Optimum manually fabricated square tube elbow joints...

[pipe2null]

I'm hoping there's a MechE or someone out there with a couple cents to spare, might point out what I'm doing wrong or what incorrect assumptions I've made.  I am definately not a MechE and am pretty new to designing/fabricating a part like this.

I'm trying to calculate the optimum cuts to form elbow joints in aluminum square tubing (Al 6063-T5), at various angles.  The important part is keeping the outer tube wall around the bend intact to keep the sections of tube aligned after bending while I'm brazing the joint.  And since I need the joint to be air/water tight, minimizing the bend radius of the outer tube wall to minimize gaps in the side walls is a really good thing...

My setup: For brazing, I'm using an oxypropane mini torch (It's just the cutest wee widdle torch you ever did see, HAH). For cutting, I have a cheapo vertical benchtop bandsaw that cuts ok-ish but the fence and guide are crap, so I'm 3D printing jigs to hold the work piece at the correct position and angle for each cut in the aluminum square tube (one for each specific location and angle = 3 per joint), and the actual cuts appear accurate enough for my needs.

I must be doing something wrong because I either get a gap between the angled cut faces, or the lowest point on the angled cut face touches first and acts a fulcrum and fractures the outer wall while bending.  Or the bend radius of the outer wall is just too small and it fractures for greater than 90deg elbow joints (partially bent back over itself).

So I'm pretty sure the main problems I have is my design of the joint/targeted cuts are just wrong or not dealing with the specific material well enough.

Quick illustration, not proportional, nor accurate, and mating faces should physically touch (but easier to see shape with small gap for illustration):
[attach=1]

I'm using 3 cuts:
- 2 angled cuts at A/2 degrees relative to joint cross section, to create the angled mating surfaces of the remaining side wall/tube top surface after the tube is cut and bent.  The angled cuts are aligned to Bz.
- 1 straight "chop" cut parallel to and centered at joint cross section but stopping short of cutting the outer wall, to remove side wall material inside the outer wall's bend radius.  For wider chop cut, I just move the work piece a little left/right over the blade to widen the cut a bit. 

My calculations so far, likely wrong, especially as rusty as my basic geometry is:
A = desired overall angle of elbow joint, where 0deg means straight pipe with no bend, and greater than 90deg starts folding back on itself.

W = tube wall thickness, in this case 1mm.

WC, WS = material (Al 6063-T5) compression zone / strain zone within the outer tube wall resulting from bending.  I am not a MechE, but AFAIK the inner side of the bend gets compressed and the outer side is strained to acheive the bend, and the actual border between the zones forms the effective arc length radius comparable to the un-bent length...  BUT, I'm not a MechE, so I don't know the right way to calculate this.  I'm guestimating WC=WS= 50% of W = 0.5mm.

RI = inner radius of outer wall bend

R = effective bend radius in outer wall bend, = RI + WC

C = chop cut width, *intended* to be equal to the arc length of the resulting bent outer tube wall.  Minimum chop cut is about 0.95mm.
C = A/360 * 2 * PI * R

Bz = z axis coordinate on joint angle bisector to aim the cutting edge at for the angled cuts.  It is not really "z axis" since the work piece is laid on its side for cutting, but it is z-axis in the un-bent illustration.  When I'm designing the 3D printed jigs to guide the cut, I align the near-edge of the cutting blade surface to this point.  Bz=0 is set at the unbent outer tube wall outer surface AKA the surface the unbent tube sits on.
Bz= (RI + WC + WS) - (C/2)/tan(A/2)

Center Point of outer wall bend radius = *intended* to be the corner between the surfaces created by the angle and chop cut.  When the tube is bent, the 2 sides of the joint should meet with this common point as the bend radius.  Emphasis on the word "intended", this has not worked out for me so far and the actual bend radius appears to be anywhere but where I think it should be, but I'm bending the piece by hand, so I can't be sure.




I've tried many iterations: Design jig->3DP->cut tube, but the results are nowhere near what I'm expecting, so either my calc's are waaay off or I've made assumptions that are flat out wrong.

Am I on the right track, do my calc's make any sense?  Any related manual fabrication pointers / math checks / etc is much appreciated.

[Robert Smith Eco Warrior]

I shall return to this and try and answer, when I don't have a stinking cold and can concentrate. Today doing maths is a non starter. I just want to sleep... 

