Virtues of Rapid Prototyping and Strandbeests

STRANDBEESTS: If you don’t know Theo Jansen, he is an eclectic Dutch kinetic sculptor and physicist, who creates moving works of engineering and art that are hypnotically elaborate. These Strandbeests are designed to be easily moved by a light wind and can be typically found roaming the windy beaches of the Netherlands. These works are laboriously hand-constructed with electrical PVC tube, plywood, and other materials. When you see how freely and effortlessly his works move, you gain a supreme appreciation of how science can be applied into creating kinetic beauty. Here’s a video from TED a few years ago of the artist himself describing his work.

A short while ago, I had told a friend that rapid-prototyping is a toy and isn’t ready for prime-time, since the typical materials have it uses has poor mechanical properties. I’ve been quite a skeptic, but recently, I have had the luxury of having access to an industrial rapid-prototyping machine and have been trying to think of way to use it in a productive way. After using it and understanding the utility of them, I am beginning to believe…

With traditional machining, one of the most difficult things about mechanical design is figuring out how to make your parts machinable. In other words, can you make a square hole or square interior corner with a mill or lathe? Realistically, no. Or, how can I get the mill tool into the right area to remove material? Is the tool long or stiff enough? Can you make something hollow? You’d be surprised how many times you would come across problems like these in designing something complicated. Rapid prototyping can solve many of these problems, since the process doesn’t care how complicated a part is or how many features there are. But this isn’t to say that this is the end-all-be-all solution. It has it’s limitations as well: non-precision surfaces, less material strength, minimum thicknesses, and certain types of objects are unprintable, i.e. non-self-supported/floating or thin/wiry objects.

The most straightforward and obvious application of rapid prototyping is being able to physically play with and quickly test an idea, no matter how complicated the part. You take an idea, model it, print it, and it’s in your hands in less than a day. In stark contrast, machining a complicated part out of a solid block of metal can take days or more than week, mainly for planning and fixturing, depending on the number of features. Suppose you forgot to add a feature or sized the part wrong, just add it or scale it up, print it, and you’re done. With machining, you adjust your CNC code, if you’re so lucky, and re-machine a new part, which still can take a whole day or more.

But, probably the single-most greatest strength of rapid prototyping is that it doesn’t care how many parts you want to make or how different they are. You can design hundreds of small parts that all different, send it to the machine, and it will create them all, likely in one run. Just try asking a machine shop to do that in a day… (Sorry Mike!)

Theo Jansen applied these strengths to rapidly create small models of his Strandbeests in one step, which simplified his process incredibly. To create something like this in the traditional machine shop methods or even injection molding or casting, it would take as much time and effort as building a full size one out of his typical electrical tubing or wood. Here, the utility and the virtues of rapid prototyping can be fully realized.

While there are many types of rapid-prototyping methods, many of these machines are outside the reach of the home hobbyist since they can require expensive materials, as in powdered titanium alloys, and high powered equipment, as in lasers that not only can make you blind, but can also burn a hole through your eyeball. With these machines, you can easily create high resolution, accurate (around +/- 1mm), strong-ish parts (about half the strength and stiffness of traditional materials), made from high strength plastics like nylon, aluminum or glass-bead impregnated plastics, or even fused steel or titanium alloys. These machines are usually reserved for industry and professionals, but Shapeways provides this service for everyone at a reasonable cost, charging by the cubic centimeter. Just upload a solid model, pick the material (even stainless-steel), wait a week or so, and you got your part! Not exactly rapid, but better than nothing!

If you’d like your own at home, the popular open-source Reprap project is great place to start, but has limited capabilities. The project revolves around the idea of creating a ‘self-replicating’ machine, which currently lays melted, extruded plastic much like a hot-glue gun in a controlled CNC fashion. The objects it can create are like what you would expect, like they were made with a hot-glue gun, except a little harder and stronger. Great to screw around with, but the parts don’t have much mechanical utility since they are relatively soft and non-uniform, meaning that you wouldn’t want to use it for anything that would be under any type of force… at least not yet. Development and popularity of the project is high and I’m personally excited to see where it goes from here.

