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5th November 2010, 08:01 PM #1GOLD MEMBER
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Aluminium straight edge - not so straight
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 sectionregards,
Dengy
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5th November 2010 08:01 PM # ADSGoogle Adsense Advertisement
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5th November 2010, 08:24 PM #2
For my home made guide, I used a $50 concretor's straight edge box section. Less deflection, but still some.
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5th November 2010, 08:39 PM #3.
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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.
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5th November 2010, 08:41 PM #4
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,
JoeOf course truth is stranger than fiction.
Fiction has to make sense. - Mark Twain
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5th November 2010, 09:06 PM #5GOLD MEMBER
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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 painregards,
Dengy
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6th November 2010, 01:17 AM #6Senior Member
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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
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6th November 2010, 02:59 AM #7Senior Member
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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.Graeme
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6th November 2010, 06:53 AM #8GOLD MEMBER
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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?
regards,
Dengy
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6th November 2010, 07:22 AM #9GOLD MEMBER
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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
Some people are like slinkies - not really good for anything, but they
bring a smile to your face when pushed down the stairs .
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6th November 2010, 07:47 AM #10GOLD MEMBER
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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 postregards,
Dengy
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6th November 2010, 08:35 AM #11
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.Cheers
DJ
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6th November 2010, 09:06 AM #12
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.
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I know you believe you understand what you think I wrote, but I'm not sure you realize that what you just read is not what I meant.
Regards, Woodwould.
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6th November 2010, 09:15 AM #13Skwair2rownd
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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.
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6th November 2010, 09:50 AM #14
What WW said go with MDF or Masonite, cheap and easy.
Reality is no background music.
Cheers John
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6th November 2010, 12:37 PM #15GOLD MEMBER
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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 terribleregards,
Dengy
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