Mill Indicators and Part Zero

A test indicator sweeps with a lever; a dial indicator measures plunger travel

Instrument	Contact motion	Setup use
Test indicator	A short stylus pivots through a small angle.	Convenient for sweeping a vise jaw or a bore where access is tight.
Plunger dial indicator	The spindle, or plunger, moves straight through a fixed mounting stem into the body.	Useful for displacement checks and alignment when the plunger can face the measured motion.
Digital plunger indicator (shown below)	A spindle/plunger still moves linearly; an LCD replaces the pointer and dial.	Useful for the same kinds of displacement checks when its range, contact geometry and specifications fit the task.

A lever test indicator mounted on a separate stand. Its short stylus contacts a cylindrical part; the stand post, arm and clamps are visible, while the magnetic base is only partly in frame. — **Lever test indicator.** The short stylus and contact point pivot at the instrument body. The post, arm and clamps are parts of a separate stand; its magnetic base is only partly visible at the frame edge. Photo: David English (Fixerdave), Wikimedia Commons, CC BY-SA 4.0. Downscaled and converted to WebP; the adapted image remains CC BY-SA 4.0. No endorsement implied.

An analog plunger dial indicator mounted on a separate stand. Its straight spindle and contact point touch a turned shaft; the stand post, arm and clamps are visible, while the magnetic base is only partly in frame. — **Analog plunger indicator.** The body carries the dial, bezel and pointer; the straight spindle/plunger reaches the contact point. The stand is separate from the instrument, and its base is not fully shown. Photo: David English (Fixerdave), Wikimedia Commons, CC BY-SA 4.0. Downscaled and converted to WebP; the adapted image remains CC BY-SA 4.0. No endorsement implied.

A digital plunger indicator on a white background, showing the LCD body, buttons, fixed mounting stem, moving spindle and rounded contact point. — **Digital plunger indicator.** The LCD and buttons change the readout, not the contact mechanism. Below the body, the fixed mounting stem surrounds the moving spindle/plunger, which ends at the rounded contact point. No stand is shown. Photo: Huugo, Wikimedia Commons, CC0 1.0. Downscaled and converted to WebP. The manufacturer mark identifies the photographed instrument; no endorsement implied.

These displays do not define the contact mechanism: the digital instrument shown here is still a plunger indicator. Test and plunger indicators can both compare alignment. Use the maker's stylus angle for a test indicator; on a common lever design the stylus lies nearly parallel to the measured surface. Align a plunger with the displacement being measured. Incorrect contact angles change the scale factor.

Keep the mounting rigid, preload within the instrument's range and confirm repeatability. During a fixed-jaw sweep, keep the spindle stopped and the contact geometry unchanged. For a bore-centering sweep, turn the spindle by hand using the approved setup method. Follow Equipment Operating Procedure for the machine state during setup.

Contact angle affects the reading

Mitutoyo: Dial Test Indicators and the Cosine Effect shows the stylus geometry and correction. Its What is Cosine Error? video demonstrates the difference using a test indicator against a plunger indicator. Do not apply a plunger-indicator correction rule to a lever indicator without checking its instructions.

An edge finder locates the spindle center one tip radius from the edge

A mechanical edge finder has a spring-loaded contact tip that becomes concentric and then kicks sideways at contact. Use its specified spindle speed and approach method. Electronic edge finders and probes use different signals and procedures; do not assume they rotate like a mechanical edge finder.

Use a clean, defined edge, repeat the approach and account for the contact-tip radius. If the tip is 10 mm in diameter, the spindle center is 5 mm outside the edge at the trigger point. Setting that center position to zero without the radius correction moves every feature by 5 mm.

Example: the left face is X = 0 and positive X goes into the part to the right. At contact from the left with a 10 mm tip, the spindle center is X = −5 mm. Enter that position in the work-coordinate setting method for the control. On approach from a right-hand face, the sign changes. Sketch the spindle center and edge before choosing a sign.

Check both repeatability and the offset sign

Repeat the touch from the same direction. On manual feed screws, account for backlash when reversing; a handwheel reading can change while the table has not yet moved. Do not use a rough saw edge as though it were a finished datum. The radius correction is geometry, independent of the control's button sequence.

MIT's mill introduction covers mechanical edge finding and feed-screw backlash.

Find an edge, choose the sign, and check where the hole lands; Interactive

Find an edge, choose the sign, and check where the hole lands

Jog a mechanical edge finder toward any of the four edges, watch the tip wobble, centre and kick, then decide what the readout should read at that position. A check hole shows where your zero really is. The radius used is half the contact diameter, which can differ from the body diameter. Re-approach from the same direction each time; reversing a feed screw adds backlash, which this model does not include.

