Monthly Archives: July 2013

DSF Build Progress Seven

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I have been surprised that the heated bed proved to be much more interesting then it would initially seemed. First I had to decide the method of heating – PCB or nichrome wire. At work we have a RepRapPro Mendel which uses a PCB heater. It’s fine but takes ages (up to 7 minutes) to reach temperature. My eMaker Huxley uses nichrome and take moments in comparison (2-3 minutes). Now obviously I have to account for the differing bed size, Mendel at 200mm square and the Huxley at 140mm. That’s a 35% difference in size which isn’t reflected in the times so there is more going on. My unscientific opinion is that I have much less nichrome wire length where the PCB has a long track to give its resistance. Bear in mind I was always taught that heating on PCBs was a bad thing! Due to this the nichrome is able to heat to a higher temperature per mm of wire than the PCB thus able to heat much more rapidly. So that is my long winded way of saying I went for nichrome for the DSF.

By no means is nichrome without its problems. It generally comes with no insulation, not many plastics can withstand the 1000’s of Celsius nicrhome can reach, so shorting on the bed is a consideration. I believe you can get it in a glassfiber but I have yet to find that in the UK. Connecting to it is another issue. You really shouldn’t solder to it, at least not with normal tin/lead solder (it would probably be hot enough to melt it).

So each problem in turn. Insulation has been provided by kapton tape. It can withstand the temperatures the wire should reach and still insulate it from the aluminium bed. To help guide the wire I designed the heat shield with a channel that allows the wire to run in a fixed pattern around the bed. This ensures that there should be reasonable heat coverage over the bed, you are never more than 25mm from it. I then covered the bed in a double layer of kapton tape where the wire will run. That completed the wire was then routed and taped down with kapton again. The connection between the 15A supply cable and the nichrome wire is taken care of with a couple of ferrules. These allow for a solid connection without solder. The supply cable was then routed through the heat shield and a cable tied used to provide strain relief.

The bed thermistor was attached and routed as well and the whole assembly wired up to the Megatronics board. I am always cautious when turning on a greater for the first time. Initially I set the temperature to get to 35C. I monitored the rise in both Repetier-Server and using an infrared thermometer. This is where I discovered something disconcerting.

The bed rose in temperature and settled at the set temperature in Repetier-Server. The infrared thermometer told a different story. It showed the temperature rocket to about 44C before settling down to 40C, 5C more than was set and what Repetier-Server was reporting. The same is true for most temperatures, the thermometer shows the temperature shoot way above the target, sometimes by 10-12C and then settle at about 5-6C above what the electronics see. I have asked for help on the RepRap forums so will see if anyone else has an idea. My current thought is that it is the temperature tables in Marlin, if so then I will need to figure out how I change it.

A safety feature on most beds is having them mounted on springs so that if there is a problem, say with a print curling, then the bed can move out of the way of the hotend. That leads to a question, how much ‘spring’ do I need? What does the spring need to support: the heat shield, aluminium bed, glass and what ever is being printed. Without the print my bed weighs in at a good 780g, adding a theoretical print of about 200g that means I need to support around 1Kg. I found a site, Lee Spring, which supplies all manor of springs. The factors to take into consideration are: the weight (1Kg), size of bolt holding the base (M5), distance to travel. On Lee Spring the ‘springiness’ of a spring is measured in newtons per millimetre. Thus if you wanted a spring of 1N/mm to move by 2mm you would have to apply 2N of force. 1N is equivalent of  about 102g. Something else to consider is that my design has 2 springs on one side and one on the other. The weight is split half way per side so the double spring side has about 500g split between the two springs, whereas the single spring has to take the whole 500 itself. I chose the LCM080DG 02 M as it fires the size I needed and has a compression of 1.5N/mm. As the springs will be compressed about 3mm anyway there is enough resistance to hold the rest of the bed up.

