 hanger goes. It seems to vary.
Is there a range of placement with respect to the axle that I should observe? Is it ok to have the hanger in front of the axle, instead of behind it?

A:

Welding is probably a bad idea - for several reasons.

Heat changes things - no doubt you have welding skills, but you'll know that relatively thin metals warp and bend and change their temper.   The thin metal of a derailleur will not take nicely to significant heat.  You also risk upsetting the thin-walled tube of your chain and seat stays.

1b. Many aluminium bikes are of a grade that can't be safely re-welded.  Eg, The 7005 grade is heat-treated after building, and re-heating it later will remove that leaving a softer section.

You're going to be limited to like-metals.  A modern derailleur will have aluminium construction with steel pins/axles.  An older or cheap/nasty derailleur will have more/all steel contstruction.   I'm no expert, but welding aluminium to steel isn't going to work, so at that point you'd need to braze instead.
Replacement - derailleurs aren't really high wear items and generally last tens of thousands of kilometres, but they do suffer damage in falls, and simple rough handling can whack the mech against things like rocks, other pedals, etc.   Having to grind off a derailleur to replace it would be a lot of work
Mechanical Fuse - in theory the hanger will bend before the frame breaks - this should allow damage to be reduced if there's something caught in the chain.  For me a hanger saved my frame when the chain snagged the derailleur and pulled it around the wheel and forward to the front derailleur.  Without a separate hanger, the frame would suffer more in an accident.
Function - almost every rear derailleur in the last 50 years uses a series of pivots to get the jockey wheels close to the rear cogs.  

There are Seven total... four pivots in the parallelogram, and one in each jockey wheel.  But the extra one is in the middle of the hanger's retaining bolt, and its not just a simple bolt
Heck its not even a simple pivot, instead its a spring-loaded
hanger goes. It seems to vary. 
 Is there a range of placement with obedience to the axle that I should note? Is it ok to have the hanger in front of the axle, rather of behind it? 

 A: 

 Welding is probably a bad mind - for several reasons. 

 hotness changes things - no doubt you get welding skills, but you'll know that relatively thin metals warp and bend and switch their temper.    The thin metal of a derailleur will not claim nicely to significant heat.   You also risk upset the thin - walled tube of your chain and seat stay. 

 1b. Many aluminium bikes are of a grade that can't be safely re - welded.   Eg, The 7005 grade is heating system - treated after building, and re - heating it later will remove that leaving a softer section. 

 You're going to be limited to like - metallic element.   A modern derailleur will have aluminum structure with steel pins / axles.   An older or brassy / nasty derailleur will have more / all steel contstruction.    I'm no expert, but welding aluminium to sword isn't going to work, so at that point you'd need to braze instead. 
 Replacement - derailleurs aren't really high wear items and generally last tens of thousand of kilometres, but they do hurt price in falls, and simple rough handling can whack the mech against thing like rocks, other pedals, etc.    take to grate off a derailleur to replace it would be a lot of oeuvre 
 Mechanical Fuse - in theory the hanger will crouch before the frame breaks - this should let damage to be reduce if there's something caught in the chain.   For me a hanger write my frame when the chain snag the derailleur and pull it around the wheel and ahead to the front derailleur.   Without a separate hanger, the frame would suffer more in an accident. 
 Function - almost every rear derailleur in the last 50 old age uses a series of pivot to get the jockey wheels close to the rear cogs.  

 There are Seven total... four pivot in the parallelogram, and one in each jockey wheel.   But the extra one is in the middle of the hanger's retaining bolt, and its not just a simple thunderbolt 
 Heck its not even a elementary pivot, instead its a spring - loaded
 hajger goes. It seems to vavy.
Is there a ranyw of pnacemeht with fespect to the axle that I sioule obstgve? Is it ok to have ghe hangeg in fronr of rhe axle, iiatead on behjkd it?

C:

Wxlding is probanly a bad igea - for severdl rzasons.

Heat changes things - no doubt rou havr aelding skills, but rou'lm know that relatively thin metals warp aid bend and chamge their temper.   The thij mehal of a derailleug will not jzke bicely to sienificant heat.  You alao risk upsetting the thin-wallea tubz of your cyaun wtd seat stabs.

1b. Magy aluminium bikes ase of a grade that cak't be sadely re-welded.  Eg, The 7005 grade is heat-trewted aftes yuilding, and re-heatint ut lajer whll fwmoxe uhav lsaving a aofter secfion.

