JamesF's observatory build

[Gina]

Ah yes, I remember now that you're making a much bigger scope room than most.  Mine is 2.4m square with one wall around 300mm and the other something like 600-700mm.

[JamesF]

Just had a rough tot up and I reckon I'm going to put about 80kg of steel on the roof plus the timber for the top and sides, plus the roof covering.  I'd guess the total weight will come in somewhere around double that.

James

[Gina]

That's not too bad.

[JamesF]

No, I thought it was pretty reasonable.  It may get a little more than that, but I can't see it getting as high as 200kg.  It's going to sit on six wheels rated for 250kg so that certainly won't be any bother.  In fact I'm now wondering whether I should just stick with 4 wheels.

James

[Gina]

4 wheels should be fine and avoids the problem of getting 3 wheels in perfect alignment.  I think my roof weighs something like 200Kg - it's a very solid wooden construction with a 4x2 framework and some 6x2 uprights.  It has 4 wheels of 120mm diameter.

[Astrokev]

I ended up going with 6 wheels and didn't have any trouble aligning them (I expected it to be more difficult than it was). The middle wheel was to provide more support, to prevent sagging of what I considered to be too big a timber span between the corner wheels, rather than to reduce the load. A steel frame may be more resistant to sagging with careful placed bracing.

[JamesF]

If I go for two wheels each side the distance between them will be 1.8m, 0.9m from each end so the roof tends to counterbalance its own weight across the wheels.  With a little bracing I can't see the steel going anywhere over that short a distance.

James

[Davey-T]

Something like this maybe, only needs to be thin steel, easier than trying to fit triangles in the joints.

Dave

[JamesF]

Rethought some of my roof design overnight, so made up a new cutting list this morning and then spent a happy afternoon with the cut-off saw turning 40-odd metres of 25mm square steel tube into this:

Almost everything is cut to size with the exception of the horizontal bracing for the "rafters", a horizontal member to go between the uprights at the warm room end, and the bits I need to extend the bottom rails (my roof being 3.6m long and the steel coming in 3m lengths).  The latter two I want to cut to size once I have most of the roof made up.  The first I've not cut yet because I want a 30 degree angle on the ends and my cut-off saw only goes from 90 to 45 degrees so I need to make up some bits to allow me to cut at the angle I want (or just cut it freehand with the angle grinder).

The next step though is probably to make up some jigs so I can weld everything together at the correct angles.

Pleasingly thus far I have very little waste material.  I have a few 100mm offcuts that I will in fact have a use for and three 300mm-ish lengths, but other than that all the waste is just small pieces where I've squared up ends or cut an angle.  Anything decent that's left over I'll probably use up for bracing once I have the main frame all put together.

James

[Astrokev]

Looks great. Look forward to seeing it welded up.

[JamesF]

My friendly builder visited this morning to do the plastering for the repairs I've made in my office ceiling and the utility room ceiling from our last two plumbing leaks so what with the clearing up and suchlike I didn't get started until well into this afternoon, but as I haven't done any welding for about two and a half years and I'm using an unfamiliar welder (and a MIG at that, whereas I've previously mostly used a stick welder) I thought I'd best get a bit of practice in.  I've spent a happy couple of hours drawing with a bead of weld on bits of scrap steel and suchlike, finishing off by joining two of the offcuts left over from last weekend.

It's not desperately pretty, I have to admit.  Only doing 25mm of weld before you have to change direction doesn't really give you a lot of time to get your eye in when you're not used to it.  I completed three of the four sides however and then ground the weld flat.  I can't flex the join and there's no sign of a break so it looks like I'm getting reasonable depth in the weld and I think I can be happy with that.

James

[Gina]

Sounds good.

[Alan White]

1 hour ago, JamesF said:

My friendly builder visited this morning to do the plastering for the repairs I've made in my office ceiling and the utility room ceiling from our last two plumbing leaks so what with the clearing up and suchlike I didn't get started until well into this afternoon, but as I haven't done any welding for about two and a half years and I'm using an unfamiliar welder (and a MIG at that, whereas I've previously mostly used a stick welder) I thought I'd best get a bit of practice in.  I've spent a happy couple of hours drawing with a bead of weld on bits of scrap steel and suchlike, finishing off by joining two of the offcuts left over from last weekend.

