Lathe threading shim calculator
Calculate thread helix angle and an indicative lathe shim from pitch, diameter and number of starts. Check the result against your holder catalog.
Roscas de taller (opcional)
None — search the catalog
Thread parameters
Result · Recommended support plate
plate tilt
3°
Catalog code · AE/AI nomenclature
AE11+2
Referencia orientativa del portaherramientas. Contrasta el angle in degrees con tu catálogo.
- helix angle
- 3.04°
- Signed
- 3.04°
- Diameter in calculation
- Ø 6 mm
- Offset vs 1st factory
- +2°
⚠ Always validate the angle and the code against your toolholder catalogue and a qualified professional before machining. Usual shim range: −2° to +4°.
Pitch/diameter nomogram
Visually check which band your pitch/diameter combination falls into.
Vista general del nomograma
How to read the chart
- Horizontal axis: effective thread diameter.
- Vertical axis: step, or advance if there are multiple entries.
- Each colour band is a shim plate (1° to 4°).
Your thread on the chart
- The highlighted point is your current combination.
- The highlighted band matches the recommended shim.
- The scales follow the unit selected above.
How to read the AE/AI code
The type codesAE16+2 o AI22-1 describen qué placa pedir para un portaherramientas con 1° de inclinación de fábrica.
1. Position
AE o AI
AE = rosca exterior. AI = rosca interior.
2. Serie de plaquita
11
Inscribed circle:11, 16, 22 o 27 según el paso.
3. Offset en grados
+2°
Addition or subtraction to the 1st standard.+2 → placa de 3° en total.
Tu resultado AE11+2: exterior (AE), serie 11, offset +2° sobre 1° de fábrica → 3° en total.
Workshop tips and common problems
Quick Tips and Common Mistakesal elegir la placa de apoyo del portaherramientas.
How to do it right
What is the support plate and when do you need it?
When threading on a lathe the insert advances in a straight line while the part rotates, but the thread follows a helix. The shim plate tilts the insert by that angle so both flanks cut equally. Most holders ship with 1° from the factory: outside of small pitches on medium diameters, another shim is needed.
Helix angle: pitch, diameter and number of entries
The helix angle is the arctangent of the lead divided by π × the effective diameter: the larger the pitch or the smaller the diameter, the larger the angle. That is why lead screws and coarse trapezoidal threads on small diameters stray furthest from the standard 1°.
AE/AI code and insert series (11/16/22/27)
The indicative AE/AI code combines the position (AE external, AI internal), the insert series by inscribed circle (11, 16, 22 or 27, chosen by pitch) and an offset in degrees over the factory 1°. So AE16+2 is series 16, external, 2° more tilted than standard (3° in total).
Multi-entry threads: use lead, not pitch
On a multi-start thread the angle depends on the lead (lead = pitch × number of starts), not on the plain pitch. A 3-start, 2 mm pitch thread behaves like a 6 mm one for helix purposes, so it needs a much steeper shim than its nominal pitch suggests.
Thread hand vs. tool hand: negative plate
If the thread hand does not match the holder hand (a right-hand tool cutting a left-hand thread), the shim is usually negative to compensate for the opposite helix. Always check the sign of the angle before ordering the shim.
If something goes wrong · symptom → cause → solution
⚠ Wear on only one insert flank
A classic symptom of a shim with the wrong angle: one flank rubs instead of cutting. Fix: recalculate the helix angle from the pitch, the effective diameter and the actual number of starts, and adjust the shim (AE/AI code with its offset) until both flanks work equally.
⚠ Torn or uneven finish on one side of the thread
The programmed helix does not match the insert's actual tilt. Fix: confirm the diameter you use in the calculation (the pitch/effective diameter rather than the major one) and the thread/tool hand; correct the shim sign if you are cutting a left-hand thread with a right-hand tool.
