Castings and forgings are the starting point for many deep hole drilling jobs. The part starts as a rough shape, and I have to put a precision bore through it. The problem is that neither castings nor forgings are uniform materials – they have scale on the surface, internal voids, and inconsistent grain structure from the forming process.
I have learned to treat every casting and forging as a potential problem until proven otherwise. The first hole tells me what the rest will be like.
The Casting Scale Problem
Every casting has a hard outer layer called casting scale. This layer forms when the molten metal contacts the sand mold or metal die. The surface chemistry is different from the interior – it oxidizes and picks up impurities from the mold material.
The scale thickness varies:
| Casting Material | Scale Thickness | Hardness |
|---|---|---|
| Grey cast iron | 1-3mm | 45-55 HRC |
| Ductile iron | 1-2mm | 40-50 HRC |
| Steel castings (WCB, WCC) | 2-5mm | 50-60 HRC |
| Stainless steel castings (CF8M) | 1-3mm | 35-45 HRC |
| Aluminum castings (A356) | 0.5-1mm | Not hard – abrasive |
The scale dulls standard carbide drills fast. I have seen a single pass through scale reduce tool life by 60%. My approach is to machine the entry surface before deep hole drilling. I take a pass that is 1mm deeper than the scale layer to get into clean material.
Parameters for Scale Removal
| Cut Type | Depth of Cut | Speed | Feed |
|---|---|---|---|
| Face mill (cast iron) | 0.5-1.0mm | 120-180 m/min | 0.15-0.25 mm/tooth |
| Face mill (steel casting) | 1.0-2.0mm | 80-120 m/min | 0.10-0.20 mm/tooth |
| Spot face (single point) | 0.5-1.0mm | 60-80 m/min | 0.08-0.12 mm/rev |
Porosity and Voids
Castings can have internal porosity from gas trapped during solidification. When the deep hole drill hits a void, the cutting forces drop instantly. Then when it re-enters solid material, the impact can chip the cutting edge or break the drill.
The risk is highest in aluminum and stainless steel castings. In my experience:
- Aluminum castings – small, scattered porosity is common. Reduce feed by 20%.
- Steel castings – porosity is less common but larger when present. Use tougher carbide grades.
- Cast iron – porosity is rare in good castings but common in poor ones. If I see long stringers of porosity, I stop and check the casting quality.
Parameters for Castings Prone to Porosity
| Material | Normal Feed | Reduced Feed (Porosity) | Tool Grade |
|---|---|---|---|
| Aluminum A356 | 0.10-0.15 mm/rev | 0.08-0.12 mm/rev | Micrograin carbide, sharp |
| Steel WCB | 0.08-0.14 mm/rev | 0.06-0.10 mm/rev | 10% Co carbide |
| 316 SS CF8M | 0.06-0.10 mm/rev | 0.04-0.08 mm/rev | 12% Co carbide |
| Ductile iron | 0.12-0.18 mm/rev | 0.10-0.14 mm/rev | Standard K-grade |
Forgings: Scale and Grain Flow
Forgings have a different problem: the forging scale is hard and abrasive, and the grain flow in the material can cause the drill to wander.
Forging scale forms when the hot billet contacts the air during forging. It is a layer of iron oxide that is harder than the base material. On a large forging, the scale can be 3-6mm thick.
The grain flow in a forging follows the die shape. If the grain flow is not symmetrical around the bore centerline, the drill will favor one direction. I have seen bores wander by 0.5mm per meter in forgings with uneven grain flow.
Forging Parameters
| Parameter | Carbon Steel Forging | Alloy Steel Forging | Stainless Forging |
|---|---|---|---|
| Cutting speed | 70-90 m/min | 60-80 m/min | 50-70 m/min |
| Feed rate | 0.08-0.14 mm/rev | 0.06-0.12 mm/rev | 0.05-0.10 mm/rev |
| Scale layer to remove | 3-5mm | 3-5mm | 2-3mm |
| Coolant pressure | 400-600 psi | 600-1000 psi | 800-1200 psi |
The Inspection Step
On every casting or forging job, I drill a test hole before running the full production batch. The test hole tells me:
- Whether the scale is as thick as expected
- Whether there is porosity in the material
- Whether the drill wanders more than expected
- What the actual tool wear looks like
The test hole costs 10-15 minutes of setup time but saves hours of rework. If the test hole shows problems, I adjust the parameters before drilling the production holes.
I also check the coolant filter frequently when drilling castings. Casting scale particles and sand from the mold can get into the coolant system and cause problems downstream.
Key Takeaways
- Machine off casting scale before drilling – 1-5mm removal depending on material
- Reduce feed by 20% when drilling castings prone to porosity
- Forging grain flow can cause drill wander – use guide bushings close to entry
- Always drill a test hole first on castings and forgings
- Check coolant filters frequently – casting debris contaminates the coolant system
For related articles, see heat treated components and valve and pump components.