Thread: Big Straight Edges.

Can you use a length of aluminium angle?

I think the ali would still be a bit bendy over that length, maybe a bit of box steel, 100x50 or something.

Originally Posted by Pup

I wasn't entirely sure if I should post this in the workbench forum or here, but “Big Stuff” certainly seems to fit the bill.

I was wondering if anyone has any ideas as to how I can build two 5m long straight edges?

I have a 4.8m long workbench at work that I use for nailing up long kickers, building reception counters on etc that I want to level and flatten. At the moment the work surface is out by up to about 3mm in places which can be a pain when you are trying to assemble things on it. The only way I can think to flatten it is by using two straight edges and a router sled, the problem is that doing so requires me to have two 5m long straight edges that are capable of bearing the weight of a router+sled without bowing.

I’ve considered making 5m long torsion boxes but it is a lot of effort for a one off job. I’ve also thought about using 6m lengths of 35X150 vic ash but that leaves me with the problem of having to joint the boards before I can use them...

Any suggestions?

forget about long straight edges and use three winding sticks instead

one winding stick at each end will allow you to determine if the ends of the bench are in wind

the 3rd one at different locations between the ends will tell you the wind and dip/hump between the ends
use a #6 or #7 hand plane to remove the high spots

the principle is you need a bench that is all in one plane which the winding stick will allow you to determine

What about dips rather than high spots ?

If the 3 winding sticks all line up then where the middle stick is is in the same plane as the ends.

if the middle of the bench has a dip, then the middle stick will be lower and probably slanted relative to the those at the ends. Putting a shim under the middle stick will tell you how much has to come off the ends to lower them to the same level as the bottom of the dip. Then the slant of the middle stick will tell you how much has to come off the back of the bench.

Find the lowest area of the bench, relative to the ends, adjust the ends (keeping them level) so that they are in line with the lowest area, then the rest of the bench will be higher than your reference points
lower the high spots one at a time

G'Day Y'all
Go to a concrete tool store and buy a magnesium screed/ float. They are about 30mm x 120mm x whatever long. They are light weight and don't bend.
Regards
Joe

Originally Posted by ian

forget about long straight edges and use three winding sticks instead

one winding stick at each end will allow you to determine if the ends of the bench are in wind

the 3rd one at different locations between the ends will tell you the wind and dip/hump between the ends
use a #6 or #7 hand plane to remove the high spots

the principle is you need a bench that is all in one plane which the winding stick will allow you to determine

The straight edges aren't for checking the surface, but rather for using as guide rails for a router sled. The idea being that you run a straight edge beside each side of the bench and level them to each other, then sit a router sled on top, drop the bit down to the lowest part of the bench and router the thing true.

I'm not so sure about using hand planes, the surface of the top is 4800X1215 (MDF) and I can't see even my #8 getting it truly flat.

I'm thinking I might have to go down the metal route.

jredburn, a screed would do the job great, but I can't find one long enough that is under $700

hi Pup
if the surface of your bench is MDF, then the best option might be to bog up the low spots, sand it all flush and level and then put a sheet of masonite over the top as a sacrifical surface

Originally Posted by ian

hi Pup
if the surface of your bench is MDF, then the best option might be to bog up the low spots, sand it all flush and level and then put a sheet of masonite over the top as a sacrifical surface

A variation on Ian's approach and requiring less skill is to put light battens around all edges of the benchtop. The corners must be watertight. Then pour some very runny epoxy over the benchtop - thinned, say, 20% with acetone or MEK - and the epoxy will self level and fill any voids. Then add the sacrificial new top.

Cheers

Graeme

PS: Wrap the battens in clingwrap to stop the epoxy sticking, or be prepared to route them off with a strait cutter with bearing.

If you are not up for hand planing, the router option might work with a sled running on straightedges. Instead of getting solid straightedges stiff enough to not sag, how about some strips of timber 90x19 or so and pin them to the long sides of your bench, parallel. you can put as many pins in as you wish to stop bowing. Then you only need something else the width of your bench.
Depending on what your benchtop is made out of I'd have a look at packing it up in the middle between frame and top, or even under the feet.

BTW
I think floor-leveler, epoxy resin or bog won't give a nice surface to work on, also will become brittle so if you do much hammering on that bench I think it won't last.

Originally Posted by ian

hi Pup
if the surface of your bench is MDF, then the best option might be to bog up the low spots, sand it all flush and level and then put a sheet of masonite over the top as a sacrifical surface

Hey Ian,

Even if I bog up the low spots instead of removing the high spots I'm still faced with the problem of getting it flat and smooth.

Regardless of how I achieve the end result I will be putting a sacrificial top on it, the work surface gets a fair amount of abuse and as such never lasts more than about 6 months.

I would have thought a hefty aluminium extrusion would exist that could do the job and would be less expensive than the float...

Or laminate/bolt a few wide ones together so there's no sideplay (4 of 100mm by 20mm for example).

Cheers,
Garth

over six meters borrow a dumpy level and / or make a water level. to find the high low spots.
There are lots of self leveling flooring products that you could use. they cure hard and will not need to be replaced nearly as often...

Two steel purlings will give a relatively flat space. flat enough that you would be bothered to check. That is how we used to flatten slabs in the shop. pretty quick going too.

Good luck and we want to see pictures of a 6 m work bench!