The Five-Step Troubleshooting Sequence
When something goes wrong during a deep hole drilling job, I teach operators a five-step sequence. This sequence solves about 80% of all problems without needing engineering support. I post this sequence on every machine in the shop.
The sequence follows the most likely cause first. Starting with the most common problem saves time. Do not skip steps and do not jump to conclusions.
Step 1: Check Coolant Pressure
Low coolant pressure is the root cause of more problems than anything else in deep hole drilling. It causes chip packing, tool breakage, poor surface finish, and oversized bores.
I check the pressure at the tool tip, not at the pump. The pump gauge can show 80 bar while the tool tip sees only 30 bar because of a clogged filter or a restriction in the drill tube. If the machine does not have a gauge at the tool holder, I install one.
| Pressure Reading | What It Means |
|---|---|
| Normal for the drill size | Continue monitoring |
| 20% below normal | Check filter, check for leaks |
| 50% below normal | Stop machine, find blockage |
| Fluctuating during cut | Chip clog forming in the bore |
If pressure is low, I check the filter pressure differential first. A differential above 1.5 bar means the filters are clogged. Next, I check the rotating union for leaks. A leaking union can drop pressure by 10-30 bar.
Step 2: Check Chip Formation
The chips are a direct indicator of cutting conditions. I look at the chips coming out of the bore every few minutes during a cycle. The chip shape tells me if the parameters are correct.
| Chip Shape | Diagnosis | Action |
|---|---|---|
| Short crescents, 2-5 mm | Optimal cutting | No action needed |
| Long stringy, continuous | Feed rate too low | Increase feed 10-15% |
| Fine powder or dust | Feed rate too high, or tool dull | Reduce feed or inspect tool |
| Blue or black chips | Excessive heat | Increase coolant flow |
| Variable thickness | Vibration in the cut | Check workpiece support |
| No chips coming out | Clogged bore | Stop immediately, retract drill |
A sudden change in chip shape during a cycle is the earliest warning of a developing problem. If I see chips change from short crescents to long strings, I know a clog is forming and I have time to stop before the tool breaks.
Step 3: Check the Guide Bushing
A worn guide bushing causes drill wandering, oversized holes, chatter, and eventually tool breakage. The bushing is the cheapest component in the system, and a worn bushing cannot be compensated by changing any other parameter.
I check the bushing ID with a telescoping gauge or a bore gauge. The clearance between the bushing ID and the drill OD should be:
- Gun drilling: 0.005-0.015 mm clearance
- BTA drilling: 0.01-0.03 mm clearance
- Replace when clearance exceeds 0.05 mm
I also inspect the bushing bore visually. Scoring, galling, or a polished wear pattern indicates the bushing needs replacement. A chip stuck in the bushing bore can act like a cutting tool and enlarge the bore.
If the bushing looks good, I check the bushing mounting. A loose bushing holder or a chip under the bushing face can tilt the bushing and cause the drill to enter the workpiece at an angle.
Step 4: Check the Tool Condition
A worn or chipped tool produces poor surface finish and high spindle load. I check the cutting edge under 10x magnification. Any visible edge rounding, chipping, or built-up edge means the tool needs replacement or regrind.
On gun drills, I check:
- The cutting edge for chips or rounding
- The tip angle (should match spec within 1 degree)
- The drill margins for wear
- The coolant holes for blockage
On BTA heads, I check:
- The cutting inserts for wear or breakage
- The guide pads for wear (replace if more than 0.10 mm worn)
- The chip opening for erosion
I replace the tool at the first sign of significant wear. Pushing a worn tool costs more in part quality than the tool is worth.
Step 5: Check Alignment
If Steps 1-4 check out and the problem persists, I check the machine alignment. Misalignment causes hole deviation, oversize bores, and inconsistent surface finish around the bore circumference.
I use a test bar in the spindle and a dial indicator on the workpiece table. The runout should be within 0.02 mm per 100 mm of travel. If the alignment is out, I adjust the headstock or the workpiece fixture.
Alignment problems develop gradually. A machine that was aligned six months ago may have drifted due to thermal changes, foundation settling, or a collision that was not reported.
When to Call for Help
The five-step sequence resolves most problems. If it does not, the operator should call for help rather than experimenting. The problems that fall outside this sequence usually involve material issues, part geometry issues, or machine faults that require specialized diagnosis.
I tell operators: if you have gone through all five steps and the problem is not solved, stop and ask. Continuing to run bad parts or change parameters randomly will only make the situation worse.
Key Takeaways
- Check coolant pressure at the tool tip first, not at the pump.
- Watch chip shape every few minutes as an early problem indicator.
- Replace guide bushings when clearance exceeds 0.05 mm.
- Inspect the tool under magnification rather than guessing its condition.
- Follow the five-step sequence in order and call for help if it does not resolve the problem.