‘Frog-Leg’ Indicator Holder

April 25, 2011 1 comment

D’oh! I forgot to buy an indicator holder when I did my last mass tool purchase. Rather than trying to track down a decent, cheap one in town (not likely here in NM) or order one and wait for a week, I built one. After talking with Machinist Mike, a ‘frog-leg’ type indicator holder would probably be the simplest and most useful, since it can articulate into just about any position. The shown design is made for a dial indicator, but another end ‘leg’ can be made to attach anything else, like a test indicator.

For material, all you need is 3/8″ aluminum round, which you can find at any hardware store. For screws, you need two 10-32, 0.5″ socket head cap screws and a 1/4-28, 0.5″ button head cap screw. I used a button head for the dial indicator mount, since it uses the same hex wrench as the 10-32. The threading placement may seem funky, but I did it so that all the bolts are facing the same direction when assembled.

Also, the milling process is designed to be easy. After cutting the round stock to length, each leg can be machined in one operation: clamp it high in the vice with a 5/8″ parallel, face the top features with an end mill, and drill the holes. Since you are measuring from the spindle to the indicator, the indicator holder doesn’t have be precise, but, by facing and drilling all of the mating faces of the parts in one step, you can ensure the movement of the legs are in-plane with the spindle axis.

Lastly, if the root ‘leg’ doesn’t fit into the 3/8″ end mill tool holder, just use some fine grit sandpaper to lightly sand down to spec.

Categories: Machining

A Small Tapping Block

April 20, 2011 2 comments

A few days ago, I mentioned to Machinist Mike about tapping a ton of holes in a tooling plate I have been planning on making. He showed me this great little tapping block he made and regularly uses. It was made of 2 inch steel round with 9 holes for #0 to 1/2″ taps, which acts as guides to keep the tap perpendicular to the surface of the part. He graciously gave me a copy of his solid model to build my own.

I built a smaller version with some scrap aluminum I had laying around. It has 5 holes for #0 to 1/4″ taps, which should cover 95% of tapping on projects with a Sherline or a benchtop mill. If I need bigger, I could always make another one. If you’d like to make your own, the drawing is shown below. Measurements are referenced from the smallest hole.

UPDATE: To get into some tighter spots, the drawing now shows all of the guide holes closer to the outer edge of the block, but still with enough meat to hold up well for a while.

Categories: Machining

Spec’ing Out a Stepper Motor for a CNC Mill

April 10, 2011 3 comments

Figuring out what kind of motor will work best for a certain application is not as simple as some may think, where there are dedicated jobs to spec’ing these things out. There are many factors that go into determining what you need and deciding is a delicate balance of all of them:

  • Motor Type – For home CNCs, there are two general classes of motors: servo and stepper. Servos require more expensive controllers and motor encoders get up and running, but provide a very smooth and quiet operation. These are typically seen on larger mills, because they can produce more output torque/power at higher rpms and require a step-down pulley system. For smaller mills, these can be too big and are not very cost effective. Stepper motors provide high torque at a slower rotational speed, but with known step increments, such as 200 steps per revolution. This allows for an open-loop control design that works just as long as your motor doesn’t stall and miss a step and is cheap to build and control. Steppers are often found in printers and scanners for this very reason. With steppers, there are three basic types of windings: unipolar, bi-polar parallel, and bi-polar series. Unipolar steppers are older, have an output torque 70% of bi-polar, and typically referred to as simple to control, but is not the case anymore with the emergence of cheap Allegro ICs for bi-polar steppers. Bi-polar steppers have a higher output torque due to using more of the available motor windings, as the name implies. They can be run in parallel (higher speed) or in series (high torque), which trades winding resistance for motor inductance. In current CNC builds, bi-polar stepper motors and controllers are most commonly used, but when choosing the type of motor to use, keep in mind that stepper motor output torque drops quickly with increasing speed.
  • Maximum Rated Torque – This is defined as the maximum continuous torque of a motor, usually given by the manufacturer at ideal conditions. In a typical application, you will rarely see this rated torque output, since there can be significant torque reductions from your motor controller, power supply selection, rotational speed, mechanical friction, and other factors. You will need to figure out how much torque it takes to rotate your leadscrews while cutting under load at the maximum feed rate and how much torque is required to start and stop the leadscrew (i.e. inertial force). Fortunately, you can infer this from manufacturer websites with their CNC kits and other people’s builds. You will just have to make sure that you take into account the rest of their setup (i.e. power and controllers) to figure this out. I will go into this further later.
  • Rotational Speeds – Motors are ‘effectively’ constant power, meaning that for a given motor, the faster you go, the less torque you get. You can only send so much electric current into a motor without burning the coils out. (There are many other factors as well, but I will not go into them here.) You just need to make sure that at the maximum rotational speed (rpm), or desired feed rate (ipm x tpi), you have enough torque at the desired maximum speed to still turn your lead screws. Many motor manufacturers provide a motor torque curve with respect to rotational speed. These are also usually given at ideal conditions, so to achieve their stated torque curve, your setup will have to be close the same as theirs.
  • Resolution – Depending on the motor type chosen, you will need to ensure that the motors have a fine enough rotational resolution, or increments per full rotation, such that you do not drive your lead screws more than your desired precision per increment. Meaning, for a stepper motor step or a servo motor encoder, each step does not translate to more than ~0.0005″ (or desired precision) to your table when your lead screw turns. For example, a Sherline mill has a leadscrew with 20tpi and a full rotation will translate to a 1/20″ or 0.050″ table movement. A standard 200 step/rev stepper motor will move the mill table 0.00025″ per full step, which is about where we want it. Any higher step per revolution, the motor will have to driven faster to reach the same feed rate, which will also reduce the running torque.
  • Motor Controller – There are a ton of motor controllers available for all types of motors. The best ones all share one common trait: both the motors and controller hardware don’t get very hot, relatively. This means that they are efficient and don’t waste energy in the form of heat. I can only speak for stepper motor controllers, but the most common good stepper motor controllers for home CNCs are made by Allegro, who manufacturers high quality integrated circuits. RepRap, HobbyCNC, Sherline, Sparkfun, and Pololu all use Allegro ICs for most of their controllers, if not all. The reason being is that they are cheap, easy to integrate, and they recycle the energy already in the windings of the motor. When choosing a motor controller, ensure that the controller can handle the peak rated amperage of the motor chosen. And for stepper motors, take care in using the correct rated amperage for the type of wiring: unipolar, bi-polar parallel, or bi-polar series. Also, keep in mind that, at least with these Allegro ICs, micro-stepping causes a reduction of output torque to 70.7% of the maximum rated torque. This applies to both unipolar and bi-polar stepper motors. (Meaning unipolars are 70% of 70% of bipolars.) This is due to how micro-stepping must power coils at different levels to get intermediate steps and still provide a constant torque throughout each step.
