Machine Design #61: Blind Holes, Relief Grooves, and Square Corners — Design for the Real Tool Path
1. Start with function and evidence
Before choosing a dimension or opening a supplier catalog, write what the machined feature must do, where the load or environment comes from, how many cycles it must survive, and what counts as failure. “Use the old drawing” hides assumptions about process, access, maintenance, and safety. A testable requirement states the input condition, operating condition, expected result, acceptance limit, and measurement method. Internal standards are a starting point; the final decision must be checked against the real model, material, process, and installation.
2. Review four layers together
Function and load
Separate nominal, start-up, impact, misalignment, and fault cases. Trace the load through the complete assembly, not only the attractive CAD section. Include inertia, thermal movement, cleaning, vibration, and the number of cycles that actually matters. State the boundary and the reason for the safety margin.
Material and manufacturing
The same geometry behaves differently after cutting, forming, welding, machining, coating, or heat treatment. The drawing must describe what a workshop can make and measure. A tight tolerance cannot compensate for an unknown process, an unprotected datum, or an inspection method that cannot reach the feature.
Interfaces and assembly
Define datums, direction, clearance, contact, fastening, tool access, and mistake-proofing. If variants exist, use geometry or markings to prevent a wrong part or orientation. Record the force, sequence, and evidence expected from the operator.
Operation and maintenance
Check access, replacement time, re-location, cleaning, adjustment, and the measurement that returns the machine to baseline. A design that works only with one experienced fitter is not repeatable.
3. Core checks for this topic
- Tool diameter, reach, holder clearance, and approach direction: design from the tool that will actually cut: its diameter, its reach, the holder that follows it in, and the direction it can approach from.
- Blind-hole depth, drill point, chip evacuation, and cleaning: state the usable depth of a blind hole separately from the drilled depth, and say how chips leave and how the hole is cleaned before assembly.
- Internal corner radius and the actual cutter radius: match every internal corner radius to a real cutter radius; a drawing that asks for a sharp internal corner asks for a process change.
- Relief groove width, depth, and burr control: give the relief groove its width and depth, and state the burr condition — a groove that removes a stress riser but leaves a burr has moved the problem.
- Inspection access for depth, squareness, and surface finish: check that depth, squareness and surface finish can be reached by the gauge that will measure them, not only by the cutter.
- Assembly clearance and the cost of a special tool: keep the assembly clearance in view, and decide consciously when a special tool is worth its cost rather than discovering it at quoting.
The checklist is useful only when every line has evidence. “Reviewed” is not “calculated”, and “calculated” is not “tested at the boundary”. Keep an evidence ID next to each requirement so an engineering change can be audited.
3b. Three tool constraints the CAD model never mentions
A 3D model will happily produce shapes a cutter cannot reach. The three constraints below account for most of the questions that come back from the shop.
Depth against diameter. The deeper a hole is relative to its diameter, the harder chips are to evacuate, the less coolant reaches the cutting edge, and the more the drill wanders. The consequence is not "cannot be drilled" but more expensive and less accurate: the drill has to be retracted repeatedly, a special drill may be needed, and the position error at the bottom is larger than at the mouth.
| Design intention | The real cost | The usually cheaper route |
|---|
| One deep hole through several stacked parts | Position error at the bottom, difficult chip removal | Short holes in each part, aligned with locating pins |
| A very deep blind hole to bury a screw head | Poor chip evacuation, hard to clean | A through hole with a plug, or a counterbore from the opposite face |
| A small hole through a thick wall | Slender tool, breakage risk, long cycle time | Start from both faces, or open up the diameter if function allows |
A drilled hole does not have a flat bottom. A standard drill leaves a conical bottom set by the point angle. That means the drilled depth must exceed the usable depth, and if the function needs a flat bottom, an extra milling operation is required — which has to be called out rather than guessed at.
A tapped hole needs room for the tap. In a blind hole the fully formed thread is always shorter than the drilled depth because the tap has a lead taper. The drawing should give the effective thread depth — the length of thread that actually carries load — instead of giving a drill depth and leaving each party to interpret it. Thread deeper than needed adds no strength but adds cycle time and the risk of a broken tap.
Relief grooves: a small groove that changes the whole process
A relief groove is material deliberately removed at a transition so the tool can run out without having to stop exactly in the corner. Three places almost always need one:
- A shaft shoulder that will be ground. Without a relief, the wheel has to stop right at
the corner and the shoulder radius is uncontrolled. That radius in turn decides whether a bearing actually seats against the shoulder face.
