SX100 Wehnelt set screws

[Jeremy Wykes]

We have some very unhappy set screws in one of our wehnelts that we will have to drill out.

Does anyone happen to know:
(1) The composition of the material that the screws are made from. If no one knows I will probe one of the screws;
(2) What the thread on the screws is. The pitch is very fine;
(3) If you can obtain taps and dies to cut that thread;

I have been informed that obtaining new screws from Cameca is somewhat expensive, which is why we are looking into having our workshop fabricate some.

[John Donovan]

Hi Jeremy,
I can help! Our machine shop made wehnelt screws for us and they are better than the original!

The thread is not standard but our shop ordered a special tap and die. I think we determined they were Be-Cu and the machine shop used a broach to make an allen key hole instead of a screwdriver slot. Much easier to work with.

When I get back to Eugene next week I'll send some to you. Please remind me.
john

[Probeman]

I confirmed with our machine shop and the screws are BeCu 3 mm dia and 0.3 mm pitch or 3mm x .3mm.

They are checking their tooling but think they would be easy to make again.

[Jeremy Wykes]

I am weighing up a couple of options. I could get some M3×0.3 screws made, or drill out the wehnelt and tap it to M3.5×0.35, which is a standard metric fine size.

Also, may I ask the SX100 community from where you obtain your filaments?

We currently get ours from Ladd, but they are mostly the style where the tungsten wire is welded to the same side of both posts, rather than different sides of each post, and it seems that we don't have enough adjustment in the wehnelt to centre some of the 'same side' welded filaments. I am thinking of getting the workshop to bore out one of our wehnelts to increase the diameter of the hole by ~500 µm to get more adjustment. Could this turn out to be a bad idea?

[Probeman]

I suspect the reason Cameca went with a finer thread than the standard fine thread pitch (0.35mm) is that the wall of the wehnelt is already quite thin and curved, so you want to make sure you have several complete threads in that situation...

Our shop guy is making some BeCu screws with the non-standard 0.3 mm pitch so I will let you know what he thinks they will cost if you want to get a few from them.

[Jeremy Wykes]

I did some measuring. The OD of the Wehnelt is 19.5 mm, the wall thickness seems to be 2.3 mm, but I did not measure it accurately.

If we drill out to 3.5 mm, we will have ~2.14 mm maximum continuous thickness.

Thus:

• M3.5x0.30 = ~7 turns
• M3.5x0.35 = ~6 turns
• M3.5x0.50 = ~4 turns
• M3.5x0.60 = ~3.5 turns

I suspect you might get away with any of those thread pitches. The coarse threads will be much stronger, but will make adjustment more challenging.

It is difficult to see why Cameca engineers would choose M3.0x0.30 over the standard metric fine M3.0x0.35 when there is that much wall thickness to work with.

[Jeremy Wykes]

Our workshop drilled out the Wehnelt to 3.5 mm and tapped to M3.5×0.50. The workshop made new BeCu screws and used EDM to put in a 2.5 mm hex socket. The threads are deeper at 0.5 mm pitch than 0.30 mm.

I sourced the tap and die from www.e-taps.com thanks to a suggestion from Peter Collett.