After BTA drilling a hydraulic cylinder bore, the bore gets honed to final size. The amount of material left for honing — the hone stock — affects both the drilling and honing operations. I have worked through hundreds of cylinder barrels across multiple material grades, and the hone stock decision has a direct impact on cycle time and scrap rate.
Leave too much stock and the hone takes too long. The honing stones load up, the abrasive wears unevenly, and the operator has to make multiple passes to bring the bore down to size. Leave too little and the drill might not clean up — the bore could have a low spot that never gets addressed by the hone, ruining the seal surface.
I typically leave 0.3-0.5mm for honing on a BTA-drilled bore. For gun-drilled bores under 50mm diameter, I leave 0.1-0.2mm. The rationale comes down to the as-drilled straightness and surface finish each method produces.
Why Hone Stock Varies by Drilling Method
BTA drilling produces a bore with a characteristic spiral feed mark left by the guide pads. The peak-to-valley height of these marks typically runs 0.02-0.05mm, depending on the feed rate and insert geometry. Honing needs to remove not just this roughness but also any waviness and taper left by the BTA head. In my experience, the boring bar deflects under cutting forces, producing a bore that is slightly barrel-shaped or tapered — sometimes as much as 0.05mm difference from one end to the other.
Gun drilling produces a straighter, more consistent bore but the surface finish can be rougher if the coolant pressure drops or the bushing wears. The single-lip gun drill head has one guide pad that burnishes the bore wall as it cuts, which can leave a better surface than a BTA head in some conditions. However, gun drilling is limited to smaller diameters — I rarely gun drill above 50mm in a production setting.
The table below shows the hone stock values I use based on the drilling method and the finished bore tolerance.
| Bore Diameter Range | BTA Drilled Hone Stock | Gun Drilled Hone Stock | Typical Final Tolerance |
|---|---|---|---|
| Under 25mm | Not typical | 0.1-0.15mm | H7-H8 |
| 25-50mm | 0.3mm | 0.15-0.2mm | H7-H8 |
| 50-100mm | 0.3-0.4mm | 0.15-0.2mm | H7-H8 |
| 100-200mm | 0.4-0.5mm | Not typical | H8-H9 |
| 200-300mm | 0.5mm | Not typical | H8-H9 |
| Over 300mm | 0.5-0.6mm | Not typical | H9 |
How Material Grade Affects the Allowance
Different hydraulic cylinder materials respond differently to honing. I have run trials on 1026 DOM tubing, 4130 honed tubing, and 304 stainless, and the hone stock behaves differently in each.
For 1026 DOM tubing, the material is soft and consistent. Hone stock on the low end of the range works fine. The honing stones cut freely and the stock removal happens fast. I use 0.3mm for BTA-drilled 1026 DOM up to 100mm bore.
For 4130 alloy steel, the material is harder and the honing stones wear faster. I add 0.05-0.1mm extra hone stock to account for stone breakdown during the honing cycle. If the stones glaze over, the operator has to dress them, which adds time to the operation.
For 304 and 316 stainless, the material is gummy and work-hardens. Honing stainless with too little stock can be a problem because the stones skid across the surface rather than cutting cleanly. I recommend 0.4mm minimum hone stock for BTA-drilled stainless bores, even at smaller diameters.
| Material | Recommended Hone Stock (BTA) | Notes |
|---|---|---|
| 1026 DOM | 0.3mm for bores under 100mm | Low end of range, stones cut freely |
| 4130 | 0.35-0.45mm | Add 0.05-0.1mm for stone wear |
| 304/316 Stainless | 0.4mm minimum | Avoids stone glazing |
| Chrome-plated bar | 0.15-0.25mm | Less stock needed, plating is uniform |
| Ductile iron | 0.3-0.4mm | Graphite aids honing, moderate allowance |
Checking Straightness Before Honing
The hone stock depends on the as-drilled straightness too. If the bore is straight within 0.1mm per meter, the minimum hone stock works. If it is drifting, I add more.
I check straightness with a precision bore gauge or an air gauge. For production runs, I use an air gauge that measures the bore at three depths — entry, mid-length, and exit. The air gauge gives a readout in seconds and I can adjust the hone stock for each part.
