# Straight edge ideas

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#### Insect

##### Member
So I need a straightedge of about 15' long to build some wing spars. I use and trust extrusions like 80-20, Misumi, kinda easy to make a jig for that and they are very straight. At work I have a 1"x2" 25' beam of 80-20 is not stiff enough to define any straightness, on it's own, not at all, so either a truss, or taller beam comes to mind... I'm drawing a blank on how to do this at a reasonable cost, any experiences?

Ultimately when you exceed 10' then gravity tends to take over, you can probably imagine the game. Any device that is truss shaped can be adjusted with a laser, not really rocket science, just fishing for ideas.

I could make a wooden pan and self levelling concrete, but, I'm not really wanting to paralyze the shop tables with 500lb of concrete to do this. I have a large pond, so any floating contraption could also work, then gravity is no longer flexing the beam, what a pain, making wing spars while in a pond, eh not for me.

#### Victor Bravo

##### Well-Known Member
HBA Supporter
Large pre-fabricated wood roof beams, which are I-beam cross section? These are probably made in a large jig fixture, so they are probably pretty straight to start with. I believe they are probably available in large lengths for larger construction projects.

THEN, run a laser down the edge you want straight, and identify any deviations in the beam cap. Sand, mill, router, or plane as necessary to whatever your tolerances are.

#### Mavigogun

##### Well-Known Member
Why not a torsion box with leveling feet? Or for temporary support, solid foam?

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
I'm drawing a blank on how to do this at a reasonable cost, any experiences?
What kind of tolerance of you really need?
VB's suggestion is probably the quickest and easiest. A couple of"I" wood floor joists from the lumber store, set up your laser to double check by rolling a hexagonal pencil down the top (shim/plane as needed) then top with some MDF. Lots of planes have been built with less and came out just fine.

#### TFF

##### Well-Known Member
Pond is flat until someone drops a rock in it or a bird poops on a flyby. Steel I beam. If it needs to be perfect, true with a grinder. Big heavy but will give you an excuse for another project to use it.

#### bmcj

##### Well-Known Member
HBA Supporter
If you want to check out the wood I-beams VIctor Bravo mentioned, they are called “engineered beams”. It helps to know the name of something when you go looking for it.

#### BoKu

##### Pundit
HBA Supporter
Taut monofilament is as straight as you'll ever need.

There are no perfectly straight lines in aircraft structures.

Everything bends all the time.

#### BJC

##### Well-Known Member
HBA Supporter
Mono is good. I like alignment wire because the stretch is undetectable and it is easier to see.

All the lasers that I have seen have a beam that is too broad.

BJC

#### Insect

##### Member
What kind of tolerance of you really need?
Wing spars, a sandwich of wood over foam, for a part 103. I would say within 1/4" will be OK, There will be dual struts with capability of fine tuning those for twist, etc. Maybe I am overthinking the accuracy a bit. I lined up the tables 40"x16'. There is one low leg, which I am making a PVC shim for now, it's 5mm low (in pic far side), otherwise they are within +-2mm, I'll bond the wings on the tables as well.

80-20 fixturing on these tables may be good enough.
Thanks!

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#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
Pond is flat until
No, even when still that pond isn't flat. My father was on a job refinishing a nice long runway. The engineers wouldn't let the blade operators use their lasers. If they had there would have been a pond rather than a dry runway.

All the lasers that I have seen have a beam that is too broad.
Thus my suggestion to use a hexagonal pencil. Catches just the edge of the beam kind of like a go/no-go gauge.

I would say within 1/4" will be OK,
Then don't bother with the laser. Just use the engineered "I" beam as it comes from the lumber yard.........Or use the string/wire to true up the tables you have. Little Scrapper had a pretty good thread a while back on building a table. A search under his name might turn it up.

#### Insect

##### Member
Little Scrapper had a pretty good thread a while back on building a table.

Yep, saw his with trusses and MDF on top, looks like a good design for light stuff. His build link is no longer, but thanks.

#### proppastie

##### Well-Known Member
Log Member
kind of depends on what you are doing with the straight edge.....I saw pictures of a table so I guess you are trying to make a table an adjust it flat......I use a a Laser on a transit mount....set it say 1" above the table and walk down with a ruler and shim so the dot is centered on the 1" mark......The dot on mine (not the one shown below) is 1/8 dia......I would guess I am within 1/32 of the center total all measurements.

Example

Edit: The harbor freight laser has some really bad reviews...The transit head looks the same as mine. I have a "Lasermark Tracer" I got off E-bay......

I also had one of these:.....uses button batteries that do not last long but worked well.....

I have a 2x2x3/16 alum angle I got at the salvage yard that I also use. It is 7 ft long.

I use 8020 for the control rod rigger it is good stuff.

