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Machining SS and First Time TIG Welding

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  • Machining SS and First Time TIG Welding

    As a physics major, one of the classes that is required is "Advanced Physics Laboratory", which is basically a glorified science fair project. It's actually one of the most fun classes I've ever taken. Not only has it been extremely educational, but its allowed me to "get my hands dirty" and put together an experiment that would be way out of my own personal budget.

    My project is the demonstration of the interference pattern generated by single electron emissions incident on a dual slit. This has only done twice before, once in Spain and once in Japan. It was an ambitious project, to say the least. There are numerous technical difficulties associated with shooting a single electron at 130,753,400 mph and hitting a 500 nm fiber and then detecting the posistion of the electron after interaction with the fiber.

    Anyway, a large part of the project has been fabricating a vacuum chamber. 304 Stainless steel was chosen for three main reasons. Ordinary materials gas off under vacuum. Practically, metals that may be used for high vacuum are aluminum, copper and SS. Aluminum forms an oxide layer and will "trap" free electrons. They will then create a very small electric field that will deflect the beam. (Speaking of which, did I mention that the Earth's magnetic field also deflects the beam?) That left SS or copper and SS is cheaper! Plus it is more easily welded.

    So here is a couple of pictures:

    This is the first flange completed. You can see the chatter marks around the outside, but that is only in the last .05" of the groove and shouldn't cause a problem.

    I squared up the flanges in the mill and drilled the bolt hole circle. I also drilled and reamed a hole in the center to give me something to indicate off of. Then it was to the lathe with a 4 jaw to cut the o-ring groove and recess the center section by .020. This is for faster pump-down time.

    I wasn't happy with welding job that a friend did so I decided to try it myself. Here is a picture of my first TIG weld:

    Here is another shot with a Viton O-ring installed:

    A little messy - I was nervous and had a hard time keeping my hands steady. I had about 5 hours in that flange and it was my first time welding. I could just see it all going up in flames!
    Last edited by Fasttrack; 11-06-2008, 01:05 AM.

  • #2
    And here is a much larger flange that will adapt my square tube to a much larger vacuum chamber:

    Those are pictures of the worst side. I had to stop mid weld and let the torch cool and get re-posistioned. This was done "all killer and no filler". I wasn't concerned with the undercut since 1) its not a structural piece and (more importantly) 2) the tube is .083" wall while the flange is .5" thick! I can loose a little meat on the flange and it won't matter
    Last edited by Fasttrack; 11-06-2008, 01:04 AM.


    • #3
      Have you helium leak checked it yet?

      When installing orings for vacuum make surr the oring does not get twisted. It can cause a annoying hard to find slow leak.

      How low are you going?


      • #4
        10^-5 Torr

        Our helium leak tester is apparently no longer functioning...
        I had it on the roughing pump earlier this evening and couldn't get below about .1 Torr. It should rough down to .001 Torr, but I think the leak is actually with the existing system. I can't recall now what the type of joint is called, but its basically a but joint with an o ring between the two pieces. The flange is cracked and the tube coming out can move around quite a bit.

        This is a student lab so alot of the equipment is hit-or-miss whether it works properly.


        • #5
          You may have some issues getting down to 10^-5 with a viton o ring on a flat flange like that. This is really getting into the range of a conflat connection.

          .1 torr is pretty good for a used roughing pump. What kind is is? Belt driven, direct drive, single or two stage.

          it would probably be best to run a bead with filler. Also it might help to find someone to grind the flanges flat. I can pretty much guarantee they are not flat after welding.


          • #6
            I'm no expert on high vacuum applications but I would think that anything organic like viton will gas unacceptably and contaminate the vacuum environment. I suggest with some hesitation to use an intersection conical seat joint seal something like a globe valve seat.

            The trick to this sealing joint it to make a 90 degree included angle conical seat recess about 1/8" wide having a bore of say 1.5" for example. The mating piece has an 89 degree conical projection having a major diameter slightly larger than the major diameter than the seat and a minor diameter slightly smaller than the bore.

            The slight difference in the seat angle results in metal deformation at the seat/bore intersection. If the intersection angle is small enough this deformation will remain within the elastic limit. The seat angles have to be true cones and the bore a true diameter.

            This is 100 year old rechnology at least widely used in globe and poppet valves. I advance it because I once machined intersection valve seats to a high degree of presision to make zero leakers under 5000 PSI air with minimal torque on the stem where the pressure was under the seat tending to force it off the seat.

            While this joint seal is expensive to execute, will not tolerate dirt or damage, and has limited reuse before reconditioning is necessary it's still a good candidate I think for your application because no organics are necessary.

            By way of determining the points of leakage I suggest a low pressurization and the old soap solution trick. It's also ancient technology but will serve to find the gross leaks and porousities. If safety considerations will tolerate pressurization with propane a keen nose will detect very small concentrations of the odorant measured in part per billion.

            I suggest the above with some humility for I know little about high, tech high vacuum applications.

            You might also look at weld distortion as Macona suggests. Many a fancy system has foundered because weld distotion interfered with the operation of some vital part.
            Last edited by Forrest Addy; 11-06-2008, 03:29 AM.


            • #7
              When I worked in research we had a spark source mass spec and that had pure gold o-rings so that the whole system could be baked at nearly 200c and 4 diffusion pumps it could go to 10^-7 torr only down side was the constant need for liquid nitrogen for the cold traps. I think it consumed 65litres on nitrogen a day for which we had our own on site reservoir.
              High vacuum is a strange beast and the only thing is it requires a lot of patience as looking for leeks that do not exist and eventually go away with time is very annoying to say the least.
              The system had 2 rubber o-rings 1 at each end of the setup so that samples could be inserted and a photoghraphic plate inserted. I dont think they were viton but I could be wrong.
              Nice work on the TIG welding though I am not sure my welding would be so good.

              I have tools I don't know how to use!!


              • #8
                Fastrack...not bad on the tig welding...but a poor choice on the proceedure.
                You need to induce a lot more heat into the piece than if you had used filler rod.
                The inner piece...straight fusion weld...ok you can do that one like that I suppose but the outer weld should have had filler.
                If you didn't warp it then who really cares but you'll find that the "pull" on high heat welds is amazingly strong.
                You did good tho...that you didn't get it hot enough to turn it all black
                I have tools I don't even know I own...


                • #9
                  For what its worth I just briefly went through all the posts and it sounds like you might be having a sealing prob. ------------- That O-ring does not look like the right size to me - it could be an illusion but it looks sloppy in the cut-out and also looks as if its in the recess too much and lacking crush...

                  What an undertaking your doing! this is going to be interesting and I admire your efforts ------ Personally I cant believe your first tig welds as I think youv done great -- But Russ is the man, When it comes to welding I listen to Torkers advise...
                  Good luck and electron speed

                  Edit; Suggestion, I know you personally dont have a mill, but since this is a school project it sounds like you might be utilizing their equipment? If so - The way I would go about getting a positive seal on an O-ring is not to use a lathe , I would get a proper size Ball endmill and use a mill and R/T table or CNC -- this leaves a nice fit on one side and if done right you can get just the right compression on the flat side to where there is nothing left for air space --- wanna get real serious with sealing, do both face plates not quite half way down -- leave just a tiny bit of crush otherwise you will physically spread the ring into the flats and compress it there and you dont want that... Yes this can be accomplished on a lathe with a special bit but the surface area is generally too broad and will induce chatter...
                  Last edited by A.K. Boomer; 11-06-2008, 09:16 AM.


                  • #10
                    Woah wait a sec guys...

                    1) Torker - Yep, I did notice the difference in heat and it certainly warped the 1/4" flanges slightly. Luckily I was able to grind them flat in a surface grinder. I was sweating bullets about the thicker one. I ended up doing little .5" beads around the corners before going back and welding the entire thing. However, I spoke with our machinist who makes vacuum chambers good for ultra-high vacuum (10^-9 torr and above) and he said that filler rod should NOT be used. There is a greater chance of introducing foreign material to the puddle or creating a void. I suppose if you were a pro, it wouldn't be a problem. It is common practice, however, not to use filler rod on vacuum system joints. Consequently, the flanges are often extremely thick to avoid warping. And, like I said, none of them are structural. The won't feel anything but a pressure differential of 15 psi.

                    I'll confess, though. After playing with the TIG welder for a while, I want to go weld some more stuff! I still like stick the best for its speed, but TIG is alot of fun just because of the control you have. I'll need to practice doing it the "proper" way sometime.

                    2) Viton is acceptable down to 10^-7 torr and standard ASA joints are also acceptable down to 10^-7 torr. A thin coat of vacuum grease was used to improve the seal with any rough surfaces. For ultra-high vacuum, gold crush gaskets are a must.

                    3) The O-ring groove is cut to ASA specifications. The largest recommended o-ring section is 3/16 and this is rarely used. 1/8 is preferred. The O-ring grooves must be cut in a lathe so that the tool marks are circumfrential and should be cut to a depth that is 85% the actual (not nominal) size of the O-ring section. The width of the groove should be 135% the actual cord diameter. Just following specs there.

                    The roughing pump is a duo-seal belt driven two stage pump. EDIT: Good call Macona. Apparently I can no longer convert between microns and torr! The pump is supposed to rough down to 100 microns or .1 torr. In fact, I'm not too sure how accurate our gauges are. For rough vacuum, I was reading a thermal couple gauge and depending on which gauge I had plugged in, it read betweeen .1 and .26 torr. The thermal couple gauges should be good down to at least .01 torr.

                    After leak checking, I plan on baking the system. There are some teflon insulators. They are small but tend to absorb moisture and can be a real pain to get dry. Teflon, however, is good to 10^-8 torr after an initial pump down of ... two days!

                    Thanks for all the replies! I'll file all those suggestions away for the next time I have to design a vacuum system.

                    Well its certainly been a crash course in vacuum systems, for me. I've still got a lot to learn though! Hopefully we'll make some progress on it today.
                    Last edited by Fasttrack; 11-06-2008, 11:35 AM.


                    • #11

                      I've been working on a vaccum mold for epoxy granite test samples and here's what I've learned: Google for the Eriks oring handbook to find out about oring grooves. oring grooves probably need to be polished as grooves that run perpendicular to the oring can cause leaks. I'm going to polish my oring grooves when I finish machining them using some abrasive sticks and pencils from Boride Engineered Abrasives. I have also seen no oring grooves in any of the handbooks that have round bottoms.



                      Hope this helps.



                      • #12
                        Thanks Cameron! Thats exactly why I cut the o-ring grooves on a lathe instead of a RT. Theoretically, all the tool marks should be circumfrential and not perpindicular to the o-ring. Of course, then there are chatter marks and scratches and etc! I did run some scotch brite through them, but pencils seem like a good idea!


                        • #13
                          Like Fastrack said, Viton is usable down to 10^-7 to 10^-8 range. Good stuff. Turning the groove is best since the scratches are circular. Also some companies only cut their flanges from sheet stock as bar stock can outgas more due to the direction it was rolled.

                          The couple turbo pumps I used to have used viton seals.

                          Those duo-seal pumps suck, and not in a good way. Too bad they dont have any direct drive pumps.

                          Filler rod is usually used on the outside but not the inside. It does take careful welding. Maybe best to wait to see how it performs under pressure.

                          One simple way to test for leaks under a vacuum is to use a good sensitive gauge. Use a small squirt bottle with IPA or Methyl Alcohol. Squirt it at the joints. If there is a leak the pressure reading will spike.


                          • #14
                            Thanks Macona, its being pumped down right now and I've got a little squirt bottle here to test for leaks.

                            I did use filler rod to lay down some tiny beads on the outside. I didn't run the bead the whole way around since I didn't figure it needed it. I was under the impression that the outside is welded for strength and the inside for a good seal.


                            • #15
                              Yep, thats pretty much it. You have very little strength with a pure fusion weld, but its the weld with the best chance os sealing.