CNC hot wire foam cutter

This CNC foam cutter was built from a kit by It basically consists of two carriages for the horizontal translations and two towers for the vertical translations. I’ve built two prior CNC foam cutters (not using this kit), and the main challenges with those DIY designs were uneven sliding friction and wobble. These problems have been mostly solved in this kit by using plastic bearing sleeves for friction reduction and two linear shafts per axis to reduce wobble. However, it became clear later that the wobble is primarily due to lack of straightness in the acme lead screws are. Any small bends in the screws will producing a rocking motion of the carriages, which will show up as ripples in the foam edges.

The stepper motors are 6-wire NEMA 23 motors with 2A drive current. The driver board was built from a kit by Hobbycnc (Model #4AUPCWHC). This board includes PWM control of the cutting wire temperature . The control signals are sent via a parallel port connector. The motors need a hefty power supply (up to 8A max), and the hot wire (30-gauge NiCr) also needs a separate power supply.

Most computers nowadays do not come with a parallel port. Although there are USB-to-Parallel converters, they will not work for CNC control because it is impossible to deliver accurate timing signals through the USB port. The solution is to get the UC100 CNC motion controller. It looks like an ordinary USB-to-parallel converter, but it is not. It has embedded circuitry and software drivers to produce the precise signals required for CNC motion control. This allows any laptop with a USB port to be used to drive the CNC. The UC100 is best used with the Mach3 software, which is one of the most widely used CNC software. However, it is a generic CNC software, so some customization (known as “screens”) is necessary. The screens for foam cutting setups can be found in many online forums. I bought mine through Ebay from

The input commands for the Mach3 software are written in G-codes. These are ascii commands that tell the motors how far to move, and how fast. Some drawing packages such as inkscape allow saving the vectors as G-codes, but none of them work straight out of the box because the foam cutter needs to drive two pairs of axes – two left axes (X & Y) and two right axes (A&B). Most conventional CNC systems use a single pair of X & Y axes. There are also software specifically created for foam cutters that can produce four axes G-codes (such as devFoam, Foamworks, Jedicut). However, G-codes for simple designs can be easily created from a python code or from an Excel spreadsheet. The codes can be viewed using a variety of software, such as camotics.

The moving hot wire melts the foam by convective and radiative heat transfer. Therefore, the width of the cut will be larger than the wire diameter. This is known as Kerf, and must be compensated in the design. Kerf is a function of wire diameter, wire temperature, cutting speed and foam type. The best way to determine wire temperature is by using an ammeter to measure the current through the wire. For a given foam type and foam thickness, the current is directly related to the current. Low density foam will cut easily and will produce a larger kerf. Slower cut speed will result in a larger kerf. Therefore, the optimum cut speed and temperature has to be determined for each foam type. The thickness of the foam also matters. A thicker foam will draw more heat away the wire, and will require a higher current to maintain the same temperature.

Dayton Daily New Article

This is an article I wrote to the Dayton Daily News, which appeared on Saturday May 5 2012.

Tom Hausfeld, the pilot of the ill-fated flight on April 1, might be alive today if not for the poor decisions to erect buildings on the approach path of incoming airplanes. While fear is being raised about the possibility of airplanes falling off the sky on innocent people, the true hazard is actually the other way around. When Tom lost engine power, he was required by the Federal aviation regulations to maneuver the airplane away from persons or property on the ground. This is exactly what he did. Not a single piece of metal fell on anyone outside the airport fence. It is also evident that he struggled to hold the airplane high enough to clear the roofs of the buildings that were on the approach path, which robbed him of the precious last few knots of airspeed that is so essential for staying airborne.

The foremost tragedy in this story is that we lost a fine citizen and a fine aviator of our community. The second tragedy is how this is being spun to advance the interests of businesses and other groups. The pilot and his passenger were the victims here. The hazard was the building. How anyone can turn that story around is amazing to me. Munroe Muffler built a shop at the very edge of the runway, and then goes on the news media complaining about low flying airplanes.

