Metadata Overview

  • Main design files: https://mdelmans.github.io/uCube.
  • Target group: Students and scientists in life sciences, physics, engineering.
  • Skills required: OpenSCAD – easy; desktop 3d printing – easy.
  • Replication: Components were produced and tested during peer review.
  • See section “Build Details” for more detail.

(1) Overview

Introduction

Optical design and assembly is difficult compared to electronic and mechanical construction. One reason for this is the lack of affordable components and common engineering standards for optical assembly. Whereas it is possible to purchase lenses for a reasonable price, or scavenge them from old devices, the optomechanical components required to hold the lenses and other components in place are relatively expensive and available from few manufacturers. Further, buying and assembling optical systems requires expertise, which, together with high prices, creates a barrier for scientists and innovators.

The advent of 3D printing allows the design and manufacture of complex objects in small batches from various types of plastics with low production costs. The 3D printing scene has been expanding since 2010, and has made an impact in the field of optical design. Numerous open-source projects related to optics have been released in recent years. Among them are OpenFlexure microscopy stage [1], FluoPi multi-fluorescent imaging station [2], FlyPi microscope [3], RamanPi spectrometer [4], SLO camera [5], Ultrascope telescope [6]. In addition, Formlabs have demonstrated that even lenses can be manufactured using 3D printers [7]. These projects are promising examples of the potential future of optical design, and call for establishment of design standards to promote sharing and further development.

In this paper we present µCube, a framework for 3D printable optomechanics, which offers a standard for design and assembly of optical devices. The framework exploits modular design, where individual optical modules can be attached through a common mechanical interface. The adopters of the framework can benefit from a broad selection of parts available in the µCube library, and can bring modularity to their designs. The µCube framework is parametric, which makes it possible to adjust generic design templates for specific needs and sizes of optical components.

Overall Implementation and Design

µCube design concept

The design consists of two conceptual elements: the µCube and µFace. The former is a hollow cube, which provides a structural support and serves as an elementary module for optical device assembly. The latter is a customisable part, which fits into a µCube and provides housing for optical elements (see Figure 1A).

Figure 1 

µCube design. A) A µCube with six µFaces. Screws are highlighted in red; screw inserts are highlighted in cyan. B) Design parameters of a µCube and a µFace. µLensFaceI is a function of cubeSize, lens and supportH parameters. Lens is a class, which groups four parameters that define shape of a thin lens. An OpenSCAD code, required to generate a µCube and a µLensFaceI is shown below.

Each side of a µCube has an indentation for housing a µFace. The indentation allows each µFace to sit flush with the surface of the cube, and facilitates protection from incident light. Each corner of the indented recess features a cylindrical hole, which holds a threaded insert for attachment of a µFace. The diameter of the holes is slightly smaller than that of the threaded insert, so that the insert is held firmly, after being heated with a soldering iron and secured in the hole.

A µFace is square in shape, and features four notches at each side, which match those on a µCube. There are four tapered holes in each of the corners of a µFace, which coincide with positions of the threaded inserts and allow attachment of the µFace with four screws, as shown in Figure 1A.

Parametric definition

The µCube framework is implemented as an OpenSCAD library [8]. OpenSCAD is an open-source 3D CAD programming language that supports parametric definition of three dimensional models. More specifically, each model can be thought of as an output of a function, where user-defined parameters specify its dimensions.

The design of a µCube is based on a generic template with two parameters. The size parameter specifies the internal dimension of the µCube and prescribes the total space available for the internal optical components. In its default configuration the size parameter is equal to 40 mm. The second parameter, d, defines the dimensions of the µCube features. For example, the section of the µCube wireframe is a square with size 2d by 2d. The dependence of other features on parameter d is shown in Figure 1B. By default, d is 7 mm, and the external size of a µCube is 68 mm.

The choice of parameter d largely depends on the size of the screws used for the assembly and can’t be smaller than the diameter of the threaded inserts. The size parameter can, in theory, take any positive value. However, the practical limits of size depend on the resolution and maximum build size of the 3D printer used for manufacturing. The smallest µCube we have successfully printed had parameter d set to 7 mm and size set to 15 mm, giving an external size of 43 mm.

Similar to µCube, µFace is a generic template, the size of which can be adjusted by varying the parameters size and d. In addition, µFace has a third parameter, which prescribes a size of a gap between an edge of a µFace and a frame of the µCube. The gap is necessary for frictionless removal and attachment of the µFaces to the body of a µCube. The details of the µFace parameters are presented in Figure 1B.

