and never with compressed air directed at yourself or others.
• Secure loose clothing, tuck in shirt, remove rings and watches before operating.
• Secure the workpiece before starting the spindle; never adjust a workholding device with the spindle running.

Inspection: Flatness and Perpendicularity

Use the granite surface plate as your reference datum for inspection.

• Flatness: Place the face to be checked down on the surface plate. Sweep a dial indicator across the face using a height gauge or indicator stand. Total indicator reading (TIR) must not exceed 0.003 in.
• Perpendicularity: Stand the block on Face 1 on the granite plate. Hold a precision square against Face 3 (or any adjacent face). Use the dial indicator to sweep vertically; deviation per inch of height must not exceed 0.002 in. Use 1-2-3 blocks as needed to support the part.

Record every measurement on the inspection report. Any face or angle outside tolerance goes back to the machine; you re-machine and re-inspect before credit is awarded.