Appearance
A deep hole drill 3 metres long and 30 mm in diameter has the stiffness of a wet noodle. Without steady rests at the correct intervals, the drill tube deflects, the workpiece vibrates, and the bore spirals out of tolerance. Steady rests are not optional — they are the difference between a straight bore and a scrap part.
Why Steady Rests Are Critical
In deep hole drilling, two distinct elements require support:
| Element | Support Requirement | Consequence of Inadequate Support |
|---|---|---|
| Drill tube (boring bar) | Must be supported at intervals to prevent whipping | Chatter, spiralling, tool breakage |
| Workpiece (long/slender) | Must be supported to prevent sag and vibration | Bore position error, ovality, surface finish degradation |
The fundamental challenge is L/D ratio. A 50 mm drill tube extending 3,000 mm into the workpiece has an effective L/D of 60:1 — unsupported, it behaves as a cantilever beam that deflects under its own weight and cutting forces.
Types of Steady Rests
Manual / Mechanical Steady Rests
| Feature | Description |
|---|---|
| Actuation | Handwheel or screw adjustment |
| Contact | Bronze or roller pads, manually positioned |
| Adjustment | Pre-set before drilling begins |
| Repeatability | 0.02–0.10 mm |
| Cost | Low ($500–3,000) |
| Best for | Low-volume, simple geometries, non-critical straightness |
Manual steady rests are economical but their limitation is significant: they cannot be adjusted during the drilling cycle. As the drilling depth increases and vibration characteristics change, a pre-set steady rest may become ineffective.
Hydraulic Self-Centering Steady Rests
| Feature | Description |
|---|---|
| Actuation | Hydraulic cylinder (1–8 MPa) |
| Contact | Three-roller design (self-centering) |
| Adjustment | Dynamic — adjustable during drilling |
| Repeatability | 0.005–0.01 mm |
| Clamping force | 200–6,900 daN (model-dependent) |
| Cost | Medium ($3,000–15,000) |
| Best for | Production, tight straightness requirements, varying conditions |
Available models from Dezhou BTADrilltools (typical of the RC series):
| Model | Clamping Range | Max Clamping Force | Max Roller Speed | Repeatability |
|---|---|---|---|---|
| RC-1 | 5–70 mm | 200 daN | 730 m/min | 0.005 mm |
| RC-3 | 10–120 mm | 1,350 daN | 930 m/min | 0.006 mm |
| RC-5 | 30–200 mm | 2,800 daN | 800 m/min | 0.008 mm |
| RC-7 | 50–300 mm | 4,500 daN | 720 m/min | 0.008 mm |
| RC-9 | 100–430 mm | 6,900 daN | 685 m/min | 0.01 mm |
Tip: Hydraulic steady rests with three-roller design provide automatic self-centering. When the hydraulic cylinder actuates, all three rollers close simultaneously and concentrically. This is superior to two-roller designs which require manual adjustment for each diameter change.
The critical advantage of hydraulic steady rests is dynamic adjustment. During a deep hole drilling cycle:
- The steady rest can be programmed to release or engage at specific depths
- Clamping pressure can be varied to adapt to changing vibration conditions
- Individual steady rests along the workpiece can be sequenced — engaging and releasing as the tool passes
- Pressure readings can be recorded as a process monitoring parameter
CNC-Programmable Steady Rests
Advanced machines integrate steady rests into the CNC control system, enabling fully programmable support.
