6' straight edge

[brburns]

I remember over at the "other place" years ago someone suggested using either cold roll or hot roll steel as a long stright edge for setting up equipment. Which was it? anything better at a decent price? I have a feeling that my DJ-20 bed is just verry slightly out of parralel between the 2 beds, and I would like to check it.

Thanks,

[TDHofstetter]

Any stick of wide aluminum extrusion will work just fine - it's surprisingly straight & true. Or, if you want something a little more robust, a stick of 1/4" (or thicker) x 2" (or wider) COLD-ROLLED steel will turn the trick. Hot-rolled is just too raggedy along any edge.

Whatever you do, don't let anybody machine it into a knife-edged straightedge for you - use it pretty much just like it is. Machine it & it'll tie itself into a knot unless it's been fully stress-relieved.

By "pretty much" I mean you may need to touch it here & there with a fine file to knock off any dings along the edges. They're usually in pretty good shape when you get 'em, but sometimes there might be a little dent someplace, and with any dent comes a little "ditchbank" that's gotta' get smoothed back down. Do it really sparingly, though - all the tension in cold-rolled steel is in the "skin".

[brburns]

Tim,

I happen to have 1.5" square extruded aluminum sitting around. Is that wide enough? I'm guessing the wider the better.

[TDHofstetter]

That should work fine for the purpose - wider is always better, but what you have - since it's square - is effectively more than 3" wide. Two 1-1/2" webs, plus the top & bottom straps.

Go over it with your fingers, feeling around for any dimples you might need to shave off. Rub your file down with sidewalk chalk, too, to help keep the aluminum from "pinning" in the file.

[boodrow]

When I was still turnin wrenchs I bought a 24" straight edge for checkin cyl heads and blocks. This tool has been of great value settin up my jointer and such.
Boodrow

[brburns]

Ya know what Tim, If you hadn't said anything about not machining the steel, I would have wanted to put it on a surface grider to straighten it perfect. Glad you said that. I know I would have eventually done that........

[larryh86gt]

Feb 27, 2010 17:56:04 GMT -6 TDHofstetter said:

Rub your file down with sidewalk chalk, too, to help keep the aluminum from "pinning" in the file.

I've gunged up more than 1 file. I never heard of this.

[cabinetman]

You can get a good 6' straight edge here:
hand-tools.hardwarestore.com/21-399-straight-rulers-and-yardsticks/straight-edge-aluminum-629085.aspx

The home centers carry them too.

[TDHofstetter]

Feb 27, 2010 20:15:31 GMT -6 larryh86gt said:

Feb 27, 2010 17:56:04 GMT -6 TDHofstetter said:

Rub your file down with sidewalk chalk, too, to help keep the aluminum from "pinning" in the file.

I've gunged up more than 1 file. I never heard of this.

Another thing that helps a LOT is the frequent use of a "file card", which is a brush thingie that looks a lot like a wool spinner's "raising card" - example shown here:

en.wikipedia.org/wiki/Carding

A file card works far better than a wire brush for keeping a file clean & sharp.

[Leo Voisine]

The factory cut edge of plywood is pretty straight.

I have used a 1/2 think by 6 foot long piece of plywood for years. I would say it is with in 1/64" on straightness.

Thick - like 1-1/2 think might be too thick to use as a guide with a circular saw.

How straight do you need to have your straightedge?

[triplefreak]

I bought a panel cutting kit at Homer Despot a few years ago.
It consists of two (2) 4.5 foot long pieces of machined aluminum.
Not extruded, not formed, machined. It has a joiner plate in the middle to join it together for cutting 8' long sheet goods.
It has been within 1/64" true since I bought it. I only paid $25 or so for it.
I use it to cut plywood, etc. all the time. No problems yet.

[brburns]

Feb 28, 2010 12:21:21 GMT -6 Leo Voisine said:

How straight do you need to have your straightedge?

I'd like it to within a couple thou, but I know I won't get that. I'm trying to make sure my jointer beds are parralel. I have plenty of things that are good enough for a circular saw and the like.

[Leo Voisine]

I used to work in an aluminum extrusion house.

