Tuesday, 17 July 2007

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.


  1. congratulations, you have reinvented the steady rest.
    ever tried extruding delrin?

  2. No not yet. It has quite a high melting point. I am working on a higher temperature extruder at the moment so I will put POM on my list of plastics to try, thanks for suggesting it.

  3. Like fenn said, that's a steady rest. Most steady rests use brass rods to contact the work, but smart folks will modify them or build their own with roller bearings.

    There's a similarly useful tool for turning small diameter work called a follow rest.

    I laughed when I saw the picture of your new tool. It's fascinating to see someone come up with the same solution to an old problem.

  4. You might want to try going close to final size with your first hole: 1mm will deviate much more over a longer hole than 3 or 4mm. In this case, it probably isn't a good idea to start with a pilot.