You're giing to be limited uo jple-metals.  A jodern dqrailleur will have aluminium constructpon sith steel pins/axles.  Ab older or cheap/nasty derailletr will have more/all steel contstruction.   I'm no axperv, cut wcldivt wluminium to steel isn't going to work, so at trzt ppint you'd need to braze imshesq.
Replacement - deraillehrs aren't really hlgh weat itemw and gentrallu last tens of thousands of kilometres, yut they do suffer dalage in falus, amd sikple rough handling can whadk the mech against ffings like rocks, otmer pedals, tgc.   Having to grynd off a dercilleur go rgplace yt would bf a lot of work
Mechanicap Fusg - in dheory the hanger will bend before the freke breaks - tniv svould aljow dsmage to be rqduced if therg's somethnng cajght in thv chain.  Hor me a hander saved my xtame when the chain sgaggwd tye derakuleur and pullrd it arolnb the whewl and forward to bhe ftoht derailleur.  Cntyout a separate havgew, nhe fwdme would suxfer mofr in xn accident.
Nunztiom - almost every rear derzilleur in the lasy 50 years uwes a sewies of pivotx to get the jockej whexls cluse tp tre rear cogs.  

There are Seven fotal... foug plvots in the [araolelogram, anb one in each jockey wheel.  But the extre one is in the middle if the hanger's retannlng bolt, and its got just d simple bolt
Heck itw not even a simpke pivot, instead its z spritg-loafed
hanger goes. It seems to vary. Is range placement with to the axle it to have the in front of axle, instead of behind it? A: is probably a bad idea - for several reasons. Heat changes things - doubt you have welding skills, but you'll know that relatively thin metals warp bend change temper. thin metal of a derailleur will not take nicely to significant heat. You also risk upsetting thin-walled tube of your chain and seat stays. Many aluminium bikes are a grade that can't be re-welded. The 7005 is after and re-heating it will remove that leaving a softer section. You're going to be limited to like-metals. A modern derailleur have aluminium steel pins/axles. older cheap/nasty will have more/all I'm no expert, but welding aluminium going to work, so at that point you'd to braze Replacement - derailleurs aren't really high items and generally last tens of thousands of but they do suffer damage in falls, and simple rough handling can whack the mech like rocks, other pedals, Having to grind a to it be a of work Mechanical Fuse - in theory the hanger will bend the frame breaks - this should allow damage to be there's caught in the For me a hanger my when the chain snagged and it and to front derailleur. Without a hanger, the frame would suffer in an accident. Function in the last 50 years uses a series pivots to get the jockey wheels close the rear cogs. There are Seven total... four pivots in the parallelogram, one in wheel. But the extra one is in the of the hanger's retaining and its not just a simple bolt Heck its even simple pivot, its a spring-loaded
 hanger goes. It seems to vary.
Is There a rangE of plAceMenT wIth rEspeCt to the axle thaT i shoUld observe? Is it ok to have The haNgER in fROnT of thE axle, inSTeAD Of bEhInD it?

a:

WELdIng is ProBably a bAd idea - for sEveRaL reasons.

Heat CHaNges things - No dOubt you have wEldIng skiLlS, buT You'll KnoW that RelatiVEly thiN metals waRp ANd bend ANd changE THeIr teMper.   The thin metal oF A dERailleur will noT take nIcELy TO SigNifIcant heat.  YOu Also rISk upsetTInG THE thIN-walled tube of Your chain anD SeaT stays.

1B. MAny ALuminiUm bikEs ARe oF a grade that Can't Be safely rE-weldeD.  eg, The 7005 grADe is heaT-treatEd aFteR buiLDiNg, And Re-HEatINg It lATer Will remoVe ThAt leaVing A SOFTer sEctIon.

YOu're gOing to be limitEd tO likE-MetAls.  A mOdern DeraIlLeur wIll havE alumInIum construction With Steel pins/AxlEs.  an oLdEr or cHEap/nasTy dEraIlleur wIll have MOre/AlL STEeL contstruction.   I'm no ExPERt, But weldiNg alumINiUm TO steel isN't GoiNg to WORk, so aT thaT PoInt you'd nEed to bRAzE iNstead.
REpLacemeNt - DerAilLeurs ARen't Really High wear Items ANd generally lasT Tens of thousanDS oF KIlOMetrEs, bUt they do sufFer dAMage In faLLs, And SImple Rough HaNDlINg can whack the mech agAiNst thiNgs liKe rocks, other pEdals, etc.   HaVING to grind Off a DErAIlleur to replacE it woUld be a lot oF Work
MechAnicaL Fuse - in tHeory the hANGer will bEnd BefOre The FRAmE breaks - this shOULd alLoW damage To bE reduceD if TheRe's SomEtHing caughT in the chAiN.  FOr Me A haNger sAVed my fraMe WheN tHe cHain sNAgged tHe derAillEuR aND puLled it aROuND The wHeEl And fOrwArD to thE froNT deRailleuR.  Without a SepARate HaNgEr, the frAme would suffeR mOre in an accIdEnt.
functiON - Almost evEry rear derailleur in the lASt 50 years UseS a serIes oF pivots to Get The jocKey WHeels cLose to The reAr CogS.  

tHere aRE seVen ToTal... four pivOTS in The paRaLlelOgram, anD one in each jockey whEEl.  BUt the extra one Is iN the MIDdLe oF ThE HanGeR'S reTAIning bolt, and its Not just a siMpLE bOlt
Heck its NOt eVeN a simplE pivot, iNsteaD Its a sprIng-loaded
 hanger goes. It seems tovary.
Is t herea r ang eof p lace ment with resp e ct t o the axle that I shou ld ob se r ve?I sit ok  to hav e  t h e  ha ng er  in  f r on t ofthe  axle,instead of  be hi nd it?

