Belts, ball chains, 35mm film, rack & pinion..electromagnets?

The current T5 timing belt thats used on the Mendel has a few shortcomings. Its expensive, can’t be printed, needs to be split with a dangerous belt spiting jig and its hard to obtained locally.

A t5 pitch timing that's used on the Mendel. This image was stolen from Stephen George who sells belts for $35AUD here.

So what are the alternatives? Here is a quick run down of what’s currently being explored.

Ball chain

The idea is simple enough, print a ball chain pulley (thingiverse link) and use the same type of ball chain you find on most blinds and shutters. Its advantages are that its dirt cheap and available just about everywhere. Its downsides are mainly over its reliability as its more likely to stretch, break or slip than the normal belt used.

A ball chain used as a belt for a Mendel z-axis. This one was made by Vik Olliver

35mm Film

Using 35mm cinema/photography film is a fantastic alternative to the standard belt and is current being explored by BalanceSeeker. You could possibly obtain it from your local cinema for free and a single role would be enough for 10’s if not hundreds of Mendel’s. Even off cuts may be long enough. It can also be joined with simple tape and should not stretch during use. If that wasn’t enough, you could also add barcode like stripes to the film which would allow the belts home position to be known at all time when read with a rotary encoder type setup.

A bed surface moved by 35mm film belts made by BalanceSeeker.

Rack and Pinion

The herringbone rack and pinion option being explored by Forrest Higgs its a fantastic option for the mendel in that its 100% printable. Its only shortcoming as far as I can see is that it would require the x-axis stepper motor on the Mendel to be attached to the extruder carriage, adding a lot of weight. Alternatively, the stepper could be stationary with the rack attached to the carriage. However this would increase the foot print of the Mendel in the same way that a typewriter extends outside its foot print as you type.

A prototype rack and pinion being developed by Forrest here.

Electromagnetic Control

This is a bit of an odd idea I had this afternoon. Why use belts at all when you can use magnets and frick’n lasers?

The general idea is this, you have a perforated surface below the print bed that acts like a air hockey table to produce a cushion of air beneath the print bed. This allows it to move in the x and y direction with very little resistance.

The bed is  then moved by4 electromagnets placed, two for each axis. By supplying current in different directions the electromagnets may either attract or respell solid state magnets placed on the bed. The bed’s position is found from two laser range finders, with one for each axis. A representation of what I’m describing is below.

In the image the bottom section is the air table, the arrows represent the directions that each electromagnet (the spirals) can move the bed and the round hockey puck shaped objects are the solid state magnets.

A representation of a 'floating' air bed moved by electromagnets.

For obvious reasons this would be a nightmare to control and most likely would suffer from very poor movement resolution and vibration. However, if the electromagnets in my CRT monitor can be controlled precisely enough to guide an electron travelling at close to the speed of light to a point less than a milimeter in size on my screen, then why not a print bed? Dozens of orders of magnitude more mass is an obvious contender, but not something that couldn’t be overcome by someone with enough technical skill.

Its simplicity of only one moving part is very attractive but this is something I wont be  attempting any time soon that’s for sure.


About Richard

I am a PhD candidate in Materials Engineering located in Melbourne, Australia.
This entry was posted in A Reprap Project and tagged , , , , , , , , . Bookmark the permalink.

9 Responses to Belts, ball chains, 35mm film, rack & pinion..electromagnets?

  1. The way it’s drawn, the table has three degrees of freedom; it can move in X and Y and also rotate. So in addition to the two laser rangefinders, you’d need to be able to measure and correct the angle…

    As well, the electromagnetic setup drawn here would probably be ineffective, because the force of the electromagnet goes as 1/distance. The planar motion stages I’ve seen used in industry are based on the Sawyer motor:

  2. Batist has also experimented with sandpaper and pulleys. The sandpaper has a high friction and when used from a belt sander it is of the type that can withstand a lot of deformation cycles. I’m not sure if the repeatability is the same as a more ‘discrete’ belt. There could be some ‘drift’ which becomes substantial in bigger objects without intermediate homing.

    • Richard says:

      Sandpaper is an idea I will try for sure. I have even seen adhesive backed sandpaper for laying on slippery floors which may be perfect.

      Thanks for the input!


  3. Belt splitter jig Dangerous? I have just split my belts for OverlapStrap, and I can hardly say that the splitter jig is a dangerous device to use! Obviously it has a sharp blade sticking out of it and has to be used with caution, as would be advised when using say… a pair of scissors. I agree with you other points about belts, and like the other innovations that have been made for alternate drive mechanisms, but the electromagnets idea is complete Pie in the sky. You should also note that some companies now offer cheap belt kits, with the width of the belt already 5mm.

    • Richard says:

      @Jacob: Controlling the rotation of the electromagnetic print bed would indeed be a big problem. Thanks for the youtube links to the linear motors, they are fantastic! So simple and yet effective. No doubt due to lack of demand they will be ridiculously expensive and hard to come by.. I will have to get stuck into some research on this topic to come up to speed.

      @Giles: Perhaps I should have phased it ‘dangerous looking belt splinter’. You wouldn’t want to fall on it that’s for sure!

      You wont get any argument that the electromagnet idea is a bit far fetched.. Just consider it thinking out load and out side of the box.

    • Wayne says:

      How about grinding off sharp end of cutter blade? I am making one and thot about this for safety.

  4. cefiar says:

    A worthwhile idea may be to use a “voice-coil” Linear motor arrangement like used in hard drives to drive the axis’s .

    In fact, really old hard disks (back in the sub 60 Meg days) used to use stepper motors to drive the read/write head in/out. The linear motors sort of started appearing around the 60-80 Meg mark, and were MUCH faster.

    BTW: The term ‘voice coil’ comes from the fact that the device is based on the same sort of magnetic design as a ‘voice coil’ in a speaker. If you open up an old drive, you could connect the head coil up to an amplifier and play music on the movement of the head going back and forth just like a speaker. I used to inspect and then try and repair basic faults on hard disks almost 20 yrs ago (part of my job at the time), and out of a bunch of dead drives we built a speaker in the office. 😉

    • Richard says:

      HDD’s into speakers, I love it!

      The voice coil idea is a good one. Like most good ideas it looks like has already been done.

      Although this setup does away with the need for belts I would assume the cost of the linear voice coils would far exceed that of the standard belt/stepper setup. I wonder if it would be possible to obtain some tubing and wind your own from scratch. Or are they a lot more complicated than they seem?

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s