Print Volume

This is the amount volume that the printer is able to print in.  Generally, the measurement is given by the three dimensions of a rectangular prism.  The thing to watch out for here is that just because a printer has a large print volume, it does not mean the printer can reliably print in all of it.  Say the design calls for a thin flat plate.  If the plate is too wide, then the edges of the plate will peel/warp during the print and the flat shape that was intended will be slightly curved.  To visualize what warping is, imagine a flat plate lying on the print bed as it is being printed.  Warping is when the corners of the plate start peeling off the base during a print.  Warping will be explained in a little more detail later on.

Resolution

This describes the accuracy of the nozzle’s positioning.  It is usually measured in microns or inches.  Clearly a smaller value means that the printer will be more accurate in its position.

Layer Thickness

Because the printer works by extruding layers of plastic, thinner layers allow for more fine details.  However, more layers means that the part will take longer to print (all other things being equal).

Print Speed

3D printing is generally a slow process, but a faster print speed could definitely help offset that.

Nozzle Diameter

A small nozzle diameter means that the extruded plastic will be narrow and thus allow for fine details.  It also improves the smoothness of the printed object.

Ease of Removing Parts off the Platform

Because during a print, the base of the part (ideally) should be firmly adhered to the print bed, one must take care when removing the part from the print bed.  In some cases, removing a part requires a little bit of force to pry the part off the print bed.  This application of force may affect the alignment of the print bed relative to the nozzle.  In lower quality printers this can permanently affect the alignment or require frequent re-calibration   Seeing how “rigid” the print bed assembly can help assess how the machine accuracy can be affect during part removal.  For example, if the print bed is mounted on really large support structure, then the affect of misalignment is less of a problem than a machine where the print bed is mounted in a smaller support.

Enclosure

Some printers enclose the part to be printed in a box and some don’t.  The reason an enclosure exists is so that the machine is able to prevent the part being printed from losing too much heat during print.  Loss of heat during a print negatively affects the adhesion of the printed plastic.  Also a larger temperature difference means the part will warp more.  Without an enclosure, heat is able to freely transfer to the ambient.  High quality printers generally have an enclosure.

Variety of Printed Materials

A common material to print with is ABS.  Other materials such as PLA can be printed as well.  The use of the particular material can be quite dependent on the application of the printed part.  The cost of material also differs.  It is worthwhile to determine what material will be printed.  In many cases, ABS should be sufficient.

Warping

This is probably one of the biggest limitations of 3D printing.  Because of the use of heat during print, it is natural for the part to shrink as it cools down.  The problem is that the shrinkage is not uniform.  Some parts of the model shrink more than others and most of the time it is very unpredictable.  While there are some general guidelines that designers can follow to reduce warping, there are definitely some machine features that are good to have to help reduce warping even further.  Because the nature of the way the printer prints is that plastic is extruded layer by layer, the plastic layer at the very bottom is the first to cool down.  Cooling down means shrinkage.  This also means that as the bottom of the model shrinks, the printer is still printing on top of a slightly shrunken part.  Sometimes the shrinkage can be so bad that the part pulls itself off the print bed.  When the part is no longer securely adhered to the print bed, the part may slide around and ruin the entire print.  Warping is the enemy of 3D printing.

Below are some qualities of a machine that help reduce shrinkage:

• Heated print bed – a heated print bed keeps the plastic hot during the print and does not let the base cool down as much as if there were no heated print bed
• Enclosure – described above

There are more ways to reduce warping that the designer can do, but that is outside the scope of this article.

Number of Nozzles

A lot of printers just have the one print nozzle, but there are some with multiple.  This allows printing with multiple colours or in some cases printing with a water soluble support material.  Generally, support material has to be manually removed from the printed part, but if the support material is water soluble, this saves a lot of the manual labor involved in cleaning up the model.

Nozzle Maintenance

Through extended use, the nozzle can become quite dirty, that is black gunk starts building up on the surface of the nozzle.  If one is not careful, this can clog the print nozzle.  It is worthwhile to find out how often the nozzle needs to be replaced.  There are also other factors that may result in the need to replace a nozzle.  This is a recurring cost of maintaining the printer.  Some nozzles are cheaper and more common than others and it is worth it to see how often the nozzle needs to be replaced and how much it will cost each time.  Machines may come with extra nozzles.

Lubrication

Over time, moving parts may have to be lubricated.  Printers should come with a maintenance manual to help instruct the user how to do this.  It is also helpful to know the cost of the lubricant and how often it will be required.