Wobble, centre, kick

Free: off-centre wobble

0.200 mm before the trigger: nearly concentric

At the trigger: the tip kicks sideways

The offset tip orbits while free, is pushed concentric by the edge, then kicks when the spindle centre is one contact radius from the edge. The wobble and kick are drawn for illustration, not as contact mechanics.

Where the check hole lands

Distance from the datum edge, magnified (mm); the dashed line is the drawing’s 20.000

Check holes: drawing says 20.000 mm from the left edge
Hole	Readout set at kick (mm)	Lands from edge (mm)	Off by (mm)
Your hole	−5.000	20.000	0.000
Radius forgotten	0.000	15.000	5.000 toward edge (1 r)
Wrong sign	+5.000	10.000	10.000 toward edge (2 r)

The sign depends on the side approached (+X right, +Y toward the back)
Edge	Spindle approaches moving	Edge position	Set readout at the kick (10 mm contact)
Left edge	+X	X = reading + r	−5.000 mm
Right edge	−X	X = reading − r	+5.000 mm
Front edge	+Y	Y = reading + r	−5.000 mm
Back edge	−Y	Y = reading − r	+5.000 mm

Worked example (left edge, 10 mm contact, r = 5 mm): at the kick the X readout shows +32.460 mm in its old, arbitrary frame. The edge is at reading + r = +32.460 + 5.000 = +37.460 mm. Setting the readout to −5.000 mm at that position makes the edge read 0, and a hole programmed at X 20.000 lands 20.000 mm from the edge. Setting 0 instead (radius forgotten) puts the hole at 15.000 mm: off by r. Setting +5.000 (wrong sign) puts it at 10.000 mm: off by 2r.

Model: top view; positions are the spindle centre relative to the part, whichever of them actually moves. The trigger occurs when the spindle centre is exactly one contact radius (half the contact diameter, not the body diameter) from the edge, on the approach side. Lengths are exact to 0.001 mm; jog steps are 1, 0.1 and 0.01 mm. A step that crosses the trigger leaves the readout past it by up to one step. The other axis is assumed already located. Backlash, tip wear, edge condition and speed are not modelled; use the edge finder’s specified speed.

Sources: MIT 2.670, Milling Machine, “Using an Edge Finder”, p. 6; Starrett 827MB and 827A product pages (how to use; 6 mm and 0.200 in contact diameters). The part, readings and drawings are original illustrations.

Home the machine, locate the part, then account for each tool's length

Machine home establishes the machine's reference positions. It does not locate the part.
Work zero is the origin chosen on the part or fixture. Its work offset relates that origin to the machine coordinates.
Tool length offset accounts for where a particular tool tip lies relative to the control's tool reference.

Choose the origin used by the drawing and program, locate X and Y, and establish Z using the approved touch-off or probing method. Account for any gauge or touch plate thickness. Confirm the active work offset, tool number and tool length before a trial path. Recheck after moving the part or changing how a tool is seated.

Use Vertical Mill Startup, Setup and Shutdown to check the sequence and G-code and Toolpaths to connect coordinates to a program. Tormach's basic-operation reference provides links to work/tool offset instructions; select documentation matching the installed control.

Two tools touch the same surface at different readings; Interactive

Two tools touch the same surface at different readings

Once X and Y are located, Z still depends on which tool is in the spindle. Predict the readout for two tools of different lengths touching the same part top, with ideal contact or through a block of stated thickness. CNC Axes and Offsets shows how a control applies a stored length value.

Two tools touched off on the same surface

Touch-off through a 10.000 mm block
Tool	Length, gauge line to tip (mm)	Z reading at touch-off (mm)	Length value vs T1 (mm)
T1 · 10 mm end mill (reference)	82.000	+10.000	0.000
T2 · 6 mm drill	109.500	+37.500	+27.500

Touch-off with ideal zero-thickness contact
Tool	Length, gauge line to tip (mm)	Z reading at touch-off (mm)	Length value vs T1 (mm)
T1 · 10 mm end mill (reference)	82.000	0.000	0.000
T2 · 6 mm drill	109.500	+27.500	+27.500

Worked example: the readout follows the spindle and reads T1’s tip height, with the part top at Z 0. T2 is 27.500 mm longer than T1. Touching T2 onto a 10.000 mm block, the readout shows Z = 0 + 10.000 + (109.500 − 82.000) = +37.500 mm. T2’s length value is that reading minus the block: +27.500 mm. With that value applied, the tip Z is the reading minus 27.500. Without it, a move to Z +2.000 with T2 would put its tip at 2.000 − 27.500 = −25.500 mm: 25.500 mm below the part top.

Model: Z reading at touch-off = surface Z + gauge thickness + (L_tool − L_T1). Lengths run from the spindle gauge line to the tip and are illustrative. The gauge is either an ideal zero-thickness contact or a stated 10.000 mm block. Controls store and apply tool length in different ways; follow the control’s documentation for the actual offset entry.