The fact that airplanes fly low just prior to landing is not a new phenomenon. This is how airplanes were flown since Orville and Wilbur. The FAA and the laws of physics require airplanes to fly in a shallow 3-5 degree glide angle during approach. At Wright Brothers airport, this means the airplanes have been crossing the fence at roughly 75 ft altitude for nearly half a century. Several years back I recall looking down during a final approach, at the construction site where the gas station and the Munroe shop now sits, wondering why in the world anyone would choose to build there knowing the risk it poses to aircraft. That choice ended up costing the lives of two innocent people.

Residents are understandably concerned about the possibility of airplanes crashing into their homes. However, it can be verified from the NTSB records that statistically this is an extremely unlikely event. Nevertheless, public perception is still important, and communities and airports need to work together to create a mutually safe environment. But that is a two-way street. Erecting buildings with no regard to aircraft safety, and then accusing pilots of flying too close to those buildings is not an environment that creates mutual trust. Communities such as Settler’s Walk can write their own rules on how airplanes should fly, but unless such rules are incorporated into the Federal registry they will have little or no effect. Airport operations are not regulated by city, state or residential communities. Airports are part of a vast national network, and operate under federal regulations. Airplanes flying here from Florida or Texas or even just from Cincinnati cannot be expected to know about the covenants of Settler’s Walk or the opinions of Munroe Muffler.


Andrew Sarangan


University of Dayton

Cockpit Voice Recorders for Instructional Flights

Published in the AOPA Flight Training Magazine

A lot goes on in the cockpit during a typical dual training flight. No one has the ability to remember every single item that was learned in the cockpit. A typical student will forget 10% of the lessons learned before the end of the flight. Another 10% will be lost before the end of the day. Only 50% will be remembered after one week. It is simply human nature to forget. Unless the experiences are committed to long-term memory through repetition, these experiences will be short-lived. This is why we take notes in a classroom, and do homework exercises. However, the intense nature of the activities in a cockpit does not lend itself to taking notes. Repetitive exercises in the form of additional training flights are expensive. A cockpit voice recorder is a seldom-used resource that can make an enormous difference to this memory lapse. It can greatly enhance retention and learning. By listening to the recording after each flight, the student will be able to recreate the flight environment in his or her mind. The exact words spoken by the CFI, the verbal expressions of the student and the exchanges with ATC can be reviewed over and over again. The student will be able to return for the next flight with a fresh review of the previous lesson, regardless of how long it has been since the last flight. This can reduce the number training hours for the student. Even the CFI can benefit from listening to his or her own instruction style and improve upon it.

A compact digital voice recorder with an 18-hour recording capacity

We are not talking about cassette tape recorders. These can be awkward and even hazardous in the cockpit. The modern day equivalent of the tape recorder is the all-electronic digital voice recorder. Digital recorders smaller than the size of a cigarette lighter can be bought for as little as $100. They can record several hours of audio, which can then be replayed or downloaded onto a computer. The electronic format has several inherent advantages over its tape counterpart. There is no need to change sides, rewind tapes or make copies. The CFI and the student can both take a copy home without having to run copies. The files can be shared through email or over a website.

A tiny condenser pickup microphone attached to the inside ear cup.

Their compact size is what makes these recorders ideal for cockpit use. They can be conveniently tucked away in a shirt pocket. All that is required is an intercom input. While it is possible to create a wiring arrangement to plug it directly into the intercom system, there is a simpler and less intrusive method. A tiny condenser microphone attached to the inside ear cup of the headset using a small piece of Scotch tape will work just fine without any elaborate wiring. These microphones are available from any electronic supply store for a few dollars. This arrangement does not affect the operation of the headset in any way, and it leaves almost no residue when removed. As long as the microphone wire is very thin, it would not lift the ear cup seal off your skin to create an air gap. The wire can be conveniently wrapped around the headset cord and into your shirt pocket away from the critical areas of the cockpit. This arrangement does not require any special wiring or modification to the intercom system. The whole contraption resides with the headset. Depending on the input sensitivity of the recorder, the microphone may also require a small volume control to prevent overloading the recorder. A little experimentation may be necessary to get it to work satisfactorily.