[pipe2null]

Looks like my last post was lost during repair...  Here are sketches again.  Please forgive their crudeness...

[attach=1]
[attach=2]

[jmelson]

Well, the problem is extruded aluminum is not real homogeneous.  I make some L-shaped brackets for mounting PC boards with power transistors, and bend them on a sheet metal brake.  Each one has to be bent to a different angle so that after springing back it ends up at 90 degrees.  Not a huge difference between them, but enough that one bend setting would leave them all different by up to 10 degrees or so.

So, there is likely to be no perfect cutout that gives you the desired bend for brazing.  Maybe if the bend was then done by pressing it into a rigid form they would come out about right.

Jon

[pipe2null]

You make a good point that the material is not very homogeneous, and springback is certainly an issue.

The bigger problem I've been dealing with is that the part fractures into 2 parts during bending, not always but nearly always for greater than 90 bends, and the material is strained/deformed to the point springback doesn't occur, which is not good.  So, I went back to the drawing board, and have been attempting to calculate the ideal cuttout to eliminate fracturing the wall and still get the surfaces to align well enough to braze.

And as soon as I attempted calculating the bend, my results got worse instead of better, thus this thread.

I redesigned my jigs from scratch and have been spending the past few evenings tweaking the acuracey of the whole thing.  Will post results when I get the next set printed and tested.

[beanflying]

There is a high tech solution and a learning curve if you haven't used Fusion 360 in the Sheetmetal environment to save the maths. There is also some YouTube videos around on making round tube cutting templates for welding with Fusion too. Won't list them here but a search will get them if you are interested. Part of the nuts and bolts of the Sheetmetal environment is you can take into consideration bend radius and deformation allowances with different materials when bending or in your case you could produce a cutting template from it.

Problem with cutting and bending extruded Aluminum sections is in part as the process introduces internal stresses when it cools this makes for problems when you want to do something accurate with it later. I have a job to do with some 100x100x6mm Angle which will need facing on three sides as the faces are slightly curved and not at 90 degrees, in my case thickness isn't essential so this trim will be fine.

Fairly common to need to overbend by 5-10 degrees when working even from flat sheet when bending to get a good 90 let alone working with extruded sections.

As you have a Gas torch then hitting the area you are about to bend with some heat will help a lot to reduce the fracture problem and also if you hold it in place at your desired angle it will tend to relax into place a lot better and make your jigging easier (less reverse spring).

[Gregg]

You may have to anneal the aluminum at the bend point to get consistent results. Different alloys react differently to annealing. The classic method is to coat the area with soot from an oxygen starved acetylene torch and then heat it until the soot burns off; in your case with propane you might be able to use a candle to deposit soot.  It just has to be annealed at the bend point for your purposes.

[Ian.M]

If you've only got propane and cant get a sooty flame, its probably easier to use the soap method:  Streak across the aluminum with the corner of a bar of white soap.  Heat till it turns mid-brown.  Dark brown or black means you've overheated it.

[Gregg]

Personally I would miter cut the tube into two pieces and jig them together in a smooth jawed vise to keep them aligned and forget about bending them.  However, calculating the exact kerf necessary for bending makes for interesting problem solving.

OTOH I would also make the fit as close as possible and tig weld without filler metal after pre-heating and use a back purge of argon.  But I can appreciate those who utilize what they have available.

[coppercone2]

if aluminum is getting funny with you prior to bending, you need to heat it up. When I need heavy aluminum bends I soot it up and heat it up until the soot is gone then do the bending, it gets rid of the weird surface skin ripping that happens when you bend aluminum.

When you work on the joints consider thermal mass, the outside cools quicker then the inside I think, but I won't get into your work there because aluminum brazing gets me into a fit, I need to try to buy a high quality aluminum braze, based on the videos I watched there are 'bad' brazes, I might have had a bad braze... but I would almost rather weld it up then try to braze it anymore lol. I basically learned to associate brazing aluminum with a bad time.

[pipe2null]

Thanks for the feedback.  Sorry it took so long to reply, been having various problems, including some connectivity issues to the forum, plus waiting for parts to get delivered...