If the bore shows more than 0.15mm of drift per meter, I add 0.1mm to the hone stock. The extra stock ensures the hone removes the low side of the bore. Without that extra stock, the hone follows the existing bore centerline and leaves a tapered surface.
For barrels over 1 meter long, the drift can accumulate. I have seen BTA-drilled bores in 2-meter barrels drift 0.3mm from one end to the other. In those cases, I either request a straightness pass from the BTA drill or accept that the honing cycle takes longer.
Honing Process Economics
The hone stock directly affects the honing cycle time, which is the bottleneck in most cylinder production lines. I have timed honing operations across different stock allowances and bore sizes to understand the cost tradeoffs.
For a 100mm bore in 1026 DOM at 500mm length, removing 0.3mm of stock takes about 4-5 minutes on a vertical hone with 320-grit stones. Removing 0.5mm of stock takes 7-8 minutes. That extra 3 minutes per part adds up over a production run of 500 parts — 25 hours of extra machine time. At $85 per hour for machine time, the extra stock costs $2,125.
The table below shows the honing time I measured for different stock allowances on a representative 100mm bore at 500mm length.
| Hone Stock (mm) | Rough Passes (320 grit) | Finish Passes (600 grit) | Total Cycle Time | Stone Life (parts/set) |
|---|---|---|---|---|
| 0.2 | 1 | 1 | 3 min | 120 |
| 0.3 | 2 | 1 | 4.5 min | 90 |
| 0.4 | 2-3 | 1 | 6 min | 70 |
| 0.5 | 3 | 1-2 | 7.5 min | 55 |
| 0.6 | 3-4 | 2 | 9 min | 45 |
The stone life also decreases with more stock removal because the stones work harder per part. At 0.5mm stock, I replace stones twice as often as at 0.2mm. Stone cost is about $80 per set for a 100mm bore, so the replacement cost adds another $0.65 per part at 0.5mm versus $0.32 at 0.3mm.
I run the numbers before every new job. The optimal hone stock is the smallest amount that reliably cleans up the as-drilled bore. For most BTA-drilled bores, that is 0.3-0.4mm. Any more than that is wasted time and stone life.
Common Hone Stock Mistakes I See
I visit other shops occasionally and I notice the same hone stock mistakes repeating. The most common error is using a fixed hone stock for every part regardless of the drilling conditions. A shop that runs 0.5mm on every bore is wasting time on good parts and not leaving enough on bad ones.
The second mistake is ignoring the effect of BTA head wear. As the BTA insert wears, the cutting forces increase and the bore straightness degrades. A fresh insert might produce a bore that is straight within 0.05mm per meter. A worn insert at the end of its life can produce a bore that drifts 0.15mm per meter. The hone stock should increase as the insert wears.
The third mistake is not measuring the as-drilled bore before sending it to the hone. I have seen shops send every barrel straight from the BTA machine to the hone without any inspection. When the bore is oversized from a worn drill, the hone has nothing to cut and the operator is running an air cut. When the bore is undersized from a drill deflection, the hone has to remove 0.8mm of stock and the stones glaze over.
I track the as-drilled bore size on a control chart. The chart shows me when the process is drifting so I can adjust the hone stock proactively. A 0.02mm trend in the as-drilled diameter over 10 parts tells me the drill is wearing and I should add 0.05mm to the hone stock for the next batch.
Key Takeaways
- Hone stock is not a fixed number — it depends on the drilling method, material grade, bore diameter, and length. I adjust it based on measured results from the first few parts in every new run.
- BTA drilling needs more hone stock than gun drilling because the bore has more waviness and taper. I budget 0.3-0.5mm for BTA and 0.1-0.2mm for gun drilling.
- Material grade changes the hone stock requirement significantly. Stainless steel needs at least 0.4mm to prevent stone glazing. 4130 needs extra for stone wear.
- I always check as-drilled straightness before setting the hone stock. A simple three-point bore gauge measurement saves expensive scrap.
- When in doubt, I add 0.05mm to the hone stock. The extra 10-20 seconds on the hone is cheaper than scrapping a $500 barrel.