Mollart Accubore (7-axis deep hole drilling centre):
- Independent X-axis programmable steady rests (3-point roller design)
- Laser ultrasonic technology (LUT) tracks the drill path in real time
- Active straightness correction applied through steady rest positioning
- Workpiece capacity: 2,180 mm long, 250 mm diameter, 900 kg
Niles-Simmons N30MC Turning Centre:
- Two steady rest slides with integrated damping units
- Patented Center Axis Alignment using eddy current sensing
- Combines turning, milling, gun drilling, gear hobbing in one setup
- 4.5 m between centres, 780 mm swing
Key features of CNC-controlled steady rests:
| Feature | Benefit |
|---|---|
| Programmable X-axis positioning | Steady rest follows the drilling axis without manual setup |
| Integrated damping | Tuned mass dampers in steady rest rollers absorb vibration |
| Load monitoring | Steady rest force sensors detect tool contact before cutting |
| Closed-loop pressure control | Hydraulic pressure adjusted in real time from CNC program |
Steady Rest Positioning
Drill Tube Support
The drill tube (boring bar) must be supported at regular intervals to prevent whipping:
| Drill Tube Diameter | Maximum Spacing Between Steadies | Notes |
|---|---|---|
| 20–40 mm | 600–800 mm | Small gun drills need closer support |
| 40–80 mm | 800–1,000 mm | Standard BTA drill tubes |
| 80–150 mm | 1,000–1,500 mm | Large BTA tubes — self-supporting over longer spans |
| > 150 mm | 1,500–2,000 mm | Very large tubes — fewer steadies needed |
Workpiece Support
Long, slender workpieces must be supported to prevent sag:
| Workpiece Diameter | Maximum Spacing Between Steadies | Typical Application |
|---|---|---|
| 20–50 mm | 500–1,000 mm | Hydraulic cylinder rods |
| 50–100 mm | 1,000–1,500 mm | Extruder barrels, shafts |
| 100–200 mm | 1,500–2,000 mm | Oil field components, tie bars |
| > 200 mm | 2,000–3,000 mm | Large rolls, heavy shafts |
Warning: The maximum spacing rule assumes the workpiece is concentric and rotates true. If the workpiece has significant mass imbalances (keyways, flats, off-centre features), reduce spacing by 30–50% and use steady rests with higher damping capacity.
Sequencing
For multi-diameter or stepped workpieces, steady rest sequencing is important:
- Load position: All steadies open — workpiece loaded
- Clamp position: Steadies close on the workpiece at programmed positions
- Drilling start: Tailstock and headstock support engaged
- Through-drilling sequence: As the drill passes each steady, that steady may be momentarily released to avoid drill collision, then re-clamped
- Completion: All steadies open for unloading
Vibration Dampers for Drill Tubes
In addition to workpiece steady rests, dedicated drill tube vibration dampers are used on many deep hole drilling machines.
| Type | Description | Application |
|---|---|---|
| Fixed bushing type | Close-clearance bush mounted in a bracket | Low-cost, fixed position |
| Roller steady type | Three-roller self-centering support | Allows axial movement during drilling |
| Hydraulic damping type | Hydrodynamic oil film between bushing and tube | Excellent vibration damping |
| Tuned mass damper | Inertial mass with viscoelastic element fitted inside the drill tube | Suppresses specific resonant frequencies |
Examples of drill tube vibration dampers on production machines:
| Machine | Damper Type | Details |
|---|---|---|
| Sunnen SHDD | Fixed bushing + roller steadies | Vibration dampeners as shaft supports |
| Premach TCS2150 | Hydraulic vibration damper steady for boring rod | Integrated into pressure head system |
| Premach TK2620 | Vibration damper steadies for both gun drill and BTA | Six-coordinate CNC machine |
| ZK21xxE | Vibration damper steadies for drill bar | Supports up to 200:1 L/D ratio |
Manufacturers and Suppliers
Dedicated Steady Rest Manufacturers
| Manufacturer | Product Range | Actuation | Clamping Range |
|---|---|---|---|
| Kitagawa | BK / RBK series | Manual, pneumatic, hydraulic | 5–300 mm |
| Dezhou BTADrilltools | RC / RX / RM series | Hydraulic | 5–430 mm |
| Röhm | Roto and hydro-mechanical | Manual, hydraulic | 5–350 mm |
| SMW Autoblok | Self-centering steady rests | Hydraulic, pneumatic | 5–400 mm |
Machine Builders with Integrated Steady Rests
| Machine Builder | Machine | Steady Rest Type | Special Feature |
|---|---|---|---|
| Mollart | Accubore | CNC programmable 3-point roller | Laser ultrasonic straightness correction |
| UNISIG | B-Series | Hydraulic with central control | Programmable pressure from CNC |
| TBT | B3S / ML Series | Hydraulic fixed and roller steadies | Concrete bed for vibration damping |
| Sunnen | SHDD | Adjustable V-block + roller steadies | Drill tube vibration dampeners |
| Shin-IL | BTA Machine | Hydraulic steady rest | Three processing methods (tool rotate, workpiece rotate, counter-rotate) |
| Premach | TCS / TK Series | Hydraulic vibration damper steadies | Dual gun drill + BTA capability |
Selection Criteria
| Factor | Manual Steady Rest | Hydraulic Steady Rest | CNC Steady Rest |
|---|---|---|---|
| Production volume | Low | Medium-high | High |
| Straightness requirement | ≥ 0.5 mm/m | 0.1–0.5 mm/m | ≤ 0.1 mm/m |
| Diameter changes during run | Single diameter | Quick changeover | Fully automatic |
| Operator skill required | Moderate | Low | Low |
| Integration with automation | None | Possible | Full |
| Initial cost | Low | Medium | High |
| Maintenance cost | Low | Medium (hydraulics) | Medium-high |
FAQ
Why are steady rests needed in deep hole drilling?