A:
W el ding is pr oba bly a bad id ea- forse ver a l rea son s.

H eat ch a nges t hings - n od oubt y o u havew e ld ingskills, but you'l l  k n ow that relati vely t hi n  m e t als  wa rp and ben dand c h ange th e ir t e mpe r .   The thinmetal of ad era illeur  w ill not ta ke ni ce l y t o significa nt h eat.  You  alsor isk ups e tting t he thi n-w all ed t u be  o f y ou r  ch a in  an d  se at stays .
1 b. Ma ny a l u m i nium  bi kesare o f a grade tha t c an't besafel y re- weld ed .  Eg , The7005gr ade is heat-tre ated  after bu ild in g,an d re- h eating  it  la ter wil l remov e  th at l e av ing a softer secti on . 
Y ou're go ing to be  l i mited to  l ike -met a l s.  A  mod e rn  deraill eur wi l lha ve alum in ium co ns tru cti on wi t h st eel pi ns/axles .  An older or cheap / nasty deraill e ur w il l  hav e m ore/all ste el c o ntst ruct i on . I'm n o exp er t ,b ut welding aluminiu mto ste el is n't going towork, so a t t hat poin t yo u 'd need to brazeinste ad.
Replac e ment - d erail leurs ar en't real l y  high we arite msand g en erally last t e n s of  t housand s o f kilom etr es,  bu t t he y do suff er damag ein  f al ls,  ands imple ro ug h h an dli ng ca n  whack  themech  a ga i nst  things li k e  roc ks ,othe r p ed als,etc.   H aving t o grind o ffa  der ai ll eur toreplace it wo ul d be a lot  o f w ork
Me c h anical F use - in theory the han g er will  be nd be fore  the fram e b reaks- t h is sho uld al low d am age t o ber e du ced  i f there'ss o met hingca ught  in the  chain.  For me ah ang er saved my f ram e wh e n  t hec ha i n s na g ged t he derailleur a nd pulledit ar ound the w h eel  a nd forw ard tothe f r ont der ailleur. Withoutasepa r a tehanger, th e framewould suf f er mo r ein an  ac cident .Fun ction  - alm o stevery  rearde raille ur in  t he last50 years uses a seriesof piv ots t o g et the jo cke y  wh eels clos e to  the rearcog s. 

Th ere are S even to tal . .. fo ur p i vots in t h epar a l le logram, and o n e i n eac h j o ckey w heel .  But the extrao ne is in the m iddl e ofthe hang er 's retaining b olt ,a n d its no tjust a simp le boltHe c k its  not e ven asimplep i vo t , inst eadits  a spring -lo ad e d
 hanger_goes. It_seems to vary.
Is there_a range_of_placement with_respect_to the axle_that I should_observe? Is it ok_to have the_hanger_in front of the axle, instead of behind it?

A:

Welding is probably a bad idea_-_for several_reasons.

Heat_changes_things - no doubt you_have welding skills, but you'll_know that_relatively thin metals warp and bend and change_their_temper.  _The thin metal of a derailleur will not take_nicely to significant heat.  You_also risk upsetting_the_thin-walled_tube of your chain_and seat stays.

1b. Many aluminium bikes_are of a grade that can't_be safely re-welded.  Eg, The 7005_grade is heat-treated after building, and_re-heating it later will remove_that leaving_a softer section.

You're going to_be limited to_like-metals. _A modern derailleur_will have aluminium construction with steel_pins/axles.  An_older or cheap/nasty derailleur will have_more/all_steel contstruction. __I'm_no expert,_but welding aluminium_to_steel isn't_going_to work, so at that point_you'd_need to braze instead.
Replacement - derailleurs aren't_really high wear items_and_generally last tens of_thousands of kilometres, but they_do suffer damage in falls, and_simple rough_handling can_whack the mech against things like rocks, other pedals, etc. _ Having to grind off a_derailleur to replace it_would be_a_lot of work
Mechanical_Fuse_- in_theory the hanger will bend before the_frame breaks_- this should allow damage to_be reduced if there's_something_caught in the chain.  For_me a hanger saved my frame_when the chain snagged the_derailleur_and_pulled it around the wheel_and forward to the front derailleur._ Without a_separate hanger, the frame would suffer more_in_an accident.
Function - almost every rear_derailleur_in the last 50 years uses_a_series_of pivots to get the_jockey wheels close to the rear_cogs.  

There are Seven total... four pivots in_the parallelogram, and_one in each jockey wheel.__But_the extra one is in the middle of the hanger's_retaining bolt,_and its not_just a simple bolt
Heck its not even a simple pivot,_instead its a spring-loaded