The short answer
A one-piece CNC design is usually the right starting point: one blank, one datum scheme, and no joint to leak, shift, or inspect. Split the part into two (or more) pieces when a single block would force extreme tool reach, a string of awkward setups, a mixed-material need, or a late-feature scrap risk that you would rather isolate on a smaller second part.
A split is not automatically cheaper. It adds a joint, locators and fasteners, an assembly tolerance stack, and often a second inspection step. Write the split plane, locating method, match-bore rule, and kit-versus-assembled delivery on the PDF you upload to quote-new.php so the quote band covers the joint you intend to buy.
What splitting a part means on an RFQ
Buyers use “split the part” for three different ideas. Name the one you mean.
1. A manufacturing split of a design that could have been one solid. You cut the geometry on a planned plane so each half is easier to mill, then locate and fasten the halves.
2. A functional assembly that was always two pieces: a cover on a housing, a wear plate on a chassis, or a serviceable insert. The joint is part of the product, not only a machining convenience.
3. Something that is not a split at all. Nesting several identical parts in one blank is a production-fixturing choice (see nest vs one-up CNC production lots). A printed preform with a CNC skim is a hybrid process choice (see SLM vs CNC hybrid part strategies). Those are different quote packages.
Typical joints are a milled face with dowel pins plus screws, a housing and cover with a seal in the joint face, or a bolted wear plate. Screws in clearance holes do not locate the pair; pins, fitted bushings, or a controlled rabbet usually do.
When a split is usually worth considering
Consider a split when several of these are true at once:
- A deep pocket or internal gallery cannot be reached with a reasonable tool length, even after you enlarge the corner radii. Deep pockets and tool reach on CNC quotes is the machining-side view; a mid-plane split can turn a long-reach pocket into two shallow cavities.
- Opposed features, internal undercuts, or crossed bores would need many setups, special tooling, or a five-axis program that is only buying access. Compare that path with 3-axis vs 5-axis CNC before you assume a split is required; sometimes one extra setup is simpler than a joint. When to call out undercuts on a CNC RFQ is the feature-level companion.
- Two regions need different materials that you cannot get from one blank — for example an aluminum housing with a steel or acetal wear face.
- A tight feature sits late in the operation list at the bottom of a large blank. If that feature is scrap, you lose the whole piece. Isolating it on a smaller second part caps the risk. How stock size changes a CNC quote and plate vs billet stock for CNC aluminum are the blank-planning companions.
- You want a serviceable insert rather than scrapping a housing when a bore or pad wears.
- Finishing requirements fight each other in a way a mask map cannot solve. A cosmetic anodized shell with a masked, as-machined sealing land can still be one part (see as-machined before Type II anodize). A hard wear plate bolted to a softer body is often cleaner as two pieces.
When you should keep it as one piece
Stay with one solid when:
- The cavity must seal, and a joint would cross a pressure or fluid path you have not designed an O-ring groove or gasket for. If you do split across a seal, O-ring groove design and O-ring groove callouts on CNC housings belong on the drawing, not in a later email.
- Position, perpendicularity, or bore alignment is tighter as one piece than any pin-and-stack assembly will hold. Datum schemes for multi-setup CNC parts, position vs profile tolerances, and perpendicularity callouts on CNC brackets are the inspection language. A split does not relax those callouts; it moves some of them onto an assembly.
- The job is an early prototype whose geometry will still change. Designing a joint, pins, and match-bores can outweigh one extra setup on a solid. Prototype vs production CNC quotes is the broader planning fork.
- The split plane would cut through a structural wall or a cosmetic show face you have not redesigned.
- Quantity is low and the only pain is a second setup, not a true access or material conflict.
A split that exists only in an email, while the STEP is still one body, leaves the estimator guessing and often produces a wide min–mid–max band. What files to send for a CNC quote and the DFM checklist before an online CNC quote both start from files that match the intent.
How a split shows up in cost and lead time
No two jobs move the same amount, and a shop cannot price a joint it cannot see. In typical practice, a split changes the package in these ways — as planning notes, not as a promise of a lower unit price:
- Two smaller blanks replace one large cube. That can cut raw-stock waste and heavy roughing, or it can add two saw cuts and two material lots.