I spent some time tweaking my 3DP'd jigs to work better with my cheapo bandsaw, generally improving dimensional accuracy of the cuts.  For the wider squared chop cut, I discovered that using screws as guide pins for the jig are not a good idea.  The screws LOOKED perfectly vertical, but they weren't, and were reducing the range of motion for the work piece against the blade, reducing the width of the chop cut.  I ordered and redesigned using steel dowel/alignment pins and now the accuracy of the width of the chop cut is within about 0.1mm.  My depth of cut is slightly too shallow, but the thickness of the extruded aluminum tube wall material is also variable, so I'm leaving it as-is and will file if necessary.  Also, I'm nervous about tightening my cheapo bandsaw belt-blade tighter than it is, so I have to start the angled cuts much slower to keep the material from pushing the blade sideways (second picture has inaccurate angle of cut due to this problem, but the first pic is dead on 45.0 deg cuts).

Here is the un-filed 90 deg cut with the calculations from earlier in the thread:
[attach=1]

BUT, when bending, instead of a nice long rounded bend that would use all the tube wall material I provided with the chop cut, INSTEAD physics is smacking me on the head by bending almost entirely in one spot with a significantly smaller-than-expected bend radius, resulting in an "L" shaped corner instead of a "C" or circular corner.  And since the actual bend radius is too small, it is starting to show strain fracturing at the surface which will likely lead to complete fracture for bends > 90 deg.
[attach=2]
[attach=3]

Unfortunately, my 3D printer decided that it would wait no longer and is demanding the long overdue overhaul I've been promising it for months...  And it decided to go on strike in the middle of printing my jigs for a 122 deg elbow, so it will be a few days before I can try that joint (that I actually need for the part I'm fabricating, not just for testing).

Stuff I'm doing/trying next:
- 3DP overhaul
- annealing prior to bend
- 122 deg joint once my printer is happy and compliant again...

Questions:
- For annealing, is it necessary to let the part cool after heating, or can I just preheat the part same as prep for brazing then bend and braze in one go?
- I've heard of the acetylene soot method, but I went with an oxypropane setup since I have enough ventilation to deal with the carbon monoxide from propane but not enough for soot, so I'm stuck with what I have.  I didn't know about the white soap method, I don't have any bar soap but simple enough to go get some.  Apparently you can use a shapie marker for the same thing (?!?), but I'm never sure how good or sketchy suggestions are from random internet sites, but a forum like eevblog with lots of enginerdy types (like myself) tend to have solid recommendations.  Are there any other tricks for annealing aluminum I might try, pref without cleanup between annealing and brazing steps?
- Slightly off topic, but any recommendations for flux for aluminum brazing air/water-tight joints?  I just started brazing the past couple months, been using the benzomatic brazing rods without flux.  Been getting mostly air tight joints but often have pinhole voids that I don't detect until I test the part with compressed air.

@beanflying: I did a quick look at the sheet metal side of fusion, not sure how to apply it to the jigs im using (see 3rd pic).  I'll check out some tutorials and if I have any Qs, ill bug you in the fusion thread...   


Thanks again @all for the feedback

[coppercone2]

i got haris aluminum brazing flux but it worked like shit but I think my rods are bad. you need to clean inside of that bent joint making this extra hard because you need to clean it then bend it and then braze it right away, the main thing with brazing or welding aluminum with anything but tig is that you need to clean it off before you start. The Al braze flows like old dog shit in comparison to silver braze or anything else , at least some types. I recommend that is Al-soldered if its strong enough and just forget the braze because that one actually kind of flows. half the time aluminum braze fakes you out not giving you any penetration or penetrates but only wets on one side. If you want to upgrade to try to acetylene weld aluminum well your in for a nasty surprise its really hard too.

maybe you can use a really fine stainless steel brush to get in there and clean it up, like a connector brush that Caig sells, or maybe put a dremel stainless brush in there on a angle or try to do this with scotch brite strips and then flush it with alcohol. you need like sand paper shims or something weird to clean that up after its hot bent unfortunately if you even want to attempt at good brazing and not the ones that look good then break when you tickle them.

The best results for aluminum I actually have are.. using aluminum solder with the aggressive flux meant for it (fluroboric acid). Is solder strong enough? I highly recommend you try to solder it with nice liquid flux then any god damn powder concoction harris sells for impossible braze work. Its alot weaker but I swear I actually saw this stuff flow before!