Critics may question if there really is a need to introduce yet another electronic device in the already crowded cockpit environment. The same question could be asked about GPS, TCAS and the myriad of other electronic gadgets that have proliferated in the cockpit. Every piece of equipment must be weighed with respect to the potential benefit it brings and the level of distraction it causes. Compared to most other equipment in the cockpit, the recorder does not require any attention from the pilot. It is turned on before engine start and turned off after shutdown. The level of distraction it causes is less than that of a transponder squawking VFR.

On the negative side, this technique does demand a significant investment of time from both the instructor and the student. The instructor has to take the time at the end of the day (or at the end of each lesson) to transfer the audio into a computer and send it to the student. The student has to spend the time to listen to the audio. Not every student will have the time or patience for this. However, experience has shown that the students who diligently listen to the audio have a higher retention rate, shaving several hours off their flight training.

Student response on the use of this technique has been very positive. Comments range from “this is an ingenious tool that has helped me a lot” to “I wonder why more instructors don’t use this”. Some students even went as far as purchasing their own recorder so that they could continue this practice after earning their certificates, or to use it with an instructor who does not use this technique.

Here are some hints on what to look for when purchasing a digital recorder. Not all digital recorders have a computer interface. A USB connection that allows fast downloading to a computer is an important feature. Recording capacity also varies greatly from one unit to the next. There are some units that can store as long as 16 hours of continuous recording. Each time the record button is pressed, the recording starts in a new file, which makes it easy to separate the audio files. Some recorders allow the user to organize the audio files into separate folders, just like in a computer system. Another important feature is the Voice Activated Recording (VOR) capability. This allows the recorder to skip over silent periods. For example, a 1-hour flight may only contain 40 minutes of actual conversation or radio chatter. The voice activation will ensure that the silent periods are not recorded. The resulting file will be smaller and easier to review. Some units also have a threshold setting for the voice activation. This is a handy feature because it can be set to the precise level to eliminate the background cockpit noise, just like the squelch control in the intercom system. Battery life can vary significantly, but it is not uncommon to get at least 10 hours of recording with each fresh set of batteries. Due to the small size, most units use AAA batteries, which typically have a shorter life than AA batteries.

These recorders are made strictly for voice recording; so don’t expect high quality stereophonic sound. The average digital recorder samples the audio 1000 times per second. For comparison, compact disc players sample 44,100 times per second. This is what gives them the high fidelity and depth. This is not a shortcoming, as the low sampling rate is more than adequate to capture every salient detail of a cockpit conversation. The recording quality is comparable to what comes out of the intercom. Any higher fidelity in the recorder will simply be wasted effort. Once the audio is downloaded into a computer, it can be transferred into any format using a number of shareware or commercial software. MP3 is one of the most popular audio formats. When encoded as a 16 kb/s MP3 file, one hour of audio can comfortably fit in a 7MB file. This is not an unreasonable size by today’s standards. The file can be transferred to a zip disk or sent over a broadband Internet connection.

This technique works so well that it is surprising why more people don’t use it. It is a small investment with high returns. It can be used even under IFR, as voice recorders as explicitly exempt from FAR 91.21. If the student is conscientious about listening to the audio after each flight, the training time can be reduced by as much as 10-20%. It also helps to preserve the memory and sentimental value of those early training flights in much more vivid detail than a logbook.

Flying to Canada

PDF file of this article

Let me start with a disclaimer that I am not the expert on international flying. My experience comes solely from having made several trips between Ohio and southern Ontario to visit friends and family. When I first checked into this, I received all kinds of totally irrelevant information, such as international flight plans, radio licenses, HF radios and wilderness survival gears. Even AOPA’s write-up did not help much.

