gantry "jumps" along the X-axis

when profiling cores the router "jumps" once in a while, which causes a gouge on the core. this happens on practically every stepdown which makes for a frustrating final pass. they seem to happen in the same general areas but do not occur everytime. I thought I solved the problem... some of the screw heads, which fasten the X-axis racks, were protruding into the rack. thinking this gave a bump for the pinion to pass through, there where even marks on the screw heads where the pinion would hit it. I removed these protrusions, but the "jumps" still occur. I have also played with the X-axis belt tension and checked machine alignment.

anyone experienced this? any fixes/suggestions?
thanks

Redbull,

Did you solve this yet? Make sure your material is held down well, I have had jumping when my core lifts up a little bit. Make sure your y axis pinion gear is snug to the rack every where in the travel. The screw holes in the rack are oversized to allow adjustment. Mine was tight at the ends but would loose in the center of the gantry. I just loosened the screws in the middle and pulled the rack out a little so there is even tension through out the travel. Make sure your set screws on the z-axis pulleys are tight. If they become loose then the Z will not travel as far as it is supposed as the pulleys are slipping. Are you Skevik skis?

Morgan

haven't solved yet. just been roughing cores with CNC and doing final pass router bridge method... no time to fool around with something that has me stumped. thought you had something with making sure the material is secure, but it jumps while just jogging (not cutting anything). I will try adjusting the Y-axis rack as it also jumps occasionally, but I'm still stumped on the X-axis. Thanks for the response. any other ideas?
Yes, we are Skevik. checked out your site, looks like you got things rollin. how long have you been around for?

Does it jump in certain spot in the x axis travel or when changing direction? I was thinking there could be a lump in the UHMWPE tape that Mike uses along running surface for the x axis bearings. Or if you gantry is out of square on the rails or if the rails are not parallel things could be binding up.


Morgan

I have left this alone for awhile now hoping to see if anyone else would chime in and offer up if they too encounter this. Thanks to romp for helping to work the problem peer to peer. Like you said there are several things to take a look at from a burr in the aluminum rub rail to trash below the vibration dampening tape used on the rails.

However what you are still describing Bull is an intermitent bounce/jump along the long axis that does not happen in the same place or at the same time and is difficult to determine which side in particular it is happening on. Going back to one thing also mentioned by Romp, is the plum or square of the gantry and rails; Also if your A motor tuning and / or home and limits settings are different from those programed for the X motor the machine can do some funky things.

I have recently found a discrepency in the section 2 users manual where the image showing the "Home and Limits" set up lists some differing numbers for the X and A motors. Check your manual and your H&L settings to see if X and A motors are exactly the same. It would not hurt to check your motor tuning window as well.

If your settings are slightly different, simply change the A motor setting to match the X exactly. The difference in numbers [if shown on yours] would be under the soft limit column in the Home and Limits settings of your config. drop down menu.

Aside from all of that there are several other things that can be checked also. A few of which will involve removing the Gantry from the rails, but let's begin with the simplest approach frist.

Have a friend jog the gantry forward and back while you closely inspect the gap between the top of each rails' bolted on angle piece and the bottom side of the gantry leg [the C channel directly over the rails]. If there are high spots in the top of the bolted on angle [the rub rail] they can catch or bind on the bottom of the gantry leg and cause a jumping of sorts. You will need to look at the entire length on each side as the gantry is moved forward and back. To "fix" any high spots on the rub rail take a belt sander [or palm sander] or even a file to the top edge of the rub rail in those spots and even the entire length.

If after inspecting the gap on both sides and that is determined to not be the issue, you will want to inspect the roller bearings and gear/pulley assemblies for each side of the gantry. Follow the steps given below to remove and rest the gantry for inspection and maintenence.

- Jog the gantry all the way forward then power down the control box and unplug the motors and switches.
- Use an allen wrench to loosen the X and A motors and drop them down in thier slots and remove the drive belts.
- Loosen the turnbuckle on each side's pivot arm so you can easily dissengage the gears from the gear racks on both sides.
- With help, roll the gantry forward until the spur gears clear the end of the aluminum rails. Then gently tilt the gantry back
resting the motors on the rails and exposing the bottom of the side assemblies. Place a spacer block beneathe the Y/Z carriage
to help support the wieght.

This now faces the underside of the gantry legs and pivot arms toward the front of the machine making them easier to work on. Carefully inspect each roller bearing looking for dust or trash that may have become embedded on the rollers' surface. Also be sure to check if they are secured to the gantry leg and are not sloppy or loose. Clean the rollers in either case.

Next inspect each gear/pulley assembly at the bottom [now front] of the pivot arms. Rotate them both ways repeatedly and see if there is any kind of bind or difficulty in the turning. Also check for any slop or play in the assembly; if they are too tight or difficult to rotate by hand it would show up in a "sticky" point in jogging or running a program. At the same time if there is excessive play in the assembly it could result in a jump or bounce. Mainly what you are looking for here is if there seems to be anything catching when you turn the pulley by hand in full rotations.

It is possible that the bolts attaching the bearing blocks [white plastic pieces] to the pivot arms are extending past the aluminum and hooking on the back-inside of the pulley. If this is the case remove one bolt at a time, hold it to the edge of the bearing block and guage how far past the aluminum that bolt protrudes. You can shorten the bolt by adding washers when you re-install it or even grind the end down with a file or belt sander. Ideally the bolt's length should not extend past the aluminum it threads into.

Once you have inspected and maintained all of this, reverse the above steps to re-install your gantry plug it in and resume tests. Let me know how you make out.

MC