A fixture is a custom workholding device designed for a specific part. Unlike standard chucks and steady rests, fixtures are built to match the part geometry exactly. A good fixture reduces setup time and improves consistency.
I’ve designed and used fixtures for all types of deep hole drilling work. The same principles apply regardless of the part shape.
Design Principles
Support the part at the bore location. The most important principle. The part should be supported as close as possible to where the drill enters and exits. Unsupported span allows the part to deflect.
Provide chip clearance. The fixture should not obstruct chip evacuation. Chips need a clear path out of the bore.
Allow coolant flow. The fixture should not block the coolant flow. The coolant needs to reach the cutting zone and return freely.
Locate from the same points each time. The fixture should have locating pins or surfaces that position the part repeatably. Every part should sit in the same position within 0.05mm.
Clamp without distorting the part. The clamping force should be directed through the rigid sections of the part. Clamping on thin sections distorts the part and produces a non-straight bore.
Fixture Materials
Fixture components are typically made from:
- Steel for structural components
- Hardened steel for locating pins and wear surfaces
- Brass or nylon for guide bushings (to prevent part marking)
- Aluminum for lightweight fixtures
I use hardened steel for all locating points. The locators wear over time, and hardened steel lasts longer before needing replacement.
Here’s a comparison of fixture materials I’ve used and when each makes sense:
| Material | Rigidity | Weight | Cost | Corrosion Resistance | Best Application |
|---|---|---|---|---|---|
| Steel | Excellent | Heavy | Moderate | Good | Structural components, bases |
| Aluminum | Good | Light | Moderate | Good | Lightweight fixtures, prototype work |
| Cast iron | Excellent | Heavy | Low | Poor | Machine bases, vibration damping |
| Urethane | Low | Light | Low | Excellent | Soft jaw inserts, non-marring contact points |
Coolant Management
The fixture should include coolant management features:
- Coolant collection channels that direct return coolant to the machine’s coolant system
- Seals at the drill entry point to prevent coolant spraying
- Drain slots for chip removal
- Pressure relief for trapped air pockets
A well-designed fixture contains the coolant and chips within the work zone. A poorly designed fixture sprays coolant across the shop floor.
Quick-Change Features
For production work, fixtures should include quick-change features:
- Quick-release clamps (toggle clamps or hydraulic clamps)
- Locating pins that drop into position
- One-handed operation where possible
- Visual position indicators
I’ve seen fixture change times drop from 10 minutes to 2 minutes with quick-change features. Over a production run of 500 parts, that saves 60 hours of setup time.
Fixture Verification
After building a fixture, I verify it before production:
- Mount the fixture on the machine
- Load a part into the fixture
- Check the part position relative to the drill axis
- Run a test hole and inspect the results
- Adjust the fixture if needed
The test hole tells me whether the fixture is correct. If the hole is centered in the part and straight, the fixture is good.
Common Fixture Mistakes
The most common fixture mistakes I’ve seen:
- Insufficient support at the drill exit. The drill breaks through the far side and hits the fixture before stopping.
- No clearance for chips. The fixture traps chips in the bore area, causing scoring.
- Over-constraint. The fixture locates the part at more points than necessary, causing the part to bind.
- Difficult loading. The operator has to struggle to load the part, increasing cycle time and risking damage.
Key Takeaways
A good fixture is the difference between a job that runs smoothly and one that causes problems all day. I’ve learned to invest in fixture design upfront rather than fighting with workholding during production. The most important principle is supporting the part at the bore location. If the part is supported where the drill enters and exits, most other fixture issues can be managed.
- Support the part as close as possible to where the drill enters and exits.
- Provide chip clearance and coolant flow paths in the fixture design.
- Use hardened steel for locating points that see repeated contact.
- Add quick-change features for production work – they save hours over long runs.
- Verify every fixture with a test hole before committing to production.
- Avoid over-constraint, insufficient exit support, and difficult loading.
For a broader overview of workholding options, see Workholding for Deep Hole Drilling.