SLA 3D Printer Project Log 6: The Up of Sketch

Creo Elements Direct is fantastic, it's similarity to Pro/Engineer and Solidworks make it an instant transition for someone in the CAD industry that's used something less archaic than AutoCAD and it basically operates on an Additive/Subtractive modelling principle rather than vertices and points like the free version of Sketchup (Sketchup Pro has Solid Tools which afford the same functionality, but costs $590 or ~£350 at time of writing).  Model Railway wagons are a no-brainer using this method given how many rivets and tumblehomes and other assorted chamfers and blends make it impossible to attain high resolutions under Sketchup.

However, It's use as a design tool for machinery like the SLA 3D Printer Project and the Reprap is severely curtailed by it's lack of visualisation features and it's very modelling system:

Initial design for the Resin Basin, made from 12mm Cast Acrylic.

As can be seen, it's a grey box with a flange, not even remotely looking like the clear acrylic intended for the production model; I also wanted to raise the flange to a level where the top of the flange will be at resin level, allowing me to make a 12mm Calibration Sheet to place in the mount to calibrate the projector - in Sketchup, this is as simple as selecting all faces you want moved, then raising them to the required level, whereas Creo will need you to delete the part, raise the hole locations on the workplanes (all four of them in this case) and then re-extrude the parts.

With this in mind, I have transferred all the geometry to Sketchup by way of drawing them again (I dread having to do this for a locomotive...), and I've made further progress by constructing a prototype for the Print Bed:

I say prototype because I seem to have blocked off the bed to the projector o.O

That's the spot!  Now we can make out which bits are Plywood, which are Acrylic and which are 3D Printed.  I know I ought to have taken the projector into account seeing as it is the most vital part of the machine, but that's okay because it's an iterative process, CAD; furthermore, it is far easier to amend a model than a drawing, which I should know given how small I like to make my concept drawings.

Until next time...

SLA 3D Printer Project Log 6: Quick Snippet

Since the last post, I have been giving thought to my projector mount - long story short, I believe it will be too flimsy with all that weight resting on an 8mm thread (wrt the vertical adjustment).  As a small update while I figure out what to do from here, I have some basic drawings to clue you in on some ideas for two of the Sub-Assemblies:

I don't draw so well...  I'm not the best calligrapher either...

I have now opted for a tighter design utilizing 3D Printed clamps on the 8mm Steel Rods; I can now drastically reduce the distance, and hence the moment arm the projector will enact upon the structure, enabling a smaller frame overall.

Never one for freehand...

This is an initial design for my Build Platform, consisting of a suspended steel grate (a fine pattern to help grip the print) suspended from four LM8UU linear bearings to stabilise the platform, taking moment stresses off of the 8mm threaded rod, which in turn positions the platform.  I have literally just thought of using two motors like the Reprap as long as I can overcome synchronicity issues.

Another thing of note is that I managed to make the Resin Basin in around 30 mins in Creo:

...Which is why I'm a CAD man ;)

So, not a complicated job, simply laser cut from 12mm cast acrylic to allow the use of heavy duty cleaning agents (Extruded Acrylic has microscopic stress marks which the agents can penetrate and cause further damage); the holes in the end and the bottom (not the flanges, I'll cover them later) are sized to take G 1/4" threads commonly used in PC Water Cooling equipment, the one in the bottom drains the tank of all fluids while the side holes act as fill levels for the Salt Water and the Resin on top - Procedure for filling is as follows:

1. Close the Bottom Valve.
2. Open the Lower Side Valve (the Upper Side Port will have no valve).
3. Fill the tank with Salt Water until it flows out the Lower Side Port.
4. Close the Lower Side Valve.
5. Fill with resin until it flows out of the Upper Side Port.

A similar procedure is done for draining:

1. Remove any Prints.
2. Open the Lower Side Valve.
3. Let The remaining Resin drain out.
4. Open the Bottom Valve.
5. Let the Salt Water drain out.

All fill leveling is therefore done by the tank if the correct procedure is followed, I have it calibrated for 20mm of Resin on top of 110mm of Salt Water, giving 120mm of Z axis Build Area alongside a proposed upgrade of 192mm x 108mm build area once I source a 1080p native projector (once I've sifted through all them "1080p" projectors with an embarrassing native resolution of 800 x 600 >:(  ) .

The other holes on the flanges are simply mounting holes to suit M10 bolts, and another idea as of present is to make a calibration sheet from more 12mm Acrylic set to the same height as the resin to position the projector accurately.  I propose using L plates for accurate positioning of the basin.

Take care :)

SLA 3D Printer Project Log 5: 200 Step Program

I've recently obtained a Raspberry Pi (and instantly overclocked it - reached 900MHz CPU, 500MHz GPU and 600MHz RAM with an overvolt of 8), as skeptical as I was at first, this £25 Set-Top Box with pretensions to being a Linux PC is giving me some real inroads in this project.

I've already got it to run a NEMA 17 Stepper Motor through an EasyDriver V4.4, and my coding prowess is as extensive as the Waterloo and City Line; I did this via WiringPi, a library for the Pi that enables you to use the GPIO Pins without using Python or typing sudo (super-user do) ahead of every command.

The commands entered were:

• gpio mode 1 pwm - this assigns Pin 1 to PWM operation
• gpio pwm-bal - this enables Balanced PWM, which is SoC default
• gpio pwmr 100 - this sets the range to 100, or 0.1ms per unit
• gpio pwm 1 - this sets the duty cycle to 1, or 0.1ms out of 10ms

Keep in mind it is possible I am hideously wrong at this stage, I don't own an oscilloscope and with that in mind I am a cretin.  I frequently used PicoScope in Electronics class in College so that'll be my first port of call.  I believe I also need to vary the frequency to change the speed, not the duty cycle; maybe that'll need a dedicated board...

