Will Trespa take a tapped thread?

[T i m]

Hi All,

I have one of those TFT arm mounts (BT7513) that I want to use for my daughters 19" screen.

The mount is adjustable in most planes except the vertical and we aren't sure exactly where she will have the screen as yet so I wanted to provide some flexibility there.

Her room was lined with a soft fibre-board (to provide a bit of insulation and be drawing pin friendly) so that's not very good for attaching things.

My BIL has some offcuts of Trespa and I fancied attaching a (say)

100mm wide by 500mm high strip of it to the bit of wall above her worktop, under her (fitted in the ceiling) bed and on top of the fibre board (I should be able to get a couple of Fischer plugs in there (top . middle).

Then, (pre fitting) I fancied drilling / tapping (6 or 8mm d) a row of suitably spaced holes up the vertical of this 'backing plate' so that we could adjust the height of the BT7513 easily? The arm mount has 3 holes (and I would use them all) so only the top and middle would be doing any real 'work'.

So, would anyone know if 18mm of Trespa would tap ok and then how strong would that tapping be please?

Oh and she's into Rock so wouldn't want to paint it pink or anything and in fact would probably like the 'industrial' look ;-)

All the best ..

T i m

[philipte]

We use Trespa benching at work, and it is (a) very heavy and (b) very strong. I once had to put a 22mm hole through a piece, and even with a sharp bit, the battery on my cordless drill was flagging by the time I achieved full penetration...... I reckon it should be fine for what you propose, though I would recommend a coarse threadform, anyone for Whitworth??

PTE

[T i m]

I bet ;-(

Hmm , I know I have a fairly good range of Whitworth taps in the workshop but can you still get Whitworth machine screws!

Maybe I'll have to buy an old Brit motorbike and nick some of the bolts off that ;-)

All the best ..

T i m

[Andy Dingley]

Whitworth, UNC, or the coarse series metric threads are fine. Polyester resin glued-in rock anchors work too.

Thread inserts won't work, as the coarse "wood thread" side of them just isn't strong enough to tap the Trespa.

[T i m]

Do they do a 'coarse' pitch metric in 6mm do you know Andy (is 'std'

1.25)?

Ok .. thanks ;-)

All the best ..

T i m

[meow2222]

even after looking at the site Im not much wiser as to what it is, or what it costs. Sounds like it just might be what I've been looking for for the bathroom.

NT

[T i m]

An acid resistant, very dense composite material used typically in lab worktops / upstands (instead of the *more* expensive / fragile marble).

A lot ;-)

My BIL gave me a bit about 4' long, 4" wide and 5/8" thick today and when you pick it up it feels like a lump of steel ;-)

Apparently it kills most ordinary tools you try to cut it with (if you aren't very careful) ;-(

I'll let you know tomorrow ;-)

All the best ..

T i m

[Andy Dingley]

Coarse M6 is 1.0 and fine is 0.75

I'd use M8, which is 1.25 for coarse and 1 or 0.75 for fine.

[Andy Dingley]

You can't afford Trespa - no-one can. But there's always a scrum over the offcuts 8-)

BTW If you dishwash Trespa a few times (I've got kitchen knives handled with it) the dye washes out of the fabric reinforcement and it develops a "woodgrain" appearance.

[meow2222]

ty, just what I wanted to know.

nt

[Aidan]

Why not drill through & secure it with nuts & washers on the rear of the Trespa? Offer up to the fibre board & counter bore the marked impressions to accomodate the nuts. No taps involved.

[T i m]

Thanks for that.

The current holes in the bracket wall plate part are only ~6mm dia at the moment but would easily open up the clear an 8mm machine screw. To stand a chance of getting the little plastic cover back on I'd either have to use pan head screws or countersink the holes (one 'hole' is round and quite deep (two layers of steel) so that would take a countersunk easily but the top one is 'slotted' (for adjustment of the vertical alignment) so that might have to be pan (as it only goes through one layer of steel).

All the best ..

T i m

[T i m]

Nice idea BUT, the object of this exercise it to provide vertical 'adjustment' for the flat screen and once the trespa is fitted to the wall I don't intend taking it down again ;-)

I could actually just fit the bracket in position with some turned nylon spacers to hold the bracket just off the surface of the fibrerboad 'lining', but if she decides she wants the scanner under the screen or sumat and therefore the screen needs to 'move up an inch' I'm stuffed?

With this pre-drilled / tapped idea and the Trespa (only using that because a) I can get some free, b) I don't need to paint it, c) it's nice and flat / strong and d) it will (hopefully) take a tapped thread to allow my incremental hight adjustment idea?

To do the adjustment you lift off the TFT, unclip the bracket cover, unscrew the two 8 x 1.25 machine screws, select an alternative position, replace the screws, repace the cover, replace the TFT ;-)

And if this 19" screen dies and we are offered a 21" replacement we can accommodate that as well (and not only can we adjust to it easily it won't pull *my* bracket off the wall!) ;-)

I suppose all the above is a function of what I am / how I think .. (I think I'm a frustrated inventor of sorts ). I don't really care how long it takes (within reason of course) or how complicated it may seem to others as long as it's 'right' for me (and there are only a few rights and hundreds of wrongs). It's also nice to have folk (who understand these things) see your effors and say 'that's a good idea' , 'how did you do that?' or 'you've made a nice job of that' ;-)

If I do something I would like to think all the options have been considered, it provides the most practical / flexible solution and I never have to do it again? If I wanted a quick solution and wasn't interested in the d-i-y part of it then I'd get someone else in to do it .. the problem with that is many 'tradesmen' today have little imagination / engineering skills and I *know* I wouldn't be satisfied with the job (present company accepted of course!) ;-)

So, "whatever you do do, do do well" ..(unless you are trying to earn a living out of it 'these days' I suppose ...) ;-(

All the best ..

