Help with sheet metal fab

[beanbag]

Hello folks, Long time no see.
My current project is making a tube for a telescope - 14" diameter and 45" long. Despite being around several manufacturers, nobody with a > 4ft roller wants to help me out. (Or they will, for an exorbitant price). My workplace has a little 2 foot roller, so I might have to make the thing in two pieces and attach them somehow. I have a set of end caps that need to fit on both ends, so the diameter has to be exactly right. So anyway, I have a bunch of questions:

• If the roller is rated for "16 gauge", what happens if I try to bend 14 ga 6061 Alu in it? I'll either use 14 or 16 ga.
• If I go the welding route, I will need them to both do a seam weld where the edges meet up, and also to stack the two cylinders on top of each other and seam weld that too. Oh, and weld in such a way so as not to mess up the heat treatment or warp the aluminum. Can this be easily done via MIG welding, or is it something that is difficult and expensive?
• If I go the route of using screws and nuts (not rivets because they seem too permanent) what's the common way to join the two pieces together? One way is to make both pieces as cones with a very very shallow taper so one fits inside the other. Another way is to butt the ends together and have another strip of metal underneath, then a double row of screws to join them. I don't know how to make an '"offset bend" on the end of a cylinder so that one just slips a fixed distance inside the other.
• Speaking of offset bends, is there a way to make one along the straight edge of the rolled cylinder so that the two sides overlap nicely? The only other tools I have access to are a sheet metal bender - the kind where you clamp the workpiece and the front lip lifts up, and a hand powered "brake" (I think that's what it's called) which has the v-shaped blocks that press down into a V groove.

Thanks

[Paul Alciatore]

Telescopes can be and have been made with square tubes. Also with open frameworks.

You can make some square to round adapter plates to mount your end caps.

And if you persist in using sheet metal in a cylindrical shape, I would not use screws of rivets. I would use galvanized steel and soft solder.

[Paul Alciatore]

I would think a pair of vise grips with the duck-bill style jaws could be easily modified to crimp an edge for the proper overlap.

[alanganes]

If you make the tube in two sections, there is probably no need to make a full weld at the seam or to to join them together. I don't know how much weight this needs to support, but I would guess that a few evenly spaced tack welds would be plenty strong to keep things in alignment. You could then use something else (tape or a band of some soft opaque material) to make the seam light-tight if that is an issue. I don't know why affecting the heat treatment of the would be of concern unless there is just something about building a telescope tube I don't understand, which is entirely possible. Distortion would be the larger concern but can probably be managed and the less welding you do, the less likely it is to be a problem.

You state that you don't want to use rivets as they seem too permanent, but they are a lot less permanent than welds. Again, am I misunderstanding something?

If you want to make the offset bends to overlap sheets, take a look at places like Eastwood that make tools for doing autobody work. They have flanging tools for exactly that use, both manual types (essentially modified vice-grips like Paul describes) and air powered ones. I am no sheet metal fab guy, but I'm guessing that you would probably want to make the flanges while the sheets are flat, then roll them.

[Captain K]

A bead roller is the tool to make offsets. There are other ways of course but bead roller works very well. I’m not sure about bead rolling 14 ga though, might work in aluminum?

[Bob Engelhardt]

Telescopes can be and have been made with square tubes. Also with open frameworks.
[snip]

This! You are clearly in a situation where commitment to a particular design is getting you into deeper and deeper complication. A one-piece rolled tube would be easy, 2 pieces is just way too complicated. Back up & re-think.

Also, if you have to ask about welding aluminum, you shouldn't be doing it on a project of this nature. Steel is your friend. A square tube frame, covered if necessary, will give you much more control over the shape.

[strokersix]

If you do aluminum in two pieces, consider soft temper. T0? Should roll very nicely I think.

Then roll a "scab" ring and epoxy the two parts together with the scab covering the seam.

Just a suggestion to consider. I know nothing about telescope construction...

[old mart]

If you fabricate the tube, remember to allow in the design for fine alignment of the optics due to the tube dimensions being imperfect.

[tom_d]

If you are going to bend aluminum I would recommend you change the alloy to 5052. It can take sharp bends without cracking and it's great material for welding. A bit mushy to machine compared to 6061 but with some cutting fluid (kerosene works fine) you will get a nice finish.