- The end of a thread. A thread running right up to a shoulder leaves several incomplete
turns. A thread relief lets the mating part seat against the face.
- The inner corner of a deep pocket. A small relief at the corner allows a larger cutter
for the whole pocket instead of switching to a small one because of one corner.
A relief is a reduction in section, so on parts under cyclic load it has to be treated as a stress raiser and not placed casually. How to recognise undercuts and their cost impact is in Machine Design #24 — Undercut.
Internal square corners: four routes in order of cost
Milling with an end mill always leaves a radius in an internal corner; the mechanism and how to state the radius are in Machine Design #106. When the function genuinely needs a square corner, four options in the order worth considering:
- Accept the radius and chamfer the mating part so it does not ride up on it — cheapest.
- Drill a relief hole in the corner, leaving room for the mating part to seat.
- Split into two parts, so each one only has external corners.
- Change the process: wire cutting or slotting. The working range of milling, turning and
wire cutting is compared in Manufacturing Engineering #02.
Which route to take is the designer's decision because it is tied to function. If the drawing stays silent, the shop will choose whatever is cheapest for them, and that does not always match the intent.
4. Tolerance stack and variation
Build the stack from the functional datum to the characteristic that must be protected. Separate nominal, worst-case, and statistical values only when the process is stable enough to justify it. Include flatness, squareness, coating or treatment thickness, clamping deformation, temperature, field assembly error, and wear.
If assembly succeeds only because a technician nudges a part, the design has no reliable capability. Assign each contributor a source and identify whether it is controlled by the supplier or verified at incoming inspection. For machined feature, pay particular attention to tool reach, chip evacuation, corner radii, relief grooves, and measurable depths.
5. Failure modes before release
Ask: “If this is wrong, what will the machine show, and what evidence will reveal the cause?”
| Failure mode | Machine symptom | Verification direction |
|---|
| Boundary selected too optimistically | Passes a demo, fails at speed, temperature, load, or cleaning | Test min/max and the defined fault case |
| Tolerance not tied to function | Difficult assembly, play, bind, noise, or leak | Rebuild the stack from functional datums |
| Manufacturing step omitted | Distortion, burr, crack, drift, or poor finish | Review the process and measure after each critical step |
| Inspection cannot reach feature | Supplier report looks complete but function is unknown | Define a reachable method and a sample plan |
| Maintenance not designed | Long replacement or wrong restoration | Run a maintenance trial and restoration check |
| Documents out of sync | Correct name but wrong revision or setting | Baseline BOM, drawing, process, and work instruction |
6. Drawing and record discipline
Specify only requirements that have functional meaning and can be inspected. For a special requirement state the scope, datum or measurement location, condition, and acceptance limit. Notes such as “accurate machining” or “assemble carefully” are not instructions. Keep the calculation or rationale, source revision, assumptions and limits, review comments, prototype or FAT result, and the point to recheck after change.
7. Design review on the real production route
Start at the input and follow the load or environmental attack to the output. Name the surface, edge, joint, thread, hole, treatment, or contact that carries the function. Mark where friction, clearance, temperature, chemicals, operator adjustment, or cleaning can change the result. If a parameter is unknown, assign an owner and measurement plan instead of hiding it in a large factor.
Ask the fabricator which operation creates the most variation, how the feature is inspected, and what happens after deburring, welding, heat treatment, coating, or washing. Compare capability with the stack. If the process cannot hold the drawing, change the process or design before ordering.
On the assembled machine, run start-up, normal duty, stop, restart, and a controlled fault. Record force or torque, temperature, vibration, noise, motion, visual marks, and replacement time as relevant. These observations often reveal an assumption that a drawing review missed.
8. Boundary conditions and calculations
Write lower and upper values for every influential parameter and include combinations that can occur together. Keep units consistent, show the equation or reference, and record catalog test conditions and correction factors. Compare at least one analysis result with a hand calculation or a measured prototype.
For machined feature, a useful sheet has columns for input, nominal, lower bound, upper bound, source, result, pass/fail, and “what would invalidate this assumption?” This makes the decision robust when a supplier changes material, cycle time, temperature, chemical, or inspection route.
9. Supplier and incoming inspection
The supplier package should include drawing revision, material condition, special process, inspection points, sample record, and rework rule. Do not outsource design intent. If a supplier proposes a different process, compare function, durability, cost, lead time, and inspection capability before approval.
At incoming inspection, measure the features that protect the function rather than every dimension with equal effort. Record actual values and instrument ID. A part can be within a drawing tolerance and still fail because of the wrong surface, burr, fit, treatment, lubricant, or orientation. Link the result to the lot or serial used on the machine.