Now I heard really good things about 'muggyweld' brand brazing rods, they have detailed youtube videos that actually make it look like they work well, the problem is they are really expensive and I would rather buy supplies to practice more gas welding then fool around with that, because it seems like the money wasted on that seems to produce some kind of slow increase in skill that I can appreciate (i am sure some people can find great applications for being good at aluminum brazing but I am not one of them so it seems really esoteric).




the first one he shows, the solder, actually works like that when you get similar things from the welding store, the 2nd part seems like black magic to me.

seriously i can weld up a hole in aluminum, weld up a box, etc with the torch... usually the main problem is cosmetic appearance, but it at least holds together somewhat well, despite too much metal usually being added, not 100% penetration along a long weld joint and other problems for the somewhat neurotic. the braze has me cursing and screaming in 5 minutes when it does not work for no apparent reason.

btw I have SERIOUS doubts about hardware store aluminum, if thats what you are using, I think it has demons that make welding or brazing even harder then it should be. When I got scrap real aluminum from a shop, when I did 2 plates together it actually looked like nice and 'buttery', when I was killing myself trying to figure out why I can't do anything with the homedepot ones. Nothing really seems to help it, including sanding, stainless brushing and even strong concentrated Alumiprep dips and stuff. It has something to do with the metal itself. IMO that crap is not suited for anything more then soldering or gluing. Maybe if you did a real heat treatment on it according to some specification in a heat treatment oven it would be right, but it might be something with the composition. As soon as you get into dissimilar thermal masses you will be most likely crying for a TIG.


May I recommend.. mild steel? . it actually lets you do what you can imagine you can do. aluminum is like throwing a party in the police station parking lot. you can use that stuff for bolts and epoxy IMO. oh yeah and you can't use magnets to fixture aluminum, its trash, if you need to go through that kind of effort get stainless, its not worth it unless your doing something with an airplane

[beanflying]

Thanks for the feedback.  Sorry it took so long to reply, been having various problems, including some connectivity issues to the forum, plus waiting for parts to get delivered...

snippity

@beanflying: I did a quick look at the sheet metal side of fusion, not sure how to apply it to the jigs im using (see 3rd pic).  I'll check out some tutorials and if I have any Qs, ill bug you in the fusion thread...   


Thanks again @all for the feedback

Mainly for learning about how to take into consideration your bend radius and the material needed for that bend. The basics are that Aluminium in this case I picked 6061 as it is common has a K (roughly speaking a bend factor) so to make a 50x50 angle you actually need less than 100mm of material as per below if you consider it as a flat 25x3mm strip. The case below is for a typical 25x2mm wall Box in aluminium.

So the problem becomes when you add the box section and try and allow for that minimum bend you get a gap as per below which is of course no good for brazing. So you cut to ignore this bend allowance and force a sharp bend you break it

So you cut to a sharp mitre at the lower face but then you introduce relief cuts to allow the aluminum to bend back away from that sharp mitre. Net result is closer meeting faces and much easier brazing. You will need to compromise a little as your bandsaw blade has a kerf but this is the general idea so aim for something like the practical option below

[coppercone2]

what gap thickness do you want for aluminum braze anyway? Silver braze is pretty specific at ~0.004 inches. I never saw a number because no one takes it seriously as a structural material, its only really detailed in catalogs about fixin radiators. I know the goal is to get a solid really cool looking aluminum scafolding up with the most minimal tools, thats the only reason I tried so hard with it. It flows so bad.. that you might need a huge gap. I do see the appeal with a small bench saw or hack saw to make somethin that looks like a roller coaster.

[beanflying]

All of those questions are to a degree how long is the string ones. Different Brazing rods, flux or no flux, material size you are actually working with and on and on with the variables.

Adjusting the gap to be larger is the easy bit reducing it without breaking the back surface is the key to the OP's original problem and also getting your head around bends in metals are not sharp things of precision but involve deformation, stretch and compression.

If you were after better weld strength across the joint then V grooving the two sides will make sense and as it will increase surface contact of the braze as well as ensuring good contact with both sides you want to join. Cosmetically you can then file/sand/grind later back to smooth.

**Pic below is not the same model as above I didn't save the file.

[pipe2null]

3D printer overhaul complete, made jigs for a 122.3 deg elbow joint, tried out "annealing", and did first attempt at brazing with (Harris) aluminum brazing flux.

FYI:  This is for 8mm x 8mm square aluminum tube with 1mm walls.  The 8x8 dimensions are pretty close to exactly what I need, aluminum is a good material for thermal conductivity, and the mostly flat sides of the square tube gives a nice surface for heat transfer.  The parts I'm making are not structural, but I need to make sure the braze is good enough for the joints to take some minor abuse without leaking.


