Friday 29 June 2007

Bob on

Here is the first RepRap part that HydraRaptor has milled :-

It is the extruder barrel clamp as shown in the previous post. As you can see it is dimensionally correct to two decimal places! It's not a fluke, the height is bob on 45.00mm as well. I have been getting pretty good accuracy with my previous milling attempts but this is astonishing.

I used a smaller end mill (2.22mm) in a collet chuck which is more accurate than the three jaw chuck I used before. The plastic is a sheet of 9mm Delrin or Acetal which I used because I read somewhere that it was a good plastic for machining. It did machine well but all the edges were left with a thin burr. This was fairly easy to remove with a fingernail.

This is not the latest version of the RepRap design but I decided it would fit my machine better. I added two more mounting holes and changed the central hole diameter to match my PTFE barrel which is 12mm rather than 10 or 16mm in the original design. The central slot is 2.5mm rather than 2mm because my bit was too big. Obviously I will have to drill the horizontal hole for the clamping bolt with a drill press.

Here is an edited video of it being made, it took around 20 minutes to make the part :-

And here is an amusing out-take if you listen to the sound :-

So having achieved perfection on this first part, things can only get worse when I attempt to make the rest of the extruder.


  1. Instead of manually removing the burr, given the accuracy you've achieved, would a second pass offset by a tiny amount, and maybe with a bevelled cutter, remove it for you?

  2. Perhaps, or it might just leave two burrs on the two new edges instead of one. I have a beveled grinder which might do the trick but would need a lot of extra programming. Also the bottom edges had burr as well I think.

    A strange thing happened to the edges of the waste material. I use climb milling round the part I am making so the surrounding scrap gets conventional milled. I expect that to be left with a rougher surface. With Delrin this is quite noticeable but also it is covered with ribbons of swarf which have to be scrapped off to even see the surface. They seem to actually be attached. This might have something to do with the fact that I was using an cutter which is cone shaped and I drop it 0.1mm with each pass.

  3. I once read that Delrin blocks have a skin that is harder than the inside. This can lead to the block deforming if the surface is removed from one side. I wonder if the burr is just the harder surface layer.

  4. Well I did wonder if that was the case because the top and bottom faces are very shiny and the burr almost looked like a continuation of that shininess.

    Playing with plastics is all new to me. I have always stuck to wood and metal in the past and looked down on plastic as inferior and difficult to work with. This project has opened my eyes to the fact that there an almost infinite variety of plastics, some with very impressive qualities.

  5. I don't know if my input will be useful as 6 years have past, but I'll give it a try anyway...

    If you are still having trouble with burrs on the top and bottom of the part you should see if you can get your hands on a compression bit. This bit is useful for finishing passes on materials that have some sort of outer skin on the top and bottom or are prone to splintering. Compression bits have a special flute design that "compresses" the chips into the middle of the work piece. The bottom flutes represent those of an upcut bit and the top flutes represent those of a downcut bit, and as a result you get a clean edge on both the top and bottom of the part.