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3D Printing – Getting What You Need Now

From need to product in less than one hour

From need to product in less than one hour

When people find out that I have a 3D printer the question that I am often asked is ‘why do you own one?’. Usually I tell them that it started as a hobby and to be able to make my own cases for my electronics projects (I have never been very good at buying a box and making it look good by drilling out holes etc!). However over the couple of years I have now owned a 3D Printer for I have evolved that thought. It was personified yesterday while I was cleaning my desk at home…

Yes, I am untidy! My desk often looks like an explosion at some weird combination engineering, computer and plastics factory. On the odd occasion even I get miffed and have a tidy up. While I was cleaning I kept finding SD cards. Having got loads for my camera and RapsberryPi, I needed a way to store them tidily and I turned to the ‘net. Sure I could have brought any number of card cases, but then I looked over at my Huxley which I was sure was bouncing there shouting “USE ME YOU IMBECILE!”. So I hit up Thingiverse and found a parametric SD card holder. Perfect for my needs. I got the Huxley up and running while I carried on cleaning. 30 minutes later it was printed and I was able to store my cards nice and neatly.

So now when I am asked why I own a 3D printer I’ll tell them that story. After all it would have cost me much more to buy a holder that probably wouldn’t have fitted my needs so well and it would have taken me days to receive it. As it stands the print cost me about £1 and took no more than an hour from investigation to final product.

Parametric SD Card Holder

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Setting Up Megatronics

Steps per mm

Possibly the most exciting thing for me when building a 3D printer is the electronics. I have built 2 kits (a Huxley from eMaker and a Mendel from RepRapPro) and am now building my own design. Until any printer gets attached to its electronics it is a mere metal doorstop. What I wanted to write up about was how I calculated the steps per millimetre for the DSF.

While in my usual ‘I’m bored lets search the net’ mood, I stumbled across the excellent RepRap Calculator3  by Josef Prusa. If you are building a printer from scratch this will greatly simplify figuring out what number the steps per millimetres should be.

Like many of the printers out there my DSF uses belts for the X and Y axis’ (T5) and a lead screw for the Z (M5). For the belt drives all I needed to change was the belt preset to 5mm – or the T5 belt. The pulley I was using was already 8 teeth and 1/16th stepping. As a result this would give me 80 steps per millimetre or a 0.0125mm resolution.

The Z axis uses an M5 threaded rod as its drive so again all I needed to change was the preset to M5. This means a step of 4000 steps per millimetre or 0.00025mm resolution. Obviously that is much finer then any of the prints I would ever do, the finest I have done was 0.1mm/layer. In reality I could set the stepper to full steps and still retain a resolution of 0.004mm.

So thanks to the calculators I have a 80 steps/mm for X and Y and 4000 for Z. Ignore the 800 for the extruder above. I have not yet got an extruder so that is just place holding!

One last thing; I have limited the feedrate of the Z axis to a low 2mm per second. Any faster and the motors stall. I need to oil the threaded rod to try and get some extra speed out of it, but I am not holding my breath.

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DSF Build Progress Six

So I have got the steppers installed, the electronics and endstops. Now all that was needed was to test it all together. Connection to the computer was successful and the machine came to life! The Z Axis homed the wrong way, but an update to the Marlin firmware solved this.

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Simple Cooling Fan for RepRapPro Mendel

Simple but very effective

At the school I work at we have a RRP Mendel. I was trying to print some students work but it kept failing. Eventually I discovered that the problem was down to curling from some overhangs on one piece. As the part had 45degree angle and only 5mm thick, the printer was catching on the curled ands and eventually cause the printer to miss some steps. I needed a way of cooling the plastic faster.

I then found an 80mm computer case fan that was being recycled. As I needed to get the part printed I simply cable-tied the fan onto the smooth rods of the X-Axis. Amazingly these are about 80mm apart and the fan fitted beautifully behind the X-Axis endstop. Now I was able to get the print to finish and have a great look as well.

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DSF Build Progress Five

Getting there...

Getting there…

Time for another update. The printer is now mechanically complete. All of the motors, belts and leadscrews are all installed. I may have to adjust the X Axis motor mount eventually as it is poking a little bit too far forward, meaning that the belt wanders a bit.

I have received the Megatronics V2 electronics that I will be using on this machine, which I will try and review at a later date. Now it is just a case of installing the wiring for the motors to the electronics and sorting out the endstops. Once that is done I will see about getting it up and running to check mechanical reliability.

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