View Full Version : Newbie Substitute for straight edge?

04-10-2012, 10:10 PM
i've come to realize that the only thing i have in my shop that is straight is not as straight... :mad:

to make matters worse, my table cross members are not the same level... some are low, some are high... guess it's the woes of dealing with 8020 assembly...

so, to line them up, would an aluminum 6061 I Beam be a decent substitute for a straight edge?

how straight can i expect an aluminum I beam to be?

here's the specs:
6061 T6 Aluminum I Beam
4" width
3" flange
0.17" web
0.29" thickness

seems that it'll cost wee north of $70 for a 5' run...

thanks in advance,


04-10-2012, 10:25 PM
Could you not use a DTI with base and position it along the main beam at each cross piece for getting a precise reading and aligning where needed?

04-10-2012, 10:29 PM
I'm sorry HHenry, but i have still not graduated from CNC101... what's a DTI?

do you mean fastening my dial indicator to the Z, measure across the cross members?

one concern i have with the 80/20 is that the slotted center has a 2 deg dip to give springing tension lock on the nuts... so would work, assuming my 80/20 cross members (Y) are perpendicular to the X and also thinking they are flat, not twisted in any way?

or am i just being anal?

would just some decent spoil board surfacing take care of the differences?



04-10-2012, 10:30 PM
Dial Test Indicator,its used mainly on lathes and mills.

04-10-2012, 10:32 PM
do you mean fastening my dial indicator to the Z, measure across the cross members?

Not fastening but using the base for the DTI and testing each cross members ends where it meets the main beam/

04-10-2012, 10:33 PM
aaah... tyvm!

but ok, using the DTI, i can get the individual cross members leveled along the Y, but aligning them with each other? i have 5 cross members at the base of my machine and one of them seems to be off by at least 1/16"

04-10-2012, 11:57 PM
Can you use a laser or a very taught string to substitute for a physical straight edge?

04-10-2012, 11:58 PM
I would have thought the surfacing would solve all of your problems. 0.063" is easily removed. The sins of the table top are as good as gone.

Another approach was given by CarveOne : he fastened the table top to the cross members, then measured the Z at each point where the table top was attached. He then went back to insert shims between the table and the cross elements to make them as level as possible, and then resurfaced. This avoids trying to force the table top sheet to follow an uneven support frame.

A bigger problem is making sure that the two long axes and rails are parallel. If they are not spot on parallel, then the table really doesn't matter because there would be no reference plane for the table and work to be true to.

04-11-2012, 12:31 AM
In the famous MadVac build, he used thin plain (unwound) piano wire pulled taut, then buried in epoxy. He then hand-scraped the epoxy to the wire, checking each section with precision straightedges.

I don't think you have to go this crazy, but the piano wire would help.

04-11-2012, 05:58 AM
As PaulRowntree mentions, shims can be used to make up for non-flat surfaces. My favorite low cost shim stock is a roll of aluminum roof flashing from Lowes. It's about 0.008" thick and however many pieces needed to fill a low spot can be used to provide a solid support. It can be cut with common household scissors. Just slide it next to each side of a hold down bolt so that the bolt pulls the table top down evenly.

To level my 12' rails (1/4" x 5" CRS flat bar mounted vertically) I used two nylon fishing lines pulled taught between the opposite corners in an X configuration to align one rail to the other "reference" rail. When the lines touch slightly in the middle of the span where the lines cross the rails are level relative to each other. You can level the reference rail with a machinist level before doing the string/wire crossing procedure to align the second rail. Sewing thread or small gauge MIG welding wire (with no kinks) will also work for this. Try to compensate for the wire thickness. The smaller the wire diameter you use the better.

If you have a place that is off by 1/16" you may want to slide a washer between the support and the tabletop instead of building uo enough shims. Place the washer where the bolt will pass through it.

04-11-2012, 06:20 AM
how straight can i expect an aluminum I beam to be?

I don't think anyone answered this, but probably not straight enough.

04-11-2012, 10:15 AM
hahaha... while i enjoyed that nice trip around from piano wires all the way to epoxy, thanks Gerry... and everyone else...

seems i have a bit of a project on hand for the weekend...

but i still have to move my router to it's "final" home in my basement.

04-11-2012, 10:39 AM
I don't think anyone answered this, but probably not straight enough.

I didn't because I didn't know the answer,I was going to suggest that he plonk it against the edge of a door in his house/shop

04-11-2012, 12:00 PM

The I-beam would be about as straight as the 8020, The piano wire will have the least cost to do, 2 pieces of 1/4x1" with a small hole near the top to suit the wire, a slot or bolt hole to mount on the side of the 8020, this is very easy to do

Move your machine first then set it up

04-11-2012, 12:11 PM
thanks mactec54,

i'll give that a try when i'm done placing my machine.

04-11-2012, 05:54 PM
so... um... why piano wire? and not any basic wire?

just curious.

04-11-2012, 05:57 PM
Piano wire has very high tensile strength. You can wind it up pretty tight before it snaps and it come in longer lengths.