Introduction: Design for a 3D Printed Culinary Centrifuge

About: Hello, my name is Toglefritz. That’s obviously not my real name; my real name is Scott, but on the Internet I use the nom de plume, Toglefritz. I like to make things and I like to share my work with others. I …

This Instructable presents a design for a 3D-printed centrifuge.  It also contains discussion of the design, and centrifuge design in general.



A couple of months ago, I read an article on Popular Science about one of their writers, Paul Adams, who was lucky enough to tour the Modernist Cuisine test kitchen.  Among other really awesome equipment the Modern Cuisine chefs/mad food scientists have at their disposal is an $8000 Sorvall RC-5C Plus High-Performance Centrifuge, which they use for culinary purposes.  It turns out that when ordinary food items are subjected to extreme centripetal forces in a centrifuge, some interesting things can happen.  Paul Adams did a follow-up article demonstrating how he used a small centrifuge to separate the solid and liquid components of pea puree and made pea butter.

After reading that article, I got really interested in what other foods could be put into a centrifuge to produce strange and enticing new flavors and textures.  I started scouring the web for other cool examples of molecular gastronomy dishes made by spinning food in a centrifuge. The pea butter in Paul Adam's article certainly looks very tasty, but I wanted to see more than just a green spread for toast.  Unfortunately, there are very, very few other examples of centrifuge recipes to be found; even after extensive searching, all I was able to find were a few random forum posts vaguely describing centrifuge experiments.  I was not even able to find any pictures (other than more pictures of pea butter).

So, decided to get a centrifuge for myself and start hacking food.  But there was a problem:  centrifuges are quite expensive.  Even low-powered bench-top models cost over $200.  Good-quality bench-top models can cost twice that.  Since I didn't want to blow my entire quadcopter fund on a centrifuge,  I set about designing my own centrifuge.  This Instructable presents that design and a discussion of the ongoing process of formulating and refining it.

So far, the design only exists virtually.  Unfortunately, I do not own a 3D printer and creating multiple developmental prototypes, and eventually a finished product, using services like Ponoko would be both extremely expensive and extremely slow.  That is not meant to be a criticism of Ponoko, I like their service a lot and I have used it for several projects in the past, it is just not ideal for creating multiple prototypes.  Over the summer, when school is out and I have more time to work, I may get a membership to a local hackerspace called Sector 67 where I would have access to a MakerBot Thing-o-matic and I could begin making physical prototypes of the centrifuge.  I will make another Instructable once I have a finished device to document the physical build process and some recipes.

In the meantime, if you are interested in making your own 3D printed centrifuge, you are welcome to use the designs in this Instructable either wholesale, in parts, or by creating your own derivative designs.


Instructable Table of Contents


     Step 1:  Background and General Design Considerations
     Step 2:  General Safety Considerations and Features
     Step 3:  Rough Calculation of Applied Centripetal Force and Sample Tangential Speed
     Step 4:  Non-3D Printed Materials
     Step 5:  3D-Printed Parts
     Step 6:  Plexiglass Cutting Patterns
     Step 7:  Centrifuge Drivetrain
     Step 8:  Rotor
     Step 9:  Safety Enclosure 
     Step 10:  Virtual Testing and Simulation

Step 1: Background and General Design Considerations

Centrifuges:  A Basic Introduction


From the Wikipedia page on centrifuges, "a centrifuge is a piece of equipment... that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis. The centrifuge works using the sedimentation principle, where the centripetal acceleration causes denser substances to separate out along the radial direction (the bottom of the tube). By the same token lighter objects will tend to move to the top (of the tube; in the rotating picture, move to the centre)."

Basically, a centrifuge is a device that spins a tube containing a liquid sample about a central axis.  This rotation causes the sample to be subjected to a centripetal force.  Since the tubes are oriented radially with the top of the tube towards the axis, the centripetal force simulates an extreme gravitational force on the tube.  It is as if you stood the tube on end in a laboratory (or kitchen) on Jupiter (only the centrifuge actually exerts much, much more force than Jupiter's gravitational pull would).  This increased force causes heavier parts of the sample, like suspended solids and fats to sink to the bottom of the tube, while lighter components, like liquids, float to the top.

