How straight does a straightedge need to be?

[siggy_7]

Having recently bought a planer thicknesser, and looking to build my own router table in the near future, I'd like to get myself a reference edge to check for table flatness with a feeler gauge. The default recommendation for straightedges is an aluminium spirit level, but how accurate are these over a 1m length? Are there ones to seek out that are particularly good in this regard?

Alternatively I was looking at either:
-The Veritas 36" steel straightedge, accurate to 0.025mm on both edges - £75.50 from Axminster
-A 48" Maun steel striaghtedge, accurate to 0.01% of the length (so 0.12mm) - £50 delivered from the bay

I'd prefer to have the longer length and £25 in my pocket, but would it be accurate enough? Also the Veritas one is wide enough to stand up by itself, which could be handy for checking table flatness.

 

[Wildman]

to fight for thousanths of an inch when wood varies day by day according to humidity is a waste of time and energy that can only leave you disappointed.. Having worked to half a thou most of my life I find woodwork quite liberating. It is a material that has life and and a mind of its own.

 

[Phil Pascoe]

If you're talking silly tolerances when machining timber, where and how hard you hold down the piece probably makes more difference than the microscopic measurement of the table.

 

[kernowjoiner24]

I would just face and edge a piece of 3x3" or similar until you get it perfect then use that to set up the bed and fences etc

 

[Jacob]

I would just face and edge a piece of 3x3" or similar until you get it perfect then use that to set up the bed and fences etc

Yes, or if you must buy one; for woodwork purposes a good quality ali spirit level is more than accurate enough , stands on edge, and is useful as a spirit level too!

 

[Cheshirechappie]

If you don't fancy shelling out, you can make your own straightedges.

Take three pieces of wood, about 3"x3/4" will do, but it's not critical, and as long as you need. Straight-grained is best, stable hardwood is good too. Designate one edge of each as the reference edge, and label them A, B and C. Plane all three as straight as you can. Now compare A against B, and note where the high spots are on each. Now compare A against C, and note again. Now compare B and C, note again. Remove the high spots (by scraper if there isn't much to come off), and repeat the comparing. Keep doing this until they match each other exactly, with no light showing through between them.

If you try to do this with only two pieces, you could end up with one slightly convex, and one slighty concave, fitting together perfectly. With the third piece, it may fit one of the curves, but it can't fit the other; so if three fit tight against each other, they must all be straight.

You now have three edges as straight as you will ever need a piece of wood.

 

[Grahamshed]

If you don't fancy shelling out, you can make your own straightedges.

Take three pieces of wood, about 3"x3/4" will do, but it's not critical, and as long as you need. Straight-grained is best, stable hardwood is good too. Designate one edge of each as the reference edge, and label them A, B and C. Plane all three as straight as you can. Now compare A against B, and note where the high spots are on each. Now compare A against C, and note again. Now compare B and C, note again. Remove the high spots (by scraper if there isn't much to come off), and repeat the comparing. Keep doing this until they match each other exactly, with no light showing through between them.

If you try to do this with only two pieces, you could end up with one slightly convex, and one slighty concave, fitting together perfectly. With the third piece, it may fit one of the curves, but it can't fit the other; so if three fit tight against each other, they must all be straight.

You now have three edges as straight as you will ever need a piece of wood.

No idea if that will suit the OPs needs but a good solid tip that I will file away for future use.
I love this place !!

 

[Andy RV]

I use a piece (18" x 1" x 1/4") of ground flat stock steel, commonly available on ebay and from cromwells in loads of slzes and relatively cheap, they are ground all over and are flat/ straight enough for my needs.

 

[No skills]

The cheaper/average spirit levels are usualy advertised with an accuracy of 0.5mm per meter of length, I have seen some very expensive ones that stated 0.25mm per meter (possably one of the stabila ranges).

If you want to work wood to that sort of tollerance then good for you and good luck, I think your mad - but good luck

 

[Eric The Viking]

I looked at the various straight-edge offerings and bought an Axminster one.

Personally, I like having machines set up properly. It's one less thing to worry about when I'm not getting the results I expect!

In the case of tablesaws, for example, it's the difference between a great, clean cut and burning or kickback, and similarly, I can just go to my SCMS and cut a mitre, knowing it's within 1/4 of a degree - not perfect but good enough for most things.

 

[Jacob]

....
Personally, I like having machines set up properly. .....

We all do, and most of us manage it quite easily without spending daft money on a straight edge.

 

[Eric The Viking]

I didn't spend "daft money." That was my point.

 

[Jacob]

Oh right. I thought they were £70 ish.

 

[Eric The Viking]

I bought the longer one of these: £25.

I think they've gone up a bit since I got mine, but it's general purpose - has to serve for non-woodwork tasks too. The shorter (450mm) one is only £15. It's milled on all surfaces, so you can use the thicker back edge for machine alignment.

I originally bought a Dakota one and sent it back because it wasn't straight enough. The Axy one is quite good enough for the purpose. I probably didn't need the longer one, but it's nice to have it.

I've got a couple of 2' steel rulers (600mm), and if they werent bashed about they'd be perfectly adequate for settingup the P/T, but they're old and the edges are chewed enough to be annoying for setting stuff up. And the extra heft of a thick straightedge is good, and, as it hasn't got a scale, I'm not tempted to abuse it, either.

E.

 

[siggy_7]

Thanks for all your responses. Like Eric, I like to play about and make sure that my machines are set up to the best of my ability, as getting them tuned up can only ever improve results. I have no illusions that I will be planing to the same accuracy of the straightedge over the same length! Although tell all that stuff about "good enough for woodworking" to those that spend lifetimes lapping the soles of handplanes to within 1 thou...

But these sorts of things will also come in handy for other tasks, like playing about with milling machines, so I don't mind a sensible amount of outlay on something of good quality that will last. I thought the Veritas one seemed rather steep and unnecessary, and from the feedback I think I've confirmed that view. The "make your own from wood" approach sounds intriguing but a little time-consuming, and also I wouldn't trust a wooden one to remain perfectly stable once made. So I think I'll probably end up getting the Maun one - the Axminster one looks nice but the longer version is 750mm, and I'd like something preferably over a metre.

 

[samthedog]

The cheaper/average spirit levels are usualy advertised with an accuracy of 0.5mm per meter of length

This. An aluminium straight edge won't harm planer blades or saw teeth if you accidently bump them. The price is reasonable and you get multi-functionality from the same tool. I have never bought a dedicated straight edge (except for mitutoyo edges for setting up / scraping machine beds). There is no need with wood as it's an "unstable" material that will flex and swell, rendering any attempt for absolute straightness a wasted effort. For machine set-up, I still wouldn't use a steel straight edge for the reasons I stated (damage to blades).

Good luck.

Paul.