Long Straight Edge options?

[Distinterior]

You can buy an 1800mm level for about £10 which will be straight enough to use to check the flatness of a workbench...??
I'm a bit confused as to what the problem is...??

 

[bp122]

You can buy an 1800mm level for about £10 which will be straight enough to use to check the flatness of a workbench...??
I'm a bit confused as to what the problem is...??

Was just wondering what people used.
I had seen the long levels at Toolstation etc, wasn't sure how good they were. That is why I was asking.
As it turns out people use different methods which I didn't know.

 

[CatIII]

Any recommendations for a 2m long straight edge for flattening a workbench top?

The workbench is (or will be) 1.8m long.

Are the long spirit levels that builders use appropriate for this application?

I’ve been using cast iron side beams off an old bed. Nice and long at over 5 foot and it’s angle iron so it’s easy to clamp. I use it to trim door edges with a router amongst other uses. One side beam I’ve cut down shorter to make it easier to use. Cos it’s cast iron, bit of a hack to cut with a saw!

 

[thetyreman]

I use a very long german made stabila and it's accurate enough in my opinion, it's around 2 meters long.

 

[Lazurus]

I have long lengths of alluminium box, great for routing and levelling footings etc

 

[Sideways]

Decent spirit levels are typically specified to 0.5mm per metre length. I guess they must be machined to that flatness.
Not perfect but not too bad.

 

[Jacob]

I use a Stabila level for levelling, and of course it will also do as a straight edge occasionally.
But if I hadn't got it, or wanted a longer one, I'd plane up the edge of a board.

 

[yetloh]

Got one on order, opted for Amazon prime one day delivery!

A joke, I know , but one of those won't take out a bow unless the opposite face is flat. I know a worshop that has a lovelyWadkin planer with 8' beds; now that would do it, but I wouldn't fancy putting my bench across it!

 

[Woody2Shoes]

Decent spirit levels are typically specified to 0.5mm per metre length. I guess they must be machined to that flatness.
Not perfect but not too bad.

I think that's more to do with the accuracy of the bubble (I seem to remember that standards normally require 3mm/m or better) the straightness of the milled edge would be better. I agree with the earlier post that the factory edge of an 8x4 sheet makes a perfectly good straight edge too. Obviously rigidity is important for a straight edge (don't titter...).

 

[J-G]

...I seem to remember that standards normally require 3mm/m or better...

That made me remember a discussion I had as an apprentice tool maker when Matrix installed their first laser test equipment to inspect the bed of a Spar Miller. This machine bed consisted of 10 castings each about 8' x 4' (we were still using imperial measure!) so over the 40' length I was told that the tolerance on the ways being parallel was 2 thou !! but of course we were not working 'wood'

 

[Rorschach]

A 6ft spirit level is a very handy thing to have. Obviously useful as a level but great for using as a straight edge in many different applications. Not terribly expensive either.

 

[beech1948]

I use a 3m length of 100x40 aluminium tube with 3 mm thickness sides. Initial checks of a line drawn against it 300mm from an edge showed that it was and remains straight enough to be considered a straight edge. Used to flatten a 75mm assembly table top and my planer thicknesser, sliding table saw and other gadgets.

At 3m its a bit ungainly but too useful to cut down.

 

[Ttrees]

But where to get these aluminium extrusions?
If the landlord wanted it back for something, it would be a priority for me to get another, as I use it all the time.

Looked at buying a similar sized level before, (with milled edges), as from looking at the really cheap ones, there's a lot more accuracy in the alu profile than those things.
Not cheap, about the guts of a hundred quid in the local.
Can't even check it as it's in the plastic, and probably would have a hard time getting a refund if it wasn't straight.

 

[Sideways]

There's a bloke on youtube :
AskWoodman / Verysupercooltools or some such (guess the nationality and again, don't snigger). He promoted the use of 8040 type alloy extrusion for saw fences etc several years ago when it wasn't so well known. He made some measurements as to the straightness of those extrusions. Very straight is what I remember, hard pushed to get a feeler gauge between two pieces held together.
I haven't tested one but always figured the back edge of a tracksaw track might be about the straightest thing in my workshop.

 

[Bm101]

One other slightly leftfield suggestion. Buy 3mm steel rectangular box section. Buy 2 x 2m sections and if you buy some old paramo /record cramp heads, drill some holes, you now have 2 (nearly free ) mega cramps for making doors. Or long stuff generally.... And two straight edges that will keep you fit and possibly put your back out on a cold winters morning.
I use a stabilia level (because I already had one)and two aldi fcramps + a section of wood that is exactly the width of the guide to blade. Mark the edge to be cut, add the width of the guide off cut and clamp sheet on trestles with the level. Bit of care it is bang on.

 

[Inspector]

That made me remember a discussion I had as an apprentice tool maker when Matrix installed their first laser test equipment to inspect the bed of a Spar Miller. This machine bed consisted of 10 castings each about 8' x 4' (we were still using imperial measure!) so over the 40' length I was told that the tolerance on the ways being parallel was 2 thou !! but of course we were not working 'wood'

The aerospace plant I worked in a had a milling machine 190'/57.9m long. There were 4 gantries with three heads on each, later upgraded to 2 with 2 heads but more rotation in the head to do horizontal work too. They got a second big machine with 2 spindles but they could scoot to the end and switch them out for two other types in a couple minutes. It was 65'/20m long. We had a number of coordinate measuring machines, the largest about 20'/6m long. Also had a laser tracker that had a measuring volume of close to a 200' radius. It was so sensitive that drafts would affect the measurements. I used it once to calibrate a granite surface table 6" x 12' long. It would be perfect for checking the flatness/straightness of a workbench. In my home shop though string straight is more than enough. I'm not building airplanes anymore.

Pete

 

[bp122]

Looks like the factory edge sheet material is the winner.

I do have a heavy duty 4080 aluminium extrusion, but it is only 1 m long.

Might make a hardwood one when I can.

 

[highwood122]

I use a Stabila level for levelling, and of course it will also do as a straight edge occasionally.
But if I hadn't got it, or wanted a longer one, I'd plane up the edge of a board.

got a decent german level myself its quite straight over 2mtrs,but how straight can you expect to make a wooden straight edge by eye, talking in thous.

 

[Jacob]

got a decent german level myself its quite straight over 2mtrs,but how straight can you expect to make a wooden straight edge by eye, talking in thous.

Easy. Don't talk in thous for starters! This is woodwork not rocket science.
If it looks straight by eye then it is almost certainly good enough.
There is one little trick if you want it to be even more precise - bevel the edge so that the straight is along a narrow edge rather than the full board thickness - it reduces any twist effect.
Incidentally this is the way to tackle very long timbers if planing by hand - you work to make one corner visually straight along the arris, rather than a whole edge or side. It's easier to do by eye just along the arris. Then take the faces and edges from that.

 

[Ttrees]

If laminating timbers, and being conservative, I think its good policy to use the bench as a reference and not relying so much on much technique.
If using this method, it will be fairly obvious. half of what's invisible to the naked eye, I'd guess.
I haven't checked what that is with a feeler, the test for that is if the timbers will sit without a gap.
The amount will likely be relative to the length of timbers you're planing most often.
i.e more forgiving on longer lengths.
I will be doing the same idea with a single shim under the middle on another bench, if need be, and expect to have to do some adjustment from time to time, and keep an eye on it,
More of an issue to get longer stock bang on, than concentrate on always taking a through shaving every time on shorter work, which is nice when its flat.