  • Power Supply – Choosing the right power supply can be tricky by itself. Most good motor controllers recycle the energy in the motor and only need to replenish the energy loss from the work done. Also, in most cases, a CNC machine will only move one or two axes at a time, while the other(s) are held stationary. As a rule of thumb, the power supply should be able to supply peak amps for all of your motors and a voltage at the motor torque rating. But, you can get away with peak amps of just 2 of 3 motors and voltages slightly lower than the torque rating. Also, consider the type of power supplies: unregulated, regulated, switching, wall wort, etc. You have to ensure that power is constant and does not turn off, if there is no load, as in switching power supplies, uses decoupling capacitors, is star-grounded, and does not suffer from voltage dips, as in wall worts. PC ATX switching power supplies can provide great 12V, high amp  power, if you install a resistor to bypass the switching sensor. These are used in RepRaps and other DIY CNC machines with great success, but in larger CNC builds, the 12V is marginally enough. UPDATE: After building my CNC, Allegro-based bipolar stepper drivers (Pololu A4984/4988) are very efficient and do no draw much power from the power-supply due to energy recycling. These output less heat at the highest possible motor driver voltage. In my case, an ATX power supply would have not worked.
  • Motor Mass – This is something you won’t often read about in CNC builds, but is really important in a structural sense. Benchtop mills don’t have much mass, especially for the Sherline mills. But, when you add a large motor mass off the end of a mill table, this can cause a lot of problems, such as stiffness and resonance. For example, suppose you have a fly fishing rod, when you cast it, it’s pretty stiff and whips around like you want it. Suppose you tape a golfball or something heavy onto the tip of the rod and then try to cast it, it rod will flop around, bending much farther than you expect and could possibly break. This same structural principle works the same as on the mill table. The larger the mass hanging on the ends of the your mill, the more it will deflect and flop around. With this, the resonance of the mill with added mass can cause the mill to vibrate at certain motor speeds. These resonance vibrations can be so large that table will noticeably flex, which is not good for your part or the mill itself. The more mass, the lower the vibration frequency and larger the amplitudes. In other words, you will need to pick a motor and mounting setup that minimizes the mass you add to the mill to a reasonable degree. This means that by buying that ‘ToolTime’, har-har, awesomely huge, overkilll motor or even adding a belt driven pulleys with a typically large servo motor will probably give you more problems than solve. However, this is all based on how stiff your mill table is and what the mass of the CNC motor assemblies are in comparison to the mill. With a thick steel/cast-iron 120lb mill, this can still be a problem, especially if you go overboard, but with the aluminum 35lb Sherline mill, it definitely can be, as someone on the internet has already experienced at finding that magic sweet spot. With the handwheel he adds, the large coupler either damps the vibrations and/or lowered the resonance frequency from the desired running speed of the motor, but I guarantee you that he probably sees the vibration moved to a particular lower speed with that handwheel on.
  •  UPDATE: Motor Rotor Resonance: During my build, I had encountered motor rotor resonance,which I did not account for. This is the vibration of the motor rotor inside the motor’s casing. Stepper casings are overbuilt to help stiffen up everything to keep internal fundamental vibration frequency higher than what the steppers are intended to run at. When the steppers are stepping at that frequency, the steppers would begin to resonate and stall. With microstepping, the stepping frequencies are doubled for each higher step resolution. So, when I had tried 1/4 step or 1/8 step, the maximum speed was about half of what I could get from 1/2 or full step mode. Structural coupling can also change the fundamental vibration of the motors too, and this depends mainly on how stiff the motor mount and leadscrew couplers are. There are a few ways to fix this: Use the smallest/lowest mass stepper you can get away with, buy a quality stepper motor with a stiff casing, use larger step sizes (as I had to do), accept running at lower feedrates, to use a damper as the guy in the video had to, or stiffen the motor mounts up (This is a bad road to go down because, to stiffen, you’re having to add more mass.) Servo shouldn’t have this resonance problem because they are smooth running, but, again, their main drawbacks are they are heavy and expensive to control.

