Aluminium straight edge - not so straight

[Dengue]

Hi, I am in the process of making a straight edge for my router , so that I just fit the edge of the MDF straight edge along the cutting line on the workpiece, and then run the router across to give an accurate cut on the workpiece. See the picture below for a view of what it looks like.

I chose a 1200 long piece of aluminium bar 32 mm x 6mm, thinking that would be substantial enough. After screwing the bar to the MDF, and then using the router to cut against this bar to give me the required a straight edge, I lined the MDF edge by eye only to see a huge bow in it along the 1200 length. I then checked out the aluminium bar and found it to be bent in the plane of the 32mm width ie., the 32 mm section was bent sideways. And that bar cost me $25 for a 4m minimum length.

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

[Master Splinter]

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

[BobL]

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.

[joe greiner]

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

[Dengue]

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 :)

[Malcolm Eaton]

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.:rolleyes:
Cheers
Mac

[bookend]

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.

[Dengue]

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?

[fubar]

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 (http://www.carbatec.com.au/progrip-fences_c8370)

[Dengue]

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 :oo:

[DJ’s Timber]

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.

[Woodwould]

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.

[artme]

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.

[munruben]

What WW said go with MDF or Masonite, cheap and easy.:2tsup:

[Dengue]

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 :rolleyes:

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 :(

[Mr Brush]

What fubar said !

ProGrip Fences : CARBA-TEC (http://www.carbatec.com.au/progrip-fences_c8370)

They are specifically designed for exactly what you are trying to do. I use them all the time for this purpose, after exactly the problems you describe with other straight edges.

I use mine all the time with a router to square up roughly broken down MDF sheets (1200mm).

For a given router, with a given bit, its easy to measure the offset exactly by making a test cut. Very easy to use this test piece to make some spacers to fit between the ProGrip and your desired cut line.

[Woodwould]

Now, where do I get a dead straight timber edge :rolleyes:
The edges of sheet goods are probably straighter than most metal bars.

I cut a strip, say 300mm wide of the edge of a new sheet and then lay it, factory edge out, on top of the remainder of the sheet, setting the strip in from the sheet's sawn edge by at least the dimension of the base plate to the router bit /saw blade. Measure from the sheet's opposite factory edge to the stepped factory edge to get them parallel. The strip is then glued/screwed to the sheet. Take the router or saw and rout/rip the sawn edge off the sheet by running the tool's base plate against the factory edge.

Insert the fence into the router/saw and again, running the fence along the factory edge, route/rip the new guide off the main sheet, leaving a new straight edge on it in the process.

[Dengue]

I cut a strip, say 300mm wide of the edge of a new sheet and then lay it, factory edge out, on top of the remainder of the sheet, setting the strip in from the sheet's sawn edge by at least the dimension of the base plate to the router bit /saw blade. Measure from the sheet's opposite factory edge to the stepped factory edge to get them parallel. The strip is then glued/screwed to the sheet. Take the router or saw and rout/rip the sawn edge off the sheet by running the tool's base plate against the factory edge.

Insert the fence into the router/saw and again, running the fence along the factory edge, route/rip the new guide off the main sheet, leaving a new straight edge on it in the process.

Many thanks for these detailed instructions, WW. I was with you right up to the 2nd paragrah ( in pink above). Unfortunately I don't have a 300mm long fence for my router to do this, and even if I did, I would not trust it to do an accurate cut over this distance.

What do you suggest as a next step, resulting in an accurate edge being left on the sheet after the jig, which is 300+ mm wide, is removed?

[Dengue]

Thanks for the clues, gents, about using timber for a straight edge. Yesterday I had purchased a sheet of 12mm MDF 1200 x 900, and ripped it into three 1200x 300mm lengths on my 10HB table saw.

I grabbed one of these and put it against my trusty aluminium 50x 50 x 5 angle straight edge, and found that the result was a perfect straight edge, much, much better that any of the 32 x 6 mm aluminium and steel flat bars I had purchased.