[Galaxie]

If you are going to bend aluminum I would recommend you change the alloy to 5052. It can take sharp bends without cracking and it's great material for welding. A bit mushy to machine compared to 6061 but with some cutting fluid (kerosene works fine) you will get a nice finish.

+1 on both counts. I often use 5052-H32 scraps and it can give a really nice finish. I have occasionally had minor surface cracking (cosmetic really) when the bend radius is smaller than the thickness however.

[oxford]

OP, depending on what roller it is and how well it’s made you may get by doing 14ga on a 16ga machine but most times it’s pushing it trying to get over capacity when you are at full width of the machine.(brake or rollers).

6061 isn’t really a whole lot easier than mild steel in those operations either. Like mentioned the 5052 would probably be easier to work with especially if you anneal it, which I don’t see a problem with what you are trying to do.

14” diameter isn’t too bad as far as tight radius’s go but anneal may be the way to go for trying offset the seam especially with hand tools.

[beanbag]

I'm purposely going with 6061 because 5000 series, while being easier to form, also dents way more easily. Maybe I will have to give up doing those little offset bends?

[mickeyf]

Back up & re-think.

Not a telescope maker, but I think that if what you care about is function rather than "looks", then it has to a) hold the lenses in correct alignment at the correct distance, b) be sufficiently rigid and c) exclude unwanted light sources. I this is true then size does not matter (as long as it's "big enough'), and shape does not matter (except to hold the lenses).

I'd look for a piece of plastic sewer pipe large enough, and just think about adapters for the ends.

[darryl]

Another option is to build the tube in a hex or octagon shape. That way you are only folding along lines. Either of these will give you flats for mounting bearings for the yoke, which can simplify that- and the same goes for mounting finder scopes, eyepieces, etc. If you wish to minimize the area of the flats, go with the octagon shape.

If you fit together 2 lengths to get to the desired length, then a smaller bending brake can be used, and the junction area would be reinforced by the piece you use to fit the two together. If you work out the math for the proper balance point, then you could make the two lengths different to place the junction at that point. I'm assuming something like say 30 inches for the front piece, and 15 inches for the rear piece.

You could piece it together from 3 lengths if you wanted to- I could do that on my bender that has only a 15 inch width capacity. Nothing wrong with having two junction points- the internal bracing at two points would make the whole thing more resistant to denting and flexing.

[tom_d]

Hello folks, Long time no see.
My current project is making a tube for a telescope - 14" diameter and 45" long. Despite being around several manufacturers, nobody with a > 4ft roller wants to help me out. (Or they will, for an exorbitant price). My workplace has a little 2 foot roller, so I might have to make the thing in two pieces and attach them somehow. I have a set of end caps that need to fit on both ends, so the diameter has to be exactly right. So anyway, I have a bunch of questions:

• If the roller is rated for "16 gauge", what happens if I try to bend 14 ga 6061 Alu in it? I'll either use 14 or 16 ga.
• If I go the welding route, I will need them to both do a seam weld where the edges meet up, and also to stack the two cylinders on top of each other and seam weld that too. Oh, and weld in such a way so as not to mess up the heat treatment or warp the aluminum. Can this be easily done via MIG welding, or is it something that is difficult and expensive?
• If I go the route of using screws and nuts (not rivets because they seem too permanent) what's the common way to join the two pieces together? One way is to make both pieces as cones with a very very shallow taper so one fits inside the other. Another way is to butt the ends together and have another strip of metal underneath, then a double row of screws to join them. I don't know how to make an '"offset bend" on the end of a cylinder so that one just slips a fixed distance inside the other.
• Speaking of offset bends, is there a way to make one along the straight edge of the rolled cylinder so that the two sides overlap nicely? The only other tools I have access to are a sheet metal bender - the kind where you clamp the workpiece and the front lip lifts up, and a hand powered "brake" (I think that's what it's called) which has the v-shaped blocks that press down into a V groove.

Thanks

You say your roller is rated for "16 gauge". Is that for steel or aluminum? Sheet metal gauge thickness varies depending on material. In your case the 14 gauge aluminum (0.0641") is almost the same thickness as 16 gauge mild steel (0.0598")

As for closing the sheet into a tube, and also when joining the two I would use a backing plate with a double row of screws. Offset bends and tapers are unnecessarily complex for joining simple tubes together.