Box Section Joints

[ukracer]

Really? I don't think you get much simpler personally. If your cut is a good, accurate 45 degree then butted up it will be square. Quick check and tack and your on your way. My saw has a gage on the side for angles and it's accurate enough to make true cuts. I like mitres they look tidy too when dressed up.

I think I know what Bede means. Its cutting the outside of the 45 to the right length. I am guessing the more you do it the better you become but when we might only build a rectangle/mitre joint once a month you guys do then day in day out.

 

[Kent]

I hate mitred joints. Too fiddly. Much easier to cap off to size then butt the perpendicular member to it, leaving a small gap as relief for moving for squaring. Always cut internal members 2mm short for this reason. You can spend forever and a day building accurately from mitres.

This can only hold up if you putting those silly plastic caps in but then you will be melting the cap because they are darn near impossible to fit flush to an abutment. Putting a steel cap on the end of SHS and grinding it will take as long as a full mitre on its own and done so it disappears and is pretty fiddly, reason being the fact you still have 4 runs to put in on a cap= same as a full mitre joint.

slapping together a square frame were it is just fine to leave an open end is quicker and less fiddly reasons are 1. quicker to cut with less critical tolerances on length and angle 2. only three runs to put in.

Square to square with steel cap is 8 runs and some extra grinding plus cutting the cap plate and tacking. It cannot be less fiddly. Cutting a mitre well is 99% of things

 

[Bede]

I'm a fabricator of 20 years, not a hobbyist!

Mitres are a crap way of assembling frameworks! If you cap and flush, crack on size, one plane of your frame is taken care of, slap your other members to it, edge butted against cap, your span is bang on size and level. Leave a small gap for ease of squaring then you only need to check across the dimension of one plane. Tack, square up, tack for rigidity and job done.

With mitring you have to check and move in all planes. If you mitre exactly to size you have no gap for movement for squaring up. With the best will in the world, if you tack each corner with an engineer's square, then measure the diagonals, the frame won't be exactly square. Steel is never perfectly straight.

You'll therefore spend forever breaking tacks and relief grinding to get it square.

If you mitre undersize, you need to check both planes and adjust simultaneously to get the frame dimensionally correct. Easier to cap off and take care of one plane and just worry about the other in assembly.

 

[Kent]

I'm a fabricator of 20 years, not a hobbyist!

Mitres are a crap way of assembling frameworks! If you cap and flush, crack on size, one plane of your frame is taken care of, slap your other members to it, edge butted against cap, your span is bang on size and level. Leave a small gap for ease of squaring then you only need to check across the dimension of one plane. Tack, square up, tack for rigidity and job done.

With mitring you have to check and move in all planes. If you mitre exactly to size you have no gap for movement for squaring up. With the best will in the world, if you tack each corner with an engineer's square, then measure the diagonals, the frame won't be exactly square. Steel is never perfectly straight.

You'll therefore spend forever breaking tacks and relief grinding to get it square.

If you mitre undersize, you need to check both planes and adjust simultaneously to get the frame dimensionally correct. Easier to cap off and take care of one plane and just worry about the other in assembly.

I am also having finished my time in the late eighties and now run my own firm, it appears we trained very differently including having to swear when typing

 

[Bede]

I too run a firm. The prospect of trying to teach an apprentice to build a simple frame using mitres is the stuff of nightmares! It'd take them forever to adjust four pieces instead of two!

Mitres are for woodwork.

 

[Kent]

you know that could possibly be due to other factors at play within the firm. All the above you mention is of course equal on either joint type done by the book.

Actually I think you will find mitres are less common in wood due to jointing methods used, have a look at a door, window frame etc

 

[Bede]

Aren't you a bundle of joy!

I give an informative, in depth alternative description to assemble a box section frame, which novices may find useful. Meanwhile you chub about the word 'crap' then take a veiled sideswipe at my firm and teaching methods?

My way and your way are both accepted joining methods. I just see more sense in my way for the reasons clearly outlined. It's neater, quicker and easier in my fab shop than mitred joints. If mitring's your bag then happy days.

 

[Richard.]

There are good points to both sides of the story here. Like bede said both joints are acceptable in the fab world and preference is down to individual. Mitres are not just for wood work though they have been used in metal work to excellent standards in industry for longer than I can remember. We all like what we like. I'm a fan of the mitred joint. I can replicate it over and over to good tolerance. That's just my preference though. Like yourselves I've served my time in this background even though I do other other things now. Bede your method is one that works for you and many others I've also done things this way with a square face, gap and cap. I just find a mitre joint suits me better. It's a good point that steel is not always straight but that will effect the squareness of any right angled joint

 

[Bede]

I suppose the common type of work also dictates. We make box section fabs that require ends capping off all the time. We've always got guillotined strips in of all sizes to cap off various sizes of RHS & SHS. It's common practice and so becomes the obvious method of construction. The only mitred joints we use are for internal diagonal bracers.

My reasons for not mitring corner joints are as explained - elimination of the need for movement in one plane.

Any 3 D frame is firstly made, squared, welded and dressed as a flat-pack pair of sides, or base and top, or whichever way is most suited to make for the easiest final assembly. Again for elimination of movement. Break everything down as simply as you can and the final assembly becomes so much more easy and accurate. If you have loads of parts to concern with adjusting and setting, then re-adjusting and setting, you'll waste time, over-complicate things and end up with poorer results.

 

[mike os]

The best joint is one that is fit for purpose, if open ends are unacceptable for aesthetic reasons, then mitre or cap, otherwise just do what you are happy doing

 

[Richard.]

I suppose the common type of work also dictates. We make box section fabs that require ends capping off all the time. We've always got guillotined strips in of all sizes to cap off various sizes of RHS & SHS. It's common practice and so becomes the obvious method of construction. The only mitred joints we use are for internal diagonal bracers.

My reasons for not mitring corner joints are as explained - elimination of the need for movement in one plane.

Any 3 D frame is firstly made, squared, welded and dressed as a flat-pack pair of sides, or base and top, or whichever way is most suited to make for the easiest final assembly. Again for elimination of movement. Break everything down as simply as you can and the final assembly becomes so much more easy and accurate. If you have loads of parts to concern with adjusting and setting, then re-adjusting and setting, you'll waste time, over-complicate things and end up with poorer results.

That's it you tailor the joint to suit your work. Some people do find a mitre more difficult. I personally think it's a lovely joint when done well but like all welding it's success is in the prep. It's no good if it's cut with a grinder at a wrong angle you'll be pi$$ing about for ages. When a mitred joint is finished well and the only visual weld on show is the inside corner I personally think it's lovely tidy job. I don't find the joint fiddly for squaring it up as I find if it's out of square there is usually a gap in the fit up. Bede it would be good if you could add some of your pics to the thread as like you say it's another option people may want to use and they may want to try the joint if they can see it on some of your work.

 

[Mr shiney]

I am also having finished my time in the late eighties and now run my own firm, it appears we trained very differently including having to swear when typing

Crap is not considered a swear word.....

 

[Bede]

I'll have a gander through my work photos to see if I have any detail of corner joints.

I also notch angle joints for the same reason. Just don't like mitres at all.

 

[Bede]

This isn't strictly appropriate as the end cap on this is 20mm thick and protrudes from the end, not flush with the cross member. As far as the discussion goes though - the end of the long member and end of the cross member join at the same point though so could be mitred. This would be a no-brainer though to mitre the frame and assemble then add the tapped endcaps. Much easier to cap off the long bearers to size and square, then assemble the frame as square cut joints. The end caps are always ground and once painted you can't tell if it's capped and square joint or mitred.

 

[Richard.]

Which is fine and you do what suits you. I don't like capped off box joints really. It's too much of a faff about cutting little squares then welding all the way round them. A mitre can be left as welded and be passable as a tidy joint. A capped off box on the other hand needs to Have welds linishing on the cap to look good. Pros and cons in all walks of life.

 

[Richard.]

This isn't strictly appropriate as the end cap on this is 20mm thick and protrudes from the end, not flush with the cross member. As far as the discussion goes though - the end of the long member and end of the cross member join at the same point though so could be mitred. This would be a no-brainer though to mitre the frame and assemble then add the tapped endcaps. Much easier to cap off the long bearers to size and square, then assemble the frame as square cut joints. The end caps are always ground and once painted you can't tell if it's capped and square joint or mitred.

Looks good. This is a example where the only joint suitable is the one you speak of. Nice work though.

 

[Bede]

Here's a decent sequence of building structures from RHS. Not a mitre in sight - apart from the diagonal bracers.

 

[Bede]

If your welding is good enough there's no difference between leaving the weld on a cap or the weld on a joint. Knock the wire feed and power down to an appropriate level and you're fine. The type of joint makes no difference if the welding is cack.

 

[Bede]

Ironically - the pile of box frames behind the trailer door in the sequence of photos - they are mitred. The only reason being that it was a run of the same item and so an assembly jig was made and it is obviously faster to mitre then drop into a jig rather than cap off shed loads of cross members. For one-offs though I'd always cap.

 

[Richard.]

If your welding is good enough there's no difference between leaving the weld on a cap or the weld on a joint. Knock the wire feed and power down to an appropriate level and you're fine. The ty pe of joint makes no difference if the welding is cack.

Agreed a naff weld is going to spoil any joint and effect it's strength. Nice photos by the way.