In its default state, µFace serves as an opaque part, which stops light from entering inside a µCube. However, it can be customised to hold optical and other components, e.g. lenses, filters, mirrors, sensors, electronic components, etc. For example, a µLensFaceI, which is designed to hold a thin lens, is represented by a function that takes a lens radius and offset from the face surface as parameters. For simplicity, parameters are grouped into classes, so that an instance of a class stores all the dimensions for commonly used objects. For example, all dimensions and focal length of a thin lens can be stored as an instance of a Lens class. Therefore, a user can define a lens instance he/she would like to use, and then supply it as a super parameter to a µLensFaceI function to generate a part with the sizes adjusted to the specified lens. See Figure 1B for details.

A µFace customised for a particular purpose is referred to as a part, whereas an assembled µCube is referred to as a module. At the time of publication, the µCube library contains 18 parts (see Figure 2) and two modules (see Figure 5 and Application section).

Figure 2 

Library of parts. Currently there are 18 parametric parts in the µCube library, grouped into seven categories. Below each part is a corresponding OpenSCAD function, showing the main parameters. The Rail parts act as modifiers on any other µFace by adding rail attachments.

The screws used for the assembly are also defined parametrically using an instance of a Screw class. Therefore, it is possible to adjust the holes for the screws and screw inserts to match the available fasteners.

Super-cube assembly

The modular nature of the framework allows joining several µCubes together into super-cube assemblies, thus making it possible to combine several modules into complex optical designs. Each side of a µCube features two additional screw inserts and two through holes. This allows joining any two µCubes together at any of the six sides using four screws in a bi-directional way (see Figure 3A). This configuration makes it possible to join up to six neighbours to each µCube, such that any structure in 3D space can be assembled. The elements are designed in such a way that a µCube can be attached to another, independent of whether it has a µFace attached at the interfacing side. This provides an option to house optical elements at the border between two µCubes.

Figure 3 

Super-cube assembly. A) Joints in a three-cube assembly. Screws are highlighted in red; screw inserts are highlighted in cyan. B) A conceptual design of a µCube assembly including light source, detectors and light path modules.

Conceptually, every optical device constitutes of three main modules: light source, light path and light detector. The same categories can be applied to µCube modules. The µCameraCube and µLightCube, described in the Application section, are examples of a detector and a light source. They can be customised to work with various optical components either as independent units or as a part of a super-cube assembly. Further, we have designed generic holders for mirrors, lenses and filters that can be used for assembling light path components, hence allowing design of complete source – path – detector optical devices. For example, in Figure 3B we show a design of a multi wavelength spectrophotometer, composed of two detectors, light source, sample holder and a light path splitter.

(2) Quality control

Safety

General safety measures should be taken when working with a 3D printer and soldering iron during the production and assembly.

Calibration

faceGap calibration

As described above, µFace template has a faceGap parameter that determines the distance between a µFace and a µCube necessary for a frictionless insertion and removal of parts. In our experience, a faceGap of 0.4 mm was an adequate value when printing µFaces on Ultimaker 2 or MakerBot Replicator Mini. However, calibration of the faceGap value may be required when using a different 3D printer. For this purpose, we recommend printing a set of µFaces with a faceGap from 0.1 to 0.5 mm and selecting the minimal value that allows frictionless insertion into and removal from a µCube. Note that the dimensions of a µCube do not depend on the faceGap parameter. Therefore, there is no need to reprint µCubes during calibration.

In order to make a calibration process faster, we have added a pair of calibration parts, µGapTestFace and µGapTestCube, which are the scaled down versions of a µFace and a single side of a µCube.

General testing

Alignment testing

In order to test the degree of alignment between optical axes in a µCube assembly, we constructed a three-cube test device, composed of a light source, featuring a µColimatorFace; a cube holding a µHolderFace45 with a beam splitter; and an empty cube attached 90° to the main light path. See Figure 4 for details.

Figure 4 

Alignment testing. Photos of the alignment testing device and the images of the focused light beam on the paper screen for A) transmitted light and B) reflected light. µFaces are labeled in black, optical components in white. Blue circles and crosses represent the position of the aperture, green circles and crosses are fitted to the image of the light beam.

