New Machine Build- Getting The Bed Frame Flat?

[johnmac]

I am starting the build on my 2nd router table. It is a 36 inch by 48 inch steel frame. I just finished tack welding the bed outer frame. Right now its just a 3x4 ft. rectangle. I'm happy with the squareness, but how can I tell if there is a slight twist? I would like to make sure its flat before continuing with the welding.

Thanks, John

[CarveOne]

A cheap, simple way to do it is to tape a length of sewing thread diagonally from corner to corner. Tape one corner securely, pull the thread tight and tape it securely. Repeat for the other diagonal corners. The error in this method is very small due to the thread diameter and very light weight. The threads should very lightly touch where they cross. Any remaining non-flatness of the steel can be checked using the thread method also. Polyester thread can probably be pulled tighter than cotton thread. Very small diameter fishing line or copper wire works also.

CarveOne

[grahamshere]

Hey Carveone, awesome, never would have thought of that. have to use that method when I build #2 machine. Thanks bud. Graham

[CarveOne]

This method is how the ancient Egyptians and Mayans did it before the advent of the laser level. They didn't see any point in waiting around for the invention of the laser and microchip. Too many things to get done before those appeared in online mail order sites anyway. (Just kidding of course)


CarveOne

[Khalid]

CarveOne.. Can you sketch it out?

[johnmac]

Thanks, CarveOne! I tried it last night and it looks like I have about 1/16 inch gap where the threads cross.

I this high tech age, I still love these low tech techniques that work good.

John

[Khalid]

John can you post pics how u did it?

[grahamshere]

Those ancient Egyptians and Mayans were very patient people, I cant wait for the replicators to arrive .

[johnmac]

Khalid, Right after checking it, I pulled the thread off.

It is rather simple. Like CarveOne says, use some sewing thread and tape it in one corner. Then pull it snug and to the opposite corner and tape it down. Now do the same thing to the other corners. You should now have an X thread in the frame. Just look where the threads cross in the center. They should just touch. If there is a gap between them, or they press into each other, there is a twist in the frame.

John

[CarveOne]

Taking this idea a little farther, suppose you now position a thread on top of a side rail and midway of the side rail thickness and tape the ends taut. This will show you any bow upward or downward or sideways if there are any gaps between the thread and the side rail, or the thread doesn't follow the center line of the side rail. It isn't five digit accuracy but it will do ok for these wood cutting machines. Below is a picture of my "String Theory".

CarveOne

[rlrhett]

The other traditional way of seeing a twist is through winding sticks. Winding sticks are esentially one light colored straight edge and one dark color straght edge (for contrast). You place the straight edges on opposite sides and sight along them. If the light colored straght edge dissapears behing the dark, there is no twist. If one corner is reavealed, you have twist and you can see by how much. See:

http://www.leevalley.com/wood/page.a...76&cat=1,41182

My real question is, now that you discovered twist in your metal frame how do you fix it?

[CarveOne]

I know of that method also. A black thread placed across the near end of the frame and a white thread placed across the far end of the frame will work the same way, although the background may allow the two threads to disappear in places. They will give very fine indication of twist for final tuning. The threads won't survive twist relaxation with a torch if left in place though. I would heat the frame while it is weighted in the direction that needs twist removal. Let it cool completely and check the twist amount again. Use only enough heat to see the frame relax a little. Do it multiple times until you are satisfied with the flatness.

CarveOne