In other news I have finished my Projector Mount for the SLA 3D Printer, images below:

Frontal View, the width of the frame (sans handles) is 580mm.

Isometric View.

Rear view, showing the main frame.

A side-on shot showing the guts of the traverser.

I'm slightly doubtful about the 8mm steel rods I'm using to support a Projector which may weigh up to 5kg, so let's do a few Beam Bending Equations :P

Let's assume that the projector is positioned dead centre of the supporting rods in both axes, and that it weighs 5kg, it's centre of mass is spot-on in the middle of the Traverse Plate and 100mm above it (to account for an early, heavy and highly bulky DLP projector).  These are the crucial dimensions (for moment calculations):

• Horizontal Rods are 560mm long, so 280mm is the moment arm
• Vertical Rods are 565.5mm long, gives a moment arm of 282.75mm
• There is 62mm from the Horizontal Rods to the Traverse Plate
• This falls to 13mm with respect to the Vertical Rods
• On the Traverse Plate there is 170mm between the centres of the bearings (for the vertical axis)
• 317mm is the distance between the Horizontal Bearings

In order to find the deflection (which will cause uneven projection distance and hence ruin our prints), we need to find the moment arms and hence the force experienced by the beams at their various points; we know the Mass of the Projector is 5kg, this multiplied by 9.81m/s² gives us 49.05N of force, which for the sake of simplicity we will disregard for the vertical axis for now, and focus on it's effect on the horizontal axis.

Mooching around on the internet found me these highly useful formulae:

Thanks to Andy Ruina of Cornell University for this PDF so I didn't have to format this pile of symbols

The formula at the end is of greatest interest to us, and the symbols are as follows:

• P = the deflecting force:  what we are hunting for
• l = the length of the beam in question
• E = Young's (Elastic) Modulus = ~200GPa for Mild Steel
• I = Second Moment of Area, details below

There are many formulas for the Second Moment of Area, as it's highly dependent on the cross-section of the beam in question:  we'll be using (Pi*r⁴)/4 since that corresponds to a solid cylinder.  Our cylinders are 4mm in radius so (3.14*0.004⁴)/4 = 2.011x10^-10 m⁴ , there being two rods so double that to 4.021x10^-10 m⁴

With this information gathered, we can deduce that the denominator of this fraction is 48*200,000,000,000*4.021x10^-10 = 3860.39 GPa.m⁴ (Dimensions will be useful later); the numerator will be 49.05*0.560³ = 8.61N.m³, therefore we get a δ_max of 0.00223m or 2.23mm down in the centre.  Keep in mind this doesn't take moments into account nor does it account for the bearing spacings.

Wow, this really is looking like 200 steps!  I didn't expect to go back into my university course this soon, but it did highlight the fact that I may need to review my reliance of Reprap-derived mechanisms especially since I don't want my threaded rods to take on an undue amount of mechanical strain.  I'll end it there for today and continue with this lark at some other point, just because I'm tired of entering HTML mode to put in ^{(Superscript text here)} constantly.  Time for a kip...

SLA 3D Printer Project Log 4: A projector mount with traversing features, potential as a garden railway traverse table?

Aside from experiments in salt, I have been working on a mounting method for my projector; one which offers versatility both in terms of adjusting projector distance to vary the build area/resolution balance and also to make way for projector upgrades.  Given that I am building a top-down design this is no easy task considering that gravity threatens to either collapse my construction or slowly inch the projector down and out of focus.  So far, this is my design which as of yet only features the vertical axis:

Looks very Reprappish, and that's because it is :D

I am keeping to Reprap parts as much as possible since every 3D printing enthusiast and their Budgie has a Prusa Mendel of some kind, so relevant parts will be cheaper; the central rod is a M8 threaded rod and the other two are 8mm plain steel rods for support and guidance, SCS8UU bearings are used throughout due to their cast metal housing and built-in M4 threads.  Another thing to note is that the Traverse Plate (highlighted in green) is not what will carry the projector - I intend to make another plate bespoke to the mounting requirements of my projector and users can make bespoke mounting plates for their respective projectors.

The bearings for the horizontal guide rods can clearly be seen.

The Backplate may look a bit skeletal for some, but this is made out of 12mm plywood; 6mm plywood may be used to allow for simplification of manufacture with other parts like the Traverse Plate, with 12mm parts made of two 6mm parts epoxied together (this also allows for half-engraved features without half-engraving, which could bring potential de-lamination issues to a single 12mm sheet).  The large amount of space also allows for a high degree of nesting to occur, saving material and space on the laser bed.

Trusses added for stylistic reasons ;)

This is the mounting cross for the Traverse Plate, where the plate interacts with the M8 rod to allow the plate to move vertically; this is done via two M8 nuts located in slots (just visible above), of which one or both may be a nylock nut to provide friction when traversing is not required.  This is made from a 12mm (or 2 x 6mm) plywood cross which interfaces with a 3D printed nut mount to create the mounting cross.

Reprap inspiration is found again...

Finally, we have the clasps for the plain steel rods that guide the Traverse Plate.  Not much to mention here other than it's 3D printed, I've decided to use Shapeways for my prints since it's WSF printers offer almost infinite flexibility in the shape of my objects within reason and my Reprap is giving me constant trouble right now, so much for saving money by printing at home :(

Good news is that design should be straightforward from here, since I only need to replicate the parts I have made here in the horizontal axis with a few minor tweaks.  Pity this is only one of five sub-assemblies I am slated to design (Projector Mount, Resin Basin, Z axis with build plate, the Mounting Frame and the Case).