Centrifuges are typically used in chemistry, biology, and biochemistry for isolating and separating suspensions.  However, they have recently begun seeing use in molecular gastronomy, a field of food science that seeks to utilize scientific principles and techniques in the preparation of food.  Little documentation exists as yet, however, of recipes for food prepared with centrifuges.  It is the goal of this Instructable to begin changing that.


Design Goals


The overall goal of this project is to create a centrifuge capable of spinning up to eight 15mL samples at high enough speeds to achieve separation of the liquid, solid, and semi-solid portions of the sample.  In addition to this function, the centrifuge must
  • be safe both during normal use and in the case of failure.
  • be easy to use.
  • be sanitary as it could be used in the preparation of food for human consumption.
  • be aesthetically pleasing and customizable in order to fit in with the decor of a kitchen.


Parts of the Centrifuge


More detail is given on the steps dedicated to each system.

Drivetrain (Step 5):  The drivetrain is the portion of the centrifuge that interfaces with a drive motor and transfer's the motor's torque to the rotor.  The drivetrain must be strong enough to withstand rotational speeds as high as the maximum of which the drive motor is capable, plus a generous margin of safety.

Rotor (Step 6):  The rotor is the portion of the centrifuge that rotates and secures the sample-containing tubes during operation.  The rotor will be capable of holding a maximum of eight 15mL tubes.  Tubes are loaded into the rotor in pivoting brackets so that, when the centrifuge is at rest, the tubes hang vertically, and during operation, the tubes lie radially, parallel to the direction of the centripetal force exerted upon them.  The rotor must be strong enough to hold the sample tubes under centripetal forces generated at the maximum rotational speed of the centrifuge, plus a generous margin of safety.

Safety Enclosure (Step 7):  The safety enclosure contains all the moving parts of the centrifuge, save the drive motor.  The enclosure is designed to do two things.  First, the enclosure prevents any foreign object from entering the centrifuge during operation.  Second, the enclosure is a shield designed to prevent any part of the centrifuge, or the sample tubes, from being ejected from the machine in the case of failure.  The enclosure is designed to protect the centrifuge operator, and anything else that could suffer damage from a mechanical failure of the centrifuge.


Sources of Inspiration


As mentioned in the introduction to this Instructable, the primary source of inspiration for this project was an article in Popular Science by Paul Adams.  Mr. Adams wrote a great demonstration piece showing how a centrifuge could be used in food preparation, specifically by making pea butter out of the fat separated from a pea puree.  It was this article that piqued by interest in experimenting with food in a centrifuge.  However, because centrifuges are quite expensive, I got to work designing my own.

Before I started the project, I had heard a story about how a 3D printer was used to produce a small centrifuge attachment for a Dremel rotary tool.  The project, called the DremelFuge, is a fantastic proof-of-concept that 3D printing can indeed by used to make a centrifuge, however, there are some serious oversights in the design, especially relating to safety.  The DremelFuge has no safety enclosure whatsoever.  If one of the sample tubes were to detach from the rotor during operation, it could cause serious, even life-threatening injuries to the operator, or others.  Even operating at the minimum rotational speed discussed on the Thingiverse page,  3000 rpm, the tubes have a tangential speed over 70 mph, which, even though the sample tubes are small and light, is fast enough to cause bodily harm.  The truly frightening bit though is the discussion of operating the DremelFuge at a much faster, 30,000 rpm.  With this rotational speed, the sample tubes have a tangential velocity of approximately 715 mph!  This speed is quite close to the speed of sound and the sample tubes traveling at this speed would have more than enough kinetic energy to cause severe damage to whatever they might hit, including a human body.

The last source of inspiration for the centrifuge design in this Instructable came from simply searching the web for other examples of homemade centrifuges.  A simple image search on Google will return many examples of homemade centrifuges.  Some are better than others, but they all offer ideas for the many different ways a centrifuge could be designed from scratch. 

