Subhole CNC Machining: 5 Checks Before You Place the Order
Subhole CNC machining covers holes that must be cut through irregular walls, internal bosses, or castings the drill cannot reach. This guide is for engineers and buyers comparing suppliers. Read it and you will know which checks decide whether a quote is real or just a number.
In this article
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Key takeaways
Which Setup Fits Your Subholes
Match the geometry to the machine before you compare prices.
| Hole condition | Typical setup | What to watch |
|---|---|---|
| Straight hole, open face, 3 axes reach | 3-axis mill or drill-tap | Cheapest route, hold ±0.05 mm easily |
| Hole on a sloped or curved wall | 4-axis or 5-axis | Breakout edge wanders if the tool deflects |
| Deep hole, L/D above 8 | Mill-turn or gun drilling | Chip evacuation, coolant pressure, peck cycle |
| Hole inside a cavity or undercut | 5-axis simultaneous | Tool holder clearance, reach, shank rub |
| Cross hole in a turned shaft | Mill-turn center | True position between two setups |
| Cast or forged skin, hard spots | 5-axis with carbide | Intermittent cut, entry angle, tool life |
| Thin wall under 1.5 mm | 5-axis, light radial step | Chatter and ovality after clamping |
What Subhole CNC Machining Actually Covers
A subhole is not a special hole type. It is a hole whose location, angle, or entry face makes ordinary drilling awkward. The bore may sit inside a pocket, pass through a boss at a compound angle, or break out of a curved casting wall. In subhole CNC machining the difficulty is never the hole diameter. It is reaching the hole without a second fixture and without the drill walking on entry.
That distinction matters when you compare quotes. Two suppliers can both write ±0.005 mm on the paper, but only one of them has planned how the tool gets to a hole buried 60 mm inside a pocket. Position tolerance on a subhole is a system result. It comes from the fixture, the datum scheme, the tool reach, and the number of setups.
Most subhole problems we see start on the drawing, not on the machine. The hole is dimensioned from a feature that will be machined later, or from a cast surface that varies by 0.5 mm. Move the datum to a machined face and the same hole becomes routine.
So the first check is simple. Read your own drawing as if you had to cut the hole. If you cannot describe the entry face and the tool path in one sentence, the supplier cannot either. Fix that before you send the RFQ.
- 1Reachable by 3 axesTreat it as normal milling. Do not pay 5-axis rates for it.
- 2Needs a compound anglePlan for 5-axis or an angled fixture. Both cost the same in the end.
- 3Sits on an as-cast faceAdd a facing cut or allow extra position tolerance.
- 4Breaks out of a thin wallExpect burrs. Specify the deburring method.
Tolerance, Position, and Surface Finish on Subholes
Diameter tolerance and position tolerance are separate promises. A Ø8 H7 bore is a diameter callout, usually reamed or bored, and Ra 0.8–1.6 μm is realistic in aluminium and most stainless. True position of that same hole to ±0.05 mm is a different job. It depends on how many setups the part needs and how the fixture locates the part.
As a rule, one setup holds tight position. Two setups double the stack-up. Three setups and you should expect to inspect every part rather than sample it. If your drawing asks for ±0.02 mm true position across a feature machined in three orientations, expect the supplier to either add a CMM check on every piece or push back on the tolerance.
Surface finish inside a subhole is harder than on an outside face. A long boring bar chatters. A reamer follows the drilled hole, which means it corrects size but not position. For a hydraulic or pneumatic bore, specify the finish and the roundness, not just the diameter. Ra 0.2–0.8 μm is achievable by boring and honing, and it costs more than reaming.
The practical question is which subholes touch a sealing surface, a bearing, or a locating pin. Those deserve tight tolerances. Holes for cable routing, oil drain, or weight reduction rarely do. Sorting your hole list into those two groups often cuts the price by a wide margin.
- 1Sealing or bearing boreSpecify diameter, roundness, and Ra 0.2–0.8 μm.
- 2Locating pin holeSpecify true position and a reamed H7 fit.
- 3Clearance or drain hole±0.1 mm and Ra 3.2 μm is enough.
- 4Threaded subholeGive thread depth and a class, not just the callout.
Materials and Wall Conditions That Change the Plan
Aluminium 6061 and 7075 cut cleanly. A subhole in aluminium usually needs no special tooling, and the entry edge stays sharp. Stainless 304 and 316 work-harden if the tool rubs. On a subhole with a long overhang the tool rubs on entry, so feed and speed need to be set so the insert always bites. That is a process detail you can ask about in the RFQ.
Titanium TC4 (Ti-6Al-4V) and Inconel behave differently again. Heat stays in the cut, so through-tool coolant matters and peck cycles get longer. A deep subhole in Inconel can take four times the cycle time of the same hole in 6061. If you are comparing two quotes and one looks unrealistically low, check what material the supplier assumed.
Castings bring their own variable. Porosity can open into a subhole and leave a void, and hard spots can push the drill off line. Magnesium AZ31B and AZ91D need sharp tools and chip control because fine magnesium chips are a fire risk. That is a shop-floor rule, and a supplier who machines magnesium will mention it without being asked.
