Long workpieces cause problems that short ones do not. A 500mm shaft supported at both ends deflects under its own weight by a fraction of a millimeter. A 2000mm shaft deflects by several millimeters. That deflection pulls the hole off-center.
Steady rests fix this. But only if they are set up correctly. A misaligned steady rest is worse than no steady rest — it forces the workpiece into a bend that the drill then follows.
Here is what I have learned about using steady rests for deep hole drilling.
When You Need a Steady Rest
I use steady rests based on the L/D ratio and the workpiece rigidity:
| L/D Ratio | Support Needed | Why |
|---|---|---|
| Under 10:1 | None | Workpiece is rigid enough |
| 10:1 to 30:1 | One steady rest | Prevents sag at midpoint |
| 30:1 to 60:1 | Two steady rests | Distributes support along length |
| Over 60:1 | Moving or multiple rests | Support must follow the cutting zone |
These are guidelines, not rules. A 30:1 workpiece in 4140 steel with a 50mm diameter may need only one rest. The same 30:1 ratio in aluminum with a 20mm diameter may need three.
The key is to check workpiece deflection at the midpoint. Clamp the part as it will be held during drilling and push up at the midpoint with a dial indicator. If the indicator moves more than 0.05mm under moderate hand pressure, add a steady rest.
Types of Steady Rests I Use
| Type | Best For | Limitations |
|---|---|---|
| Fixed three-point | Rigid workpieces, moderate L/D | Contact points can mark soft materials |
| Roller-type | Soft or finished surfaces | More expensive, needs maintenance |
| Open steady rest | Large diameters, BTA work | Less support than closed types |
| Moving (traveling) rest | Very high L/D, deep holes | Complex setup, limited availability |
For most gun drilling work on shafts between 20-100mm diameter, fixed three-point rests with carbide or bronze contact pads work well. The pads wear over time and need periodic adjustment.
For workpieces with a finished OD that cannot be marked, I use roller-type rests. The rollers spread the contact force and do not leave marks.
Placement Strategy
Where you place the steady rest matters more than how many you use.
For a single rest, I place it at 40-50% of the unsupported length. This is the point of maximum deflection for a workpiece supported at both ends.
For two rests, I divide the unsupported length into thirds — one rest at 33%, one at 66%.
The error most operators make is placing the rest too close to the chuck or tailstock. The workpiece does not need support where it is already stiff. It needs support where it is most flexible.
For very long workpieces (L/D over 60:1), a fixed steady rest at the midpoint is not enough because the portion between the rest and the cutting zone is still unsupported. On these jobs, I use a traveling rest that moves with the drill head. This keeps the support close to the cutting zone at all times.
Alignment Procedure
A steady rest that is not aligned to the spindle axis will push the workpiece off-center. The drill will follow that offset.
I align steady rests using a test bar:
- Mount a test bar or a straight reference bar in the workholding (chuck or collet).
- Indicate the bar near the chuck. Zero the indicator.
- Move the indicator to the steady rest position.
- Adjust the steady rest pads until the bar runs within 0.02mm TIR at that position.
If the bar runs out more than 0.02mm at the steady rest, the rest is pushing the workpiece off-axis. Loosen the rest mounting, adjust, and retighten.
I check alignment after every steady rest adjustment and after any machine maintenance that could affect the spindle or way alignment. I cover machine alignment in Machine Alignment and Concentricity.
Common Problems and Fixes
| Problem | Likely Cause | Fix |
|---|---|---|
| Hole drifts toward one side | Steady rest pushing workpiece off-center | Realign rest to spindle axis |
| Vibration marks deepen at midpoint | Rest not engaging, workpiece vibrating against pads | Adjust pad pressure, check pad condition |
| OD of workpiece marked by rest | Fixed pads on soft material | Switch to roller-type rest |
| Hole is straight but position shifts | Rest is correctly aligned but workpiece is bending between rest and chuck | Add second rest closer to the bend |
| Scored or galled steady rest pads | Insufficient lubrication on pads | Apply lubricant to pads before engagement |
| Hole diameter increases at depth | Workpiece deflecting away from cutting zone | Move rest closer to entry or add traveling rest |
Pad Material Selection
The steady rest pads contact the workpiece under pressure. The wrong pad material damages the workpiece or wears too fast.
| Pad Material | Best For | Wear Life |
|---|---|---|
| Bronze | General purpose steel | Moderate |
| Carbide | Hardened or abrasive materials | Long |
| Nylon/plastic | Soft or finished surfaces | Short |
| Roller bearings | All materials, no marking | Long (highest cost) |
I use carbide pads for most deep hole drilling work. They last through multiple setups and do not gall on steel. For aluminum or brass, I switch to nylon pads to prevent marking.
Key Takeaways
- Use steady rests for workpieces with L/D ratios over 10:1. The longer the part, the more support it needs.
- Place the first steady rest at 40-50% of the unsupported length — the point of maximum deflection.
- Align each steady rest to within 0.02mm TIR of the spindle axis using a test bar. A misaligned rest is worse than none.
- For very long workpieces (L/D over 60:1), a traveling rest that follows the cutting zone provides better support than fixed rests.
- Match the steady rest pad material to the workpiece material — carbide for steel, nylon for soft or finished surfaces.
- Check steady rest pad condition regularly. Worn pads allow the workpiece to vibrate and produce poor holes.