Machine Lifetime Cost

Aside from the cost of the machine, operating a 3D printer can also incur the following costs:

• Shipping
• Insurance
• Brokerage fees
• Tools for removing support material
• Currency conversion
• Sales tax
• Print material (recurring)
• Replacement nozzles (recurring)
• Lubricants for moving parts (recurring)

As it can be seen, the extra costs can be quite significant.

Conclusion

Hopefully this has given a good general overview of some of the considerations in buying a 3D printer.  Different people will value different things in the printer and hopefully this subset of possible considerations for buying a 3D printer helps people make a decision as to what to look for in a printer.

I’ve been tracking my sleeping and over the past little while and I’ve accumulated 12.7 hours of sleep debt (as of 5 Oct measured by the Sleep as an Droid app).  I got this value by taking the difference of my actual time slept and the actual amount of time I need for each day and added up all those numbers over two weeks.  I noticed that this sleep debt over several days stacks up more and more.  I ended up feeling less and less rested everyday.  It seems obvious, but I never really paid attention to it until now.

What I’ve been doing now is try to sleep earlier to “pay back” this debt (probably the right thing to do).  I did a quick search on Wiki about sleep debt and as it turns out, the article basically told me the same thing that I noticed myself.  Sleep debt indeed does accumulate and catches up.  For me, one or two days, it’s probably okay, but anymore than that, then I start feeling less rested everyday.

For a bit, I tried sleeping at 11pm and waking up at 7am to pay back this debt, but I soon came to realize that I still didn’t feel better even though I was in fact paying back this debt (albeit slowly).  I came to realize the problem was in my sleeping pattern.  I didn’t actually know how much sleep I needed or when I needed to sleep (i.e. circadian rhythms).  I had completely forgotten about it (I wrote something similar on the subject awhile ago).  I always knew it existed, but I guess I lost track of what it was after I came back from Germany.  As of right now, my sleep debt for the last two weeks –7.9 hours, which is a lot better than before.

Circadian Rhythms

After I bit of trial and error, I found my optimal duration and time to sleep.  Apparently, I feel best if I sleep for 7.5 hours at 2:30am and wake up at 10am.  When I woke up after doing that, I felt great.  Kind of a night owl’s sleeping pattern.  Apparently some people can in fact sleep for 6 hours a day and sustain this daily without the need for naps, but that is a small minority of the population.  I on the other hand, fall within the average.

This doesn’t really work for me since I need to be up at 7am to go to sleep.  So now, my solution is to shift my circadian rhythms forward.  I’ve read a method for people with DSPS (delayed sleep-phase syndrome) that involved pushing back my sleeping time later and later until I reached the time I wanted.  If I slept at 2:30am usually, I’d sleep at maybe 5:30am the next day, then 7:30am the next etc. until I reached my goal of 11pm.  It would mess up an entire weeks schedule but apparently it works.  I’m not saying that I have this syndrome (I’m not about to self diagnose here), but this kind of treatment seems kind easy to do at home without great risk.  I don’t really have a week to spare for this right now so that is automatically out of the question.

Another method involved bright lights.  The Harvard Medical School Guide to A Good Night’s Sleep (really good book on sleep by the way!) states the following:

This treatment grew out of the discovery that exposure to bright light within a few hours of the body’s low point in body temperature—which occurs during the overnight hours—shifts the circadian rhythm. A dose of bright light after the temperature minimum advances the circadian rhythm, while a dose before it delays the rhythm.

Knowing this, I can shine a bright light in my face every morning when I wake up at 7.  Hopefully, my circadian rhythm will slowly shift to this new time.  Once I do that, maybe I can wake up at 7am feeling well rested.

Although I try to sleep and wake up at the same time everyday, school work always tries to disrupt my schedule, and as a result, I haven’t paid off my sleep debt and my sleeping time is kind of inconsistent at times.  Life always finds a way to break it.

Random Analysis on My Sleep Cycle

As an aside, I tried to find out how long one of my sleep cycles were.  On average, they are 90 minutes long, but differ from person to person.  The app that I use for measuring sleep graphs out how deep/light my sleep is throughout the night.  Since the human goes through cycles of light and deep sleep overnight, I thought I could measure the time duration between.  I looked at some of my sleep graphs (similar to this one):

In this graph (from 9 September – there is a typo in the picture because I mixed up the dates initially), the blue graph shows how deeply I sleep through the night. The sampling rate for that measurement wasn’t very good with the trial version that I used at the time.  Three regions are shown where values of the blue curve above the blue line mean I’m awake, values between the green and the red line mean I’m in a period of light sleep, and any value below the red line means I’m in deep sleep.  As it is shown in the plot above, it seems that a huge chunk of my sleep is in deep sleep, which is really strange since I’m supposed to be going through cycles of light and deep sleep.  Here, I only see one instance of light sleep.  Not really sure what to make of it other than the fact that the app is not a robust way of doing such measurements.

