Backlash in the tool saddle — the saddle moves slightly before the tool engages the cut — is a precision problem that gets worse over time. The cause is almost always a worn saddle nut on the cross-slide feedscrew.
What Causes Saddle Backlash?
The tool saddle rides on the cross slide and is driven by a leadscrew through a bronze or steel nut. As the nut wears, clearance develops between the nut threads and the leadscrew threads. This clearance shows up as:
- Backlash: When you reverse the feed direction, there is a dead zone before the saddle starts moving
- Positional drift: The saddle position is inconsistent between passes
- Poor surface finish: The tool chatters or leaves witness marks because it is not rigidly positioned
How to Measure Backlash
Mount a dial indicator on the column with the probe touching the saddle. Rotate the feedscrew handwheel forward until the saddle just moves, zero the indicator, then reverse. The indicator reading before the saddle moves in the reverse direction is the backlash. OEM specification is typically less than 0.001" — anything over 0.003" warrants replacement.
Saddle Nut Variants
W&B used several saddle nut variants depending on the machine series and configuration:
- WB730 — most common variant (6 orders on file)
- WB731 — alternate variant (6 orders on file)
- WB921 / WB922 — later production variants
- WB734 — saddle locknut (often replaced at the same time)
Always confirm the correct variant with the machine serial number.
Order Saddle Nuts
We stock common saddle nut variants in the USA. Provide your serial number and we will confirm the correct part.
Related Parts
- WB730 — Saddle Nut
- WB731 — Saddle Nut (variant)
- WB734 — Saddle Locknut