Deep hole drilling tools and workpieces have high L/D ratios (often exceeding 50:1 or 100:1). Without support at regular intervals, the drill tube whips and the workpiece sags, causing bore deviation, chatter, and surface finish defects.
What is the difference between manual and hydraulic steady rests?
Manual steady rests are pre-set before drilling and cannot be adjusted during the cycle. Hydraulic steady rests can be adjusted dynamically — clamping pressure can be changed, and the rest can be opened and closed under CNC control at specific drilling depths.
How far apart should steady rests be placed?
For drill tubes: 600–1,500 mm depending on diameter. For workpieces: 500–3,000 mm depending on diameter and material stiffness. As a general rule, support at intervals equal to 20–25× the diameter of the element being supported.
What is a CNC-programmable steady rest?
A steady rest whose position and clamping pressure are controlled by the CNC program, enabling automatic support changes during the drilling cycle without operator intervention.
Can steady rests correct bore straightness?
Yes. Advanced systems like Mollart's Accubore with Laser Ultrasonic Technology (LUT) use steady rests as active straightening devices — applying corrective forces based on real-time drill path measurements.
What is a drill tube vibration damper?
A support device specifically for the rotating drill tube (boring bar), typically a close-clearance bushing or hydraulic damper that prevents whipping and suppresses vibration at critical L/D ratios.
What industries use hydraulic steady rests for deep hole drilling?
Hydraulic cylinder manufacturing, oil and gas (drill pipe, downhole tools), aerospace (landing gear struts), automotive (shafts, tie rods), power generation (turbine shafts), marine (propeller shafts), and defence (gun barrels).
How accurate are hydraulic steady rests?
Repeatability of 0.005–0.01 mm is standard for precision hydraulic steady rests from Kitagawa, Röhm, and Dezhou BTADrilltools.
Can steady rests be retrofitted to existing machines?
Yes. Hydraulic steady rests can be added to existing lathes and deep hole drilling machines with a hydraulic power unit and CNC or manual control. Retrofit cost: $5,000–25,000 depending on size and complexity.
How is steady rest clamping pressure controlled?
In hydraulic systems, a pressure reducing valve sets the clamping force. Advanced systems use proportional valves controlled by the CNC, allowing pressure variation during the drilling cycle.
Conclusion
Steady rests and workpiece support systems are indispensable for deep hole drilling of long, slender components. Three levels of technology are available: manual steady rests (low cost, pre-set before drilling), hydraulic self-centering steady rests (dynamic adjustment during drilling, 0.005–0.01 mm repeatability, 5–430 mm range), and CNC-programmable steady rests integrated with active straightness correction systems. The correct choice depends on production volume, straightness requirements, diameter variation, and automation level. Hydraulic steady rests offer the best cost-performance balance for most production applications, providing the critical advantage of dynamic adjustment as drilling conditions change with depth. Proper positioning — at intervals of 600–1,500 mm for drill tubes and 500–3,000 mm for workpieces — is essential regardless of the steady rest type chosen.