Thread: How straight does a straight edge need to be?

Originally Posted by Sir Stinkalot

So after that long winded story I would be interested to hear what others may be doing. I am confident that the Veritas would offer the most accurate out of those mentioned, however is a raw length of aluminium going to be just as effective for the tasks I am looking at undertaking?

I have no idea how absolutely straight it is but use a 3m long length of 50 x 25 mm ally extrusion to mark out sheet goods.
I bought a 6m length from Capral and sold the other half at cost to a WW friend.
Something like 35 x 85 would be better.

Thanks Bob. Do you roughly remember the price of the extrusion?

The local Bunnings have 50x30x2mm hollow rectangular Al in 1m @ $17.10 and 3m @ $51.30. I had intended to check out the local Capral place, they have the sizes listed on their website but I would need to check on the prices.

It would be interesting to know how straight that the Al sections would be given they are directly off the mill with no refinement. I have a feeling they would be perfect for a general straight edge for cutting down sheet goods, but I am not sure if they will be accurate enough for machinery setup.

Originally Posted by Sir Stinkalot

Thanks Bob. Do you roughly remember the price of the extrusion?

I thing it was ~$60 for 6m

Originally Posted by Sir Stinkalot

...The thought of being able to get a 6m length and being able to create a number of smaller straight edges is very appealing. ....

Capral Aluminium Extrusions, Suppliers - Security Windows & Doors - Capral will sell some of their 6M lengths in halves.

I bought half a 2" x 1" with a .01" wall thickness for about $30 & the bloke chopped it up into 1200, 900, 600, & 300mm lengths for me.

At work we (well, I) have a Veritas 36" steel straightedge for machine setup, mill finish ally extrusions are fine for everything else.

Just as a side note, we bought one of those 'proper' 2.7m ally straight edges (white with a red label, I think the brand was called Master Finish) ... had a 2mm bow over the length. Got a 2.4m as well, same deal. It's now labelled the 'not straight edge'

I use my Veritas 2' long steel for setting the planer tables at the same hieght for a blade change, all I need is 2" over onto the outfeed to tell me up/down/OK
For other machinery setup I use the above or if I need a bit longer I have a 1000mm steel rule



Pete

aldi is your friend , or will be when they get to SA

I've got the 38" alloy LV. Effectively it's longer than the nominal length as in some applications I can slide it along the surface to be checked.

For short-piece checking I use an expensive UK-made 150mm rule that I've checked against my LV 12" steel job.

To mark out some sheets for cutting I got a long level from Alltools/TotalTools/whatever for maybe $40 and checked it against the long LV; it's concave by about a thou. Good enough.

Alloy is prone to dings so take care with it if you want not only to check but to mark.

There is also the local product from Michael Connor Woodwork:
Workbenches and Accessories

scroll to the bottom of the page.

It depends on what you want to do. If you are jointing edges to glue together, it pays to have an accurate straight edge. Blackwoods (which has offices in every state) carries a range of steel straight edges from 600mm to 3m in length. https://www.blackwoods.com.au/search...dges/202004872

I have a 1500mm one and use it often to check for flatness. They aren't cheap, but are accurate. I could use a shorter one at times, but it is nice to have the length when joining boards.

I don't know how good the aluminium ones are, but they would be lighter (but I suspect easier to damage). As you say, being thicker, they could stand up on their own, but they are that shape to give enough strength to keep their straightness.

For accurate work I just don't think that buying a piece of extrusion would be accurate enough, but you might get lucky.

Agree that fitness for purpose is the starting point.

How do you know the Blackwoods are accurate starr?

When I plane two edges and the straight edge shows them to be flat, they go together without any light visible at the join, so that's straight enough for me.

I usually put the straight edge on the board and rotate it back and forth a bit. If it rotates at the centre, the centre is too high and similarly for the ends. I then use a thin piece of paper to make sure it doesn't slip under the edge anywhere along the board being checked.

The company should be able to tell you how accurate the straight edges are guaranteed to be (I'm surprised the website doesn't state this.)

Yeah.

OP asked for something reliable at the get-go as the Yanks say.

I was surprised to get my result with a cheap level.

But could only know that with the benefit of quality gear. The maker guarantees 'The reference edge is machined flat to within 0.003" over the entire length.' re V. alloy straight edges.

Re fitness for purpose, my second bandsaw turned out to be somewhat agricultural.

The wheels could not be adjusted to be co-planar (the motive for getting the long LV/V straight edge).

The blade guides were a pox to reset after a blade change.

Well as it turned out it was perfectly possible to cut a straight line without co-planarity.

And blade changes could easily be reduced by using a bimetal blade and sharpening it in situ.

Horses for courses.