The collimated light, produced at the light source, can be focused by a lens on a screen, and the resultant image of the light spot can be used to evaluate the alignment of the optical axes. For this purpose, we attached a Thorlabs SM1-threaded standard cage plate to a modified µLensFaceI, featuring an adapter with four Thorlabs rods and a plano-convex lens. We then used this face to obtain images of the light beam at various positions on the test device by sliding the cage plate along the rods.

First, we ensured that the collimated light is aligned with the center of the light source cube by attaching the modified µLensFaceI directly to the light source cube opposite the µColimatorFace. Then, we attached the light source cube to the remaining optical system and obtained images of the light beam either after being transmitted through (Figure 4A) or reflected from (Figure 4B) the beam splitter.

We quantified the alignment of the light spot by taking a picture of the screen and calculating the distance between the centers of the circles fitted to the cage plate aperture and the image of the light spot. The resultant misalignment was ~0.14 mm for the transmitted light and ~0.44 mm for the reflected light. Given the total light path of ~195 mm, angles of the misalignment were ~0.04° for the transmitted light and ~0.13° for the reflected light.

(3) Application

Use case(s)

We used µCube framework to design a detector (µCameraCube) and a light source (µLightCube) module.

µCameraCube

The µCameraCube, is a camera module, based on the Raspberry Pi and Raspberry Pi camera. It is composed of one µCube, three blank and three customised µFaces: µRaspberryCam2Face, for holding a Raspberry Pi camera module; µRaspberryPiFace, for holding a Raspberry Pi board; and a µFace for holding a lens. µCameraCube comes in three versions, which vary in the type of lens used. The lens can be either from a commercial photo camera, M12 CCTV camera lens or a thin lens. See Figure 5A for details.

Figure 5 

µCube Modules. A) Assembly of a µCameraCube. Left to right: thin lens version, photo lens version, M12 CCTV lens version. B) Assembly of a µLightCube. Left to right: collimator version, light guide version, point source version. Black arrows label parts; white arrows label components that serve as input parameters. Screws are highlighted in red; screw inserts are highlighted in cyan.

For attaching a commercial photo camera lens, a µTMountFace is used, which features a T-Mount adapter ring, obtained from a commercial T-Mount adapter. In the M12 CCTV camera lens version, both the lens and the Raspberry Pi Camera are held together by a single part.

The version of the assembly can be defined by a type parameter, which can be either ‘thin-lens’, ‘M12’ or ‘photo’. In addition, the distance of the lens and the Raspberry Pi Camera from the corresponding µFaces is calculated automatically based on the focal length of the lens and cameraOffset parameter. Therefore, during the design stage, it is only necessary to specify the focal length and size of the lens in order to generate the models ready for 3D printing.

µLightCube

µLightCube is a light source module, which is composed of one µCube, three blank µFaces and three customised µFaces: µLEDFace or µCollimatorFace for holding an LED with heat sink; µApertureFaceP with a hole for attaching a DC power socket; and either a µLightGuideFace or µApertureFaceP, which serve as a light output. See Figure 5B for details.

Similar to µCameraCube, the µLightCube is provided in three versions. The point light source version uses a µLEDFace for holding an LED without any additional optics. In contrast, the collimator version uses a µCollimatorFace, which allows attachment of a collimating lens. Finally, a light guide version uses a µLightGuideFace designed for focusing collimated light onto the end of a fibre optics cable. The exact dimensions of the collimating and focusing lenses, as well as fibre optics cable and LED are provided as input parameters.

Reuse

We have implemented parametric methods for designing and compiling 3D models of the µCube parts. This programmatic approach to part design allows one to modify, extend, reuse and share parts in the same way that one would share open-source code in software development. Furthermore, the parametric aspect of the library allows users, even with a limited knowledge of OpenSCAD, to modify sizes of the parts by specifying the function parameters.

Apart from offering a modular structure, µCube provides a standard for assembly of 3D printed devices, independent of whether they were designed using the µCube framework or not. An existing design can be extended to comply with the assembly standard by either directly adding a µCube compatible adapter or by introducing an adapter µFace. For example, we have repurposed an illumination arm of an OpenFlexure microscope to serve as an adapter face, and embedded a µCube adapter into the design of a 3D printed camera [9]. See Figure 6 for details.

Figure 6 

Examples of µCube adaptations. A) Extension of an OpenFlexure microscope with an adapter µFace, which serves as a support leg. B) µCube compatible adapter incorporated into the body of a SciFiCam allows using µFaces for lens attachment.

Commercially available kits, e.g. Optical Cage System by Thorlabs, offer comprehensive set of optomechanical components and allow manual adjustments at the build stage using a sliding rail system. Inspired by the Thorlabs design, we introduced two µFaces featuring rail clamps. The mµRailFaceA and mµRailFaceT act as modifiers that can be used to add rail attachments to any existing µFace. These µFaces not only give an opportunity to create low-cost alternatives to the commercial rail systems, but also allow to introduce custom-built 3D printed components into the existing systems, like those produced by Thorlabs.

(4) Build Details

Availability of materials and methods

3D Printing

3D models of all the parts presented in this paper were produced using OpenSCAD 2015.03 software. STL models were then imported into Cura 15.04.6 or MakerBot Desktop 3.10.0.1364 slicer software, and 3D printed using Ultimaker 2 or MakerBot Replicator Mini correspondingly.

For Ultimaker 2, 2.85 mm PLA filament was used with the following printing parameters: 0.1 mm slices, 100% infill, build plate adhesion set to ‘Brim’ and enabled supports for µFaces; 20% infill, build plate adhesion set to ‘Raft’ and disabled supports for µCubes.

For MakerBot Replicator Mini, 1.75 mm PLA filament was used with the following printing parameters: 0.2 mm slices, 100% infill, enabled raft and supports for µFaces; 20% infill, enabled raft and disabled supports for µCubes.

In general, we recommend printing µFaces with the external side up. However, when extrusions are present on the internal side, e.g. µLensFaceI in Figure 2, it is advised to print internal side up to eliminate the need for supports.

Assembly

Once 3D printed, µCube elements were completed by embedding 36 M3 4 mm diameter, 4.78 mm depth screw inserts (six per side). The embedding was performed by heating the screw inserts with a soldering iron and pushing them into the pre-printed holes. µFaces were attached to the µCubes with M3 × 6 mm hex socket countersunk screws. The detailed assembly instructions are available at the DocuBricks portal.

Ease of build

Production and assembly of the µCubes and µFaces only requires a 3D printer and a soldering iron, no special skills are necessary. The design is implemented as a parametric template of three parameters, hence easy to modify.

Operating software and peripherals

Compilation of the STL files requires OpenSCAD 2015.03-3 and µCube library files available through GitHub.

Dependencies

Hardware documentation and files location.

Archive for hardware documentation and build files

Name: µCube at DocuBricks

Persistent identifier:www.docubricks.com/projects/ucube

Licence: CERN Open Hardware License

Publisher: Mihails Delmans

Date published: 14/12/17

Software code repository

Name: µCube at GitHub

Identifier:https://github.com/mdelmans/uCube

Licence: GNU General Public License v3.0

Date published: 24/08/16

(5) Discussion

Conclusions

Here we present µCube, a standard framework for 3D printable modular optomechanics. We have designed 18 parts, assembled two modules and released the source code at GitHub, along with the assembly instruction at the DocuBricks portal.

Since the designs of every component are implemented in a form of a written code, it is easy to adjust, extend and share them between the members of a community. Furthermore, the project exploits parametric design principles, making it easy to create functionally identical parts with a wide range of dimensions. Therefore, every optical module design created using the µCube framework is more of an abstract concept, rather than a fixed blueprint. These features make µCube an attractive platform for creating, sharing and manufacturing open-source optical devices and instruments. The low cost of production, and ability to reuse parts for different projects, opens up potential use of µCube for educational purposes, hobby projects or low cost scientific equipment.

Future Work

In its current state, µCube presents a solution for standardising design and assembly of 3D printed optical devices. However, the fixed nature of current parts, together with the insufficient precision of current desktop 3D printers, can result in misalignments of the optical parts during assembly. Although this does not prevent using µCubes for educational and preliminary design purposes, the precision of assembly could be improved for scientific instruments. In order to enable construction of high quality optical devices the manufacturing method could be changed to machining, molding or other type of precision manufacturing. In addition, adjustable parts could be designed to allow alignment after the assembly is complete. This can be achieved by designing parts for holding micromanipulators, or as has been shown by Salazar-Serrano et al. [10], with 3D printed kinematic mounts. The use of 3D print-based construction will facilitate integration of such complementary approaches to assembly of high precision optical prototypes.