This article is written in the hope that it might be of some use to pilots looking for first-hand information about flying to Canada. This is probably the easiest international flight any pilot could make. Canadian aviation is almost identical to ours, except for a few minor differences.

The first airport of landing in Canada must be a designated airport of entry, and you must arrive during their normal operating hours. Most Canadian airports near the U.S. border as well as larger ones further north are most likely to be designated airports of entry. Canada Customs operating hours varies from one airport to another. Smaller airports might only offer weekday service, but busier airports will have customs service at all hours including weekends. A complete listing can be found in the Canada Border Services Agency website. There is also a special program called CANPASS which allows pilots to land at additional airports outside normal operating hours, but that requires a special application and an annual fee, and is really only intended for frequent travelers.

Flight plans are mandatory and the aircraft must remain in contact with ATC with a discrete squawk code while crossing the border. Filing IFR is the best way to go. IFR flight plans are handled seamlessly across the two countries regardless of where you file. I am told that VFR flight plans are not handled the same way, and may require two separate flight plans to be filed with each country, with separate activation and termination as you cross the border.

Detroit, Lake Huron, Montreal and Halifax Sectional charts cover large parts of Canada

Obtaining aeronautical charts is the most frustrating part. Canadian charts are not available on the internet due to copyright rules. I am not even aware of any paid service where you can download their charts. It is a major pain in the neck. If you are planning the flight the night before, as I often tend to do, you will be out of luck. But you can get away without buying Canadian charts if you are willing to take the unofficial route. All areas of Canada below the 49th parallel are covered by U.S. charts. That includes Toronto, Ottawa, Montreal, Quebec City, and the entire provinces of New Brunswick and Prince Edward Island. What a deal! The FAA has a disclaimer that the information outside the U.S. may be unreliable, but that is up to you. However, if you insist on buying Canadian charts, it is cheaper to get them mailed from a Canadian store. In addition, U.S. FBO’s near the border might also be a potential source for Canadian charts.

Before departing from your home airport, call Canada Customs on their toll-free number 1-888-CANPASS. You will be asked for an ETA, passenger names, citizenship and dates of birth. Once you depart, it works just like any other domestic flight. The Canadian and U.S systems are virtually transparent under IFR. You won’t even know which country’s airspace you are in. It was interesting that most of southwestern Ontario is under Cleveland Center’s airspace. You will be flying in Canadian airspace talking to a U.S. controller. Go figure. Handoffs were a nonevent. Cleveland hands you off to Toronto Center somewhere near London, Ontario.  Toronto Center might hand you off to Montreal Center if you are going further east. There are some minor differences in the ATC language. They say “radar identified” instead of “radar contact”. They call “Terminal” instead of “Approach”. Canadian aircraft identifiers begin with a “C” followed by four letters and contain no numbers.

Upon landing, you call the same 1-888 number to report your arrival. It is best to do this from the aircraft with a cell phone. They will give you an arrival record number which you should keep in a safe place. I’ve been told that customs agents make random inspections and may show up to check your documents. However, of all the times I have landed in Canada, I have never seen a customs officer.

Canadian rules require VFR flight plans for all flights greater than 25 NM. The format is quite different from ours and it could be very confusing. The best thing to do is confess to FSS that you are not familiar with the format. They were more than happy to talk me through the flight plan. There are other differences. Tower will automatically open and close VFR flight plans. At non-towered airports, unless you tell the briefer otherwise, the flight plan will be activated at the proposed time. If you do not depart as planned, it is important to call FSS and amend or cancel the flight plan. They are very serious about search and rescue up there.

Airspace designations are somewhat different too. Class C airspace is treated like our class B. Class D looks much like our class C. They also have a class F. It can get pretty confusing. On the plus side, they don’t appear to be as rigid about the airspace rules as we are here. It was a more relaxed atmosphere. We filed IFR as much as possible, so this was not an issue for us. An interesting observation is that I was able to file using DUATS and still get my clearance from the local ATC.