Step 2: General Safety Considerations and Features

One of the primary considerations in this project, and in all projects, is safety.  Designing a centrifuge in particular requires careful thought and testing in order to make certain that it can be operated safely and that, should the centrifuge fail, it will do so in a manner that does not place the operator or anybody else in harm's way and avoids damage to property.  The centrifuge design in this Instructable was created with great care given to safety. 

Safety Features of the Rotor


The primary load bearing part of the centrifuge is the rotor.  The rotor experiences significant stress in the radial direction resulting from the centripetal force acting on the sample tubes.  The rotor must resist the force pushing the sample tubes away from the axis of rotation.  In order to ensure that the rotor had the necessary strength to withstand the forces placed upon it, the piece was virtually tested in SolidWorks.  Successive iterations of the design were tested and the resulting force diagrams observed.  Then, appropriate reinforcements were applied to areas lacking in mechanical resilience.  This testing is described in detail on Step 8.

The rotor also incorporates a backup safety feature to minimize damage in the case of rotor failure.  The brackets that hold the sample tubes are, in addition to their normal attachment to the rotor arms via hinges, are attached by a cable to the center of the rotor.  If the brackets were to detach from the rotor under load, the cable would prevent the brackets and tubes from being ejected at high speed from the centrifuge.

Pending the production of a physical prototype, the rotor design will be tested in real-world conditions and in conditions exceeding operational norms. 

It should also be noted that, in order for the centrifuge to be operated safety, the rotor must be balanced with respect to weight distribution about the axis of rotation.  The unloaded rotor is perfectly balanced, however, it is the responsibility of the operator to ensure that samples are loaded into the rotor in a balanced fashion.  For example, if only two samples are to be placed in the rotor, they must have equal weight (equal volumes of fluid) and be placed directly opposite from each other across the axis of rotation of the rotor. The rotor cannot ever be used with an odd number of samples since this would, by definition, result in an unbalanced rotor.  If, for example, only one sample is to be spun in the centrifuge, it must be balanced by an equal volume of water in a tube placed opposite the sample. 


The Safety Enclosure


As mentioned in the previous step, the safety enclosure serves two functions related to safety.  First, the safety enclosure creates a physical barrier preventing foreign objects from entering the centrifuge during operation.  Since the centrifuge rotor spins at about 3000 rpm during operation, the entrance of any foreign objects could result in the destruction of that object, the rotor, or both.

The second function of the safety enclosure is to act as a shield in the case of rotor failure.  During peak operation at 3000 rpm, the sample tubes travel with a tangential speed of approximately 125 mph (see the next step for calculations).  Thus, should a sample tube detach from the rotor, or the rotor itself experience catastrophic mechanical failure, preventing pieces traveling at such significant speeds from impacting anything sensitive to impact is an absolute necessity.  The primary component of the safety enclosure is 12" plexiglass in panels. 


Step 3: Rough Calculation of Applied Centripetal Force and Sample Tangential Speed

The most important measure of a centrifuge's performance is the force it is capable of exerting on the samples, in the form of centripetal force.  This capacity is determined by two factors:  the rotational speed of the rotor, and the radius of the rotor.  The centripetal force is usually measured in multiples of the earth's gravitational pull (g).  The following is a rough calculation of the tangential speed of the samples and the centripetal force exerted on the samples.

Summary


Rotor Rotational Speed:  3000 rpm
Rotor Radius:  7 in

Sample Tangential Speed:  125 mph

Relative Centripetal Force:  1789.4 g



Calculations


Given:

Rotor Rotational Speed:  3000 rpm
Rotor Radius:  7 in


Rotor Circumference:

C = 2πr

where

     C is the rotor circumference
     r is the rotor radius

C = 2π(7 in)

C ≈ 44 in


 

Sample Tangential Speed:

v = NC

where
     C is the rotor circumference
     N is the rotor rotational speed (in rpm)

v = (3000 rpm)(44 in/rev )
v
= 132000 in/min


v ≈ 125 mph


Relative Centripetal Force:


RCF = (rω2) / g


where
     RCF is the relative centripetal force
     r is the rotor radius
     ω is the rotor angular velocity
     g  is the earth's gravitational acceleration

This can be simplified to:

RCF = 1.11824396x10-5r N2


where
     r is the rotor radius
     N is the rotor rotational speed (in rpm)

RCF = 1.11824396x10-5(17.78 cm)(3000 rpm)

RCF = 1789.4 g


So, the centripetal force experienced by the samples is equivalent to about 1790 times the earth's gravitational pull.  Needless to say, suspensions will separate much more quickly in the centrifuge than they would under earth's gravity.

Step 4: Non-3D Printed Materials

The centrifuge in this Instructable will be constructed almost exclusively from 3D-printed components.  For practical and financial reasons, however, the centrifuge does incorporate a number of non-3D-printed parts.  Some of these parts simply cannot be produced on a 3D printer with current 3D printing technology, others would be impractically expensive to produce, and others could be 3D printed, but the resulting quality, in terms of mechanical strength or sanitation, would be insufficient for safe use.  Below is a list of non-3D-printed centrifuge components:


ComponentQuantity
Drive Motor 1
15mL Centrifuge Tubes 8*
4-40 x 0.75" Machine Screws 112
4-40 Nuts 112
0.5" ID x 1.13" OD Ball Bearing** 3
Plexiglass (see Step 6)



*  The 3D design files provided in this Instructable were design to use Cole-Parmer 15mL centrifuge tubes with tapered bottoms.  The centrifuge can hold as few as two tubes and as many as eight tubes but can only hold even numbers of tubes so that the rotor will be balanced.

** The centrifuge was designed to use 0.5" ID x 1.13" OD x 0.38 H Shielded Ball Bearings from McMaster-Carr (Part Number: 6384K61)

Step 5: 3D-Printed Parts

Other than the parts listed in the previous step, every component of the centrifuge presented in this Instructable can be fabricated by 3D printing (except the food put into the centrifuge, which can't be 3D printed yet).  All of the components listed below can be printed on a MakerBot Replicator or Replicator 2, as well as high-end 3D printers through services like Ponoko.  Many of the parts had to be designed in sections, to be assembled with standard metal hardware since the build area of the MakerBot Replicator is not large enough to print many of the pieces whole.  The following is a list of 3D-printable centrifuge components, the .zip file attached to this step (and to the intro step) contains all of the design files:


3D-Printable Parts

Bottom Bearing Holder (quantity: 1):
The bearing on the bottom of the drive shaft is affixed to the bottom of the centrifuge with this part.
 
Corner Rod (quantity: 8):
This piece is part of the system of components that holds the plexiglass and forms the safety enclosure.
 
Drive Motor Bit* (quantity: 1):
This piece allows the drive motor to interface with the drive shaft and turn the rotor.
 
Enclosure Horizontal Rods (quantity: 8 of each):
There are four horizontal rods, two for the top of the safety enclosure and two for the bottom, that are part of the system of components that holds the plexiglass and forms the safety enclosure.
 
Enclosure Joining Rod (quantity: 16):
This component joins the enclosure horizontal rods with machine screws and nuts.
 
Handle (quantity: 2):
In order to put tubes in and take tubes out of the centrifuge, the top of the safety enclosure simply lifts off. These handles make removing the lid easier.
 
Rotor Section (quantity: 8):
The rotor is constructed from eight identical sections, fastened together with machine screws and nuts.
 
Shaft Lower Sleeve (quantity: 1):
This tube simply acts as a spacer between the bearing in the center of the drive shaft and the bearing at the bottom.
 
Shaft Lower (quantity: 1):
This is the lower half of the drive shaft.
 
Shaft Sleeve (quantity: 1)
This part simply acts as a spacer between the bearing at the top of the drive shaft and the rotor.
 
Shaft Stabilizer (quantity: 1):
This part provides additional stability for the drive shaft. It supports the bearing in the middle of the drive shaft.
 
Shaft Upper (quantity: 1):
This is the upper section of the drive shaft.
 
Top Bearing Holder (quantity: 1):
The bearing on the top of the drive shaft is affixed to the top of the centrifuge with this part.
 
Tube Holder (quantity: 8):
This part acts as a bracket to hold the sample tubes in the rotor. The tube holders hinge on the rotor so that they hang vertically when the centrifuge is off and, when the centrifuge rotates, the tube holders lift into a radial position.