- Each half may be easier to hold and may use a shorter, stiffer tool, which is often kinder to thin walls and pocket floors. How workholding changes a CNC quote still applies — now on two parts.
- You add joint features: spotfaces or counterbores, tapped holes or inserts, dowel bores, and sometimes a rabbet. Thread inserts vs tapped holes in aluminum and press-fit pins and bushings for CNC housings belong in the joint notes.
- If a functional bore crosses the split, the shop may need to assemble, clamp, and match-bore or ream the pair. That is an extra operation and an extra inspection, not a free side effect of “two pieces.”
- Lead time can shorten when each half is a short, standard job, or lengthen when the critical path is match-boring, assembly, and a second inspection pass. What affects CNC machining lead time and what drives CNC machining cost are the general maps; the joint is a line item on those maps, not a discount by default.
Do not ask the estimator to invent an assembly time from a one-piece model.
Designing the joint so it can be quoted
Treat the joint as a designed feature, not a saw cut through a solid.
Split plane
Put it on a mid-plane or a face that is easy to mill flat, away from show surfaces and, if possible, away from seals. Say which half carries the primary datums so the mate has a clear reference.
Location
A typical pattern is two dowel pins — or one pin and a fitted bushing — plus screws. Call press or slip fits the same way you would on any bore; see press-fit vs slip-fit CNC bores. Screws in clearance holes do not restore pin location once the pair is taken apart.
Fasteners
Name thread size, usable depth, and whether aluminum holes get a thread insert. Counterbore vs countersink on CNC RFQs and thread depth callouts for blind tapped holes keep screw heads from sitting proud or running out of usable thread.
Match-bore rule
If a bore, port, or alignment diameter crosses the joint, write “match-bore assembled” or “each half bored separately, alignment by pins only.” Silence here is a common reason two quotes describe two different parts.
Sealing and finish
If the split is in a wet or pressure path, add the O-ring or gasket callout, which face carries the groove, and the surface finish on the joint. Edge-break notes should not nick the seal land. Say whether joint faces stay as-machined, whether anodize or plate is masked at the split, and whether marks may land on the joint. Laser marking before vs after anodize and paint mask and plug notes are the finish-side companions.
Delivery
“Ship as a kit, not assembled” and “assemble, tighten to the drawing note, and inspect as a unit” are different packages. Write the one you are buying.
RFQ checklist for a split
Before you submit on quote-new.php, confirm:
- The STEP matches the PDF: two bodies if you want two pieces, with a named split plane
- Which half carries datums A, B, and C, and how the mate is inspected
- Locators: dowel pins, bushings, or a rabbet — including fits
- Fasteners: size, depth, inserts, counterbore or countersink
- Match-bore or match-ream rule for any feature that crosses the joint
- Seal or gasket, if the split is in a fluid or pressure path
- Finish and mask notes on the joint faces
- Kit versus assembled delivery, and whether you need an assembly inspection
- Revision of both halves, and whether they must stay a matched set
Common buyer mistakes
- Uploading a one-piece STEP and mentioning a split only in the email
- Using screws without locators, then calling a tight position on a bore that crosses the joint
- Leaving the match-bore rule unwritten, so one shop bores the pair assembled and another does not
- Splitting through an O-ring groove or a sealing land
- Forgetting that the assembly stack-up consumes part of the tolerance you used to hold in one solid
- Asking for a cosmetic, paint-ready show face on a joint that also has to be a precision locator
- Mixing revisions of the two halves, or omitting whether the pair is a matched set
- Treating a split as a way to avoid a five-axis setup without checking whether one extra setup on a solid would have been simpler
Bottom line
Split a CNC part when the split removes a real access, material, scrap, or service problem that a solid cannot solve at a reasonable tool reach and setup count. Keep it as one piece when sealing, alignment, early prototype speed, or a simple second setup favor a solid. Design the joint — plane, locators, fasteners, match-bore, seal, finish, and kit versus assembled — and put that paragraph on the PDF with a STEP that actually contains the two bodies. Upload both files on quote-new.php so the quote band covers the joint, not a guessed saw cut. Ready to price a split or a one-piece alternative? Start on quote-new.php.