It's not desperately pretty, I have to admit.  Only doing 25mm of weld before you have to change direction doesn't really give you a lot of time to get your eye in when you're not used to it.  I completed three of the four sides however and then ground the weld flat.  I can't flex the join and there's no sign of a break so it looks like I'm getting reasonable depth in the weld and I think I can be happy with that.

James

James,
as long as its sound, who cares what it looks like as you have a grinder to 'finish' things.
Look forward to the roof taking shape.

[Gina]

My observatory build is pretty rough in places but it works and looks alright from the outside so IMO that's all that matters.

[JamesF]

Doesn't feel like it's a lot to show for a day in the workshop

Whilst the welding of the cut ends at the top of each of the four "trusses" went well, the same could not be said of those for the crossmembers where the side of the tube really didn't want to take the weld.  Even applying the grinder to the side of the tube beforehand didn't really seem to help much.  The metal laid down looks badly contaminated, presumably by whatever it is coating the outside of the tube.

I need to find a better way to clean the surface before I continue.  I may even grind down the crossmember welds to see how they look and make them again if necessary.

James

[Davey-T]

What power welder and what gas are you using James ? the coating won't stop it penetrating.

Dave

[JamesF]

Just using CO2.  Without going to the workshop to check I can't be certain, but I think the welder is 130A.

Welding the cut ends was absolutely fine, but on the sides of the tube it looks like a textbook picture of a contaminated weld, I assume because of whatever coating is on the steel burning off.  The weld still takes nicely on the clean cut steel, but really struggles on the smooth side.

James

[Davey-T]

Ah, never got on with CO2, if you were a bit closer I could lend you a bottle of Argon, works much better.

130amp should be plenty.

Dave

[JamesF]

For what it's worth, the coating isn't what I'd recognise as mill scale.  It's more like very fine soot, in that it turns anything that touches the steel black.  I assume it's some sort of anti-corrosion coating.  It might even be slightly oily, but I'm not sure.

James

[JamesF]

I emailed the vendor to ask what the coating is and what I need to clean it off.  Impresively they have already responded saying they'll find out and get back to me.

I might have a go with some isopropyl alcohol tomorrow anyhow.  Or even just washing up liquid.

Not sure what I'll do with the waste if I do get it clean though.  This stuff seems like there's enough of it to turn the entire south west black

James

[Davey-T]

Most steel comes with this black stuff on it, I think it's just as you say an attempt at temporary rust proofing but may be residue from the manufacturing process,  you can buy bright steel without it on which either means it's been cleaned off or added on at the steelworks.

They use charcoal in the finishing process so I guess it could be residue from that.

Dave

Edited September 9, 2018 by Davey-T

[saac]

It is a corrosion protection - it should be removed abrasively prior to welding.   Cold bright steel is simply black bar  that has been given further processing, generally such as cold rolling, to improve dimensional tolerances. Black steel has no further machining done on it so is not as dimensionally stable as bright steel - generally used where dimensional tolerance is of less of a concern.  Easiest way to remove the black coating is to use an angle grinder and surface skim it. 

Jim 

[JamesF]

This afternoon I found I could get rid of the coating and together with turning the wire speed right down everything started to go right again.  I remade a few of the welds I was less happy with (possibly unnecessarily) and then continued with assembling the rest.  Not a huge amount more is done, but it's moving in the right direction.

They actually feel quite light.  I can comfortably pick them up one in each hand.

Hopefully I can get some more done tomorrow.  I need to join these two bits together, but I think I might make the vertical sections up next, then fix these two in place on top of them before putting the remaining horizontal sections in.  Welding will have to be done outside however, so it's a bit weather-dependent.

James

[JamesF]

Got most of one side made up today, but lost a couple of hours due to a swimming club committee meeting.  I hope I can get the rest finished during the week though there seems to be so little time.  I'm going to make up both sides and then clamp them together at the correct spacing for the rails, then weld the rest of the roof to them.  It's a bit more of a faff to do it that way, but ensures the bit I really need to be spot on is actually correct.

I also sorted out how I'm going to mount the wheels

Which reminds me, I must order some more bolts, washers and nyloc nuts (which I'll use instead of the plain ones in the photos).  I might get them in stainless, come to think of it.  Might be useful if they didn't corrode in the damp.

James

[Gina]

SS definitely!!