Jul 122012

(also written as Ockham’s razor, Latin lex parsimoniae) is the law of parsimony, economy or succinctness. It is a principle urging one to select from among competing hypotheses that which makes the fewest assumptions and thereby offers the simplest explanation of the effect. —Wikipedia

This project is an electric scooter, for Jamo’s EV Design Class.  I have decided to go the simplest, and slightly oddball route.  Occam’s Razor [scooter] will have no onboard electrical parts.  It will use a channel for the frame, a mechanical throttle and cable, and a single rear disk brake adapted from a bicycle.  Whats the catch, and how does it go?  Well, I’m glad you asked!  I saw the following youtube video, and decided that I have to power Occam’s Razor with a Dewalt Hammerdrill:

I won’t be purchasing his expensive adapter, but making my own instead.  Stay tuned for more awesome EV-ness as the Occam’s Razor is constructed!

Jun 032012


3D Printing Today

The proliferation of affordable home and open source 3D printers has been followed by an explosion of freely available things to print.  Thingiverse is one of the easiest and most friendly place to obtain printable objects, and has boosted the confidence of many designers to create things to print.  Most of the popular 3D printers in the affordable and hobby price range have a relatively limited build envelope.  The popular Makerbot kits normally on the order of 120x120x120 millimeters, or 210x210x220 millimeters for the impressive Ultimaker kit.  To make bigger items, a designer must either design parts that are small enough to fit in the build envelope and assemble them later, or choose not to design things that are bigger than the build envelope.  When the small envelope of the Makerbot is the design constraint, then generally the simplistic extruder designs employed at this level are reliable enough to complete most print jobs.  If a print does fail, often the part is small enough that the material loss is relatively insignificant.

What I Want Out of My 3d Printing Experience

The Crux of the Matter:  I want to print really big stuff.

Really big stuff is a bit vague, so let me expand upon that.  I want to print out full size helmet designs, maybe for movie props, such as a Star Wars stormtrooper helmet.   I want to print out model airplane designs, with lengths as big as six feet long, and wingspans reaching as wide as eight feet.  I want to print out high power rocket parts, such as nosecones that are 8 inches diameter and 3 feet tall, or tailcones with the fins attached reaching footprints of 24×24 inches.  I want to print out new headlight bezels and other assorted custom parts and pieces for my car.  I want to be able to print these things in as few pieces as possible, some are only feasible if printed in a single piece.

Mean Time Between Failure

Experience with a Makerbot Thing-O-Matic and a Rev 3 Ultimaker have indicated some of the problems with the open source designs, specifically and most alarmingly in the extrusion mechanism.  The Mean Time Between Failure (MTBF) of the extrusion tool on these machines is approximately 5 hours.  This means that if I print for 100 hours, I can expect 20 extrusion failures.  Not all extrusion failures are fatal to the job, but recovery always requires the operator to notice the problem almost immediately and perform a corrective action.  For large jobs, this leaves an operator nervously monitoring the extruder for hours, rather than doing something productive while the job is printing.  If the job needs to run overnight, monitoring may simply not be an option.  Unfortunately, the price of failure both lost time and wasted material.  Not a big deal on small parts. but a twelve hour job that fails sometime during the night, but near the end of the job, could be significantly wasteful.  By increasing the MTBF, an operator is freed up to do other things while less printable material and machine time is wasted.

Failure Modes

The most common failure for open source printing extruders is full or partial loss of extrusion.  Full loss of extrusion occurs for two basic reasons.  Either the filament has been stripped and the effective diameter reduced beyond the extruders ability to grip it, or the end of the filament strand has been reached.  Partial loss of extrusion generally occurs because the extruder is slipping against the filament, so the software thinks it is pushing more plastic through the extrusion nozzle than it actually is.  Partial loss or even excess extrusion can occur because the filament diameter does not have a tight tolerance, and it changes enough to effect the quality of the print.

Design Solution

The biggest issues generally come from lack of instrumentation and open loop control systems, and sometimes also from inadequate design.

Gripping the Filament

The Makerbot Thing-O-Matic’s MK7 Stepstruder and the Ultimaker’s Ultistruder both suffer from a similar design flaw that is relatively easy to fix.  I have seen the significant difference between the Makerbot MK7 and the upgraded spring loaded replacement from Thingiverse.  For the same reasons, it is easy to see why the Ultistruder MK2 was also uploaded to Thingiverse.  The use of a spring loaded pinch mechanism will make the extruder tolerant of filament diameters and increase the likelihood that the extruder can recover from slipping on or stripping out the filament.

Filament Odometer

The favored method for defining filament usage is by defining length of filament consumed, and allowing the Cartesian firmware to adjust the extrusion speed to match the accelerating or decelerating Cartesian velocity.  Currently, there is no known open source extruder that measures the filament’s progress; the filament is always assumed to have correctly traveled the specified amount due to the open loop control system.  By adding an encoder that measures the filament’s movement, the extruder can automatically calibrate the extrusion rate, which can vary by material.  The encoder can also be used as a watchdog to ensure the filament is moving and has not stripped out, restarting the filament in the case of failure.

Diameter Measurement

Filament diameter on a spool is inconsistent enough to affect the print quality.  The accepted method is to measure your filament several times over the length of filament you expect to use, then put the average diameter in the model slicing software.  The result of adjusting this tends to be fairly good, because the slicing software uses a volumetric calculation to determine the desired length of filament to extrude, but the resulting file is not really reusable.  By adapting the extrusion speed on the fly using feedback from a diameter sensor, the part does not need to be re-sliced to account for filament nuances, saving time.

