Here's a picture of the welding jig with the trunnion tube centering/hold down aid. The aid is machined to 2.25"OD.
There is a lip on the top peice to help hold the tube firmly in place. The stud and all holes are very tight fitting so as not to permit movement.
If your trunnion tube is less than 4.125" tall you will need to add shim stock accordingly to the underside of the tube before using this jig for welding.. If it is more than 4.125" tall you need to machine it to that height.The relationship of the top of the tube to the 3/4" rod should be as close to the plan's dimensions as is sanely possible. Shoot for +/-.001 as this is your only control over the vertical axis alignment of the miter gears.(aside from trimming the 1"tube to compensate.)
DISCLAIMER I am not a certified weldor. The following information is only my opinion. Do your own research and form your own opinions before attempting to weld parts for an aircraft. Before attempting to use welding equipment you should receive instruction in the proper techniques and safe use of such equipment. Misuse of welding equipment can result in injury or death.
As you know all of the steel parts in this replica are 4130N aircraft steel. Welding 4130 aircraft tubing and plate demands the very best of a weldor's skill. Getting a good weld in 4130 requires you to follow every rule in the book. Most of my experience is with mild steel. Mild steel is not as sensitive to hot and cold and certainly does not crack as easily as 4130. Mild steel is also not as sensitive to variables such as filler rod type, filler rod diameter and electrode(tungsten) diameter.
I was in for quite a surprise when I thought getting welds in 4130 to pass the bend test with flying colors was going to be easy After a week of part time welding and despair I got it licked! In my pursuit a durable proper weld in 4130 I have learned more about welding science than a lot of A&Ps know!
A water cooled TIG torch and some .125" thick test pieces.
Before welding on anything that is going on your plane you need to do a few test samples. The samples should be of the same material, thickness and type of weld (lap, butt, outside corner Tee weld etc...) Even if you are an accomplished weldor and think that you equipment is functioning properly there may be a variable that you have overlooked. Perhaps the shielding gas is CO2 instead of argon or maybe you forgot to replace that O- ring in your TIG torch. At least you can find out on the test piece before ruining some really expensive stuff. When you order your 4130 get a few feet of strip steel (aka. strap) for your tests. This will speed up the fabrication of your test pieces.
Welds can look pretty, have the proper penetration and still fail. There are several reasons (which are preventable) that this may happen. The only way to know if a weld is good aside from an X-ray test or some other expensive means is to do a bend test. To perform a bend test you simply weld a sample together, let it cool, stick it in a vise and hammer it 90 degrees. Consult a welding textbook for pictures of which way to bend various welds. It stands to reason that if a test piece passes the bend test the real item which underwent the same set of variables in welding will pass also. What you are looking for in a bend test is the parent material to break NOT the weld.
ALL of these samples failed ! I took a flat lap weld and bent it 90 degrees. The weld on the left strarted cracking at the root. (back side)The only weld that passed was the one in the center. It passed because it was mild steel and was easy to weld.
I experimented with several types of welding rod and found the best one using the bend test as my guide. I broke a lot of samples to find the best filler rod. (picture above) Another interesting thing I learned was that the diameter of the filler rod really has an effect on how fast the weld puddle chills. Usually you can dip a smaller filler rod a little longer and chill the weld puddle enough to progress to the next one. Since 4130 is so sensitive to heat the filler rod diameter needs to be the same as the base metal thickness. Another important variable is electrode diameter which needs to be close to the plate thickness. On my main page metal advice area I have suggestions for equipment and a book. Performance Welding by Richard Finch is a must have.
Most suppliers of equipment and certified weldors have only cursory knowlege of 4130's characteristics. Many of them are not aware of the proper filler rod and techniques used to make sound, application proper welds in 4130. One shop I visited tried to sell me silicon bronze filler wire when I asked for something to weld 4130 with! Not all 4130 filler rod is for TIG use! 4130 wire used for acetylene is of a different composition than 4130 TIG wire. If they don't know which it is don't buy it!
Here's a website with some 4130 wisdom.(Lincoln site below) They will tell you that 4130 rod is brittle but I believe the context of the page refers to thin wall tubing. Engine mounts and other thick 4130 tubs/plates should use 4130 "MC" grade welding rod from. United States Welding Corp. (click here)
While it's not one of my better welds, this weld did pass with flying colors!
A list of the elements for a succesful TIG weld (LAP WELD) in .125" 4130 N
Sanded all surfaces to bare metal with emory paper. Top plate of lap weld beveled.(critical)
Acetone wiped filler rod and metal to be welded.
3/32" two percent thoriated tungsten 1/8" stickout and sharpened to point
Gas lens with # 7 ceramic cup
1/8 (.125) diameter filler rod
"MC"grade 4130 bare filler rod.
$56.00 per pound and worth every penny! See below for address and phone #.
120-150 amps (will vary with each machine) DCEP high frequency on "start"
Despite what you hear TIG welding is easy to learn. Perhaps the hardest thing about learning TIG is getting that intuitive "feel" of how far your weld puddle is penetrating. You don't want to burn through and/or blow holes in your work. Seeing the and controlling weld puddle is easy wth TIG. If things are getting too hot you simply back off on the foot control and watch the puddle shrivel up. Acetylene is much less responsive in this respect and seeing the puddle is not as easy as TIG.
If you are considering buying a TIG machine see metal advice on mymain page. Make sure the machine you get has high frequency start and does not require you to touch the electrode(scratch start) to get the arc started. Make sure your machine has a foot control for your heat. Using the thumb operated on/off switch is no way to weld aircraft parts! Not an absolute requirement but nice to have is AC high frequency welding ability. (You need this to weld aluminum.) You may want to make your own gas tank out of aluminum.
Weld the .100 strap and tube onto the trunnion tube before making the final trimming on the trunnion horns. Even though I made the gap between the horns wider my weld still overlapped the center. I used 1/16th 4130 filler for the strap and it's tube to get a smaller bead. Don't ream the 1/2" tube until the welding is done. It helps to have the extra thickness. Even if your horns aren't perfect this jig will make their alignment right which is all that matters.
