Circles not concentric

Hello, the circles I tested in the wood are more elliptical than circles, I search in this forum and tried some of solutions but still can not make it circles.
The steps I already tried are:
1- check the screw and if nuts are not loose
2- check if the GT2 tooth pulleys are tight no screw loose.
3-I adjusted all GT2 belts

I grabbed the bottom of the Z axis and was able to flex in one direction to another with no more than a 1mm loose but I could not fix it, a link to a 5sec movie to explain in the link below, I found some of the lower bearings are not touching the conduits.

The measures of the opposite corners are 894x896mm I did not try to fix this.

I am using Marlin and dual end stops, the machine is 820x680mm (X/Y) Hight of the foot = 280mm hight of the Z conduit = 270.

I made a video for the gantry loose if interested the link is this:
https://www.dropbox.com/s/ecmq54g7l2ksoq0/IMG_51291.mov?dl=0

How far off are your circles? Are you raising the material closer to the top of z travel? How much wiggle when you grab the bit with the z down vs up? Can you make circles with the pen mount? Have you checked that your x and y travel are calibrated?

All the bearings should be touching the conduit. That’s what’s probably causing your z play. I can’t imagine you’ll get perfect circles with z play combined with out of square x and y.

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1- About 1cm between each one
2- Yes the Z conduit is not long enough
3- Yes long time 3 months ago, did not check now
4- it is about 1mm less than 2
5- How can I check X & Y calibration?
6- Conduits are very good and expensive, still some of the bearings are out of the conduits, What do you suggest?

What endmill, what depth of cut, what speeds?

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You check how far it actually moves when you tell it to move a certain distance. A simple way is to measure how far the roller moves when you tell it to move 100mm for example. Another simple way is to mill or draw a square pattern and measure whether the dimensions are correct. You check this for both X and Y axis.

I’d suggest tightening the tension bolts. Check out the assembly instruction for details. Personally I tighten them just enough so that they don’t move freely anymore when I try to rotate a bearing by hand.

All that said, you have a very long Z travel and it will be hard to make it very stiff. I would try to play around with different CAM approaches. For example, make sure to use a finishing pass.

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X&Y are calibrated square is perfect.

All the Bearing bolts are tight, more than this will brake the PLA.

I am not sure the Z are long, Z travel is 9cm and Z conduits is 28cm long.

You need to make sure that all the bearings are touching the tubes. You don’t need to make it tighter than that.

This is very long. It means that the vertical distance between the moving tubes and the tip of the end mill is large. If you get some force on the end mill during cutting, this will tilt the router a lot and make it very inaccurate. Ideally, this distance should be as short as possible. If you don’t want to make the legs shorter, you can also raise the height of the material you’re trying to mill by putting a (rigid) box under it or something.

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I will make a second wast board or third, I really don need this high I need only cut plastic maximum 5 mm.

About bearings, I really don’t know what can I do to adjust, I though the only adjustment necessary was the screws and bolts to hold the bearings, if I can not find a solution I can print the gantry again.

@Peter - how can you reply my question one by one from the same reply??? teach me please :slight_smile:

If I were you I would try to shorten the legs, a lot. The steel part of my legs is only 85mm, and that gives me around 7cm Z travel or something (you can double check that with the MPCNC size calculator). My Z tubes are 30cm, but I never have to use the whole length.

Besides the inaccuracy of the Z axis, shorter legs also help with reducing movement of the whole frame.

To reply to (parts of) a comment, just select that part with the mouse. Then you get a button called “Quote”, click that. Then that part is added to the message that you are composing. You can do that multiple times for the same message.

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There are two kinds of “bolts with bearings”. One type is where a bolt goes through plastic, then has a bearing, and then has a lock nut. The bottom three bearing of a roller is one example. You just tighten these ones such that there are not lose. You cannot adjust anything else with these.

The second type has plastic on both sides of the bearing. The top two bearings of the roller is an example, and there are more on the center assembly. With these ones, when you tighten the nut, you also squeeze the plastic from both sides. Typically, when you tighten this kind of bolt, the other two bearings will be squeezed together a bit more, so that all three bearings will touch the tube.

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Mine are this ones with plastic. I will try to put the box and drill higher.
Thanks for your inputs very helpful

This is the approach I took. I stacked multiple pieces of MDF on my waste board. Right now I’m stacked two high. The reason I did this is sometimes I want to mill the top of very thick boards. Having multiple waste board heights allows me to add/remove layers to keep the distance from the end mill to the gantry a minimum for each project.

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I will try that :slight_smile: