Metallury help needed for a MYOG Flash 30

[Tad Englund]

I needed a well fitting pack that could hold up to 40 pounds of fish, while fishing in Alaska; but would be small, very light and stay out of the way, handle gale force rain; as I travel up to 5 miles up and down the river, all the while fly fishing for salmon (without taking the pack off and wearing full waders, wading jacket and wearing or carrying enough clothing to stay warm in a 30* to 40* mild typhoon).
After many years of trying many different types of packs, I decided to try one more option:
I made a Frame for a REI Flash 30 pack I wanted to try out. I needed the frame for support when carrying fish back to camp at the end of the day (sometimes up to 40 pounds of fish) and a frameless pack just doesn't cut it given the conditions distance walked and the extra weight.
I made the frame out of some Aluminum 6061-T6 Bare Drawn Tube 0.25" x 0.049" x 0.152" .
I made a form (seen in the picture below) and screwed it to a piece of plywood. I then slowly bent it around the form putting drywall screws in the plywood to hold it in place as I went along. After I got the whole thing to fit the form I heated it up with a Mapp gas torch and then dumped some water on it to "temper" it so it would maintain its shape.

The frame broke at the bend (see the duct tape at the bend)after carrying a large load of fish. I didn't notice the break until I was emptying the pack for the day to dry out.
What caused the frame to break? Should I have heated it as I did the bending. Was the aluminum compromised by bending or was it something else. I don't think it was the weight because even after the fix the other side did just fine.
I used 2 pieces of wire hanger inserted in the tube, pushed it back together and duct taped it.
I want to make it again, but the last time it was sheer luck I found the hanger. I don't want a failure again.

Here is how I fitted the frame to the pack:
Opened the seams and sewed in some web loops:

At the bottom I just folded over some webbing and sewed it to the inside of the hip belt. In a place that I wouldn’t feel the frame:

I attached Velcro to hold the pack to the frame top. I left the ends long because while constructing I didn't know the exact needed length. I did this the night before I left and didn't get around to cutting them to length. Now I'm glad I didn't becasue I have to remake the frame and the needed lengths might be a little different.

I also bent the frame to match my back. This worked very well and the fit of this pack turned out excellent. I needed a pack that I could wear all day while fishing. I usually carried about 7 pounds of gear, supplies, fish gear, extra reels and line, 3 fly boxes of heavily weighted salmon flies and food. Other than the break the pack worked the best of all I’ve tried.

[Andrew F]

I think the problem was heating up the aluminum then quenching it. Who knows what that did to the heat treatment and could have made it quite brittle. 6061 with a T6 heat treat should be able to be bent 90 degrees around a large radius without requiring a heat treat afterwards. It will work harden a little but shouldn't be significant for a backpack frame. You might also want to use a mandrel bender next time to make sure you don't fold the tube when bending.

[Greg Mihalik]

6061 T-6 Drawn Tube

6061 Aluminum Round Tube, is an extruded product that is widely used for all types of fabrication projects where lightweight and corrosion resistance is a primary concern. The interior is smooth with no seam and is available in an extruded structural or a drawn seamless aluminum round tube for higher precision applications. Call us for additional sizes or the 6063 grade.

Specifications: ASTM B221 (structural), ASTM B483 (drawn), QQA-200/8, 6061-T6
Applications: frame work, support columns, gates, fencing, handrails, etc.

Workability: Easy to Weld, Cut, and Machine. Not recommended for bending or forming as cracking may result.

Mechanical Properties: Brinell = 95, Tensile = 45,000 +/-, Yield = 40,000 +/-
How is it Measured? OD (A) X Wall (B) X ID (C) X Length
Available Stock Sizes: 1ft, 2ft, 4ft, 6ft, 8ft, 10ft, 12ft, 20ft or Cut to Size

[Colin Krusor]

Tad, why not try straight aluminum or carbon tubes that meet at 90 degree angles? You could probably find Delrin or aluminum fittings for the joints, or you could fabricate fittings, or you could just cut the tubes to butt against eachother and epoxy it with some kevlar roving (w/epoxy) wrapped around the joints. These options would be heavier than bent tubing but much stronger.

[Tad Englund]

Colin, that is a great idea, that way I can still use the frame I made and not have to buy a new one (saving the cost and shipping).
But I don't have a source for the kevlar roving (I assume I can use normal epoxy). I don't need/want to buy a whole bunch for just the two bends in the frame. I could probably use two 4"x4" pads or smaller.
Is there a source for a small amount that you (or anybody) knows of?

[Greg Mihalik]

Fittings?

[Colin Krusor]

Tad, kevlar roving (or "tow") is just a bundle of parallel fibers. It is usually available in small spools on ebay, but I don't see any right now. I do see small amounts of Vectran and carbon tow for about 5 and 10 USD, respectively, and they would work just as well.

