Manual machining
Lathe cross slide and compound infeed calculator
Radius dial: move half the diameter change; diameter dial: move the full change. Going from 1.250 in to 1.180 in is 0.035 in on a radius dial.
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
Cross slide: diameter to dial move
Radius-reading dial
in
dial move = (D1 - D2) / 2 = (1.250 - 1.180) / 2 = 0.0350 in
Feed in 0.0350 in (0.889 mm).
Diameter-reading dial
in
dial move = D1 - D2 = 1.250 - 1.180 = 0.0700 in
Feed in 0.0700 in (1.778 mm).
Compound: thread depth to compound travel
Compound travel to full depth
in
compound travel = h / cos(A) = 0.0472 / cos(29.5°) = 0.0542 in
h = 0.613435 x P = 0.613435 x 0.076923 = 0.0472 in. Travel 1.377 mm.
Z shift of the tool
in
Z shift = h x tan(A) = 0.0472 x tan(29.5°) = 0.0267 in
How far the tool point moves along Z by the time it reaches full depth.
Compound: fine feed in tenths
Tool movement on the radius
in
X move = travel x sin(B) = 0.0050 x sin(5.7392°) = 0.0005 in
0.0010 in on the diameter.
Source: Machinery's Handbook (thread cutting; compound rest setting); ASME B1.1 and ISO 68-1 for thread depth
Dial readings include backlash. Always approach the setting from the same direction.
How it works
Cross slide dial
Radius-reading dial: dial move = (D1 - D2) / 2; diameter-reading dial: dial move = D1 - D2
| Symbol | Meaning | Unit |
|---|---|---|
| D1 | Current diameter | in or mm |
| D2 | Target diameter | in or mm |
Compound infeed for threading
compound travel = h / cos(A); Z shift = h x tan(A); at 29.5°: travel = h x 1.1490, Z shift = h x 0.5658
| Symbol | Meaning | Unit |
|---|---|---|
| h | Radial thread depth (single depth); 60° external thread: 0.613435 x P | in or mm |
| P | Pitch, 1 / TPI for inch threads | in or mm |
| A | Compound angle from the cross slide axis; at 30°, travel = h x 1.1547 | ° |
Fine feed from the compound
X move = travel x sin(B); at B = 5.7392°, sin(B) = 0.1
| Symbol | Meaning | Unit |
|---|---|---|
| travel | Distance fed on the compound dial | in or mm |
| B | Compound angle from the lathe axis (Z) | ° |
| X move | Tool movement on the radius; double it for the diameter | in or mm |
Worked example 1: diameter to dial
Turning from 1.250 in to 1.180 in.
- Diameter change: 1.250 - 1.180 = 0.070 in.
- Radius dial: 0.070 / 2 = 0.0350 in.
- Diameter dial: 0.0700 in.
Result: 0.0350 in on a radius-reading dial, 0.0700 in on a diameter-reading dial.
Worked example 2: compound travel for 1/2-13 (inch)
1/2-13 external thread, compound at 29.5°.
- Radial depth: h = 0.613435 / 13 = 0.0472 in.
- Compound travel: 0.0472 x 1.1490 = 0.0542 in.
- Z shift: 0.0472 x 0.5658 = 0.0267 in.
Result: feed the compound 0.0542 in to full depth; the tool moves 0.0267 in along Z on the way.
Worked example 3: compound travel for M16 x 2 (metric)
M16 x 2 external thread, compound at 29.5°.
- Radial depth: h = 0.613435 x 2 = 1.227 mm.
- Compound travel: 1.227 x 1.1490 = 1.410 mm.
- Z shift: 1.227 x 0.5658 = 0.694 mm.
Result: 1.410 mm on the compound, 0.694 mm of Z shift.
Worked example 4: fine feed
Compound set 5.7392° from the lathe axis (Z), fed 0.0050 in.
- sin(5.7392°) = 0.1000.
- X move on the radius: 0.0050 x 0.1 = 0.0005 in.
- On the diameter: 2 x 0.0005 = 0.0010 in.
Result: 0.0050 in on the compound takes 0.0010 in off the diameter.
Threading with the compound on a manual lathe
- Set the compound to 29.5°.
- Touch off and zero the cross slide and compound dials.
- Each pass, feed in on the compound only. Back the cross slide out at the end of the pass and return it to zero.
- Stop at the compound travel for full depth, then check the thread with wires or a gauge.
Shop notes
- Dial check: move the cross slide 0.010 in by the dial and measure. If the diameter drops 0.020 in, the dial reads radius; if it drops 0.010 in, the dial reads diameter.
- Compound protractors are marked differently from lathe to lathe. Confirm which axis reads 0° before setting 29.5°.
- Thread depth and pass schedules live on the threading formulas page; this page turns a depth into dial moves on a manual lathe.
FAQ
How much do you move the cross slide to take 0.070 in off the diameter?
0.0350 in on a radius-reading dial; 0.0700 in on a diameter-reading dial.
How far do you feed the compound for a 1/2-13 thread?
0.0542 in at 29.5°: 0.0472 in radial depth x 1.1490.
Why set the compound at 29.5°?
Rule of thumb: the half degree lets the trailing flank shave lightly instead of rubbing. At 29.5° the compound travel is the radial depth x 1.1490; at 30° it is x 1.1547.
How do you get tenths from a 0.001 in dial?
Set the compound 5.7392° off the lathe axis. sin(5.7392°) = 0.1, so 0.001 in on the compound moves the tool 0.0001 in on the radius.
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