Cheap surface plate ?

[Ian S CParticipant @iansc]

The glass from an old scanner/ photocopier could be placed on a bit of MDF, possibly with some cloth between would be good. I remember reading in an old ME of placing a peice of glass on a tray filled with some sort of tar to support the weight of things placed on the plate, and prevent it from flexing. One thing, if you get a granite one, when you "go", them that are left can get it engraved, and polished, reddy made headstone. Ian S C

[jason udallParticipant @jasonudall57142]

Posted by Ian S C on 27/12/2012 11:46:12:

One thing, if you get a granite one, when you "go", them that are left can get it engraved, and polished, reddy made headstone. Ian S C

Like it. Sort of taking it with you.

If you didn't have one already get yer headstone made up now and than get some use out of it first…starting to like this idea..maybe even do the M I your self ( strip in the date later…)…quite facied 50 mm toughened glass for mine ( head stone) laser etched inside ..( spooky floating images /text ) solar lamp in base to light it … Doubt the powers that be would allow it…..

[Alan JacksonParticipant @alanjackson47790]

Jason,

Perhaps you could have this en"graved"

Here lies Jason Udall

Below this flat sheet

On which is this doodle

[NJHParticipant @njh]

What are you guys using these "chopping boards" for?

I have a cast iron plate but ( I think on the recommendation of our esteemed Editor) some time ago bought a granite "chopping board" from J Sainsbury to keep handy on the bench. After the remarks here about inaccuracy I just went out and ran a dti over it. The max. variation I can find is less than 0.02mm. Now that's flat in my book! I know it cost me less than £5. Useful for bits of trial assembly and a much better place to rest your tea mug than the CI variety!

Headstones,epitaphs and immortality?

Remember the old adage – " Die and leave someone £1000 and they will remember you for a short while. Die owing them £1000 and they will remember you for ever!

Cheers

Norman

 

 

 

Edited By NJH on 27/12/2012 13:16:53

[Billy MillsParticipant @billymills]

The last generation of flat CRT's have very thick front glass panels. The implosion protection is by having the very thick glass and by the steel "rimband" which holds the outer panel edge in compression. A lot of tubes have been scrapped due to shorted scan coils which have not been made for at least 7 years, the factory in Mexico has been flattened.

You need to be VERY careful in handling CRT's- think of it as around a ton of force acting on that glass bottle. I would suggest that anyone not trained in handling CRT's stays well clear.

Whatever you do don't chuck a brick at it or drop it in a skip, the glass will fly very fast and at random. Never pick up a large CRT by the neck- it will snap off very easily as it implodes.

All in all -unless you know what you are doing with CRT's -reclaiming the front panel is a lot more work and risk than using an offcut of marble or getting a second hand cast iron plate.

Billy

[Sub MandrelParticipant @submandrel]

> around a ton of force acting on that glass bottle.

Good estimate!

Front area of a tube, say 18" x 20" = 360 square inches

1atm = ~14 psi so 14 * 360 = 5040lbs – about two tons.

> don't chuck a brick at it

I was definately NOT recommending that course of action

I have a large chunk of 'epoxy granite' worktop which is very flat but cost me an angle grinder to cut off a VERY heavy offcut!

Neil

Edited By Stub Mandrel on 27/12/2012 16:43:39

[magpieParticipant @magpie]

I am thinking of starting a thread on tangents. When i started this thread i never thought for one second that gravestones would come into it. Fantastic !!! best of it is I have a cast iron one.

Cheers Derek.

[DMBParticipant @dmb]

I obtained a real, small CI surface plate, £10. May I suggest that anyone using thick glass obtains a piece of felt to go under it? Probably cheaper than baize and just as good.

[NJHParticipant @njh]

Hey Derek

A Cast iron gravestone – now that's unusual ( especially as,presumably, you are still with us!)

Norman

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Early televisions had glass safety screens for three main reasons :

(1) To protect tube from accidental knocks .

(2) To limit the dose of X-Rays that viewers received . Early CRT's were 'harsh' in their action and certainly produced X-Ray emission . Probably quite a small emission but potentially long viewing times could give rise to high dosages . Later tubes were 'softer' in action and produced fewer X-Rays .

Screen was high lead content glass .

(3) Purely for furniture design – allowed a nice frame etc to disguise rather ugly early CRT's .

Michael Williams .

[magpieParticipant @magpie]

Nice one Norman. A senior moment on my part brought on by an invasion of six grandchildren all at the same time. That's my excuse anyway.

Cheers Derek.

[NJHParticipant @njh]

Ah Derek

Excuse accepted – I recognise the situation – although not quite to the same degree!

 ( I mean quantity of grand brats – not senior moments – or should that be and senior moments – I really can't remember)

N

( P.S. Simultaneous delivery of sextuplets must be a rare occurance! )

 

Edited By NJH on 27/12/2012 20:33:15

Edited By NJH on 27/12/2012 20:44:41

[Peter G. ShawParticipant @peterg-shaw75338]

Thanks to all for the very interesting info re CRT's

Peter G. Shaw

[Ray LyonsParticipant @raylyons29267]

Sorry Peter,

Since my posting on using the glass from a TV screen I have been in bed with the mother and father of all colds. A disaster for Christmas. During the 90s my son ran a TV repair business and he had many different types of CRT sets in for repair. A few had a sheet of plate glass in front of the CTR which as many have remarked is made of very thick glass and unlikely to to be damaged in an accident with a broom handle.