My suggestion would be to hang six foot lengths of tow from tape in a doorway, use your fingers to saturate them with slow-curing (4-hour+) epoxy, and wrap them around the tubing joints until you have a good bundle. If you want it to look more polished, you could lay up a piece of epoxy-saturated satin weave (8-harness) carbon cloth over the carbon or Vectran lashing. Cut strips from a plastic grocery bag and wrap the joints with them, then wrap the plastic-covered joints tightly with a generous amount of Teflon tape. Wipe away any excess epoxy that squeezes out. The plastic strips and Teflon tape will peel away pretty easily once the epoxy is cured. If you choose to use carbon tow or a layer of carbon cloth, you can sand this after the epoxy has cured and give it a clear coat. You can't sand Vectran or Kevlar to a nice finish (they just get fuzzy).

It's a bit messier and more work than bending a tube, but it could easily be done in one afternoon.

If you can find sturdy premade elbows of the right size, that seems like the simpler way to go, though.

[Jon Fong / Flat Cat Gear]

6061-T6 is heat treated aluminum. There is a very careful process of heating and annealing it to get to the T6 state. But just heating it up and bending it you will probably crack the frame. What you would need to do is look up the metal working process for 6061 and find out what temperature and times are required to soften the material to a workable state (like T0). After you get the shape you want , you would have to take it to a metal shop that would heat and anneal your frame back up to the T6 state. This may not be too hard to find as a lot of people build bicycle frames out of 6061-T6 and the have to braze the fittings together. After the braze processes, the frames go through the thermal cycles to bring it back up to T6.

Good luck – Jon

[Delmar O’Donnell]

Agree with commentary above. Heat then quench makes a very brittle metal. Which BTW is not tempering. Tempering is a subsequent step where the metal is heated slowly to a much lower temperature and held there for awhile, to restore some of the toughness it lost during the quench.

If this were my project, I'd machine some aluminum connectors to attach the tubes. Without that ability, I'd either take it to a machine shop and have them do likewise, or I'd experiment with…yes…rivet and/or epoxy attachment methods.

One method I've used to good effect, when fastening aluminum tubing in a home shop environment, is to pound an epoxied, short, tight fitting hardwood dowel (a "plug") into the end of an aluminum tube. When dry, drill center and insert a threaded nut insert. (A TNS that is wood screw thread on the outside and metal screw thread on the inside, as most are.) Now you have a way to screw a bolt into the end of the tube. A 90-degree attachment is now easy, merely drill holes in uprights and attach the horizontals that have been "plugged." I built a heavy use workbench with this construction and it has held up magnificently for a decade and still going strong. To avoid swelling wood from contact with water, be certain to use a waterproof epoxy, and coat the entire exposed end of the wooden plug.

In addition, this fab method will hide your wood plug and look "pro." If you really want to up your game, you'll need to gently dish the faces of the rod ends so they mate tightly with the rods with which they connect. That's best done with a round file. If you go this route, be certain to epoxy those plugs down into the tube a quarter inch or so, so they're not flush with the face.

Clear as mud? Good luck and have fun!

[peter vacco]

do we really want the frame equal stiffness in all directions (as with tubing) ?
or, can it be flat stock that's easy to work and fun to weld ?
if not actaully needing to keep things in a hard 90° structure, you can simply bolt the spreader piece in place and call it a day.
Aarn lets things flex all over the place, and his packs carry great.

with flat stock you can have stiffness the plane you want it on, and flex the other ways.
just a thought,

and yes of course, it needs TIG welded, but it's just lap joints, so no big deal.

if not really up against a wall weight wise, might try 3/8" stainless hydraulic tubing. stiffer than xhit, and bends (using a bender) quite sweetly. looks cool too.

keep us posted.

[Delmar O’Donnell]

Flat should work, too. If flat frame, and no TIG, then rivet the overlap and be done. But Tad may want to keep his existing cloth pack as is, which was built for round.

BTW Tad, that was a clever field fix you did, congrats, you get the McGyver award for that.

[Roger Caffin]

Hi Tad

> After I got the whole thing to fit the form I heated it up with a Mapp gas torch
> and then dumped some water on it to "temper" it so it would maintain its shape.

Er … big mistake! As others have said, never ever try to do heat treatment on aluminium alloys unless you have a large budget and a large workshop (and a lot of time). Seriously wrong idea. It's very different from steel!

Cheers

[Sam Farrington]

Tad,

+1 on Roger's cautionary advice. Leave the metallurgy (heating etc.) to the pros.

Fabricating with metals is another matter. 6061-T6 is a good metal for bending if the walls are thick enough for the strength you want. The stock is sold in metal displays in many good hardware stores.

