looking for strong steel

Lacwood

Hi there.
I'm trying to make a pair of clamps with 1200mm long jaws to stretch paintings but im having trouble finding information about which metal would suit. I was thinking of combining aluminium angle with some kind of flat bar which wont bend easily but finding out details on the net gets more frustrating all the time, like looking for the sugar in a cup of tea,
Any help would be greatly appreciated, Also where could I buy in such a small quantity

Thanks

bnt

The shape of the section is going to be important: a flat section is far more likely to bend, whatever it's made of. I'm pretty certain aluminium can do that job if you pick the right section: if it can hold up a row of seats on an aircraft filled with fat people through clear air turbulence, it can stretch a painting. :pac:

I haven't dealt with any suppliers here in Ireland, but these guys look like they may be able to help: Amari do cut lengths (under "sections").

Lacwood

Thanks bnt. I was going to use aluminium for structural purposes and bar, possibly 50mm on flat to take the pressure. I agree, some kind of aluminium extrusion would probably be the best, but finding something suitable proved hard. I will check through amari again.
Also without the technical knowledge it is very hard to judge aluminium specifications. As for high tensile steel or something like that I just could not find it

DublinDilbert

How are you going to apply the clamping force? Is it a compression or expansion force.

As the poster above says the shape of the section is what's important.

I would use square/rectangular steel box section. Drill holes in the end of each piece. The use 2 threaded bars with washers & nuts, to apply the clamping force. You can literally apply tons of force with this...

Lacwood

the machine is built and operational except aluminium angle clamps allow canvas to slip in the middle. It all operates on pneumatic air cylinders (12 in total) it is designed for speed of operation so nuts and bolts are out im afraid (thanks anyway). clamping pressure is applied at ends of jaws by two air cylinders and this is the reason I want to try steel which will not bend. Believe me, I've spent countless hours on this thing and sleepless nights(not really), but to achieve the result I want, it seems the only alternative. P.S. it is a compression force

apoeiguq3094y

The strength of the bar you're talking about will depend on the thickness of the bar in the direction you are applying the load... for this reason angle iron would be a much better bet than flat.

Instead of trying to find higher strength steels, could you just buy angle sections with thicker walls?

A photo or a sketch would be great, cos its hard to visualise your set-up.

As you are only buying such a small quantity.. you would be better to try a local engineer/ fabrication company. e.g. someone who makes farm machinery as they might have an offcut you could use

Mad_Mike

Definitely a sketch or photo would help, but from what you describe, the aluminium is fine only its slipping. Then all you need to do is rubber line the aluminium to stop any slipping?

dave2pvd

Is this a canvas-over-frame stretching machine?

I've made one....or rather, played a part in its design and build.

Lacwood

'dave2pvd', yes it is a canvas over frame stretcher, maybe you can be my saviour, how did ye get the clamps to grip the canvas with no slippage? I've tried rubber and I've tried leather slips glued to 40x40x6mm aluminium but it just seems to slip. There seems to be some mysterious polymer tape that some commercial designers use but I could not locate anything and they're not inclined to communicate their details. I have never actually seen one of those machines except on the net, so maybe I am missing something basic. I think the issue has more to do with a straight clamp and good grip than with any massive clamping pressure,which in the long term causes distortion in a lightweight assembly. I'm going to try to add a sketch to this in a while as soon as I work out how, All comments are appreciated (especially yours dave)(maybe theres a mislaid blueprint lurking in adrawer somewhere)

Lacwood

Hi. Heres an attachment of some rough sketches canvas stretch machine.jpg Hope someone is inspired.:rolleyes:

you cant see it from the sketch but jaw on right is supported at 5 points along its length. I cannot use another angle section here because painting needs to be stapled on rear of stretcher for paintings which will not be framed.

P.S. I have tried engineering workshops and i've tried their suppliers, but the stuff isnt really used outside of mass production it seems. I thought I would get it handy enough but not the case.

dave2pvd

Our machine was similar in principle. The operator placed the canvas on a fixture. The canvas was clamped and stretched outwards (4 sides). The frame was then placed on the canvas. A press plate pushed the frame down into the canvas (clamps floated). The clamp assemblies then moved in, pressing the canvas flat against the sides of the frame. The clamps opened, while the canvas was still being pressed against the frame sides. The operator flipped the remaining canvas over to the back of the frame (it is facing up) and stapled it on.