I remember the one from which I salvaged my surface plate was from a set where the the colour had gone and a new tube cost more than a (then) modern TV. Most of the later CRT sets did not have protective glass screens. I guess the change to a more "square" tube made it neccessary to use thicker glass and therefore no need for the additional protective screen.

Ray

[TakeawayParticipant @takeaway]

I had been using a marble chopping board as a surface plate, I think it may have come from Dunhelm Mills. Placing an engineers straight edge over both sides I was able to get a ten thou feeler under the middle on the concave side and on the opposite convex side it was similarly distorted. In truth it was bent. While being probably OK for marking out on non critical machining jobs, it was unreliable for measuring say with a vernier height gauge. and slip gauges with grinding in mind.

Next step, I have a glazier friend and he kindly cut and dressed a piece of laminated glass

30 x 35cm x 12mm thick. I expected much more accuracy with this but although it is a great improvement and will probably be ok for general machining it would still not be good enough for precision measurements.

Just a suggestion, a large thick piece of say, half inch thick good quality gauge plate would probably afford a better degree of flatness but the cost might be creeping toward a new, small commercial surface plate.

After all that, I have discoverd, a bit late in the day that the heavy duty length of kitchen worktop I use for a bench is flatter than the marble and the glass over a similar surface area – funny old world init?

Stuart

[Michael GilliganParticipant @michaelgilligan61133]

A great story, Stuart … and there's a moral for us all.

Within reason; the choice of material is fairly arbitary

Flatness is what really matters.

and … Shiny does not always equate to Flat.

MichaelG.

[Ian S CParticipant @iansc]

I was lucky, back in the 1980s the NZ Govt was selling off various departments, and when the Post Office Workshops in Christchurch went, i got a 14" x 12" cast iron surface plate for $NZ15, freshly back from being checked for flattness, the other one they had, had not been checked, and was known to be a bit out(but it had handles) went for$NZ50, go to auction- 2 items same, buy the second one, wait for the auctionereto drop his price, well it worked for me. Ian S C

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

One of the problems with improvised surface plates is that they are not thick enough . For any surface plate there is a minimum thickness below which it can never be accurate . For a Granite or Iron plate 300 mm by 300 mm the minimum thickness is about 40 mm if supported on legs at the optimal positions or on a compliant bed and minimum thickness is 50 mm for a randomly supported plate

It need not be solid as such – a thick top and ribbed construction underneath is almost as good if done well .

High accuracy Granite plates are not only very thick but they have supports underneath at exactly specified locations – usually the same locations as were used for final finishing the plate during manufacture .

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Billy Mills on 27/12/2012 14:14:58:

You need to be VERY careful in handling CRT's- think of it as around a ton of force acting on that glass bottle. I would suggest that anyone not trained in handling CRT's stays well clear.

A bit off topic but when I was working at the Mullard research labs in the 1970s we had some special CRTs sent over from Eindhoven for testing. The customs paperwork specified that they should be returned or destroyed after a certain date.

One day a customs officer came into the lab where I was working and asked to borrow a hammer. A couple of minutes later there was a loud explosion from across the corridor!

Russell.

[Sub MandrelParticipant @submandrel]

I have just checked my 'granite worktop' plate and it seems to have developed a significant 'bow' several thou deep in the centre.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Neil,

This is worth a look

and, incidentally, many "Granite" worktops are reconstituted material; which will probably flow more than the natural stone.

Certainly worth checking from time-to-time

MichaelG.

[Donhe7Participant @donhe7]

Posted by Russell Eberhardt on 28/12/2012 14:04:29:

Posted by Billy Mills on 27/12/2012 14:14:58:

You need to be VERY careful in handling CRT's- think of it as around a ton of force acting on that glass bottle. I would suggest that anyone not trained in handling CRT's stays well clear.

A bit off topic but when I was working at the Mullard research labs in the 1970s we had some special CRTs sent over from Eindhoven for testing. The customs paperwork specified that they should be returned or destroyed after a certain date.

One day a customs officer came into the lab where I was working and asked to borrow a hammer. A couple of minutes later there was a loud explosion from across the corridor!

Russell.

I personally have destroyed a number of CRTs, and have NEVER had any trouble with "implosion" or flying glass, and have always released them "to air" by gently tapping the neck of the tube after removing the deflection coils etc, until I hear the gentle "Hisssssss" of the air entering the tube, after which the tube can be converted to whatever suits your purpose, in my case, landfill!!!

donhe7

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Done7 ,

Do you remember the blue-red flash of light which happened sometimes when older types of valve were broken and the getter contacted the air ?

Michael Williams

[Billy MillsParticipant @billymills]

donhe7

I spent some 30+ years in a Company that ran about 2,500 CRT's in all sorts of equipment. Have always broken the vacuum on tubes that went to the skip- which over the years was around a thousand. I used the re-gunners method of letting the air in slowly which is not what you describe.

Tapping the neck on one type of tube will cause a crack to run around the cone and the tube to immediatly implode, this is a very widely produced Philips ultra flat type with a 30" screen.

When writing a previous posting I considered describing a safe method of breaking the vacuum however there are so many other factors and there is nothing worthwhile to be gained. But there was the risk that someone would have been hurt. I would not advise ANYONE to break or even handle a CRT without being trained.

I would not advise anyone to try to recover parts from a CRT. You will not recover anything useful.

One day four years ago a new employee in a Company was told to skip a 19" CRT. He placed it in a skip and chucked a brick at it. He had multiple facial cuts from shards of glass but was very lucky not to loose his sight. Implosions can be very dangerous.

Billy

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