So, WW, I would agree with your comment below:

The edges of sheet goods are probably straighter than most metal bars.

Many thanks for all your advice everyone, it has been a very interesting exercise, with many lessons learnt

[Woodwould]

I described the last process just as a way to leave the sheet with two parallel straight edges, but it's not necessary or relevant to making the jig.

Having said that, once you've made one of these jigs, you can make future jigs double-sided. I have one for the router to suit a 19mm bit on one side and the other side is for a 12mm bit. Like wise, the jig for my 7" Bosch circular saw has another on the other side for a 9" saw etc. etc. etc.

[Dengue]

Thanks for the suggestion on the Pro Grip fence from fubar and Mr Brush. Believe it or not, I have one, so will make a few spacers and try it out.

My preference has always been to eliminate any chance of error by putting a straight edge put directly on the cutting line, as in my original photo, but the problem has been the need to clamp the workpiece and the straight edge together. That is the big advantage of the Pro Grip fence, with it's great clamping arrangement

[Woodwould]

JillB, with the jigs I described, you will have a perfectly square, sharp and straight edge to place on a cut line and be guaranteed of an accurate cut.

As for clamping, I use a couple of the large Chinese-made steel spring clips which, IIRC, the Beloved One bought at either Bunnies' or M10. They're profile is nice and low and they don't move during cutting.

[Dengue]

I use a couple of the large Chinese-made steel spring clips thanks WW, any chance of a photo please? I am always on the lookout for better ways of doing things

[Dengue]

Mr Brush and fubar, you might be interested in this attachement for the ProGrip fence for doing dados. See it here
(http://www.youtube.com/watch?v=rYR-B87GO8A)

[Woodwould]

thanks WW, any chance of a photo please? I am always on the lookout for better ways of doing things

I snap a picture of my set-up for you tomorrow.

[joe greiner]

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

I meant the proportions of the shape, not the deflection itself.

And to add to Malcolm's description, the mechanical straightening is usually done by yanking the ends. The spaghetti is a good analogy for aluminum extrusions, and to continue it, rolled structural steel looks like pie crust edge as it exits the rollers.

Cheers,
Joe

[fubar]

Mr Brush and fubar, you might be interested in this attachment for the ProGrip fence for doing dadoes. See it here
(http://www.youtube.com/watch?v=rYR-B87GO8A)
thanks jill for that link :2tsup:. this is what i use these days torqueworkcentre.com (http://torqueworkcentre.com/) its very accurate and incredibly versatile from gigantic overhead router, huge drill press and able to cut down full size panels all in one table havent found its limits yet and to rout out dadoes I just set the stops on the rear track lock the workpiece down then move router repeatably and accurately up and down the workpeice:D

[Woodwould]

JillB, here's one of my double sided guides and spring clamp. Make sure you write clearly on each side of the guide exactly what tool and/or bit it is intended to be used with!

[Malcolm Eaton]

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 :rolleyes:

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.

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

Its a matter of check and check again.:D
The way to check a straight edge is to place the piece to be checked on a flat surface and drawer a fine line onto the surface using the straight edge to be checked as the guide, now you roll over the proposed straight edge piece and line the ( so called ) edge to be checked with the line you had previous drawn if it lines up with out a gap showing along the line then it is straight ( as straight as you will get). If it does not line up you will need to look for another piece to check.
It is a matter of checking each piece until you find something that meets your requirements.
A rectangular aluminium box section is your best bet, check out a local aluminium window manufacturer as they may some offcuts or other wise check out the local salvage yard as old discarded aluminium sliding fly doors are a potential source of sections of material suitable for a straight edge. You will still need to check each piece by the above method.
If all fails contact a local sheet metal manufacturer who handles heavy gauge material and ask them to cut of a length of material aluminium or steel which would be suitable for a straight edge. Say something around the 3 to 4 mm thick and 70 to 80 mm wide.
Best of luck.
Cheers
Mac:)