For your design, don't use aluminium anywhere on the clamps. Use carbon steel channel and plate, as needed. You need the stiffness, afterall. Also, for the clamp pads you could use high density, high durometer polyurethane foam, or maybe high-friction conveyor belting. Screws and glue to adhere it onto the clamps' working faces. The screws can be countersink type, so that they don't contact the canvas.

I wish I had photos for you, but it was 1998 when we built this. No pics on the server from that far back...

Edit: try using leverage for your clamp mech. Let the claps pivot about a point. You could do with some mechanical advantage there.

Lacwood

Thanks a lot for that input 'dave2pvd'. Theres a few technical terms for me to play with on google, this project has been mothballed for about 2 years while I struggle to keep my regular biz afloat but I need to apply myself again as theres quite a bit of funds tied up in it. I will definitely try to get that p.u. foam you mentioned and look for carbon steel

dave2pvd

Lacwood, carbon steel may be a US-centric term; I'm not sure. At least I used the correct spelling for aluminum....;)

Carbon steel is basically the common stuff. Welds easily, machines easily, holds a good thread, has plenty of stiffness, etc.

If you really want to go aluminium, you could try an extruded type that takes T-nuts, like 80/20 or Bosch. In fact, that might simplify things for you. Especially since some of the manufacturers carry sections that are linear slides. The stuff is expensive, but it goes together so easily.

Lacwood

Aha !! are you stateside dave2pvd?. I lived there too inthe nineties and anyone can get anything, anywhere, its a great country. Its impossible to get anything outside the ordinary in ireland, unless you are involved in an industry and know the inside track. Even that carbon steel you are talking about, maybe I will get it on the net but certainly not anywhere locally, I always have returning emigrants complaining to me how frustrating it is to get anything done in Ireland. If I remember correctly, americans are almost honour bound to return a phone call, if only to tell you they dont want to talk to you. not so the paddies i'm afraid, paddy 'doesnt know' and 'doesnt know who might know' and 'who do you think you are wanting to know anyway'. (enough!, dont start me on the politics).

I will definitely check out all you said and see how I get on, Thanks

ZRelation

Have you tried http://radionics.rs-online.com/web/ for materials? They're expensive enough but they supply tool steels etc if you think you'll need them. They also do a selection of polymer sheets if you want them for clamps.

dave2pvd

Lacwood wrote: »

Aha !! are you stateside dave2pvd?. I lived there too inthe nineties and anyone can get anything, anywhere, its a great country. Its impossible to get anything outside the ordinary in ireland, unless you are involved in an industry and know the inside track. Even that carbon steel you are talking about, maybe I will get it on the net but certainly not anywhere locally, I always have returning emigrants complaining to me how frustrating it is to get anything done in Ireland. If I remember correctly, americans are almost honour bound to return a phone call, if only to tell you they dont want to talk to you. not so the paddies i'm afraid, paddy 'doesnt know' and 'doesnt know who might know' and 'who do you think you are wanting to know anyway'. (enough!, dont start me on the politics).

I will definitely check out all you said and see how I get on, Thanks

Lacwood, there are definitely cultural differences! Each country has its pros and cons as far as doing business goes. I am aware that Ireland lacks 'critical mass' when it comes to getting certain supplies. You end up with less choice and higher prices. It's a real economic disadvantage. But then again you are a lot more insulated from the myriad rip-off schemes that we have over here.

For the steel, you might want to check out a scrap merchant. Or what about a weld/fabrication shop? Even a college/tech/uni might have some stock you could use.

Pity you don't have McMaster-Carr over there.

Lacwood

You had to gloat didnt you(just kidding):) with your mcmaster-carr. That is one serious website.
One example of the difference between here and U.S., when I needed air cy;inders I checked everywhere over here, the options were, buy them new at about 40 euro each or pick up maybe 2 at a time on ebay for about 10 euro each. I ended up getting a lot of about 40 from U.S. for $50 + $20 shipping.


'ZRelatiion', I know I looked on radionics at some stage but i'll give another look, Thanks