Build Envelope

The end goal of making a more reliable extruder is obtain the ability to print very large objects with minimal waste or failed attempts.  The goal for this project is to build a 3D printer with a 50 x 50 x 100 centimeter build envelope into an enclosed box.  The enclosed box allows for the ability to regulate the temperature of the build environment for increased precision and reliability.  The entire project will still need to fit through a standard North American interior door, 32 inches in width.

Filament Reload

Considering the vast volumes that can be printed within the proposed build volume, the probability of running out of filament on a reel becomes quite significant.  It is relatively easy to have the printer pause in software and wait for the operator to intervene, but a better alternative for boosting reliability would be to have the extruder automatically changeover to a secondary filament.  This would be analogous to having a hot spare in a RAID array: the operator can then replace the spent reel when it is convenient, without increasing the printer’s downtime.

Contingency Plan

The basic plan is to pipeline the acquisition of raw materials as much as possible, by ordering the parts needed for the next step or two while working on the current step.  The steps will be carried out approximately in the following order:

  • Design and build the mechanical extruder
  • Test extruder with prototype/breadboarded electronics
  • Design and fabricate extrusion control circuit board
  • Design and build Cartesian Robot cabinet
  • Write/Integrate firmware such that mainstream open source 3D Printing tools can be used to make things!


May 252012


I tend to have at least three different sound sources running on my desk, usually computers.  Only one will be playing music, but I do want to get the audible notification noises from all of them at the same time.  Additionally, I don’t want to suck up valuable bandwidth on the network with network based audio.  On top of that, I want at least one of the computers to be able to adjust the receiver, so the control side of this includes automated volume and preferable tone controls for everything that can be adjusted both manually and remotely.

Project Requirements

  • 4 stereo inputs
  • 1 master send/receive (effects loop before the master fader)
  • RCA/Dual Phone/Stereo Phone/stereo line inputs
  • No individual tone circuits, but leave lugs/terminals to add them later
  • Each channel selectable (or mutable)
  • No Phase inversion between input/output
  • Master balance control
  • Individual channel gain trim (this would be a trimpot, not normally adjusted)

Preferred Features

  • Direct Injection for Hi-Z musical instrument
  • Turntable/RIAA circuit accommodation
  • 48V phantom power
  • Individual balance control
  • Relay and aux 110V outlets, for controlling an external power amplifier
  • Rack mountable case
  • Network enabled

Resource Links for the Mixing Receiver


Apr 022012

I have been doing some research on archtop guitars and luthierie, because I would like to build myself an electric archtop semi-hollow guitar.  Being that I believe all reasonably modern fabrication usually involves CAD, I have been trying to model the guitar in Autodesk Inventor.  With any luck, I can avoid becoming a master craftsman at woodworking and use CNC techniques to fabricate my guitar with pretty good quality.

Archtop guitars are just what they sound like, Arch-topped!  So, unlike your flat-topped acoustics that most people think of, we have a side profile of the guitar front (and back) that is not flat.

The trick was, I did not know what the shape of that arch really is, which is important to being able to model it.  I did have a topo map of the guitar top on the plans, but I do not really trust it.  It might be sufficient if one is only using hand tools and simpler power tools, to fabricate the guitar, but I am going all out modern CNC.

The Curtate Cycloid

After googling around a bit, I discovered this picture, showing this shape is a curtate cycloid.  A regular cycloid line is formed by drawing a line from a point on a circle while rolling the circle along a straight line.  Unlike the regular cycloid, a curtate cycloid is formed by drawing a line from a point on a smaller, concentric circle inside the larger rolling circle.  Mathworld has a really good graphical explanation of how it works:

After watching that neat gif for a few minutes, it becomes readily apparent why Cremonese violin makers would choose this shape, the geometry is easy to draw.  I found a tool for generating the shape, but its meant for printing out templates, not for adding to a CAD drawing.  I have recently read that Autodesk Inventor 2013 has a new feature allowing mathematically defined equation curves, but since I just got 2012 installed, I think I will put that off a bit.  Instead, I created a spreadsheet to generate points for a spline in Inventor.  The math is pretty simple, read about it on the Mathworld website.

Download the Spreadsheet: CurtateCycloid

F-Hole Geometry

I had previously thought that the shape of the F-hole was largely an aesthetic thing, and I didn’t care to particularly for the shape of the F-hole on the set of plans I have.  So I thought, “gee, maybe I can look up some F-hole placement and geometry and find something I like.”  Well, to my surprise, the shape and position are apparently quite effective in shaping tone, but the method of determining the best shape is largely empirical.  Since I am not really interested in the PhD level research involved for analyzing sound hole shapes, plus my guitar is semi-hollow and electric, perhaps it is a bit less critical.

I tend to like the Stradivarius shape, but I also like the diamond in the middle of the original pattern.  I will probably combine them to get a design I like for my guitar.  For comparison, here is the Gibson ES335 shape, the Stradivarius, Del Gesu, and Baumgartner F-hole hapes.

This really awesome violin building forum thread is where I got most of the links and info on cool F-hole stuff:

Maestronet Forum Thread

For Reference, I bounced through these interesting sites while trying to figure out how to model an archtop guitar: