Scrap in deep hole drilling is more expensive than scrap in most other machining operations. By the time the hole is drilled, the part has already been turned, heat-treated, and faced. A scrapped bore means all that upstream work is lost.
I have worked on reducing scrap rates across multiple shops. The approach that works is not complex — it is systematic.
Track What You Are Scrapping
The first step in reducing scrap is knowing what you are scrapping and why. Without data, you are guessing.
I keep a simple log for every scrapped part:
| Date | Part # | Operation | Defect | Root Cause | Cost |
|---|---|---|---|---|---|
| 6/1 | 45 | Gun drill | Oversize hole | Worn bushing | $184 |
| 6/2 | 47 | Gun drill | Broken drill | Chip pack | $212 |
| 6/3 | 52 | Gun drill | Hole deviation | Material hard spot | $156 |
After 30 days, I review the log. The patterns are usually obvious. One shop I worked with found that 60% of their deep hole drilling scrap came from a single cause: worn guide bushings not being replaced on schedule.
The log does not need to be fancy. A spreadsheet or a notebook works. What matters is consistency — every scrapped part gets recorded.
The Most Common Scrap Causes
Based on my logs across multiple shops, here is how scrap causes break down:
| Scrap Cause | Typical Percentage | Root Cause |
|---|---|---|
| Oversize hole | 25-35% | Worn bushing, excessive runout, wrong drill size |
| Hole deviation / out of position | 20-30% | Bushing wear, misalignment, entry condition |
| Broken drill in part | 15-20% | Chip packing, coolant pressure drop, material variation |
| Surface finish out of spec | 10-15% | Worn tool, coolant issue, parameter drift |
| Other (cracks, wrong depth, etc.) | 5-10% | Various |
The top two causes — oversize holes and hole deviation — account for 50-65% of all scrap in most shops. Both are directly related to bushing and alignment condition.
I cover oversize holes in Drill Walking Systematic Diagnosis and bushing wear in Gun Drill Entry Bushings Guide.
Target the Biggest Source First
Once you have the data, do not try to fix everything at once. Focus on the single largest cause of scrap and eliminate it.
In a shop where 60% of scrap is from worn bushings, replacing bushings on a fixed schedule will have more impact than optimizing feeds and speeds across all jobs. In a shop where 40% of scrap is from coolant-related chip packing, the priority is the coolant system.
I set a target: reduce total scrap by 50% within three months. If the main cause is responsible for 60% of scrap, fixing it alone gets most of the way there.
Track Trends, Not Just Absolute Numbers
Scrap rate as a percentage of parts run is the standard metric. But I also track trend direction.
| Scrap Rate Trend | What It Means |
|---|---|
| Stable or declining | Process is under control |
| Gradually increasing | Tool wear, bushing wear, or coolant degradation |
| Sudden spike | A specific change — new material batch, new operator, new tool supplier |
| Cyclical | Related to maintenance intervals or shift changes |
If the scrap rate increases gradually over two weeks, I look for a consumable that is wearing out — bushings, filters, or tooling. If it spikes suddenly, I look for what changed.
I cover trend analysis in SPC for Deep Hole Drilling.
Prevention: The High-ROI Investments
There are three investments that consistently reduce scrap in deep hole drilling:
| Investment | Typical Cost | Typical Scrap Reduction | Payback Period |
|---|---|---|---|
| Bushing inspection program | $0 (labor only) | 15-30% | Immediate |
| Coolant filter maintenance schedule | $200-500/year | 10-20% | 1-2 months |
| Operator training on chip reading | $500-1000 | 10-15% | 2-4 months |
I have implemented all three in multiple shops. The bushing inspection program consistently delivers the highest ROI because it addresses the most common scrap cause at minimal cost.
When to Accept Some Scrap
Not all scrap is worth eliminating. I use a simple cost-benefit check:
If the cost of preventing one scrap part exceeds the cost of the scrap itself, accept the scrap rate and move on.
For a $50 part that scrap at 2%, the cost of scrap per part is $1. If the fix requires $5,000 of new tooling, the payback is 5,000 parts — which may take years at low volumes.
For a $500 part that scraps at 10%, the cost per part is $50. The same $5,000 investment pays back in 100 parts.
I use this calculation to prioritize which scrap causes to address and which to accept.
Quality Documentation as a Scrap Reduction Tool
Maintaining quality documentation — first article reports, in-process inspection logs, material traceability — directly reduces scrap. When every part is measured and recorded, parameter drift is caught before it produces scrap.
I cover documentation requirements in Deep Hole Drilling Quality Documentation.
Key Takeaways
- Track every scrapped part with defect, root cause, and cost. Without data, you cannot systematically reduce scrap.
- The top two scrap causes — oversize holes and hole deviation — account for 50-65% of all deep hole drilling scrap. Both are bushing-related.
- Focus on the single largest cause first. Fixing 60% of scrap with one change is better than making ten changes that each fix 6%.
- A bushing inspection program costs nothing and typically reduces scrap by 15-30%.
- Use cost-benefit analysis to decide which scrap causes to fix and which to accept.
- Quality documentation catches parameter drift before it produces scrap.