10. Commissioning and maintenance feedback
Create a baseline at commissioning: cycle time, force or torque, temperature, vibration, noise, position, surface condition, or appearance as relevant. Define the alarm limit and the action when it is reached. After the first service interval, compare the trend with baseline and update the maintenance instruction. Replacement should restore the baseline, not merely make the machine move again.
Threads in a blind hole: usable engagement is shorter than the drilled depth
In a tapped blind hole the usable thread length is always shorter than the drilled depth: you lose the drill-point cone, the first few incomplete threads, and the chip room needed at the bottom. If the drawing only says "M8 deep 20" without separating the full-thread depth from the drilled depth, the shop has to guess.
A few rules:
- Enough thread engagement matters, but longer does not add strength: the load concentrates in the first few threads, so tapping deeper than needed only wastes work. A soft material (aluminium) needs a longer engagement than steel for the same bolt size.
- If you need full thread up to a shoulder or the bottom, add a thread relief (undercut) so the tap can run out without leaving stub threads.
- Blind tapping needs chip evacuation: use a spiral-flute tap to pull chips out and leave chip room at the bottom — otherwise packed chips break the tap or damage the thread.
| Wrong note | Consequence | Correct note |
|---|
| Only the thread depth, no drill depth | Shop guesses; stub threads or breakthrough | State both the full-thread depth and the drilled depth |
| Full thread demanded to the bottom | Cannot tap to the bottom | Add a thread relief or allow a few incomplete threads |
| Deep blind hole, small thread | Tap breakage from packed chips | Add a chip-evacuation note, choose a suitable tap |
11. MINATA release checklist
- [ ] Function, load path, duty cycle, and fault or environment boundary are written.
- [ ] Material, process, surface condition, and inspection method are agreed.
- [ ] Functional datums and tolerance stack are visible.
- [ ] The six topic checks above have evidence and pass/fail criteria.
- [ ] Assembly, tool access, orientation, and mistake-proofing were tried on hardware.
- [ ] Failure modes have an owner, evidence, and re-test condition.
- [ ] Supplier and incoming records link to the revision and lot.
- [ ] Commissioning baseline and maintenance response are defined.
Frequently asked questions
What exactly does the blind-hole depth refer to?
State which part is meant: the depth of the full cylindrical portion, or the depth including the conical bottom left by the drill. For a tapped hole, the functionally meaningful figure is the effective thread depth; give that and let the shop work out the drilled depth.
Why does the shop query a deep hole?
Because depth relative to diameter changes both the tool and the process: the drill has to be retracted to clear chips, a special drill may be needed, and the position error at the bottom is much larger than at the mouth. If the function only needs a short guide hole, splitting it or opening the diameter is far cheaper.
Does a relief groove weaken the part?
Yes, because it reduces the section and creates a stress raiser. On parts under cyclic load, place reliefs in low-stress areas and round the groove root. But omitting the relief at a shoulder that will be ground trades that risk for a loss of control over the shoulder radius, which is also a risk.
Does a flat-bottomed hole need to be stated separately?
Yes. A standard drill leaves a conical bottom, so a flat bottom is an added requirement and an added operation. Call it out only when the function truly needs it, because it adds cycle time.
The mating part rides up in the corner — which side should be changed?
The side with the external corner. A chamfer or a relief on the external corner is far cheaper than forcing the part with the internal corner to achieve a perfectly square corner.
12. Closing note
Good mechanical design is a chain of explicit assumptions that survives manufacturing, assembly, operation, maintenance, and change. For machined feature, the right question is not only “will it work?” but “what evidence will show it continues to work when the boundary moves?” That is the standard MINATA uses to turn a drawing into a dependable machine.
Frequently asked questions, continued
Is a deeper thread always stronger?
No. The tensile load concentrates in the first few threads, so beyond a certain engagement length more thread adds almost no strength — it only wastes work and hole depth. What matters is enough engagement for the material (aluminium needs more than steel), not "as deep as possible".
Why is a thread relief needed in a blind hole?
Because a tap cannot cut full thread right up to the bottom or a shoulder. If the design needs full thread to that point, a thread relief lets the tool run out its full travel; without it, stub threads remain and the mating part does not seat.
Conclusion
Blind Holes, Relief Grooves, and Square Corners is not a matter of picking a single number. Good design ties function, load, material, process, tolerance, assembly and maintenance into one contract that can be checked. An internal standard preserves the experience already accumulated; the designer's job is to understand the conditions under which it applies and turn it into a clear decision on the product at hand.
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