⚠ The angle goes outside the plate range (−2° to 4°)
Very coarse pitches, very small diameters or multiple starts can produce a helix angle outside the range of standard shims. Fix: check against the actual toolholder catalogue; it may need a special shim, a different insert geometry or a holder made for coarse pitches.
Other threading tools
Technical guide: support plate (shim) for lathe threading
Reference article for this section. Switch goal above (drill, identify, CNC, method, repair or base plate) to see the guide for that tool.
A badly chosen shim plate is a common cause of uneven wear on a single flank and a poor finish when threading on a lathe with an insert, especially on coarse pitches, small diameters or multi-start threads. This guide explains what it is, how the angle is calculated and how to read the AE/AI codes in the toolholder catalogue.
Use pitch, pitch diameter and number of starts to calculate the helix angle. Confirm the indicative shim code against the holder catalog.
What is a backing plate and why do you need it?
When threading on a lathe the insert advances in a straight line while the part rotates, but the real thread follows a helix: the larger the pitch (or the lead, on multi-start threads) relative to the diameter, the larger that helix angle. The shim plate tilts the insert by that angle so both flanks cut equally, instead of one rubbing and wearing out first.
Most lathe threading tool holders come factory prepared with a fixed inclination of 1°, sufficient for small pitches in medium diameters. Outside that range, a different plate is required.
How helix angle is calculated
The helix angle is the arctangent of the pitch (or lead) divided by π times the thread's effective diameter: the larger the pitch or the smaller the diameter, the larger the angle. That is why coarse threads on small diameters — typical of lead screws and trapezoidal threads — are the ones that most need a shim other than the standard 1°.
The diameter used in the calculation is not always the largest: for greater precision it is advisable to use the pitch diameter or the effective diameter, especially if you are going to check the plate on an already started thread.
AE/AI code and insert series (11, 16, 22, 27)
The indicative AE/AI code combines the position (AE for an external thread, AI for internal), the insert series by inscribed-circle size (11, 16, 22 or 27, smallest to largest, chosen by pitch) and a degree offset from the holder's factory 1° (for example, AE16+2 means series 16, external, 2° more tilted than standard).
These codes are indicative: every manufacturer has its own shim catalogue and not all use exactly the same nomenclature, so always confirm against your toolholder's actual catalogue before ordering a shim.
Multi-entry threads and opposite tool hand
On a multi-start thread the helix angle depends on the lead (lead = pitch × number of starts), not on the plain pitch: a 3-start thread with a 2 mm pitch behaves like a 6 mm pitch as far as the angle is concerned, so it needs a much steeper shim than its nominal pitch suggests.
If the thread hand does not match the holder hand (for example, a right-hand tool cutting a left-hand thread), the required shim is usually negative to compensate for the opposite helix: always check the sign of the angle before ordering the shim.
Frequently asked questions: support plate (shim) for lathe threading
What is a support plate in lathe threading?
It is a wedge or angled plate fitted under the threading insert to tilt it so it matches the thread's real helix angle, preventing one insert flank from rubbing and wearing out before the other.
When do I need a plate other than the 1st standard?
When the helix angle calculated for your pitch and diameter moves away from 1°: coarse pitches, small diameters or multi-start threads are the most common cases. If you see a poor finish or wear on just one insert flank, this is the first cause to check.
What does a plate code like AE16+2 mean?
AE indicates an external thread (AI would be internal), 16 is the insert series by inscribed-circle size, and +2 is the offset in degrees from the holder's factory tilt (normally 1°), that is, a 3° shim in total.
How does the number of thread entries affect the angle of the plate?
The angle depends on the feed (lead = pitch x number of entries), not the pitch of a single thread. A 2 or 3-way thread requires a much steeper plate than a normal thread of the same nominal pitch.
What if you threaded with the wrong backing plate?
The insert flank that does not match the real helix rubs instead of cutting cleanly: the typical result is a torn or uneven finish on one side of the thread and premature wear on that face of the insert, even though the programmed diameter and pitch are correct.
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