Speed: http://en.wikipedia.org/wiki/Orders_of_magnitude_(speed)

Mass: http://en.wikipedia.org/wiki/Orders_of_magnitude_(mass)

Concept

I was given the circuit shown below (from Microelectronic Circuits 6th Edition by Sedra and Smith):

I was asked for the voltage of the output terminal and the current flowing through the diode.  Finding the current was easy, but I didn’t quite get the voltage.  I thought to myself, “from what two points do I measure the potential difference?”, because after all, voltage measurements are always taken relative to two points.  It took me the longest time before I could just use ANY two points and the measure the voltage.

What I figured is that, for part a, since the diode is in forward bias, the diode will act as a short circuit.  I then can say that V – (-5) = 0, since the voltage difference along a short is 0.  From there, I can easily determine that V = -5.  I can apply that same logic to the other three.

Nodal Analysis

I was given the circuit shown below (from Microelectronic Circuits 6th Edition by Sedra and Smith):

From class, I remember the professor saying that since we cannot tell right away which diodes are on and off, we can only make an assumption about the states of the diodes and then apply the usual analytical techniques.  In the event that the result is unreasonable, we know that our assumption wrong.  I made my assumptions, but afterward, I didn’t know where to go next.  I had forgotten how to analyze these circuits.

Later, I realized that I could use nodal analysis.  Since I assumed the two diodes in part a were both on, I replaced them with short circuits.  I used the intersection right before the current splits as my node.  For part a, the current into the node was 3 V/5 kΩ = 0.6 mA.  The current leaving was some unknown current for diode 1, and a current for diode 2.  I could determine the current of diode 2 by taking the potential difference between the node and the -3 V output and dividing that by the resistance in the path.  The result was (0 – (-3))/ 10 kΩ = 0.3 mA.  From the nodal analysis equation 0.6 mA = current from diode 1 + 0.3 mA, the current from diode 1 is 0.3 mA.  And as for the voltage V, the potential difference between V and the -3 V output is equal to the voltage across the resistor, (0.3 mA)(10 kΩ) = V – (-3).  Therefore, V = 0.  Alternatively, I can use the potential difference between the node and the output, which equals zero, thus yielding the same result.

Boolean Logic

The next issue I had was determining the Boolean outputs of a diode logic circuit.  Again, the picture below is from Electronic Circuits 6th Edition by Sedra and Smith.

The issue I had here is that I did not know current sources have no voltage if there is no current running through them.  Looking back that sounded pretty intuitive.  Finding all possible combinations of inputs A and B for this logic circuit, we get the truth table below:

A B X Y
0 0 0 0
1 0 0 1
0 1 0 1
1 1 1 1

The outputs of X reveal an AND gate.  Setting A or B to a logic 0 in (a) will cause current to flow through that path.  The potential difference X – A or X – B (depending which one was set to 0), will yield 0, thus the output X will be zero as well.  When A and B are both set to logic 1, it causes the diodes to go into reverse bias, and node X will be powered by the current source.

The outputs of Y reveal an OR gate.  Setting A or B in (b) to have different inputs, either 1,0 or 0,1, will cause the diode to be in forward bias for the input that has the logic 1.  For the input with logic 0, the diode will in reverse bias.  Since one of the diodes is on, current will flow and output Y will yield a logic 1.  In the case of both diodes being off, no current will flow, thus causing output Y to yield logic 0.

Other Notes

In the case of multiple parallel diodes in the same direction, if one of them is on, chances are they are all on (provided that the voltage across them is positive).  However, there may be a case where one parallel diode will want to flow into the other because of a greater voltage difference.  For example, if there were two parallel diodes, one with a 3V input and the other with a 2 V input both going to the same node, current from the 3 V input will want to flow into the diode connected to the 2 V input.  This will cause a reverse bias in the diode connected to the 2 V input, thus turning it off.

There is also another case where there are parallel diodes with different output voltages.  If current is flowing toward it, they will all be on, so they will all be a short circuit.  Which path will the current go to?  The path with the larger current.

We first went to the Koerner Pub, but found that there was too many people, so we went to Mahoney’s instead.  We sat down at a table with about 20 people just literally just talked while we drank.  I had about two pints and started feeling a little bit dizzy.  The rest of the people there had at least 5 pints.

I rarely participate in social events like this.  I’m not sure why I agreed to go.  Either way, the I thought experience was pretty good.  I got to know my roommates better and finally acquired a taste for beer.