Centrifuge Shaker Mount
An orbital shaker will hold anything you can clamp to it, and a 225 mL centrifuge bottle is not something you can clamp to it. It is round, smooth and far too fat for a flask clamp, so it either sits loose on a sticky mat and creeps to the edge, or it doesn’t go on the shaker at all. This is a printed rail that cradles five of them lying on their sides, snapped in from above, and bolts to the platform with one screw per half.
What the part is
One rail, printed as two halves that slide together end to end. Assembled it is 389 mm long and 59 mm tall, standing on a base 20 mm wide, with five cradles on an 80 mm pitch. Each cradle is a 59 mm bore with 5 mm walls, open at the top, and the opening narrows to 44.3 mm between the arm tips — well inside the bore, which is the whole trick. The arms spring apart as the bottle goes past its widest point and close behind it.
That 20 mm is the whole grip. Each bottle is held by a single band round its middle and hangs out well past the rail on both sides, so the row needs clear deck either side of it, not just under it.
The joint is not in a gap between cradles, it is through the middle one: cradle 3 is split down bottom dead centre with one arm on each piece, so the bottle in position three is sprung between the halves and squeezes the seam shut. Below the bore they interlock with a zigzag of angled faces cut with about half a millimetre of clearance. That keys the two bases level and square with each other; it does not lock them lengthwise. The bottle does.
The flat strip along one long edge is the base, and it is the only face with a fastener hole through it. That puts the cradles facing up, the bottles lying on their sides, and the underside of each bottle 9 mm clear of the platform, so nothing drags on the deck.
Each half prints as a 199 × 59 mm footprint 20 mm tall, which is why there are two of them: the whole rail on one bed needs 389 mm of X, and half of it drops onto a 220 mm bed with room to spare.
Parts
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- 225 mL centrifuge bottles — five, and the thing the cradles are drawn around. Measure yours before you print
- eSUN PETG filament, 1.75 mm, white — print it in this; the print settings section is why
- SUNLU PLA+ 1.75 mm, black, 4 × 1 kg — for a single test cradle sliced out of one end, to check the fit before committing to a long print
- M6 × 16 mm stainless socket-head cap screws ×2 — one per half. Socket head, not button head: the pocket the head sits in is 10.0 mm, which is exactly a cap screw’s head, and a button head is 10.5
- Loctite 242 blue threadlocker — a shaker is a machine whose entire job is vibration. Two screws with nothing on them will be finger-loose in a fortnight
- Neiko digital caliper, 0–8 in — for the bottle you actually stock
- AFA deburring tool — one pass round each cradle mouth and the hold-down hole
- Needle file set — for the joint faces if your printer runs on the fat side and the halves won’t slide together
- MITREAPEL CA glue with activator — only if you decide to bond the two halves permanently
- Solimo 99% isopropyl alcohol — cut to 70% for wiping the rail down, same as everything else on the bench
If you are not allowed to drill or tap the shaker’s platform, a rubber-coated magnet base with an M6 male stud will hold the rail to a steel deck without modifying it — a 4-pack, you need two, 40 lb of pull each, and the rubber face won’t scratch the finish. The stud comes up through the 6 mm hole and takes an M6 nut inside the cradle; the pocket it lands in is 10.0 mm and an M6 nut is 11.1 mm across the corners, so open the pocket out with a needle file first. It is a substitute for bolting it down, not an equal: it will hold at normal mixing speeds and it will not hold five full bottles at 300 rpm.
Numbering the five positions is worth the two minutes if the rail feeds a timecourse — a Brother P-touch and laminated TZe tape survive alcohol; a paper label doesn’t.
Measure your bottles first
The cradle bore is 59.0 mm and the mouth narrows to 44.3 mm. Grip comes from the arms closing past the bottle’s widest point, so the number that matters is the diameter of the body of your bottle, not its nominal volume. Two brands of 225 mL bottle are not the same bottle.
If yours measures more than a millimetre off 59, change it in the Fusion file —
there is an .f3d and a STEP in the download section — and not in the slicer’s
scale box. Scaling the STL scales the 80 mm pitch, the 5 mm walls and the 6 mm
hold-down hole along with the bore, and you will end up with a rail that fits
the bottles and no longer fits an M6 screw.
Print settings
| Setting | Value |
|---|---|
| Layer height | 0.2 mm |
| Perimeters | 4 |
| Infill | 30% |
| Supports | None |
| Material | PETG |
| Orientation | Flat on the bed, as exported |
Orientation is the one thing you cannot get wrong. Print it lying flat, the way it comes out of the STL, so each layer is a copy of the C-shaped profile. The arms then flex along the extrusions and the snap loads nothing across a layer line. Stand it up on the bed to save space and every arm becomes a stack of layers being peeled apart every time you push a bottle in, and it will crack at the root.