T i m

[T i m]

Good job you mentioned that Andy ...

I just checked out the replacement 'Blue Wizard' 8m x 1.25 tap I bought the other day and noted it was a 9mm ;-(

So I just popped back to the very well equipped tool shop (I leave my wallet at home and only take the right money with me or I know I'll spend all of what little I have in there .. I know I will!) and swapped it for the 8mm .. oh and some quality 14tpi HSS hacksaw blades .. ( and a little fold up magnifier, 3 expanding rules (never find one when you want one) and a 'Mole' style table quick clamp for the pillar drill !).

Then it got me thinking re do I lube or not? For steel and these forward swarf ejecting 'speed taps' I generally used them in the DeWalt cordless (on low) and some cutting paste (and then it's a 'knife through butter') ;-)

Apparently if you try cutting Trespa too fast it heats up quickly and causes all sorts of issues (and the material expands etc). ;-(

I think with some 'plastics' you can lubricate with water?

Maybe I'll just take it slower than usual and see what happens ..

All the best ..

T i m

[John Schmitt]

Possibly worse than you think. 9mm AFAIK is not part of the ISO standard series. Some years ago I had to make a replacement part with a 9x1.25 thread and dies were hard to find. I think that after 8M the sizes in the standard series go up in steps of two. Machine screws and nuts and bolts in that size may well be as hard to find as rocking horse doo-doo. You may have to buy a whole box, in which case it may be cheaper buying a new tap. Good luck in any case.

John Schmitt

[T i m]

Go on .. ;-)

Ooo err .. funny as I was swapping it I was considering keeping it .. you know .. for the collection but couldn't actually remember ever seeing a 9mm bolt?

Ah .. ;-(

That seems to make sense. I'm pretty sure I have a selection of 10m bolts etc ..

Swaped already ;-)

Cheers. I don't know if I should drill / tap a test thread first and see how hard I can tighten it (with a pozi headed screw to be 'realistic') before:

1) I can't tighten it any more ;-) 2) It strips the thread ;-( 3) I shear the screw off! ;-)

The problem with these 'L' shaped arm wall brackets is the top mounting is screw nearly completely in tension (when the arm is at 90 deg to the bracket), both from the holding force and the load on the arm?

I'm sure someone here can do the sums for me ..

The TFT weighs 6.5 kg and at full extension is approx 500 mm from the bracket. Assuming the 160 mm high bracket is completely rigid (which it isn't I know) and the top fixiing is actually 30 mm from the top of the plate (therefore and possibly more importantly 130 from the bottom where I assume it would try to pivot?), what sort of tension would be in the top fixing please? Would a std grade 8 x 1.25 hold it ;-)

(I guess it should as they provide 3 x No10 x 40mm screws / plugs in the kit)

(fwiw the arm minus wall bracket weighs another 1.2kg?)

All the best ..

T i m

[John Schmitt]

If the Trespa is as tough as has been mentioned, you will probably find that the driver bit will "cam out" before any of those happen to an M8 screw.

Rule of thumb: 6mm steel rod is good for an ultimate load (i.e. the metal fails) of approx 1 ton(nne). 8mm 1 3/4, 10mm 2 3/4. Exactly how much overkill are you after? ;-)

that I an being conservative on the larger diameters. If you fix it to the wall and then do can a pullup on it without it failing, this would suggest a factor of safety in the region of 10 or greater.

In direct tension and well installed that combination should be good for at least 150 kg in direct tension, probably rather more. Dont forget that most telly brackets are designed for something like 21" tubed TVs which come in at about 25kg, with the weight concentrated towards the front.

John Schmitt

[T i m]

That would be my wrist discolating then (some of us still like the feel of a screwdriver for some jobs) ;-)

;-)

But (and brings us back to the op) I felt there was more chance of the thread ripping out of the Trespa than the steel screw failing (as strong as Trespa *might* be)? But like balsa wood, it's very strong for it's weight but not *strong* in the traditional sense of the word?

And how I often test such things but this will end up under a raised bed and over a worktop ;-)

Ok ..

Indeed .. and most of the telly brackets I have used have a distinct inverted 'channel' section to the wall mouting plate to spread the load from the fastenings over the base. *This* wall mounting bracket is only 3mm thick steel (with some 'hinges' welded through (rather than 'to' making the hinges stronger but the plate weaker)) suggesting a much lower expected load (than a telly) ;-)

As mentioned the TFT is 6.5kg, the arm another 1.2 (so worst case

7.7kg) and the bracket (and presumably supplied fastenings) is supposed to be good for 15kg (with some in reserve no doubt).

However I'm not sure I'd trust the fastenings given even if I was going to mount the thing directly to the wall of this 1897 house (where most of the mortar is now sand!) ;-)

I think I'll start with my design and simply test how well one screw bits down and use that to predict how well it will all hold up in the real world (assuming the thread taps out ok I'm sure it will be fine) ;-)

All the best ..

T i m

[John Schmitt]

A little bit of web browsing suggests that trespa is about 1/5 the tensile strength of mild steel, suggesting that a single 6mm threaded fastening will pull out in the near neighborhood of 100 kgf, 8mm 150 kgf. Creep is the next consideration.

John Schmitt

[Andy Dingley]

It's part of the series (ie. they define the thread form and pitch) but it's one of the "second choice" size recommendations so it's not widely used.

Volvo probably use it. I've never known a car with so many of those odd "even" metric spanners needed. I've had to use spanners from sets I've owned for 20 years and never used.