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#### Insect

##### Member
Thx for the suggestions. I do run a self levelling green laser. Today added one PVC shim to table (leg was 5mm short, far side, middle leg of pict.), now the whole pair of tables is within 2mm with very little work, still going to make an 80-20 system to control both axis' of the ribs I make. I rather be accurate than regret it later. Not like wood and foam is expensive, but hey. I can't claim I need any better than 2mm for a part 103 wing

#### BBerson

##### Light Plane Philosopher
HBA Supporter
String is almost ideal. Yes, the sag will be evident so just support the string every 10 feet or so. Sight with eye to get sag to zero. The eye is accurate. Leave the string, check with eye each day before assembly.

#### proppastie

##### Well-Known Member
Log Member
String is almost ideal
Someone here a long time ago (can not remember who) suggested dental floss.....I have also used it, worked well.

#### Geraldc

##### Well-Known Member
The eye is accurate
I worked for years on steel fabrication and when joining long beams together the eye was just as accurate as any other tool.
I did a test with a laser and a detector at 100 yards and even though the laser beam had spread to half an inch the detector still beeped in the middle of the dot.

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
You say you need a straight edge. I think--reading between the lines--what you need is a flat surface. Can you clarify what you need please?

#### PIK-21

##### Member
When i built the fuselage of the PIK i used a straight back like the boat builders use. and backed up with a cross laser from Lidl. i was 1 mm out over 5 meters. I couldnt argue at that. The same with the spar.

#### robertl

##### Well-Known Member
So I need a straightedge of about 15' long to build some wing spars. I use and trust extrusions like 80-20, Misumi, kinda easy to make a jig for that and they are very straight. At work I have a 1"x2" 25' beam of 80-20 is not stiff enough to define any straightness, on it's own, not at all, so either a truss, or taller beam comes to mind... I'm drawing a blank on how to do this at a reasonable cost, any experiences?

Ultimately when you exceed 10' then gravity tends to take over, you can probably imagine the game. Any device that is truss shaped can be adjusted with a laser, not really rocket science, just fishing for ideas.

I could make a wooden pan and self levelling concrete, but, I'm not really wanting to paralyze the shop tables with 500lb of concrete to do this. I have a large pond, so any floating contraption could also work, then gravity is no longer flexing the beam, what a pain, making wing spars while in a pond, eh not for me.
My work table is only 12.5 feet long and the spars for the CH-701 I was building were 12 feet. I popped a chalk line on the table and used small wooden blocks screwed to the table on each side of the spar to keep it aligned. If my table had been longer, say 15 feet, I'm sure that method would have worked as well.
Bob

#### wktaylor

##### Well-Known Member
The eye is a magnificent straight edge... you can see even minor perturbances/deviations in alignment... and You will 'see' that even the best extrusions have obvious bowing/warpage that is actually allowed by aluminum extrusion specification 'tolerances'.

May I suggest a machinist or 'pavers' straight edge. I found a 1.00 X 2.00 X 192.0 aluminum straight edge blade . Aluminum is subject to 'nicking' and 'kinking' but with proper care/handling [2-or-3 pairs of hands]/storage could last indefinitely. Even with a 1.0 X 2.0 cross-section this part will droop dramatically from its own weight [estimate 39#]/flexibility.

Marshalltown 25543 16' Aluminum Straight Edge - Blade Only...
16' Alum Straight Edge - Blade Only Paver's Straight Edge Kits Straight edge measures 1 X 2" (25 X 51mm)

Marshalltown 25543 16' Aluminum Straight Edge - Blade Only

A SStl 'machinist' straight edge would have superior alignment and resistance to damage... and the tapered/fine edge that a draftsman could appreciate... longest that I found was 72-inch length. HOWEVER... my 0.25 X 3.0 X 60-inch SStl machinist straight edge is a hefty chunk of metal. I suspect that longer ones are available.
Suburban Steel Straight Edge, Square Edge, 72" - SE-72-TS
Extremely accurate reference line. Can be used for checking surfaces for straightness. Working edges are precision ground and finely finished. Perfect for drawing or scribing straight lines. Graduated & non-graduated models available. Stainless steel models longer than 24" have hang-up hole and all stainless models are flame hardened. Working edges are straight within 0.0005" per 12" (T.I.R.).

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NOTES.

IF You wish to 'trust' any straight edge for tooling-alignment then You MUST VERIFY!

There is only one aluminum extrusion [large Dia tube] that I am aware of that has an incredible 'Dia/wall-thickness/alignment' [dead straight] specification tolerances... over ~25-Ft length... and the raw tube is many thousands/tube and getting more/more expensive per procurement.... before polishing/anodizing and SFL application.

A trick I learned years ago about validating straightness/alignment with flat/symmetrical parts... place [2] parts back-to back and the outlines should perfectly match. You WILL BE astonished at what distorting/bowing is present in most 'perfect' parts with this simple test.