Thread: Aluminium straight edge - not so straight

For my home made guide, I used a $50 concretor's straight edge box section. Less deflection, but still some.

Originally Posted by JillB

Is this experience of others? In future I shall be using steel bar section

Usually I find ally stuff is pretty straight and it's usually no worse than steel. If you are selecting from a bin or rack I lay one strip on the floor of the hardware and then match each side of another against one side of the one on the floor till I find a straight one, or go to another store.

1200/32 = 37.5. Much too flexible.

In bridge design, we aim for an initial guess of 1/24 depth-to-span ratio. In steel, a span of 1200 would require a minimum depth of 50mm. For aluminum, x 1.5 = 75mm. And even at that, deflection is expected.

I'd consider a bar of 32x6 to be merely raw material for smaller projects.

What Master Splinter said.

Cheers,
Joe

No joe, it did not deflect 32 mm - only one or two mm, but far too much for a straight edge.

The best straight edge I have, one that really impressed the builder doing some renovations to the extent he borrowed it a few times, is a 1800mm length of aluminium angle, 50 x 6mm - dead straight!

I also use a piece of 50 x 50 U-channel section aluminium which has stood the test of time, but clamping it to the woodwork is a bit of a pain

If you where present to see how aluminium extrusions are produced you would be very cautious in selecting a length of extrusion for a straight edge. It needs to be checked thoroughly.
As the heated aluminium billet it forced through the die to form the extrusion profile it spews out like spaghetti onto a table and from there is mechanically straighten and checked partly by eye, the material is very soft and pliable and has not had a chance to harden, from there it is cut into lengths.
Perhaps it would be preferable to have the straight edge cut from sheet material where the straight edge is formed by cutting by a guillotine.
Of course you could always file the length of material straight by hand checking as you go.
Cheers
Mac

I second BobL's advice. That is what I did to get a 1500mm straight edge when I mislaid my Veritas aluminium version.

Note: I was inspired by a brickies level as I have heard that some people use these as straight edges. I bought a 50 x 10mm piece of extruded flat bar. When I found the Veritas straight edge, the extrusion was dead straight.

Malcolm Eaton, thanks for this information on how they manugfactuire aluminium extrusions. Do you know if they do it the same way for steel flat bar? Would steel be more likely to be straight and more suitable as a straight edge?

gday Jill before switching to festool guides and saws I used the range of progrip fences quite successfully they are reasonably priced and the 50inch is very straight although I did check in store for straightness before buying . You can find them here ProGrip Fences : CARBA-TEC

thanks for this , fubar, but I am after a flat metal straight edge that can be screwed to a piece of timber for permanent use in this role. Using the idea as shown in the photo in my original post, I then don't have to do any measurements for using the router, just sit the timber edge along the line of cut on the work piece, clamp it down and and Bob's your uncle

Once this piece of timber gets chewed up, and it will, I just screw the metal straight edge to another piece, run the router along it, and away we go again.

I always thought that aluminium extrusions were the most accurate, but not so, especially after reading Malcolm's post

I'd be using some box extrusion like 50 x 25 if you're only looking for 1200mm long bit.

I've got a couple here, 1@ 100mm x 25mm x 2.4mt and 1@ 100mm x 50mm x 5mt and they are straight. Flats in ally and steel are generally not as rigid as box sections.

JillB, why do you particularly want to use a metal edge? Flat steel merchant bar won't be 'straight' either (ground stock would be perfect, but cost a fortune). Any DIY saw guides I've seen or read about use the factory edge of a thin sheet of MDF or Masonite. After making mine, I checked it with a known straight edge and it is perfect.

I have a straight edge that is a piece ifperspex off cut.

As WW said Masonite or MDF is fine, With MDF just laminate the adeg you are using to prevent wear.

What WW said go with MDF or Masonite, cheap and easy.

Thanks for the suggestions gents. I am coming around to your way of thinking about a timber edge instead of a metal edge to run the router against. Now, where do I get a dead straight timber edge

My original thinking was for a metal edging with would be strong enough to stop the 1200 mm long board from bowing lengthwise ie., I wanted a flat jig. I suppose I should just hang it up to do that

I have actually ducked out to our local steel supplier and spent $3.50 on a 1m length of 32 x 5 mm stell flat, and it looks dead straight, just what I wanted for a straight edge for this board.

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Actually, I checked it against a good straight edge just now, and found it to be terrible