Cheap Chinese gas lens

Recently I got my TIG welder up and running again. I have a WP17 size torch with a variety of collets, cups and electrodes. Mostly I seem to need the larger 2.4mm diameter electrodes with the occasional need for something smaller. The consensus seems to be that gas lenses are more forgiving (to stickout), giving better shielding at the slight cost of a wider body. 

The std gas lens for the WP17 is a 19mm ID cup with a narrower mouth. But it's possible to fit a smaller gas lens system and they are widely available on ebay, Aliexpress etc for peanuts. The basic parts are fairly simple, so surely it shouldn't be possible to screw them up?

So I acquired a small lens kit from ebay for £12. This uses the Pyrex cup which is trendy these days as it allows better visibility of the arc. Obvious good things (and crap things?) come in pairs, so I also got a pencil torch body from Aliexpress. 

Not wanting to be ripped off on simple, things like Pyrex glass tube, I got myself some borosilicate tube of both diameters (for the large and the small lens systems). When you buy from a specialist glass supplier like Tuffnell Glass, the cost is orders of magnitude cheaper (£6.50 for 0.5kg, which is about 1.5m length for the larger size). 

My first trials with the smaller lens system was a disappointment, after getting used to the original, large system. Lots of spitting and brown/black sooty deposits on the weld, which indicates air/oxygen getting to the weld zone. 

Small Chinese lens:

 Large, ceramic lens:

 Large ceramic lens:

 Small Chinese lens:

There was no problem with the gas flow as such, so clearly it was drawing air into the argon stream, which rather defeats the point of a gas lens. Going back to the larger system showed what was possible and should be expected.

One key difference between the large ceramic cup and the smaller Pyrex cup is the shape - the (small) glass cup is parallel walled, whereas the (large) ceramic cup has a narrow mouth (the new torch body is on the right):

So was the problem due to the parallel walled "Pyrex tube" cup - or some failing in the smaller copper lens body? One simple way to find out is to fit a Pyrex tube cup on the large body using my newly acquired bulk tubing. I can't try the small lens system with a ceramic cup - because I don't have any small ceramic cups.

The tungsten "knife" I bought with the glass tubing is supposed to make cutting a length off quick and simple.

I was somewhat sceptical and that scepticism was justified in the event. I suspect I just need to man up but there we are. I got the result, sort of - or at least good enough for the purposes of this test. Some practice and investigation will be required....

To get a nice even score along a consistent diameter I thought it would be rude not to make use of the lathe, turning at about 50rpm.

 Hmm.

 This is the larger tube size:

Simple enough - the large Pyrex cup works fine with no evident air contamination. It seems that the smaller copper body doesn't diffuse the gas sufficiently so that it doesn't achieve even flow rate across the width of the cup. To use these small things, some additional diffusion is required. The original gas lenses had several layers of diffuser and market leaders like CK still do so. 

Here's the end result:

So - I have a functional large gas lens system with my own Pyrex cup. As for the smaller system, I'd need to modify the diffuser body before I could make use of it. I'll give it some thought.....