When I started in deep hole drilling, I didn’t understand why the experienced operators spent so much time looking at the chips coming out of the hole. They’d pick up a chip, look at it, and then adjust a parameter. It seemed like magic.
My mentor explained: “The chips tell you everything. Short and broken means you’re cutting. Long and stringy means you’re rubbing. Fine powder means you’re forcing it.”
The First Time Chip Reading Saved a Job
About three months into my training, I was running a 12 mm gun drill in stainless steel 316. The job was 150 parts, and I was on part number twelve when I noticed the chips coming out looked different from the first eleven. Instead of the usual small, consistent chips that looked like needles, I was getting long, curled ribbons about 100 mm long mixed with some dust.
I stopped the machine and checked the coolant pressure. It was fine. I checked the flow rate. Also fine. Then I remembered what my mentor said about stringy chips meaning rubbing. I pulled the drill and inspected the cutting edge under a loupe. The outer corner of the insert had a small chip missing — about 0.3 mm of carbide was gone. If I had let that run for another twenty parts, the drill would have scraped the bore surface and I would have lost the whole batch.
Replacing the insert took two minutes. The job finished on time. That experience made me a believer in chip reading.
How I Categorize Chip Types
Over the years, I have developed a mental catalog of chip types and what each one means. I teach this system to every new operator I train because it gives them a concrete reference rather than a vague instruction to “look at the chips.”
| Chip Type | Appearance | Root Cause | Action to Take |
|---|---|---|---|
| Needle chips | Short, 2-5 mm, uniform | Ideal cutting conditions | Maintain parameters |
| Loose washer chips | Curled, 5-15 mm, spiral | Moderate feed, good coolant | Monitor closely |
| Long ribbons | Continuous, 50+ mm | Low feed rate or dull tool | Check tool condition, increase feed |
| Fine dust | Powdery, no visible chips | Rubbing, not cutting | Stop immediately, check tool and pressure |
| Bird nest clumps | Tangled mass at exit | Chip evacuation failure | Increase coolant pressure or check flute |
| Blue or discolored chips | Heat discoloration | Excessive cutting speed | Reduce RPM or improve coolant delivery |
I check the chips at regular intervals during every deep hole drilling job. The first check happens about 20 mm into the bore, once the drill is fully engaged. That tells me the initial contact is correct. The next check is at 100 mm, which confirms the chip formation has stabilized. After that, I check every 200 mm or whenever I hear a change in the cutting sound.
Reading Chips by Material
Every material produces chips differently, and I had to learn the idiosyncrasies of each one the hard way. In low-carbon steel like 1018, chips come off as tight, consistent needles when the parameters are dialed in. If the chips start feathering at the edges, I know the coolant concentration has dropped below the recommended range.
In stainless steel grades like 304 or 316, the chips naturally curl into tight spirals. A sudden change to straight, stringy chips means the edge is starting to break down. In stainless, I can trust a chip change about ten to fifteen parts before a surface finish failure shows up on the bore gauge. That head start saves me rework.
In high-alloy tool steels like H13 or D2, the chips are naturally shorter and more fragmented. I watch for chip color more than shape in these materials. If the chips come out silver or light straw, the temperature is fine. If they come out dark blue or purple, the cutting zone is too hot and I need to back off the speed or check the coolant flow at the tip.
The Sound and Chip Connection
I have also learned to tie chip reading to what I hear from the machine. A healthy gun drilling operation produces a steady, high-pitched hiss from the coolant passing through the drill flute. When the chip formation starts breaking down, that hiss turns into a lower-frequency rumble with occasional crackling sounds. The crackling means larger chip fragments are being forced through the flute clearance, which is the first sign of a potential jam.
When I hear that crackling sound, I check the chips immediately. Nine times out of ten, the chips will show some change — either they are getting longer or there is more dust mixed in. That is my cue to stop and check the tool before the problem compounds.
