Mistake 1: Trusting the Pump Gauge
I see new operators check coolant pressure at the pump gauge and call it good. That gauge can read 80 bar while the tool tip sees 45 bar. The difference comes from clogged filters, long hose runs, or a worn rotary union. I teach every operator to read the pressure at the tool-side gauge, not the pump.
The fix is a simple habit: glance at the tool pressure gauge before hitting the cycle start button. If it’s below the minimum listed on the setup sheet, stop and investigate. This one check prevents more drill breakages than anything else I’ve taught.
Mistake 2: Ignoring Chip Formation
New operators stare at the machine cycle and don’t look at what’s coming out of the bore. Chips tell you everything about the cutting process. Good chips are consistent in size and shape. Bad chips mean something changed.
| Chip Condition | What It Means | Action |
|---|---|---|
| Small, broken chips | Normal cutting | None |
| Long stringy chips | Feed too low or material changed | Increase feed |
| Powder or dust | Drill is rubbing, not cutting | Check speed and coolant |
| Blue or burned chips | Heat problem | Check coolant flow and speed |
I show new operators the difference between healthy and problematic chip samples. Once they’ve seen what good chips look like, they start noticing problems before the machine alarms. It takes about a week of conscious attention before it becomes automatic.
Mistake 3: Not Stopping When Something Feels Wrong
The most expensive mistake is running a part that doesn’t sound or feel right. New operators are afraid to stop production. They think stopping means they failed. I tell them breaking a drill saves nothing — scrapping a part plus a drill costs ten times what a preventive stop costs.
I’ve had operators tell me, “I heard a noise but kept going.” That noise was a drill rubbing against packed chips. The drill snapped 20 seconds later. The part was scrap and the drill was gone. A stop-and-check would have saved both.
I set a rule: if anything seems off, stop the cycle, retract the drill, and inspect. No questions asked, no blame. This cuts drill breakage by about half in the first month of a new operator’s training.
Mistake 4: Running Feed Too Low
New operators think slower feed means safer cutting. In deep hole drilling, low feed causes more problems than high feed. Feed below 0.02 mm/rev makes the drill rub instead of cut. Rubbing generates heat, causes built-up edge, and leads to chip packing.
For a 10 mm gun drill in steel, I run 0.03-0.05 mm/rev. Below 0.025 mm/rev, the chips come out as powder and the bore surface looks burnished rather than cut. I let new operators feel the difference — the machine sounds smoother at the correct feed rate.
Mistake 5: Skipping the Bushing Check
New operators assume the guide bushing is fine because it was fine yesterday. Guide bushings wear progressively. A 0.02 mm wear increment doesn’t show up visually but it changes hole location and surface finish.
I have operators check bushing ID with a bore gauge every Monday morning and record it on a wall chart. If the ID is 0.03 mm over nominal, the bushing gets replaced. This simple weekly check prevents a lot of positional scrap.
Expanded Mistake Reference Table
Over the years, I have compiled this list of mistakes I see most often in deep hole drilling operations:
| Mistake | Consequence | Cost Impact | Prevention |
|---|---|---|---|
| Trusting pump gauge over tool-side gauge | Running at insufficient pressure, chip packing | Scrapped part + broken drill ($200-$2,000) | Install and use tool-side gauge |
| Ignoring chip formation changes | Delayed diagnosis of tool wear or coolant problem | Scrapped part ($50-$500) | Check chips every cycle |
| Running with low feed rate | Drill rubs, heat buildup, built-up edge | Premature tool failure ($100-$800) | Maintain 0.03-0.05 mm/rev minimum |
| Not stopping when something feels wrong | Catastrophic drill breakage | Part + drill + downtime ($500-$5,000) | No-blame stop culture |
| Skipping guide bushing inspection | Positional drift, oversize holes | Rework or scrap ($50-$300) | Weekly bore gauge check |
| Ignoring return coolant pressure creep | Chip packing in bore | Snapped drill ($200-$1,000) | Watch return gauge during cycle |
| Using wrong coolant concentration | Reduced lubricity, tool wear | Shortened tool life ($100-$500/month) | Weekly refractometer test |
| Not verifying drill runout before setup | Oversize holes, poor surface finish | Scrapped part ($50-$500) | Dial indicator check before each job |
| Rapid feed at drill entry | Shock loading fractures cutting edge | Broken drill ($200-$800) | Program reduced feed for first 5 mm |
| No pilot hole for depths over 12xD | Drill walks, chatter at entry | Scrapped part ($100-$1,000) | Drill pilot hole 1 diameter deep |
| Inadequate workpiece clamping | Vibration, intermittent cutting loads | Oversize bore or scrapped part ($100-$500) | Verify clamping before each cycle |
| Using dirty or contaminated coolant | Seal wear, clogged passages, tool failure | Multiple repairs ($200-$2,000/month) | Maintain 10-micron filtration |
Training Checklist for New Operators
Based on what I have seen work, here is the training checklist I use for every new deep hole drilling operator:
Pre-Operation Skills
- Read and interpret setup sheet parameters
- Verify coolant pressure at tool-side gauge before cycle start
- Check and record guide bushing ID with bore gauge
- Inspect drill for visible damage, wear, or built-up edge
- Verify workpiece clamping is secure
- Confirm tool runout is under 0.01 mm with dial indicator
During-Operation Skills
- Monitor chip shape and color every cycle
- Watch return coolant pressure for signs of chip packing
- Listen for changes in cutting sound
- Recognize the feel of correct feed rate vs. rubbing
- React to any anomaly by stopping and inspecting
Post-Operation Skills
- Inspect drill for wear after each part
- Document any chip formation changes in the log
- Report coolant pressure trends to setup technician
- Clean and inspect guide bushing at shift end
I have found that operators who complete this checklist for their first 40 hours on the machine make 70% fewer errors than those who learn by trial and error alone.
For more on training operators to read process signals, see deep hole drilling operator guide. For diagnosing chip-related problems, see chip clogging in gun drilling.
Key Takeaways
- Check coolant pressure at the tool tip before every cycle — the pump gauge lies
- Train operators to read chips — it’s the best process indicator
- Encourage stopping for any concern — it’s cheaper than breaking a drill
- Feed too low is more dangerous than feed too high in deep hole drilling
- Weekly bushing ID checks prevent positional scrap
- Use a structured training checklist for the first 40 hours of operation
- Track cost impact of common mistakes to justify training investment
- Listen for changes in cutting sound — the machine tells you when something is wrong
- Never rapid-feed at drill entry — program reduced feed for the first 5 mm
- Return pressure creep is an early warning of chip packing