Tuesday, 28 December 2010

Round robin

I have been making a few small tweaks to my host software to improve quality recently. One such tweak is the order in which islands of an object (or objects) are visited. By "island" I mean a closed outline and the holes and infill that it encloses. Skeinforge seems to always go for the nearest island, so when it finishes a layer it starts the next layer on the island it has just done and revisits the others in the reverse order.

This means that the plastic is added to the hottest island first and the coldest last. When an island is small it can mean that the layer below is still molten when the next layer is added. I simply reverse the order of every second layer so that the islands are visited in a round robin order. That means they all get the same time to cool down before the next layer is added.

The only downside is one extra long head move each layer from the last to the first island. If your machine leaves strings that is not ideal but mine hasn't since I started reversing the extruder. That also makes the Comb and Tower modules of Skeinforge redundant.

Sunday, 26 December 2010


My wife has assembled her own Christmas crackers from kits in recent years. She puts in much better gifts than even the more expensive commercial ones contain. It did backfire one year when she put a handkerchief in one and it ended up with a powder burn from the explosive!

This year she asked me to make some reprapped boxes instead to contain the usual cracker contents and look decorative on the table. The explosive element to be provided by a party popper. This is what I came up with: -

Having zero artistic ability myself: the star is Christmas star by andrewar from Thingiverse and the tree was grafted from the frame vertex of the Holiday Prusa Mendel by kliment.

My contribution to the design is the box. The base dimensions were determined by the hats my wife wanted to use and the height by the party popper diameter. This one also contains a magnetic bookmark, two chocolates, two PLA snowflakes and a charade instead of the usual bad joke or motto.

The lids had to be printed hollow side down because of the raised design on top. The gap is too big to be spanned without a lot of droop, so I used the support facility in Skienforge. I set the "support gap over extrusion perimeter ratio" to 10 to make it easier to remove and waste a little less plastic. I have no idea why the ends of the support are all in slightly different places.

It was still quite tedious to remove, so I tried Adrian Bowyer's technique of using oil to reduce the bonding. I knew the roof of the lid started at 8mm, and my host software prints the height of the current layer, so I just waited until it had finished the support and painted it with machine oil using small paint brush, while dodging the head. It worked very well and made the support easy to remove.

Here you can see the scars left behind, probably where I missed with the oil: -

I removed the scars by waving a hot air gun over the plastic.

The unsupported area sags a little and that makes a visible pattern on the top as there are only three solid layers. I think that actually makes it look more decorative by adding a textured border: -

The removed supports could be glued together and used as streamers.

These cracker replacements went down very well with both our families. They make a lot less mess on the dinner table and could also be reusable, but they all asked to keep the boxes, which was of course our original intention.

The files are available here.

Merry Christmas!

Saturday, 18 December 2010

101 Mendels

From March up until a week ago I have run my Mendel as close to 24/7 as I can and it has printed 101 Mendels, with a bit of help from HydraRaptor. During all that time I have been able to sell them as fast as I could print them but there has been a dip in demand running up to Christmas, so I stopped printing on Monday, having built up a small stock.

I have shipped parts to England, Scotland, Isle of Man, Ireland, Sweden, Denmark, Belgium, Netherlands, Germany, Austria, Poland, France, Spain, Portugal, Italy, Tenerife, USA, Canada, Australia and Singapore.

It seems weird now to have a quiet house and not have to stay up until midnight every night to start the overnight build. It does mean that I have time to blog again though, and print things that are not Mendel parts.

I have been printing parts of a Milestag laser tag gun for a friend of mine. I recommended CoCreate to him and he has taken it and run with it. His first design is way more sophisticated that anything I have managed so far. It is a large device broken up into parts that just fit on my 200mm bed. Here is one of them: -

You can see the rest in Tony's blog http://funwithelectrons.blogspot.com/2010/12/milestag.html.

I think a machine printing 101 copies of itself must be a bit of a milestone in the RepRap project. That is about 100kg of plastic and not far off 4800 hours of printing in about 6000 available. It is testimony to the reliability of the mechanical design and if anything, the quality of the parts is getting better as I tweak the settings.