Teaching Others to Read Chips
I now run a two-week chip-reading exercise for new operators. I have them collect chip samples from every job and tape them into a logbook with notes on the material, tool, and parameters. After about forty samples, most operators can identify a problem chip before the machine indicates any trouble.
I also put together a reference board near the coolant station with chip samples mounted on labeled cards. New operators can walk over and compare their chips to the reference samples without guessing. That board has saved me hours of questions during training shifts.
A Chip Reading Log I Still Use
I keep a chip reading logbook for every machine I run. It is a simple spiral notebook with columns for date, material, drill diameter, depth of cut, and a description of the chip shape at three points during each bore. I started this habit after the stainless steel incident and have filled seven notebooks since.
The logbook serves two purposes. First, it forces me to look at the chips deliberately rather than glancing at them. Second, it creates a reference I can look back at when a new job behaves unexpectedly. If I am drilling 4140 steel with a 14 mm drill and the chips look different from the last time I ran that combination, I know something has changed. Maybe the material batch is different. Maybe the drill has been resharpened and the geometry changed slightly. The logbook gives me a baseline to compare against.
I also log the coolant pressure and flow rate alongside the chip observations. Chip shape is influenced by coolant delivery, and having both data points helps me separate a chip problem caused by tool wear from one caused by coolant issues. If the chips change from needles to ribbons but the coolant pressure is steady and the flow rate is normal, the problem is the tool. If the chips change and the coolant pressure dropped by 5 bar, the problem is the coolant system.
The Biggest Chip Reading Mistake I Made
About two years into my career, I misread a chip change and scrapped a part as a result. I was drilling a deep hole in D2 tool steel, a material known for hard machining. The chips were coming out as fine powder mixed with small fragments. I assumed the powder meant the tool was rubbing and increased the feed rate to force a proper cut.
The powder was actually caused by a micro-crack in the carbide tip. Increasing the feed rate loaded the cracked tip and broke it completely. The drill seized in the hole at 400 mm depth. Extraction took four hours and the bore was oversized by 0.15 mm at the bottom. The entire part was scrap.
I learned that day that not all powder means rubbing. In hard materials, fine chips can indicate micro-spalling at the cutting edge, which requires a tool change, not a feed adjustment. I now stop and inspect the tool under a loupe whenever I see an unexpected change in chip form, regardless of what I think the cause might be. The loupe inspection takes thirty seconds and costs nothing. That feed rate adjustment cost me a part worth three hundred dollars.
Reading Chips During Tool Break-In
One area where I see inexperienced operators make mistakes is during the first few bores with a new drill. A fresh cutting edge produces chips that look different from a worn-in edge. The chips are often smaller and more fragmented during the first three to five bores as the micro-geometry of the cutting edge stabilizes. I have seen operators pull a perfectly good new drill because they saw fragmented chips and assumed the tool was defective.
I now tell every trainee to ignore chip shape changes during the first five bores of a new tool. Instead, I tell them to watch the trend across the first ten bores. If the chips start fragmented on bore one and trend toward consistent needles by bore ten, the tool is breaking in correctly. If the chips stay fragmented or get worse, then it is time to inspect the tool.
Key Takeaways
- Chip reading is the fastest diagnostic tool on the shop floor, faster than any sensor or gauge.
- Each material has a normal chip signature. Learn it before you try to diagnose problems.
- Check chips at fixed depth intervals, not just at the end of the bore. Early detection prevents scrap.
- Blue or discolored chips mean excessive heat. Stop and investigate — heat damage propagates fast.
- The sound of the cut and the chip shape are connected. Learn to use both senses together.
- A reference board with physical chip samples is the best training tool for new operators.
- Never assume chips are fine because the parameters are “correct.” The chips tell you what is actually happening, not what you think should be happening.
- Keep a chip reading logbook. It creates a baseline for comparison across material batches and tool conditions.
- When chips change unexpectedly, inspect the cutting edge before changing parameters. A thirty-second loupe check can prevent a scrapped part.
- Do not judge a new drill by the first three bores. Let the edge break in before making a tool change decision.