Plastics are the quiet trap. POM and PEEK move after machining, so a subhole measured at the machine may not pass inspection a day later. For tight plastic subholes, ask about stress relief or a temperature soak before final inspection.
- 1Aluminium 6061 / 7075Simple. Watch for burrs on thin breakout.
- 2Stainless 304 / 316Control rubbing. Do not let the tool dwell.
- 3Ti-6Al-4V, InconelExpect longer cycles. Coolant through the tool.
- 4POM, PEEK, PAAllow for movement after machining.
Quote Input, Lead Time, and Order Quantity
A useful quote needs four things: a 3D model in STEP or IGES, a 2D drawing with GD&T, the material and temper, and the finish. Send a PDF only and the supplier will guess at the model. Send a model only and the threads, fits, and surface callouts are lost. Both files together is the standard we work to, and it is why we can return a quotation and a free DFM analysis within 12 hours.
Quantity is the second half of the price. There is no minimum order quantity here, so a single prototype and a 10,000-part run use the same route. What changes is the fixture. A machined soft jaw or a dedicated plate adds cost once and pays back over the run. For prototypes, a 5-axis setup often avoids the fixture entirely.
Lead time is where quotes diverge for reasons that have nothing to do with price. Production can start within 24 hours of a released order, and parts ship in 3–5 days for most work. Deep subholes in hard alloys are the exception. If a supplier promises the same lead time for Inconel and for aluminium, ask what the schedule assumes.
Confidentiality belongs in this section too. Drawings for new products leak value if they travel. Uploads are handled as secure and confidential, and an NDA is available on request before you send the files.
- 1Send STEP plus PDFModel for geometry, drawing for fits and finish.
- 2State the temper6061-T6 and 6061-T4 do not machine the same.
- 3Say the quantity band1, 50, or 5,000 changes the fixture decision.
- 4Ask for the DFM notesA supplier who flags risks early saves a rebuild.
Five Steps to a Quote You Can Compare
Run these in order. Skipping step 2 is the most common cause of a quote that cannot be compared.
- 1Sort the hole listSplit subholes into function-critical and clearance. Mark critical ones with diameter, position, and Ra. This alone often removes 30 percent of the tight tolerances from the drawing.
- 2Fix the datum schemeDimension subholes from a machined face, not an as-cast surface. Use three datums that a fixture can actually touch.
- 3Send STEP plus a marked 2D drawingHighlight every subhole that needs CMM reporting. Note the material temper and the finish on the drawing, not in the email body.
- 4Ask two process questionsHow many setups, and how is position verified. The answers tell you whether the ±0.005 mm on the quote is planned or copied.
- 5Compare on the same basisPut quantity, material, tolerance class, finish, and inspection scope side by side. A lower price with no inspection scope is not a lower price.
Subhole CNC Machining Questions Buyers Ask
Can a subhole be machined on a 3-axis machine?
Yes, when the hole axis is parallel to the spindle and the tool can reach it without the holder touching the part. A hole on the top face inside a shallow pocket is a normal 3-axis job.
It stops working when the hole sits on a sloped wall, passes under a boss, or needs a compound angle. At that point a 3-axis setup needs an angled fixture, and the fixture usually costs more than the 5-axis time it replaces.
What position tolerance is realistic for a deep subhole?
In one setup in aluminium, ±0.02 mm true position is achievable and we hold ±0.005 mm on the features that need it. In two setups, expect ±0.05 mm unless the fixture is designed for it.
Depth makes it worse. Once the length-to-diameter ratio passes about 8, the drill deflects and the exit position drifts. Gun drilling or a mill-turn operation with a piloted start brings it back.
How do you inspect a subhole that is hard to reach?
Pin gauges and bore micrometers cover diameter. Position needs a CMM with a long stylus, or a fixture-based check with a dial indicator and a master pin.
We inspect 100 percent of parts before shipment, with raw material check, in-process monitoring, and final inspection. Inspection reports are available on request.
Does the material change the price of subhole work?
Yes, mostly through cycle time and tool life. Aluminium 6061 is the baseline. Stainless 304 and 316 typically run slower because of work hardening. Titanium TC4 and Inconel can take several times longer, and they consume more inserts.
The price difference is not a supplier mark-up. It is the cutting time and the tool changes that the material forces.
What file format should I send for an accurate quote?
Send a STEP or IGES model together with a 2D drawing in PDF. The model gives the geometry; the drawing carries the thread classes, fits, datums, and surface finish callouts that a model usually loses.
If you only have a drawing, send it and say so. We will quote with assumptions listed, and you can confirm them before production starts.
How many subholes can be machined in one setup?
On a 5-axis machine, every subhole whose axis the table can present to the spindle is cut in one setup. That is often the whole part. Reach and holder clearance set the limit, not the count.
On a 3-axis machine, one setup covers one face. Each additional face adds a setup and a position stack-up. That is why the setup count is the first question worth asking about any subhole quote.
Send Your Subhole Drawing and Get a Plan, Not Just a Price
Upload your STEP file and 2D drawing. You get a quotation and a free DFM analysis within 12 hours, with the setup count and the inspection method stated in writing.
12-hour quote100% inspectionNo minimum order quantity
More from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