Four perimeters rather than two, because at 0.4 mm each that is 1.6 mm of solid wall on both faces of a 5 mm arm. The perimeters are the spring; the infill is just what stops them buckling.
PETG, not PLA. Two reasons and they are both about the arms being under load all the time. A snapped-in bottle holds the arms permanently sprung, and PLA creeps under a constant load — more so if the shaker lives in a warm room or an incubator, where a 37 °C ambient is most of the way to PLA’s softening point. And PLA crazes with repeated alcohol contact, which this thing gets. PETG is also simply tougher in bending, which is what a snap fit is.
The STL holds both halves in their assembled positions, so their bounding boxes overlap and some slicers will import it as one object sitting on top of itself. Run split to objects first, then arrange the two parts side by side. They are separate closed shells, so the split is clean.
Assembly
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Deburr both halves. One pass with the deburring tool round each cradle mouth and round the hold-down hole, and take the elephant’s foot off the bed-side edge. It runs the length of the base, which is the face that has to sit flat on the deck.
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Clear the hold-down hole. It is a 6 mm hole printed on its side, so its top will have drooped a little, and 6.0 mm is nominal on an M6 shank rather than a clearance fit. A 6 mm drill turned by hand takes ten seconds; a round needle file does the same job.
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Slide the halves together. The joint has about half a millimetre of clearance all round, so it pushes together by hand. If it is tight, file the angled faces on one half only and check the fit every few strokes — take it off both and you lose the alignment the joint is there for.
Leave it dry unless the rail is going somewhere it will be picked up and carried a lot. A dry joint means a half that snaps can be reprinted on its own. If you would rather it were one part, run CA along the seam.
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Bolt it down. Flat-bottom each screw pocket first (see below) — it is the difference between a flush head and a bottle sitting on one. Then an M6 cap screw into each pocket, a spot of blue threadlocker on the thread, and into the platform. The two holes end up 140 mm apart and each sits 10 mm to one side of its half’s middle cradle.
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Snap the bottles in. Press each one straight down into its cradle until the arms close behind it. Position three goes in last — it is the one that spans the joint, and having it in is what stops the halves working apart.
Mounting it to the platform
The hole through the base is 6.0 mm and opens out to 10.0 mm where it breaks into the cradle, 10 mm to one side of bottom dead centre. 10.0 mm is an M6 socket-head cap screw’s head, and the intent is clearly that the head sinks out of the way.
It doesn’t, as drawn. The step between the two diameters is a radius rather than a flat shoulder, so a head with a flat underside bottoms on the curve about 2.5 mm down and stands 2–3 mm proud inside the cradle. Give the pocket a flat bottom and it disappears, and you cannot overdo it: the pocket runs into the 6 mm hole at 5.75 mm, and an M6 cap head is 5.7 to 6.0 mm tall. A 10 mm end mill turned by hand is the clean way; a flat needle file is the slow one.
Then run a fingertip round the inside before you load a bottle. There is no clearance between a 59 mm bottle and a 59 mm bore, so a head still standing proud is what the bottle lands on.
What the screw threads into is your platform’s problem, not the model’s. Most universal shaker platforms are drilled on a grid; if neither of the two holes lands on one, drill and tap the platform where it does land, or fall back to the magnet bases above. Do not be tempted to run it unfastened. A 20 mm wide base holding five bottles 38 mm up is stable on the bench and is not stable at 250 rpm.
Bottles lying on their sides need their caps done up properly. Upright, a loose cap leaks nothing. Horizontal on an orbital shaker, a loose cap is a bottle emptying itself into the machine over twenty minutes. Check every one, and load the row symmetrically — three bottles in positions 1, 2 and 3 with the far end empty will walk a light shaker across the bench.
What it is not
Do not autoclave it. PETG softens around 80 °C and an autoclave runs at 121 °C. It will come out as a puddle with five bottle-shaped dents in it.
It is not sterile and it cannot be made sterile — an FDM part is porous by construction and no wipe reaches into the layer lines. It holds closed bottles on the outside and never touches anything that goes in them. Wipe it down with 70% on the same schedule as the shaker itself.
Files & downloads
Printable parts
CAD source
The design itself, if you want to change it rather than print it. A STEP file is exact geometry and opens in almost anything; a Fusion 360 archive keeps the modelling history, so you can go back and edit the sketch that made the part.
- 225ml_centrifuge_bottle_shaker_mount_v3.f3d Fusion 360 archive — the editable design, with its history
- 225ml_centrifuge_bottle_shaker_mount_v3.step STEP — opens in Fusion 360, FreeCAD, Onshape, SolidWorks