If you saw the way aluminum is extruded - and stretched - you would understand why it just might not be so straight - especially over 6 feet. It will be wavy at best.

Within a couple of thou over 6 feet really is asking a lot.

I have a Stanley 4' level that I use to check my jointer. No it does not span the entire length of the bed.

I suppose a 6' aluminum lever might work pretty good - but again - it is extruded and not going to be straight within .002. I am sure it would be good enough.

I think the plywood would also be good enough. Not as pretty, but it would show where the jointer tables are not coplaner.

What is happening at your jointer that makes you want to be checkin it? Is there maybe a technique thing going on?

[brburns]

Sometimes I get a slight gap in edge jointed boards. Also depending on where I place the fence, it varries slightly on 90 degrees. Very very slight, but it does and I'm anal. I get too used to setting machines at work within microns and sometimes get too picky in my shop. But I like to put my mind to ease and get things as good I can.

[Leo Voisine]

Feb 28, 2010 18:46:08 GMT -6 brburns said:

Sometimes I get a slight gap in edge jointed boards. Also depending on where I place the fence, it varries slightly on 90 degrees. Very very slight, but it does and I'm anal. I get too used to setting machines at work within microns and sometimes get too picky in my shop. But I like to put my mind to ease and get things as good I can.

Yeah - Ben - You do know wher I was going.

Yep I know what you do for a livin, and yes it is likely that your trying to get these little home hobby shop machines to perform like big heavy industrial machines.

You can shim the gibbs and stuff like that to get them in dead. It's a hasstle, and an excersize that I am not convinced has any real benefit.

This is wood that we are working with.

As to the 90 degree, part.
Did you put a 90* try square against the fence?
You can move the fance to different locations across the bed to see if the bed varies in perpenticular to the fence.
You can compare the infeed to outfeed.

Ahhh yes - I do know what it means to be anal too

[dcarter636]

I really can't see a need for better than a few thousandths of precision here. I think the sag over 36" of cantilevered unsupported length of any truly flat/straight bar of a manageable weight would amount to many thousandths of an inch. As soon as that adequately rigid and stable 1/2" x 6" x 6' precisely ground bar gets dropped, dinged, or leaned upon, it's precision is gone anyway.

My version of overkill here is to moving a 24" steel straight edge for along the tables to gage the flatness of the jointer tables and a .0001" dial test indicator on a heavy goose-neck base to measure the table parallelism, and the out feed table to cutter height, alignment.

[TDHofstetter]

Mar 1, 2010 13:06:22 GMT -6 dcarter636 said:

I really can't see a need for better than a few thousandths of precision here. I think the sag over 36" of cantilevered unsupported length of any truly flat/straight bar of a manageable weight would amount to many thousandths of an inch. As soon as that adequately rigid and stable 1/2" x 6" x 6' precisely ground bar gets dropped, dinged, or leaned upon, it's precision is gone anyway.

My version of overkill here is to moving a 24" steel straight edge for along the tables to gage the flatness of the jointer tables and a .0001" dial test indicator on a heavy goose-neck base to measure the table parallelism, and the out feed table to cutter height, alignment.

That's a pretty fine test indicator - are you sure you didn't slip a decimal place? 0.001" is plenty fine, and readily available. Indicators that read in ten-thous are a little less plentiful & far more expensive. Overkill, too.

EDIT: Ah. Missed your point, didn't I?

[dcarter636]

Yeah Tim, my main point was that a six foot straight edge is impractical for gaging .001" in our hobbyist shop context. ;D

But I do use a .0001" indicator to evaluate the jointer, table saw, and hand plane soles, because as a hobbyist, anything worth doing is worth overdoing. I don't use my .00005" indicator much because it's a PITA to set, the range is too small.

My thinking is that the time required to make well made things ridiculously precise pays off over time, they seldom get better with use and the screwing around to attain near perfection using good measuring tools is not a lot more time than making them "good enough for now", which seldom is good enough a year later. I can certainly appreciate Prof Burns' desire to get it right the first time, but not with the long straight edge method.