If you are flying in the Toronto area, a visit to the City Center airport is highly recommended. It is a magnificent airport on an island just walking distance from downtown. It has a landing fee, but it is cheaper than parking a car in downtown. The view of the downtown area with its famous CN tower is breathtaking, and you can circle over the city without being pursued by fighter jets.


Coming back into the U.S is a much more serious affair. Similar rules apply, such as having to land at designated airports of entry during their business hours. There is no central toll-free number for U.S customs. You have to contact the individual customs office listed in the CBP website. There are two kinds of airports: international airport of entry, and customs landing rights airport. I am still unclear on the practical difference between these; they both seem to be the same. To avoid complications in case of a diversion, delay or emergency landing, it is important to clear customs as soon after entering U.S airspace as possible. We normally land at Sandusky, OH for two reasons: it was very close to the Canadian border, and it has an on-site customs office. We were told that on-site officers are more relaxed and less cranky than off-site officers who would have to drive to meet us. Upon landing, it is imperative that you stay in the airplane until the officer comes to meet you. There is a customs declaration form, and is best to have this form filled out ahead of time. The officer will ask for passports, aircraft registration, pilot certificate and medical. We found the officers at Sandusky to be friendly and courteous, but I have heard horror stories about other locations.

There is a $25 per calendar year fee for U.S customs services. You can pay this online, and you will receive a decal in the mail which should be attached to the aircraft door. If you don’t have time to wait for the decal to arrive in the mail, you can bring the printout of the online receipt as proof. In some cases, you can purchase the decal directly from the customs inspector, so it is best to check all of this ahead of time. If you are paying the inspector, it is best to have the application form filled out and have exact change before you arrive.

One last thing. NavCanada will send you a bill for ATC services. It is about $12, and is good for three months.

VOR as a course Instrument

By: Dr. Andrew Sarangan (October 2003)

PDF file of this article

The vast majority of pilots use the VOR as a command instrument – turn left when the needle deflects left, and turn right when the needle deflects right. Then they learn about reverse sensing. If the OBS is set to the reciprocal course, the commands become reversed – turn right when the needle deflects left, and turn left when the needle deflects right. Naturally, most prefer to stay away from reverse sensing. As a result, they do a lot of knob twisting to keep the desired number at the top of the OBS. Those who use the five-T’s mantra would recall that one of the T’s stands for twisting the OBS knob. This technique works fine for the most part, but there is a simpler and more elegant way.

It is a less known fact that the original VOR receiver was designed as a course instrument. It was not designed as a “fly left” “fly right” indicator. This is why the needle was called a Course Deviation Indicator. This is also why there is a full circle of numbers on the face of the VOR. The needle points to the hemisphere where the selected course lies. The triangle pointer (also called the TO/FROM flag) points to the hemisphere where the station lies. Together, they point to a quadrant of headings that will intercept the desired course. Perhaps this is best illustrated through a few examples.

Let’s say we are flying a VOR approach, and the final approach course is 210. And let’s also assume that the VOR station is on the field. You want to intercept the 210 radial and fly inbound towards the station. When we select 210 on the OBS, the needle deflects as shown. Which heading should we fly? Pause for a moment and think about how we normally do this. Most pilots would turn the OBS until the needle centers, get their current radial position, form a mental image of their relative position to the station, and then determine which heading to fly. This works fine, but it is a lengthy process and takes too much mental effort. The alternative method is a lot simpler, and requires less handwork and brainwork. Look at all the numbers along the needle-side (left-side) of the VOR face. The needle is pointing to a hemisphere of headings between 030 and 210. Turn to any one of those headings, and the needle will eventually center. Yes, it is really that simple! The fastest way to get there of course is to fly 120, which is directly against the needle. This will make a 90-degree intercept to the desired course. All other headings will intercept the course at a shallower angle. Now look at the triangle pointer. It points to a hemisphere of headings between 300 and 120. This is where the station lies. In order to fly towards the station and intercept the selected course, we need to pick a heading from the bottom left quadrant of the VOR. For example, 080 would be a good heading to fly. Once the needle centers, its hemisphere collapses to just two numbers – 030 and 210. Of these two numbers, only 030 lies in the direction of the station. This is the heading we need to fly to track the course towards the station.

Let’s look at a second example: We want to intercept the selected course and track outbound from the station. This time we have to look at the needle and the tail of the arrow. They point to a quadrant of headings between 280 and 010. Pick one and fly it. Whether we turn left or right is immaterial. What matters is that we turn to the desired heading. When the needle centers, fly 280 to track outbound.

Notice that it doesn’t matter which number we put at the top of the OBS. We could put the desired course, or its reciprocal, and the indications will not change. There is no reverse sensing.

Here is another scenario. We are cruising along an airway. VOR1 is set to the airway radial, and VOR2 is set to an intersecting radial from another VOR. We want to know whether we have passed that intersection or still headed towards it. Here is how to do this in less than 5 seconds. Take a look at the panel. Our present heading is 060. That number is on the needle-side of VOR2. Therefore, we are still flying towards the intersection. Bingo. There is no need to twist any knobs, do any math or visualize our position. Why work hard to find the answer that is already written on the instrument?

The next example highlights the most valuable use of this method. Let’s say we want to fly to an intersection of two VOR radials as shown below. We tune both NAV radios and set the OBS to the required radials. Both CDI needles hit the stop. What heading should we fly to get to the intersection? Using the conventional method, this would take several minutes of OBS-twisting and mind-bending visualizations.

Here is how to do this in less than five seconds. Look at all the numbers on the needle-side (left-half) of VOR1. Then look at all the numbers on the needle-side (right-half) of VOR2. Find a number that is common to both. How about 360? Fly that heading. No mind-bending visualizations are necessary. If VOR1 centers before the VOR2, we are left with only two choices – 330 or 150. Only one of these is still on the needle-side of VOR2. That would be 330. Fly that heading, and eventually both needles will center. Like before, it doesn’t matter whether we put the radials at the top of the OBS or at the bottom. The results will be the same.

The same technique works on a localizer. Put the localizer front course at the top of the OBS and fly it like a course instrument. Interpret the top numbers for the front course and the bottom numbers for the back course. There is no reverse sensing. Our brain will be free to attend to more important matters. The same technique works on CDI’s driven by a GPS. On the Garmin 430/530 unless you set the OBS to the selected track, it will keep popping up a reminder message.

Notice that we did not have to figure out our current position in any of these examples. We instantly knew where to point the airplane without twisting any knobs or doing any math. Once the airplane is headed in the right direction, we can leisurely attend to the task of locating our current position. This embodies the true utility of the VOR receiver. It presents the information in the order of their importance – heading first, position later.

Given the simplicity of this technique, it is somewhat mysterious why this is seldom taught during flight training. Most pilots are taught to turn towards the needle and ignore its numeric indications. This method works only when the heading indicator is aligned with the OBS. If they are different, the indications will produce meaningless commands. Additionally, turning towards the needle is more likely to promote needle chasing, especially when the pilot is under pressure. When the VOR is used a course indicator, the pilot must read the numbers and respond with an appropriate heading. It reinforces the importance of finding and holding a constant heading. Pilots trained under this system are more likely to be disciplined about their headings.

It is unfortunate that some avionics manufacturers have failed to recognize this important VOR feature. Some have dropped the station arrow and replaced it with the TO/FROM flag. Some displays have all the numbers tucked behind a plastic sleeve except the top and bottom numbers. The worst ones are the digital VOR displays. They have an LCD bar scale to simulate the needle deflection. There is no compass rose on the face of the instrument. It is interesting to note that technological improvements have actually made the VOR more difficult to use. There might be a lesson in this. Some things are better left the way they are. The VOR system might be a 50-year old technology, but it is one of the greatest inventions in aeronautical navigation. It is really too bad that we won’t have them for much longer.


The VOR” by Joe Campbell, December 1995. Unpublished article available from