I tried out the "sharpie permanent marker" trick for annealing aluminum, just mark up the bend with the marker and torch it until the marking fades to nothing.  I have no idea if the part reached actual annealing temp, but I bent it while it was still hot and the bend did not have the surface fracturing it used to, even for the 122.3 deg elbow joint, so tangible improvement there.

The facing surfaces of the joint still end up too far apart for a decent braze.  First, the material tends to bend only at it's weakest point which is rarely in the middle of the intended bend.  Even when introducing axial force to try and get the bend to spread across the whole intended bend radius, it either pinches in one small area or the bend ends up being too shallow (as if  the radius was much larger than intended).

Basically, because I'm manually/air bending (not sure term for bending without a specific tool/brake press/etc), there is nothing to force the material out into the corner of the joint so instead of the intended nice rounded circular corner, there is one or more small radius corners and portions of the bend that are close to being straight.  Basically, uneven bending that separates the joint faces outward but still bends the intended number of degrees...  There are a million different off the shelf clamp/jigs for 90 deg joints, but roughly half the joints I need are weird angles like 122.3 to get the tube ends to align with other parts.


@beanflying:  I went through some Fusion tutorials on the sheet metal CAD tools.  Now I finally get what you were saying, and that was a good suggestion to use flanges to model all of my part's joints and bends, and then use measurements from the Flat Pattern to get the adjusted lengths and offsets of the joints.  I still need to figure out what my bend radius will be for the material I'm using, maybe even be fancy and try to calculate a reasonably accurate K factor, but then I just need to update the Sheet Metal Rule.
For your suggested target cut patterns, I think any of those would solve my remaining problems fabricating these joints, but I do not have the means to make the horizontal cut along the inside of the wall, I think the term is "relief cuts", if my understanding is starting to match what you were saying.

@coppercone2:  Yup, I just tried out the Harris aluminum brazing flux powder (cheap and quick delivery).  I'm still very new to brazing so I probably was doing something wrong, but my brazes got worse instead better using that stuff, at least when in powder form and dipping the rod in.  Next time I'll try and make up a paste and see if that does any better.  Thanks for the suggestions, if I have no luck with other things, I'll probably give the more pricey "easy aluminum brazing" stuff a go.


So...  Next up I'm going to try smaller bend radii with the psuedo-annealing permanent marker trick and see what I get. 

[beanflying]

I hadn't really caught on that you were using that small a section  So there will be no way of cutting a relief with a sawblade at that scale even with say a jewelers fretsaw really. The best compromise might be a simple mitre to a fairly crisp point at the back face then drop a thin file cut to make a square gap.

Quick and dirty model below with some idea of dimensions is now at 8x1 with Aluminium K factor. Fusion all but ignores it but is giving a 2mm OD Radius for the back. I would think the Aluminum braze should be able to fill the small gap near the back without to much issue.

[coppercone2]

btw have you made a test lap joint on something similar to see the strength?

and have you tried cleaning the rod? with my solder, using stainless wool on it right before use helped drastically. I have a feeling those rods are coated with some kinda shitty flux, and maybe the harris flux interacts with the flux on the rod to do something bad, or the rods are just oxidized. I never really thought of this before and always used the shitty rods I got from the welding store, that are flux coated, but I am thinkin maybe what happened is they are mad old and it oxidized under the flux and it would benefit more from cleaning and using external flux then hoping the surface is good.

[pipe2null]

So I tried the same cutting strategy as before but with a much smaller (0.25 * Wall thickness) inner bend radius.  Previous inner bend radius was 1.0 * wall thickness.  The result left the facing surfaces almost close enough for a decent braze, but not quite good enough yet.  Heating the joint with the permanent marker trick and bending while it is still hot left the 90 deg joint with no surface fracturing but the 121.3 deg joint had a little due to the much smaller inner radius of the bend.

The main problem keeps coming back to not having anything to force the wall material out toward the corner of the joint and as result the material bends unevenly and more or less unpredictably...  Well, technically it is predictably a crappy bend.

I'm thinking my best bet is to redesign the cut to purposely result in a CRAPPY joint.  Basically, design the joint as 2 very small bend radii and a straight section, and adjust the chop/mitre cut accordingly.  If I do it right, should be able to use the inside edge of the facing surfaces as a fulcrum, but can I do it without fracturing the part?!?  Heh.  I guess I'll find out.


