Check out this thread. http://www.cnczone.com/forums/showthread.php?t=51485 (Make your Gantry rock solid!)
I wsa piddling with my machine [basically a Joes 2006] yesterday, squaring up the bed .I ran a pointed cutter just deep enough to make a shallow furrow in the bed in the "y'axis.Then I brought the cutter back to the side and did the same in the "x" axis.the two furrows were not even close to being square [at 90 degrees to each other].I am using a single central screw to drive the gantry,do i need two outoard screws to prevent racking orare there some tricks to alleviate this problem. regards mike hide.
Took me a while to figure out but finally the penny dropped.What a great solution to an aggravating and potentially expensive problem....thanks for the great solution ,iam off to Lowes in the morning......regards mike hide
Thanks to this thread, I also have a "sliding knot" setup on my Joe's 2006. I found some steel, ball bearing "Pulleys" that are sold as sliding door guides at Lowe's. Six of those, some 1/4 inch thick aluminum plate for mounting platforms, and some 1/16" galvanized wire rope and swages, and I was in business. I cut some pieces of the aluminum plate about four inches by seven inches, drilled and tapped each with a 1/4 x 20 tap, and screwed the door guides down to the plate. I cut the bolts off flush with the bottom side of the plates, and then used polyurethane construction adhesive in a caulking gun to glue the plates to the upper surface of the gantry spreader. I drilled through the length of two 1/4 inch bolts for the 1/16" cable.... and used the little round cable "clamps" (little round donuts that you crimp onto the cable) to secure the cable at the fixed ends of the cable runs. The cable runs around the four pulleys under the gantry... out holes in the end plates of the machine, and around two more pulleys to come together.. connected by a six inch extension spring and turn-buckles. It took me a bit of staring at the drawings of the sliding knot setup quoted in the link above... but basically what this thing does is apply spring tension in both directions to both sides of the gantry, and prevents it from twisting or "racking" as it travels the length of the bed of the machine.
The modification was easy to do... and very effective!
can you post some pictures of your setup now it will help others by seeing it in person.
Good idea Joe...... I wish I had taken some photos before I installed this thing, but I got excited about it, and wanted to try it out. In these pix, you'll see the pulleys mounted on the plate on the upper surface of the gantry joiner box.... the pulleys mounted on the ends of the machine, and the drilled out bolts that support the cable where it goes through the "fixed" end of the machine. The spring, and turnbuckles stay pretty much stationery, and obviously, the cable tension is adjustable with the turnbuckles.
TC, I see that your four pulleys that are on the gantry spreader are side by side instead on top of each other. Do you have any problem where your cables cross each other under the table? Seems that the cables would be riding on each other (which may not hurt anything).
So far, the cables crossing each other is a non-issue.... I don't think there is much pressure between the two of them. A thousand hours of operation from now, I may have to replace the cables..... It shouldn't take over five minutes, and the cable is .16 per foot from Lowe's.......
I was really surprised how much smoother the machine runs (at least in the long axis) by having that "pre-load" tension on each side of the gantry.
Naturally, my machine is down again. I replaced the computer I was using with one that had a "real" parallel port, which my CNC foam hotwire machine requires. The replacement ran fine for about a week.... and then went ZAAAAAAAP when I booted it up a couple of days ago. It is on the way back to the dealer for a warranty replacment.
Gilda Radner was right...... "It's ALLLLLWAAAAYS something.... "