Stop Choosing One Process for the Whole Build

Most prototype quotes arrive as a single line item: print the whole thing, or machine the whole thing. Neither is usually right. A working prototype is a mix of parts with completely different jobs, and forcing all of them through one process means overpaying for the easy parts or under-building the hard ones.

A typical benchtop assembly might have twenty parts. Two of them carry load, spin, or need a bore held to a couple of thousandths. Eighteen are covers, brackets, spacers, and housings that only have to be the right shape. Printing all twenty gets you a fast build that flexes and rattles. Machining all twenty triples the cost and adds three weeks. The right answer is to split the bill of materials by requirement, not by preference.

Which Part Goes to Which Process

Print the part when its geometry is complicated and its tolerances are loose. Enclosures with internal ribs and snap features, ducting, cable routing channels, ergonomic grips, mounting shells around a board, anything organic, and anything you expect to change twice before the design settles. Printing also wins whenever a machined version would need five-axis work or three setups to reach undercut features.

Machine the part when one of these applies: a bore, shaft, or slide surface has to hold under 0.002 in (0.05 mm); the part sees real load or a press fit; the surface finish matters for sealing or sliding; the material has to be a specific aluminum, steel, brass, or engineering plastic with certified properties; or the part is a flat plate, a shaft, or a simple prismatic block, where machining is genuinely cheaper than printing. Read 3D printing vs CNC machining for prototypes if you want the cost curves behind those rules.

A useful shortcut: if you can name a tolerance on the drawing and you would be upset if it were missed by 0.010 in (0.25 mm), that part gets machined. Printed parts drift with humidity, orientation, and shrinkage, as covered in 3D printing tolerances, and no amount of slicer tuning turns FDM into a milling machine.

The Seam Is Where Hybrid Builds Fail

The interface between a printed part and a machined part is the single most common failure point in a hybrid prototype. Two different processes hold two different tolerance bands, and if you dimension the joint as if both were tight, nothing assembles.

  • Put the tight side on the machined part. Machine the pocket, print the boss slightly undersized, and let a shim or an adhesive gap absorb the difference. Never ask two printed surfaces to locate each other precisely.
  • Design in adjustment. Slotted holes instead of round ones, 0.020 in (0.5 mm) of clearance around fasteners, and one deliberate shim stack per interface. It costs nothing at prototype stage and saves a rebuild.
  • Locate on three points, not four. A printed surface is never flat enough to sit evenly on four pads. Three pads plus a clamping screw gives a repeatable position.
  • Handle fasteners deliberately. Screws going into printed plastic want heat-set inserts, not tapped threads, per threads and inserts in 3D printed parts. Screws going into aluminum can be tapped normally.
  • Watch the press fits. A bearing pressed straight into printed plastic creeps and loosens within days. Press it into a machined aluminum carrier and bolt that carrier to the printed housing, using the interference values in press fits and clearance fits.

Machining Onto a Printed Part

The third option is one part that goes through both processes: print near-net shape, then face, bore, or ream the few features that matter. This is standard practice for SLS nylon and for large FDM parts where a machined billet would waste ten pounds of aluminum.

It works, with conditions. Leave 0.040 to 0.080 in (1 to 2 mm) of stock on the surfaces you will cut, and make sure the printed part has enough solid material underneath: cutting into a 15 percent infill lattice opens voids that no reaming will fix. Print those regions at 100 percent infill or with a thickened solid boss. Also give the machinist a way to hold the part. A printed sacrificial tab or a flat datum pad you can clamp is worth adding, because workholding on a curved printed shell is the reason these jobs get quoted high.

Materials matter here too. Nylon and polycarbonate machine cleanly. PLA melts and gums up at anything but low speeds. Resin parts from SLA chip at the edges. If a part will be post-machined, choose the print material with that in mind rather than after the fact.

What It Does to Schedule and Budget

The honest tradeoff is coordination. Printed parts come back in two to four days; machined parts in five to ten business days, sometimes fifteen with anodizing or heat treat. A hybrid build finishes on the machining clock, not the printing clock, so release the machined parts first and let the printed parts follow. Founders routinely get this backwards, finalize the housing, and then wait two more weeks for the shaft.

On cost, a representative twenty-part assembly might run $900 to $1,600 all printed, $4,500 to $8,000 all machined, and $1,800 to $3,000 split sensibly. The split version usually performs closer to the all-machined build because the parts that mattered got the process they needed. Cost drivers on the machined side are covered in CNC machining for prototypes, and the material decision itself in plastic or metal for your prototype.

What to Send Each Vendor

Print vendors need STEP files plus a note on orientation and material. Machining vendors need STEP plus a dimensioned drawing calling out only the features that are critical. Toleranced drawings on every dimension of every part is the fastest way to inflate a machining quote. Mark three to five critical dimensions, leave the rest at standard shop tolerance, and state the material and finish plainly.

Get the Split Decided Before You Quote

Projects House builds hybrid prototypes for clients whose first assembly has both a housing and a mechanism: we sort the bill of materials by process, design the interfaces so the tolerances land on the right side, and manage both vendors on one schedule. Send your CAD and what the prototype has to prove through our contact form.