Not clear from your statement of the dimensions what they are. Tube dimensions are not like lumber where the description is standard and just numbers can be provided. It is helpful to use "D" for diameter, OD for outside diameter, and if the number is for wall thickness, so state. From your OP, I'm guessing this was 1/2" OD, but just guessing.

Before getting too deeply into the bending process, you might want to consider just buying the U-shaped frame sold by GG at: http://gossamergear.com/packs/aluminum-curved-stay.html
It is $25 US, but might end up being cheaper than buying a DIY setup, not to mention the time saved.

If you decide to go ahead, a tube bender is critical, because it holds the walls of the tube in their circular shape during bending, so the tube does not flatten or break. The best ones I've found for home use are the Ridgid brand. But much cheaper ones are available from the online hardware companies, and so long as they are tube, and not rod benders, they should be good for a small job or two.

It is also helpful to pack the tube with fine sand before bending. Hardware stores have flexible plastic tube caps for furniture and other apps that will hold the sand in place. Be sure to tamp it down well in the tube so it is packed tight before capping and bending.

Using the above, I've bent ALU ski poles more tempered than T6, and thinner than the hardware store stock, to a radius about the same shown in your photos. Most recently, for a lightweight camp chair – thread posted this September on this site.

I'm working on a frame now made of ~3/8" OD T9 tubing that will not bend to a 2.5-3" radius, so obtained ferruled prebent elbows from Quest Outfitters, and bonded with JB Weld suggested by Dale W on this site, which is sold in Walmart and proved a terrific adhesive for ALU tubing. The T9 can be bent to a larger radius, but won't go there as it is not your issue.

Bending or elbowing the tubing has a much cleaner appearance than using Tee or L fittings, but if you want to go there, you can spend a lot of time at plumbing supply departments and stores looking for what will fit the tubing sizes you have available. There are also kite fittings, but most are made from a rubbery material that does not handle stress well, and almost never fails to disappoint. Been there. Drilling holes in the smaller diameter tubing to hold the fittings in place is not recommended for reasons you don't need to be a metallurgist to grasp. So then you have an issue of finding an adhesive that will bond the ALU and whatever type of plastic the fitting is made of – not so easy to find, given the different materials. You might have to settle for a very tight press fit.

So there are some approaches that I hope will be helpful.

[John Donewar]

I had good results using this Harbor Freight tool on 6061 aluminum tubing while building my wife's daypack.

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=52594

Party On,

Newton

[David Thomas]

If you're looking for a source of strong, pre-bent aluminum tubing, look to fishing net frames and tennis rackets at the thrift store.

[Roger Caffin]

> look to fishing net frames and tennis rackets at the thrift store.
Ooh …
MYOG UL snow shoes!

Cheers

[Redacted k]

Definitely some room to improve while maintaining the concept.

6061-T6 is very sensitive to being bent too tightly. Use a bigger radius on the bends, it looks possible based on the photos you posted. Pack the tubing with powdery sand before bending it, this will keep the tubing from developing little kinks that weaken it. Using a tube bender as shown is a very good idea.

IF you are going to use heat, use it before bending. Note the approximate locations the bends will be, heat the metal very hot, and then let it slowly cool to room temp. I think this may be wasted effort though, due to the precision available to you.

[Tad Englund]

Colin and everybody else thanks for the help. I decided to salvage the existing frame (save money) and went with the fix Colin suggested; using Vectran yarn saturated with epoxy.
I used the existing fix of coat hanger rod inside the tubing and 5-min epoxy to keep everything lined up and in place so I could wrap with the Vectran/epoxy.
As you can see be the pictures everything turned out great. I decided to do both sides (solving a potential problem before it came up on the other side). I didn’t think I needed to make the wraps too large because a good portion of the strength was in the hanger pieces inside the tubing.
Next time maybe I'll ask how to do something before a guess how to do it (causing the problem in the first place)

As you can see I had a little help from my grandson.

Broken tube

Otherside

Finished

[Delmar O’Donnell]

interesting fix! Where did you source the vectran yarn? And what brand is the white epoxy?

[Tad Englund]

Delmar, I got the Vectran yarn off E-Bay (I was looking for the kevlar by this ia all I found). I purchase 100 ft figuring I would probably need it for some other mistake later on.
The epoxy is: Loctite 2 hr marine epoxy. I like the color also.

[Vick Hines]

Tad,
Looks like you nailed it. In future, you can bend any AL tubing with nothing more than a spring that fits over the area you want to bend and a form with the radius you want … as long as the radius is not too tight. AL work- hardens Bending hardens it. As others have cautioned, annealing it properly takes some care. With a mild radius, most tubing won't get too brittle.

[Author]

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