@beanflying:   Heh, yep, it's a pretty tiny section.  A couple of the parts I'm trying to make with this aluminum square tube use about 150mm of (uncut) tube with 4  joints each, and the adjacent joints toward each end of the tube are close enough to each other that brazing one joint is impossible without the metal filler material in the other joint melting.  Well, "impossible" might not be completely accurate, I picked up some tiny clamps to try out as heat sinks between the joints, but outlook is not good on that front.

@coppercone2:  The current quality of my brazes are not up to the point where doing a strength test is worthwhile.  I still have practice and learning to do, like how to better clamp my work piece so gravity quits pulling filler material out of the joint, but that should improve when I get the cut dialed in.  I've been using 2 pneumatic tools for filing and buffing.  They look like 2 really oversized pencils will a little tube connected to the air compressor, but those tools are fun to use.  The reciprocating one makes very detailed filing easy and somewhat quick with a tiny diamond file.  The spinny one (that's a technical term BTW) I'm using with a dremel-sized wire wheel, which works really great until the wheel gets a tiny bit off balance and the high RPM of the tool spins it fast enough the steel shaft just bends in half pretty quick. Which is exactly what happened yesterday. And since I'm still waiting on a new pack of wire wheels, I didn't try the Harris flux as a paste yet, but that's top of the list once I can effectively (AKA lazily) wire brush the work piece.

[coppercone2]

yeah, just be sure to make sure that the braze is possible in a split piece that the rest of the project is viable, I just have a feeling you might be disappointed when you get the whole bending setup configurable/dependable you will find that the braze just does not flow how you want it and have to switch technologies

I would strongly suggest getting some angle piece samples into a vise and work on getting a good braze to figure out what space you need, and if you need to invest in other rod. Substitute the bend for a right angle clamp and see if you can get good results IMO. This stuff had me fuming mad before I realized that I have other technology I can use (I was obsessed with filling a hole in a PCB alignment light I made, I could not get the braze to make it 'nice' to mushroom over, but it filled the hole, so I had to epoxy* on top and sand it down.)

If you have the angle pieces and a saw setup to make them, try to braze 2 long ones together and cutoff the bad material after each attempt until you figure out exactly what you need.

or try to braze 2x hollow rectangle pieces together so you can cut and inspect the insides to see if its doing what you are thinking its doing, you might be surprised when the nice top side joint looks like on the inside where you think it wet.

Also when you are brazing or gas welding aluminum, be SERIOUSLY careful using files on it, because they will 'blend' the material over the hole and make it look great, but there might be major posority and a weak joint inside. When you do it, you need to get aggressive with a sharp pick to see if you can pry anything off after blending or using a die grinder like you are using, it also does this thing (imaging smearing mayonnaise on a piece of rye bread with a hole in it, looks solid right?). When you start gouging and poking it, and trying to 'claw' into it with a heavy right angle pick, you will learn the truth about the joint. I would try to braze a right angle to a right angle then flip it over after washing, cut that bit off, then start messing with that. I had Aluminum blending disks, Aluminum specific file and carbide aluminum burrs (I only buy decent quality ones from reputable welding vendors, i.e. walter, radnor, not the ebay ones, so its not a question of the tools being bad) all do the same thing where they 'hid' bad weld joints.. the only thing that makes them come out with out aggressive chemical etching seems to be a sharp steel pick and some aggression. With gas welding you can get these 'buttery' looking welds (like you poured butter in there and it solidified) that are pretty reliable optically, but anything apart from that might either be good weld with crap on top or a bad weld with crap on top and then your air tools or whatever will just 'stuff' it with displaced aluminum that has no structural strength but looks good/is polish able. If you keep on blending it with proper blending techniques (stepping down the grit), you might eventually see a kind of micro fissure show up that indicates the truth about the joint, but it takes some work to get there, the coarse easy grits all seem to hide it very well, so I recommend the minimum for flattening then picking at it hard. Its like the fucking devil compared to a normal metal joining process. Almost bought a tig but decided I might as well try stainless and I am happy, its much more to my liking/skill level/tools.

[G7PSK]

I have found tghe only accurate way is to make a complete cut allthe way through the section , bending can cause twisting and fractures, so often as not you end up welding/brazing the outside corner anyway. So I just go the whole way and jig such joints up, it's quicker in the long run and gives greater finnished accuracy.