Threading

Khiraly asked me to explain how I manage to put a thread on stainless steel, so here goes.

Aluminium and brass are fairly easy to thread, but stainless steel is very tough. In order to make it easier you need to use a split die and a holder designed for one.



By tightening the middle pointed screw you can force the die to spread and increase the diameter of the thread a little. That allows you to make a first pass that doesn't cut as deep, so does not require as much force. By loosening the middle screw and tightening the outer ones you can reduce the thread diameter and make a second pass.

Another thing that makes it easier is to use cutting compound to lubricate it. I use Trefolex on Adrian Bowyer's recommendation. It is a sort of green lardy gunk.



To start off you need to align the rod or tube that you are threading orthogonally to the plane of the die. The easiest way to do this is with a lathe. You put the work piece in the headstock chuck and mount the die in a die holder that slides along a bar held in the tailstock.



You then turn the chuck with one hand and the die holder with the other. I use the handy chuck grip that I RepRapped, but a chuck key can be used to turn the chuck in 1/3 turn increments.

You need to go about half a turn forward and then one third of a turn backwards to break the chips off. If you don't it may jam.

When you start you need to feed the die against the workpiece with some force, but once the thread is started it feeds itself.

It is unlikely the chuck will have enough grip for cutting a stainless steel thread from scratch. You may have to file some flats on the stock.

If you don't have a lathe, the next best thing is to put the workpiece in the chuck of a drill press and put the hand die holder flat on the bed. Let the weight of the head press the work into the die and turn the chuck by hand. Once started you can put the work in a vice and spin the die holder.

Using a die to extend the thread on a hex head bolt is much easier because you start on the existing thread and you can hold the head in a chuck or a vice.

Going green

Getting a bit bored of natural ABS, which is cream coloured, I ordered some black and some green from New Image Plastics in the USA. Despite the high cost of shipping it still worked out cheaper than buying it in the UK. I chose green because it should be easier to photograph than black or white and it the RepRap logo colour.

I was surprised to find that it does not behave the same as the natural that I got from the RRRF. It does not seem to stick to itself, or to the base material, as well as the RRRF ABS did at the same temperature. I have raised all the temperatures by 15°C and, although it is better, I think it still needs bit more. It's appearance is more glossy, so that might account for it being less sticky, or it may just have a slightly higher melting point. I have no idea if the green dye has an effect or whether the base ABS is different. It will be interesting to try the black.

I made a new version of my screw top pot in order to perfect the thread before posting the STL files on the RepRap Objects wiki. It turned out to be nightmare to get it to print. Besides the problem of it not sticking to the bed, my extruder kept breaking GM3 gearmotors, but that's another story.



The ABS version came out about 1mm bigger in diameter than the HDPE version due to lower shrinkage.

I modified the thread to make it easier to start. The original thread started abruptly at the top of the pot and the bottom of the lid. I modified it in CoCreate by continuing the helix past the top of the pot and then milling it back to 2mm below the rim. I did the same to the lid's thread. That allows the lid to overlap 4mm before the threads engage, making it easier to line up. The thread now tapers out to a thin sliver to make it easy to engage. The result works a lot better than the previous attempt.

Screw Top Pot

A day before going on holiday I decided I needed a container to store the lens adapter for my camera. The lenses have caps on each end, but they require and adaptor which is a bit delicate: -



I knocked up an HDPE pot with a screw top lid that just the right size to hold it: -



The outside diameter of the adapter is about 48.5mm so I made the pot I.D. 50mm to allow some clearance. It actually shrank to 48.5mm, so it is a snug fit. Lucky it didn't shrink any more!



I made the thread using the helix tool in CoCreate. You draw a 2D profile and then use the helix tool to spin that round an axis, specifying the pitch. The dimensions were just a stab in the dark: I made the crest of the thread 0.8mm as that is two layers (Nyquist sampling theorem) and made the sides 1mm long 45° slopes, so that made the crest height 0.7mm. The minimum pitch with this profile would be about 3mm so I made it 3.5mm to give some clearance. I also made the the lid 1mm bigger radius so there is 0.3mm clearance from peak to corresponding valley.



A couple of things I missed which would make it easier to engage: -

1. The thread starts abruptly, but it should have a tapered lead in.
2. Both the pot and the lid should have a few millimetres with no thread to aid lining them up before the thread engages.

Despite this it works surprisingly well for a first attempt with an arbitrary profile and dimensions.



I am slowly homing in on getting rafts peel-able for HDPE. I made this with the first layer outline hotter than its infill :-

HDPE.raft_temp = 215
HDPE.first_outline_temp = 230
HDPE.first_layer_temp = 205
HDPE.layer_temp = 240

Most of it peeled with a little encouragement from a chisel at one side.