Thickness of Lexan/Polycarbonate Sheet for safety screen?

[chipmunk]

Hi All,
For public demos I have a Jet mini lathe (1014) but I think it'd be a good idea to have a polycarbonate screen to safeguard the public.

Does anyone have any recommendations on how large and thick a sheet of polycarbonate I should use?

Any help gratefully received - thanks
Jon

 

[Random Orbital Bob]

I would think absolute minimum 1/4" and probably half. If it flexes in the middle, add a rebated muntin or cross bar of some description to stiffen it. I guess you have to imagine how it would perform if a bowl came off and hit it!

 

[chipmunk]

Thanks very much for the reply Bob.

What do you think about silicone sealing the polycarbonate into a rebate in a wooden frame to improve the catching abilities?

Jon

 

[Random Orbital Bob]

Yeah...great idea...anything that binds it to the frame will only add to its strength.

I'm pretty sure the last wood turning exhibition I attended (which was the Kent one earlier this year) one of the demo guys had a 1/4" screen. I remember thinking how flimsy it seemed because he hadn't braced it in the middle using the muntin idea we've already discussed.

I keep thinking....what if you really have a piece fly off...what if it really hits the screen at full pelt. You know, in a public place, you're going to get crucified if you've not taken reasonable precautions. So I think the screen has to be really well anchored and really stiff....or much thicker. Wonder if there is any published health and safety guidelines on that? Can't imagine there would be??

 

[chipmunk]

This is interesting statistic though...

Taken from http://www.epse.org/faqs/what-is-the-impact-strength-of-polycarbonate/

"a 3 mm-solid polycarbonate sheet withstands the force of a steel ball of about 4 kg dropped from a height of 9.5 m, 3 times in a row."

Something dropped from 9.5 m is travelling at ~13.5 m/s or 28 mph when it hits the ground.
My Jet mini has a 10" swing. At 1000 rpm the outside of a 10" bowl (0.25m) is travelling at ~12.5 m/s - pretty close to the steel ball. So a 4kg blank at 1000 rpm hitting a 3mm screen should still be ok.

And that sounds a pretty big blank on a Jet mini to me !

So my feeling is that it may look flimsy but if well bonded into a good frame then this would suggest that even a thin sheet ought to do the job?

Thoughts or comments welcome
Jon

 

[Random Orbital Bob]

well...that's entirely rational Jon...sounds reasonable. The 1/2" stuff is pretty expensive too so if you can use a thinner stock but remain safe then all power to you.

 

[Phil Pascoe]

I would think absolute minimum 1/4" and probably half. If it flexes in the middle, add a rebated muntin or cross bar of some description to stiffen it. I guess you have to imagine how it would perform if a bowl came off and hit it!

The bowl would fly at you not the screen, in my experience. The screen would be there for chippings and legal neccessity.

 

[chipmunk]

I agree Phil but I suppose the possibility the bowl could bounce (off me? ) needs to be taken into account...

 

[Random Orbital Bob]

Aye that's true...they do tend to come forwards don't they. Happened twice to me, luckily no contact.

 

[jpt]

Below is a picture of my set up for demoing at shows.



It might seem over the top but it protects the public and myself from the public.

It is large enough so that any shavings etc wont hit people standing in front of the lathe and the sides are there to stop the bits that go off to the side from hitting people and can be closed more to stop shavings going on to the stand and on to the stand next to me. It protects me from people because when I just had the front shield a lot of people, a high preportion kids, would look around the side of the screen this prevents it happening.

A lot of items that come off a lathe go away from you first before hitting a wall or shield and bouncing back, in my experience.

The thickness is to some extent not very important what is more important is the EN or ANSI strength you need one of at least medium impact resistance but preferably high impact resistance, it will cost more but if the worst did happen and something came off the lathe and the shield prevents it hitting someone it will seem cheap.

john

 

[DMF]

Hi,

i know nothing about turning yet (to many things to try!) but this may be relevant, one of my few wood projects i have done was to make a screen for a friends run of indoor archery demos. Wish i took some pics now but basically after a couple of experiments we found a curve on the screen definitely deflects smaller objects much better, i made him a u shape screen with ply top and bottom and 2x3 uprights with 45 degree braces coming off those, rebated in but not siliconed so he could take it apart, when we fired smaller stuff at it the object followed the path of the screen depending on the angle so make sure people aren't in line with that maybe?!

Different scenarios i know but maybe of some use, sure there were different grades as well, i know we used a 3mm that was more pricey than an other for some reason?

Dean

 

[Lons]

Can I chuck in my twopennerth?

Polycarbonate is made to flex - that's its strength and it really will take some breaking unless it's been fixed incorrectly. There are some very good technical working sheets available from the various manufacturers and distributors such as Amari Plastics (no connection). For any sceptics, just get a small piece, lay it flat on a smooth(ish) surface and belt it with a hammer. It'll dent but won't break unless faulty!

There are minimum size frame rebates recommended and if drilling for mechanical fixing I'd seriously recommend holes no closer to the edge than 12mm. The holes have to be carefully drilled also as micro cracks are a weakness and always oversize so the poly can expand around the fixing. Note also that some silicones attack and degrade the polycarbonate and you should always use one suggested for the material.

It's many years since I was in the industry but at the time there was a 3m brand d/s tape available about 10 wide especially for fixing into older bus shelter frames and it was exceptionally strong and reliable.( I might still have some in a drawer ) There was one instance where yobs tried everything to break a large 6mm bus shelter panel without success so used a blowtorch which was, to my great amusement left stuck firmly in the hole they had managed to burn throughthe poly. #-o :lol:

For demonstratiopn purposes (Lexan) we used to fix a 1200 x 600 x 6mm sheet into a timber frame. 20mm rebates and held in place only by pinned mouldings then offer all and sundry the opportunity of attacking it with a very large mel / hammer. It would mark and scratch very easily but never had a breakage.

Normally 4mm would be quite adequate but use 6mm if you want belt and braces. If you make small screens which will fit into a domestic oven they can be heated until pliable and clamped around formers or even "blown" to shape btw. Don't normallytry to heat with a flame or element as it just degrades the surface, though it is possible with care.

I might have some info tucked away in the loft if you're stuck.

Bob

PS Just done a very quick search and here's one I found for Makrolon http://www.curbellplastics.com/technica ... krolon.pdf

 

[Phil Pascoe]

I've found with the plastics I've ever worked that it's better to burn holes than drill them - you don't seem to get stress cracks.

 

[chipmunk]

Dear All,
Thanks very much for all your help.

I've just got to put it all into practice and I'll have the perfect screen. 8)

Jon