Later, I will go through the process of reverse engineering the Sherline CNC kit and how I ended up selecting the motors, controllers, power supply, etc.

Categories: CNC

Segway Jousting?!

April 9, 2011 Leave a comment

This is absolutely hilarious. I have no idea who these guys are or where the picture was taken, but if I had a couple suits of full plate armor and Segways laying around, you best bet that I would be out in some random field, jousting someone into the ground, on my trusty electric steed too.

Categories: General Ramblings

Mini MAME Arcade!

March 25, 2011 Leave a comment

Ok. Building a dedicated MAME arcade cabinet is something that has been on the list of things to do for a long long while, but I have always pushed it off since it would cost a lot of money and time to build or convert a full-size cabinet. That is… until I had ran across this at, who built this mini, tabletop, portable, full-featured arcade cabinet that is powered by plug or a battery.

Ed hacks a cheap netbook, separating the LCD and using it as the arcade screen, and keeping the main body with its battery packed inside a small portable-ish MDF enclosure, complete with graphic decals and plexiglass shiny-ness. Along with a simple arcade joystick, he adds an external USB port and power button to keep the cabinet fully enclosed. This mod doesn’t require much, just a spare netbook or old laptop, joystick, some soldering and wiring skills, and access to basic woodworking tools. I have say, this is way freakin’ cool, especially the fact that it can be completely battery operated.

Categories: General Ramblings

“The machine is just the down payment for the tools.”

March 23, 2011 Leave a comment

Before I purchased the mill, a machinist friend mentioned:

The machine is just the down payment for the tools.. Ain’t that the truth.”   -Machinist Mike

I thought he was joking, until he later helped me compile the list of things I would need to get a basic manual mill setup and running. So for those of you endeavoring to create your own milling machine shop, here’s the list of things you will need.

Measurement and Setup Tools:

  • Dial Indicator (1″ travel, 0.001″ min graduation)
  • Test Indicator (0-15-0 travel, 0.0005″ min graduation)
  • Dial Caliper(s)
  • Micrometer(s)
  • Depth Gauge Base (For accurate caliper depth measurements)
  • Parallels Set (3″ lengths for Sherline mills)
  • 1-2-3 Blocks
  • Machinist Square
  • Edge and Center Finder
  • Steel Rule(s) and Protractor
  • Step Block Hold-Down Set
  • Milling Vise

Mill Tooling:

  • Drill Bit Set (Screw Machine 115-pc recommended for Sherline, HSS or better)
  • Center Drill Set (Pre-drills and chamfers holes)
  • Fly Cutter (Large surface machining)
  • Mill Collets and Holders
  • Drill Chuck
  • End Mill Set
  • Boring Tool and Bars (For machining large precision holes.)
  • Specialty End Mills (Reamers, Ball-End, Dovetail, etc. as needed.)

Things You Will Eventually May Need:

  • Surface Plate (Ground granite metrology block for accurate measurement)
  • Surface Gauge
  • Indicator Holders
  • Height Gauge and/or Gauge Blocks (To determine height from known flat surface)
  • Scribing Tool (For layout techniques)
  • Grinder (To sharpen dull bits and tools)
  • Air Compressor (Makes cleaning chips much easier. Needed to fabricate a Kool-Mist type spray lubricant for CNC.)

Ex cetera:

  • Machinist’s Handbook (The bible for all machining referencing)
  • Tap and Die Set
  • Cutting Stick Lubricant (For cutting metals on bandsaw)
  • Super Lube (Teflon-based spray lubricant for leadscrews and tables)
  • Coconut Oil (or your choice of a milling lubricant)
  • Stiff Brushes (For cleaning and applying lubricant)
  • Vice (For rough cutting material and holding parts not on the mill.)
  • Hand Files of Various Sizes (Chamfering milled sharp edges)
  • Bandsaw (and/or a Hacksaw)
  • De-burring Tool
  • Eye Protection
  • Someplace to put everything, i.e. Tool Cabinet.
  • General Home Tooling (Hammers, screwdrivers, wrenches, rags, etc.)

AND, don’t forget to buy the stock material that you will inevitably need.

AND, this is not including the cost and equipment needed for a CNC mill conversion.

Even going with nearly all imported and the cheapest available equipment, which still is more than sufficient for a home machine shop, the cost of everything is easily equal to or beyond the cost of the mill itself…. Ain’t that the truth.

Categories: CNC, Machining