*  This part was designed specifically for the Cuisinart hand blender I plan to use as the drive motor for the centrifuge.  You may need to customize this part so that it works with the drive motor you choose.

Step 6: Plexiglass Cutting Patterns

In order to prevent injury to the user, or anybody else, the centrifuge is surrounded by a safety enclosure made from 1/2" plexiglass held together by a 3D-printed frame.  The plexiglass used in the safety enclosure must be cut in order to fit into the 3D-printed frame.  Holes must also be drilled in to plexiglass to mount certain 3D-printed parts.  There are three different plexiglass components that must be manufactured for the centrifuge safety enclosure:  the top piece, the bottom piece, and the side pieces (of which eight are required).  The pictures on this step are cutting patterns that can be used to fabricate these pieces.   

Step 7: Centrifuge Drivetrain

The centrifuge drivetrain is the system of components that transfers the drive motor's torque to the rotor.  The centrifuge design presented in this Instructable has an exceedingly simple drivetrain.  The drive motor interfaces directly with a vertical shaft that acts as the axis of rotation for the rotor.  As the drive motor rotates, it rotates the shaft at the same speed, and thus rotates the rotor, also at the same speed as the drive motor.  Careful consideration was given to the drivetrain design, and the design presented here was chosen for a number of reasons:
  • The simple design does not require any gears, which make it cheaper, more compact, more durable, and easier to put together.
  • The design minimizes the number of bearings used, which reduces cost.
  • The design minimizes the material used for 3D printing, which reduces cost.
  • Should any components fail, the drivetrain is easy to repair.
  • The drivetrain is exposed for observation, showing off the 3D-printed parts.

Step 8: Rotor

The rotor is essentially a disk, which holds the sample tubes at a distance from the axis of rotation, which results in the generation of a centripetal force on the tubes during rotation.  Due to limitations with the build envelope of the MakerBot Replicator 3D-printer, the rotor must be printed in sections, which are fastened together with machine screws and nuts.

The rotor holds the sample tubes in brackets mounted around the perimeter of the rotor.  These brackets are free to rotate so that, when the centrifuge is at rest, gravity makes the tubes hang vertically.  This position facilitates loading and unloading sample tubes.  When the centrifuge is in motion, however, the centripetal force generated on the tubes forces the brackets into a horizontal position.  This is beneficial as it allows the centripetal force to act on the tubes in a way that simulates the force exerted by gravity.  As the centrifuge slows down at the end of a cycle, the tubes settle into a vertical hanging position gradually in order to avoid agitating and recombining the separated contents inside.


Importance of Balancing the Rotor


It is critically important that the rotor be balanced!  Before running the centrifuge, the operator must ensure that the weight distribution around the axis of rotation is balanced.  Otherwise, the rotor will develop strong vibrations that would become increasingly violent as the centrifuge reaches increasing angular speeds, eventually leading to catastrophic failure.  For this reason, the centrifuge cannot, ever, be loaded with an odd number of sample tubes.  Even if an odd number of food samples are to be separated, they must be balanced by an appropriate number of tubes filled with water.  For example, if only one tube full of a food sample is to be spun in the centrifuge, a tube filled with water must be placed in the rotor opposite the sample tube. 

Step 9: Safety Enclosure

With any design, for any device, the most important consideration is safety.  Given the fairly extreme forces and velocities at play inside a working centrifuge, safety played a large role in the design of the centrifuge presented in this Instructable.  Although safety features permeate the entire design, the primary safety feature is the plexiglass enclosure surrounding the centrifuge.  The enclosure has two functions:
  1. Prevent foreign objects from entering the centrifuge during operation.  The centrifuge spins at high speeds so it is critically important for safety that nothing enter the centrifuge while it is turned on, to prevent damage to both the foreign object and to the centrifuge.  The enclosure acts as a simple barrier against anything that could enter the centrifuge and disrupt the rotor.

  2. In the case of rotor failure, prevent parts of the centrifuge from being ejected into the surroundings.  During operation, the sample tubes have a tangential velocity of approximately 125 mph.  Needless to say, the tubes, when traveling at this speed, could pose a safety concern if they were to be ejected from the machine.  Should the rotor itself break, other parts could be thrown at high speeds.  The safety enclosure is constructed from 1/2" plexiglass mounted in a 3D-printed frame, which has the necessary strength to prevent sample tubes or other parts of the centrifuge from escaping the device and harming the operator or others.

Step 10: Virtual Testing and Simulation

In order to estimate the performance of the rotor and the sample tube holders (the parts of the centrifuge that experience by far and beyond the most stress) a simple FEA stress analysis was performed on each part.  Results were obtained for both the stress acting on the parts and for the scaled displacement experienced by the rotor and sample tube holders.  The results for both tests indicated that the parts would have sufficient mechanical resilience to withstand the forces placed upon them during centrifuge operation.


Rotor Stress Analysis


For a perfect disc, rotation generates a constant state of stress across the surface.  So, for the centrifuge rotor, the stress distribution is determined exclusively by the geometry of the part.  In the stress diagram for the rotor, the more green a region is colored, the greater the stress acting on that region, and the more blue a regions is, the lower the stress.  We can see that the peak stress in the part occurs in the spokes near the drive shaft.  This seems intuitively correct since these spokes have the smallest cross-sectional area of any region.  However, even with the 1.75 displacement scale common to all of the diagrams in this step, the part experiences little deformation.  However, should problems with structural integrity occur, it is clear that those problems will occur first on the aforementioned spokes.  Thus, if the rotor fails to perform in real-world tests, those areas will be the first targeted for reinforcement.


Rotor Displacement Analysis


If one were to imagine spinning a soft clay disk about its geometric center, one would expect to see the clay material forced away form the center of rotation, towards the perimeter of the disk.  This intuitively-expected behavior is clearly illustrated in the rotor displacement diagram.  The color scale used in the diagram ranges from blue, to green, to yellow, to red, in order of increasing displacement.  We can clearly see that displacement increase with distance from the axis of rotation.  We can also see that, even with a displacement scale of 1.75, the part experiences minimal deformation.  In theory, should the rotor experience plastic deformation as a result of applied force (which it should not, given that it will be made from ABS, PLA, or some other thermoplastic which has a high yield point) a metal band could be placed around the rotor in order to add additional strength against displacement.


Sample Tube Holder Stress Analysis


Note in the diagram of stress in the sample tube holder, that the part is attached to the rotor by the pegs at the top of the piece.  The force exerted on the sample tube holders is generated by the rotation of the rotor.  As with the rotor stress analysis, regions that are more green in color experiences greater stress than bluer regions.  We can see that the peak stress occurs at the base of the hinge pegs.  This seems consistent with the intuitively expected results.  Fortunately, the test indicates that the magnitude of the stress acting on the hinge pegs is not great enough to cause mechanical failure.

The second highest area of stress is in the center of the piece next to the window cut into the piece.  This result is also to be expected as this region is the thinnest region of the part.


Sample Tube Holder Displacement Analysis


In both the stress and displacement analyses on the sample tube holder, the centripetal force acts down (or in the direction from the hinge pegs to the tip of the holder).  So, as was the case with the rotor, the greatest displacement is experienced by the point farthest from the axis of rotation of the centrifuge.  However, the section that experiences the greatest deformation as a result of the applied force is, again, the thinnest region of the sample tube holder, the middle area.  However, there are two factors to consider when analyzing the displacement diagram of the sample tube holder:  first, the diagram was created with a displacement scale of 1.75 to ease visualization, and second, during operation, the sample tube holders will contain centrifuge tubes that will contribute additional strength to the middle portion of the sample tube holder.


Conclusion

 
According to the studies conducted on the computer, both the rotor and the sample tube holders have the mechanical resilience to withstand the forces placed upon them during centrifuge operation.  Although, to confirm that the parts have the necessary strength to survive, real-world tests must be conducted.  Nevertheless, the studies analyzed in this step provide valuable information as to the weakest portions of each component, and thus, useful information if it proves desirable to reinforce the parts.