Bowed Table Problem

[BanduraMaker]

My machine is made with 8080 extrusions for the long axis. One of the "features" of 8080 extrusion is something called the "Drop Lock" which can be seen here: 80/20 Inc. - The Industrial Erector Set?

The problem I'm having is that when I bolt down my t-slot extrusion table to my 8020 extrusions, the middle of my table raises about .008 and the ends drop about .003 for a total "bow" of the middle being about .010 higher than the ends. This happens with just the smallest amount of pressure at the ends. I Didn't measure but it's probably even worse when both screws at each end are tightened down. I'm thinking that this is due to the drop lock feature.

I've checked the table extrusion and it's very flat when it's not being bolted down at the ends. just barely tightening it down to the extrusions it's still flat. By barely, I mean when you take up the slack and the screw just touches the metal. Anything beyond that (like an 1/8th turn) and the middle starts to bow. when the middle is at +.008", the screws are still loose enough to move the table top easily.

This is a problem. Any suggestions on what could be done to remedy this?

[louieatienza]

My machine is made with 8080 extrusions for the long axis. One of the "features" of 8080 extrusion is something called the "Drop Lock" which can be seen here: 80/20 Inc. - The Industrial Erector Set?

The problem I'm having is that when I bolt down my t-slot extrusion table to my 8020 extrusions, the middle of my table raises about .008 and the ends drop about .003 for a total "bow" of the middle being about .010 higher than the ends. This happens with just the smallest amount of pressure at the ends. I Didn't measure but it's probably even worse when both screws at each end are tightened down. I'm thinking that this is due to the drop lock feature.

I've checked the table extrusion and it's very flat when it's not being bolted down at the ends. just barely tightening it down to the extrusions it's still flat. By barely, I mean when you take up the slack and the screw just touches the metal. Anything beyond that (like an 1/8th turn) and the middle starts to bow. when the middle is at +.008", the screws are still loose enough to move the table top easily.

This is a problem. Any suggestions on what could be done to remedy this?

I've contemplated this many times, and is the reason I opted to use mic-6 for the table, and add t-track to that. Also, I think the t-table squeezes somewhat, twisting the rest of the extrusion. I think what you CAN do is bore through your long rails to make access holes, and bolt
the t-table from the bottom.

The other things you can do? add a couple more 8080 longwise to attach the t-table to, or simply affix a spoilboard and level that...

[gio666]

The problem is the 8080 profiles are not prefect flat , and bow in a little on the T slot ends to help lock stuff together and lower vibration. i found this way a lot better to mount top

[gio666]

[BanduraMaker]

Well, I was hoping for something easier but I guess that's the way I'll have to go. Unfortunately it means re-machining my table extrusions but oh well.

George, What size angle is that?

Louie, I've been thinking about using MIC6 for a table top for a while but the thought of all that drilling and taping isn't that appealing. I could see doing that upgrade down the road when I have a better grip on exactly how I'm going to be running my machine.

[BanduraMaker]

I think for now I'm going to go the angle route for mounting the table.

In any case, I'm going to use 1.5" angle and I'm trying to decide between 1/8" hot rolled steel and 1/4" aluminum.

The aluminum sure would be easier to tap but the steel would hold longer.

Any thoughts on whether one might be flatter than the other?

[ger21]

I have some 1/4" x 1-1/4" aluminum angle and it's not 90°. It's off about 1/2-1°.

[BanduraMaker]

eeeenteresting. I found a short chunk of 2"x2" .250 angle aluminum in the basement and it's quite flat and seems to be dead nuts 90 degrees. I checked it with a square and a digital angle finder.

I'm leaning toward the Al route just because it will be much easier to drill and tap. Theoretically it should expand at the same rate as my extrusions too although I do have 20mm steel rails bolted to it so I guess that's somewhat irrelevant.

[ger21]

Theoretically it should expand at the same rate as my extrusions too

How much temperature change do you get in your basement?

[BanduraMaker]

I don't know. It seems that my ears and face get all of sudden hot from time to time when using my machine so it seems that there are wild temperature swings in the basement that seem to coincide with breaking bits and ruining parts....weird!

[BanduraMaker]

I have some 1/4" x 1-1/4" aluminum angle and it's not 90°. It's off about 1/2-1°.

Ger,

Thanks for bringing this up. I stopped at Metal Mart today to pick up the angles and it wasn't until I went to the architectural Al that I found the square stuff.

Glad I brought my square!

[JerryBurks]

Maybe I don't understand correctly how the T-slot profiles are installed....but it looks like they are pretty much hanging free over the x-screw for the entire width?

If that is the case I would be concerned about the flex of these slats. You can have the most rigid gantry in the world but if the table itself gives under weight, clamp and machining force that is no good. The table on my old shark pro machine was made from 3/4" thick extrusions, 30" free hanging, and it contributed about as much to the horrible flex as the flimsy HDPE gantry. That is why I went with 1545 size 80/20 extrusions for my bamboo machine table and these are bolted flat to a solid plywood torsion box (96 bolts M8). Maybe that was a bit overkill, though