Dinner and Delrin

I am used to being inundated with HDPE milk bottles but it's something when your dinner guests bring you a bottle of wine and a carrier bag of Delrin scraps! Very welcome of course.

Not posted for a while because, apart from entertaining, I have been puzzling over how you drill a centred hole in the end of a long workpiece using a lathe. Several of the RepRap extruder parts need axial holes drilling down the middle of them. On a normal lathe these parts would not be considered long. They would be pushed back inside the chuck, so that the end which meets the drill mounted in the tail stock is well centred. On the scale of watch parts they are massive so they are too big to fit down the spindle of my watchmaker's lathe. If I put the far end in the chuck then the near end drifts out of centre. Normally you would support a long item with the tail stock when turning it, but how do you support it when drilling?

Here is a picture of a 1mm pilot hole being drilled in M5 threaded rod with a drill guide. Although this was only slightly off centre each successively bigger drill wandered further out.



The problem is not unique to my small lathe as it would occur with larger parts on a larger lathe. I searched around the web for a solution but I did not find one, so I decided to solve the problem by making a new part for the lathe. Not sure what to call it but it is a sort of inside out live tailstock. I.e. rather than having a bearing which meets the end of the workpiece, it has three rollers which support the outside of the workpiece, keeping it centred but allowing it to turn freely with access to its end for drilling.



This is the largest thing I have milled so far, so not wanting to risk wasting time and material, I decided to add a tool path simulation view to my Python script. As I have come to expect with Python this was trivial. A quick Google found me the Tkinter package. A quick scan through the tutorial and ten lines of code later here it is:-



I don't know if the new device works yet but it looks promising. Here it is with some M5 threaded rod spinning in it:-



I don't know if I have invented a new tool or just reinvented the wheel, or three wheels to be precise.

Peck it till it squeaks

Not much progress recently as I spent the weekend at the British F1 Grand Prix. The next part of the extruder that I made is the motor holder. Surprisingly this gave me the most trouble.



Obviously, as it is a 3D structure with overhangs, it cannot be milled out of a solid block and would be very wasteful of material if it could. Instead I made it out of three pieces of 6mm Perspex sheet. I always think of this as being pretty flat but in fact its thickness varied by about 0.25mm across a six inch square cut from a larger sheet. This wasn't a problem however.

The first problem I had was getting the dimensions from ArtOfIllusion. For the other bits I have been clicking on sub components of the object to get the size and position. In this case the model seems to have been constructed in a different way so I could not select sub parts. Also there seems to be no rulers or dimensions in AOI and the grid labels are poor. In the end I redesigned it in Visio taking measurements from the motor itself. The shape of mine is a bit simpler but I think it holds the motor in the right place so it should work.



Milling these three parts was straightforward, the hard part was drilling the holes along the plane of the sheet. There are four M3 clearance holes which are 24mm deep. These are very hard to drill in Perspex without getting the drill stuck and very time consuming. The way I did it was to make a paper template with cross hairs in Visio. I cut it out and aligned it with the edge to be drilled. I then marked the centre by drilling a few mm with a 0.5mm drill in a hand held pin vice. I then drilled the holes using a very slow drill in a drill press and the piece in an improvised drill vice. I started with a 2mm drill then opened them out with 2.5mm and 3mm drills.

You can only drill a few mm at a time before the drill starts to bind because the swarf cloggs the bit and then the Perpex melts and sets again incarcerating it. You can't then turn the drill backwards or forwards. The first time this happened I thought I was going to have to smash the workpiece to recover the drill and start again. However, I did find a less drastic way to get the drill out. I put it in a spare chuck which is the good old fashioned type with a chuck key. I tightened this as much as I could, rested the workpiece on top of vice jaws with the chuck below and tapped it with a hammer. This does work, I had to do it several times.

You have to use a pecking action while drilling. I drill until I hear the Perspex squeak which indicates the onset of binding. I then back out and brush off the swarf and repeat. Very tedious, perhaps a lubricant would help but I have no idea what to use on Perspex.

To fasten the pieces together I chose M2.5 screws and decided to tap the Perspex. The only problem there is that I have a set of large taps that go down to M3 and a set of small taps that go up to M2. I thought tapping M3 in 6mm Perspex was likely to crack it so I decided to try using the screws to cut their own thread. That was a nightmare with binding and the heads shearing.

Anyway, eventually I got it done and am quite pleased with the result.



At some point I will mount a drill vice on my X-Y table, add a laser centre finder and create a highly accurate semi-automatic drill press. Hopefully I can monitor the motor current to detect the onset of binding and make the pecking automatic.

Now I need to do some lathe work to make the moving parts, barrel and nozzle.

All hands to the pump

I finished milling the polymer pump casing this evening. Here it is :-



The bearing gaps gave me some problems because they require a small ball end mill with a relatively long reach. As I said before all my small milling bits have shafts bigger than their heads.



Rather than order one from the US and wait for it to come I decided to try grinding down the shaft of one of the ones I already had. I put it in the lathe but it was too hard to be turned down with a cutter. Instead I put a small grinder in my Minicraft drill and held it against it while it was spun in the lathe. This worked quite well as you can see :-



The parts aren't exactly to the ArtOfIllusion models provided by RepRap. Obviously internal corners have a minimum radius due to the tool being a finite size so I will have to bevel the outside edges of the bearings a little. Both the polymer channel and the bearing gaps have a slight overhang in the models. While this would theoretically be possible with a larger ball mill it would make the programming more complicated. Hopefully I can get away without it.

I also increased the bore of the polymer channel from 3 mm to 3.2 mm as my reel of HDPE is a bit over sized. I am still a bit worried about this being too tight. Also the material I used is not as slippery as say PolyMorph so there may be too much friction. I think I will open it up slightly with some fine emery paper and then polish it with wax.