Can't Flatten Table??

[wardsa]

I have posted this problem a few times and although Brady has helped a bit with it I am still having issues that leave me scratching my head. I have owned my machine for about 10 years so I have a good grasp on how things work. Here is my issue...I have a 2004 PRT 48X96 .My cuts are about .015 deeper inside 12"-16" from the edge of my table. It is consistent all the way around my table. I have attached a pic. so you get the idea of what I mean. I have replaced the spoil board many times and have even re-surface/flattened my plenum. I just pulled my spoil board off again and ran many passes with a v-bit and I get the same results on the plenum. You would think that if I surface/flatten the spoil board, even if there were variations in the table it would all transfer through. No matter how many times I flatten I get the same results. 95% of the time I am cutting cabinet parts so it doesn't matter but when I want to VCarve, the letters are cut "deeper' at X20,Y20 then they are at X5,Y5. My first thought and what Brady suggested was the gantry was sagging but then why wouldn't it when I flatten the spoil board. I did run threaded rod to reinforce the gantry as Brady suggested. This is driving me crazy! Any suggestions would be great.
Terry

[cnc_works]

Have checked that your rails are equi-distant so that the rollers are not riding up on the rails in spots? You should actually be able to see .015" with a straightedge. Flatten the board, then immediately afterward lay a straightedge down and view with a flashlight behind. Is the v-carving being clamped down (or vacuumed) in the same manner as the flat work? Good luck, Terry.

[Gary Campbell]

Is your spoilboard glued down?

[Burkhardt]

Wild guess...If you are using a large fly cutter bit for surfacing at high speed, that may apply higher vertical forces to the gantry than a small v-bit. That in turn may make the z-carriage move more (flex or play) when the spindle is in the middle than when at the ends. But that would assume that something is indeed unusually flexing or loose.

You can clamp a dial gage in the collet with a 1/2" rod as adapter and scan the table surface. That should give you a hint where to look.

[wardsa]

Thanks guys.

Wheels seem to be riding fine. The VCarve is held down with the vacuum. I have a quad plenum.

Spoil board is siliconed to the plenum. The first few times it happened I thought maybe I put too much silicone on and it was compressing and springing back so then I started just putting it on real thin and even. When I put it on I usually turn on the Vac. and put 5 or so sheets of ply on it for weight.

I use a 1.25 cutter for flattening. I wondered the same thing so tried fast, slow, climb,conventional...every way I could think of. Same results.

So..I just discovered this. If I put a straight edge across the X and Y, the table is dead flat in the middle and tappers down the .015 around the whole perimeter of the table. This I am checking on the plenum and I know for a fact is transferring to the spoil board. Wow, now I really confused! I have attached a drawing of what I mean. It is obviously not to scale and exaggerated but just wanted to give a visual of what I mean. I'm not even sure where to go from here??

http://www.talkshopbot.com/forum/attachment.php?attachmentid=29314&stc=1

[guitarwes]

Does the rise only happen in the Y direction or along the X direction also? Like a plateau or a pyramid?

[wardsa]

Mostly at the ends of the X. A little at Y. I am resurfacing my plenum right now. It looks to be cutting the whole table flat. Moving slow and taking off just a bit. When it's done I'm going to drop the Z .010 and run the same program again. I'll seal the plenum and attach the new spoil board tomorrow. Nothing really different then I've done many times, except I've never run the surface program twice in a row. We'll see??? Maybe I have been trying to remove too much at a time???