Machine Design #92: Split Parts — Accuracy Lives in the Joint Face and Tightening Method
Split part must keep function, accuracy, manufacturability, inspectability, and serviceability when the boundary changes.
Function before geometry
Write the input condition, expected result, acceptance limit, failure symptom, and measurement method before selecting a pair, joint, draft, or wall. Assign an owner to every value and change.
Core checks
- Functional reason for splitting the part: record value, source, method, owner, and pass/fail evidence.
- Joint face, datum, and locating feature: record value, source, method, owner, and pass/fail evidence.
- Fastener pattern and controlled clamp load: record value, source, method, owner, and pass/fail evidence.
- Distortion, gap, burr, and cleanliness: record value, source, method, owner, and pass/fail evidence.
- Inspection after assembly: record value, source, method, owner, and pass/fail evidence.
- Disassembly, reassembly, and repeatability: record value, source, method, owner, and pass/fail evidence.
| Failure mode | Symptom | Verification |
|---|
| Pair or joint undefined | Wrong fit or replacement | Audit identity and assembled function |
| Process omitted | Warp, gap, sink, or mismatch | Walk the process and measure |
| Documents out of sync | Correct name, wrong revision | Baseline every reference |
What splitting a part buys, and what it costs
Split parts — split bearing housings, two-piece covers, split bushes — solve problems a one-piece part cannot:
| Reason for splitting | Example |
|---|
| Fitting and removal without withdrawing the shaft | An intermediate bearing on a long shaft, a crankshaft bearing |
| The inner part cannot be threaded through | A housing around an already assembled unit |
| The internal features are easier to machine while separate | Grooves, deep pockets, oil passages |
| The part is too large to machine or transport in one piece | Large machine bases, big fabricated housings |
The cost is that all the accuracy is concentrated in the split face and in how the two halves are located. A one-piece part has no such problem; once split, every deviation of the joint face becomes a deviation of the bore or the profile after assembly.
Locate with pins, not with bolts
This is the most important principle and the one most often broken.
| Function | Which component does it | Why |
|---|
| Locating the two halves correctly | Locating pins | Pin holes are machined accurately and repeat every time the joint is opened |
| Clamping the halves together | Bolts | Bolt holes have clearance and cannot locate |
Bolt holes always carry clearance so the bolts can be fitted, so relying on bolts to locate means the two halves sit slightly differently after every disassembly. On a bearing housing, that becomes an out-of-round bore and a bearing carrying an uneven load.
Two pins are enough to lock the position; place them far apart to minimise angular error, and place them asymmetrically if the halves may only be assembled one way round.
Sequence: machine the functional features AFTER assembly
For a split part with a bore through the split face, the correct sequence is almost always:
- Rough machine the two halves separately.
- Machine the split faces of both.
- Assemble, fit the pins and tighten the bolts to the specified torque.
- Finish machine the bore and the functional surfaces in the assembled state.
- Mark the set, and separate again for any remaining steps.
Steps 3 and 4 are decisive: a bore reamed in the correctly torqued condition is the one that stays round when the machine runs. Reaming each half separately and assembling afterwards produces a distorted bore, because each half deforms slightly when tightened.
The consequence that comes with it: the two halves become a set and cannot be replaced singly. Mark the set number and state it on the drawing — the same principle as in Machine Design #90.
The split face: four things to state
- Flatness of the joint face, and whether it carries its own roughness requirement.
- Where to put it: through the bore centre is the most common, but an offset can be better for carrying
the load or for the removal direction.
- Whether there is a shim — with a shim, its thickness enters the tolerance chain of the bore.
- The tightening torque at finish machining and at final assembly, because the two must be the same.
The joint decides whether a split part works
Splitting a part into two halves opens up an extra mating joint, and that joint is where error creeps in. Three things must be controlled at the split line itself.
- Repeatable relocation: bolts alone let the two halves shift slightly each time they come apart
and go back. Add two locating dowels (taper or reamed parallel pins) so the halves return to their original position every time. Bolts hold clamp load, dowels hold position — two different jobs.
- Flat, sealed faces: the two mating faces must be flat so they neither gap nor form a step. For a
precise joint the faces are ground or scraped; for an oil-wetted joint, add a groove and a gasket so it does not leak.
- A step at the joint: if a bore runs across the split line (like a bearing bore in a split housing),
even a small step between the halves makes the bore go out of round. That is why the bore is reamed after the bolts are tightened to working load, not on each half separately.
The machining order decides the joint bore's roundness
| Approach | Bore roundness result |
|---|
| Ream each half loose, then join | Bore distorted at the split, hard to fit a bearing |
| Join, tighten to full load, then bore | Bore round in the working state |
| Join, tighten lightly, bore, then tighten hard at assembly | Final tightening distorts the bore again |
Tightening the bolts deforms the whole assembly, so the bore is only round at the same load it was bored at. Call out the tightening torque and a "bore after assembly" note on the drawing, so the shop does not bore each half for convenience.
MINATA release checklist
- [ ] Function, boundary, pair or feature, and failure symptom are written.
- [ ] Datums, ownership, process, and mistake-proofing are clear.
- [ ] Six topic checks have evidence and pass/fail limits.
- [ ] Manufacturing, assembly, inspection, and maintenance were tried.
- [ ] Revision, supplier, material, and configuration records agree.
Good engineering is an explicit chain that survives manufacture, operation, maintenance, and change. For split part, evidence that accuracy and function remain reliable is the MINATA standard.
Frequently asked questions
Can the halves be located by the bolts?
No. Bolt holes carry clearance so the bolts can be fitted, which means they clamp but do not locate. Use pins to locate and bolts to clamp — confusing the two functions is why a housing sits differently after every disassembly.
Ream the bore before or after joining the halves?
After joining, with the pins fitted and the bolts torqued correctly. Reaming each half separately produces a distorted bore because each half deforms slightly when tightened. The torque at finish machining must equal the torque at final assembly.
Can the two halves be replaced singly?
No, once the bore has been finish machined in the assembled state. They become a set and must carry a set number stated on the drawing. Replacing one means remachining the bore together with the surviving half.
Should the split face pass through the bore centre?
Through the centre is the most common because it is easy to machine and easy to assemble. But if the main load acts along the direction of the split face, consider offsetting it so the joint does not sit where the load is highest.
How many locating pins are needed?
Two are enough to lock the position. Place them as far apart as possible to minimise angular error, and place them asymmetrically if the halves may only be joined one way round — that also prevents assembling them the wrong way.
Frequently asked questions, continued
Why must a split bearing housing be bored after joining?
Because tightening the bolts deforms the whole assembly slightly, and the bore is only round at the load it was bored at. Reaming each half loose and joining them gives a bore distorted at the split, so the bearing runs hot and wears quickly.
If the bolts already clamp it tight, why add dowels?
Bolts hold clamp load but not lateral position; after a few disassemblies the halves drift. Two locating dowels return the halves to their original position each time, preserving the joint bore's accuracy.
Do the mating faces need grinding?
It depends on the accuracy. A precise bearing joint is ground or scraped so it neither gaps nor steps. A general machine-body joint only needs a milled flat; an oil-wetted joint adds a gasket and a groove to stop leaks.
Conclusion
Split Parts is not paperwork done to make a file look tidy. It is how intent becomes a result that can be manufactured, assembled and measured repeatedly. A good drawing does not need the designer standing beside it to explain it; the structure of the information has to do that work.
View all MINATA technical articles