Alignment issues with passes creating gaps!

I had this issue before with my old MPCNC build but it would always go away when I tightened things down this time however everything is really rigid (But still very mobile) and im seeing a similar issue where passes get spaced out maybe I’m losing motor steps? Anyone got any advice?

 

First image is a test I’ve used to calibrate things in the past and quickly shows the problem.

Second image is the obligatory crown run from the example 12/mms file which also shows the issue.

I don;t see it on the crown. Is this with your vacuum hose on? What bit are you using? You sure the wood is not moving?

Wood isnt moving, tried removing everything on the middle assembly but the drill to remove that as a possbility, ran it again on the crown which this time it was much more obvious, birch wood.

I can;t tell from your builds pics but maybe your belts are loose, and they look like thick zipties which can kinda act as a spring if you aren’t careful? Try making sure they are bent square and don’t spring when it moves. You built a really tall machine which doesn’t help but it should not be this bad.

It is not the same as building it shorter but try stacking material to get the wood to just under the gantry to reduce the z axis sickout. See if that helps.

Alright, I’ll give those a shot, the belts feel pretty tight and I just checked for squareness again and its pretty dang square, worse case I can drop it an inch shorter, and the gantry can definitely be shorted (lots of wasted space there) but I don’t think that would be causing this kind of issue.

In what direction is the issue, it is not happening in them all. It looks to be one direction only, that makes it much easier to find. Is it a diagonal, or purely x or y?

As for the super long Z, when driving the bit through the wood it gets pushed opposite the direction of travel. The z stepper motor gets swung in the direction of travel, it s a counter weight that makes it worse. So yup the shorter the better. This is usually a direction change (acceleration issue) not a straight line issue. Rapid constant oscillation can get nasty.

So half the z stickout makes the machine twice as rigid (Linear), + all the gains from shorter distance the z stepper weight is away, + the rigidity gains for shorter legs (x’s 4). 1" extra height is significantly less rigid.

 

 

It seems like its occurring on both axis, but now that you explain the gantry a bit more it sort of makes sense the issue could be caused by that because all of the issues seem to arries from pivoting in the direction that would pull at the bottom against the motor at the top the most, so after this next test, I think its time to cut an inch or two off the gantry!

Stick out is the key not total length. Making the tip of the cutting tool to the center of the tube crossover point distance shorter.

Trying to understand that, you are saying, shorter legs help not shorter gantry, because that would put the center of the drills mass in the center of the middle assemblies mass and there for make it more rigid?

 

Seems it has improved greatly, the belts were tightened (some were actually loose) and I also noticed that one side of the Y axis isnt staying square to the other side (half an inch off or so) so before I started it I gave it a shove on the side that wasn’t lined up right, would this have caused the issue? Also, what is the source of this offset? is it in the middle assembly? My corners are very square compared to this offset.

Nope. Nothing to do with mass. Grab a pencil, wrist on the table, sign your name, now move your hand further away from the tip grab the eraser, sign it again.Which one is more accurate, this represents how far the tip of the tool is from the center of the gantry assembly. Now while holding it by the eraser pick you wrist up off the table and only use your elbow, sign it again. This represents long legs. The shorter every single part is on the machine the more rigid the entire machine gets.

Maybe a better way to think of it is this. Pick a foot, move the spindle as far away from it as possible and set the tip of the tool on the table. From the bottom of the foot (at the table surface) measure from the foot to the center of the corner, from the center of the corner to the center of the gantry, from the center of the gantry to the surface of the table. The smaller this number the more rigid the machine, the more rigid, the more accurate, the more accurate the faster you can move and maintain the desired accuracy.

 

This is caused by you not moving both steppers with both hands, dragging it around by the gantry. This is normal. You should be checking before you start a cut to make sure they are even. This could cause your issue 100%. Of it could be any of the other things I mentioned, or other things including what bit you are using, how fast your spindle is, how it is mounted, how sharp.

 

Ahhhhh I get it, its a pivot point issue with the gantry, I totally get that!

 

As for the Y axis, Yea I always did grab the gantry and move it around I guess that was my issue whats odd is that the whole time I used my other cnc it never caused a problem, very weird!

Its very resistant to go into alignment so I feel like something else is wrong like some amount of warp in the middle assembly or something.

I guess moving the motors around and checking for square is just my life now haha…

Lever arm…close though. http://hyperphysics.phy-astr.gsu.edu/hbase/torq.html

 

Yes, it is an issue for every machine. Some more than others. That is why we put so many months worth of work into the dual endstop firmware and hardware. You can watch some other CNC machines and how this only gets realized when they try to do some fine adjustments/cuts/tool changes.

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Not warp, assembly tension. You should have been checking for square during assembly. I have a few pictures of this in the instructions.

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And don’t take any of this in the wrong way. I do my best not to critique peoples builds until asked (think cable chains on the Z axis, I stopped warning them). You seem genuinely interested and have asked for solutions, so I am trying to give complete answers. I enjoy helping people have the best experience with CNC. All of this is kinda small stuff but each little bit adds up.

If it is overwhelming amount of info just ignore it for now, you will eventually figure it out with more test cuts.

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Definitely not taking it the wrong way! I’d prefer more information then short answers because they help me understand how you arrived at that answer so I can later answer these questions for myself!

I wouldn’t call it overwhelming in fact…re-reads the middle assembly instructions oh…I’m an idiot lol…its all square now!

So, next question;

I’m noticing a slightly aggressive ‘pop’ as the dril cuts into an already cut into space, this sounds like the kind of thing that is caused by the drill not being more centered with the middle assembly (IE the legs are to long for this kind of rate/feed) does that sound right?

What kind of endmill are you using (not all are meant for plunge cutting)? Is it dull? Is your tool perpendicular? Are you cutting too fast (plunges should be pecked or spiral) and not making a hole within tolerances? Why are you going back into a hole? Are you using a finishing pass?

Too broad of a question. You will need to do more test cuts, put on the safety glasses and get your face in there. Watch everything.

2 straight flute long; bought it from you so I hope its not dull :stuck_out_tongue: (feels like if I applied to much pressure when touching it, it would break the skin, so its pretty sharp) 12/mms feed rate 3mm depth per pass.

It occurs when going along paths and meeting up with the start of the path (the troopers outline makes it most obvious).

After tightening a few things down (mostly the feet) it seems to have gone away, I guess just about anything can cause that.

Okay. So 12mm/s 3mm DOC might actually be too much for that bit. A straight flute does not clear the chips very well it’s job is to not mess up either surface, not a fast material remover. The cuts you keep testing are slotting operations, even worse. If you use a slotting operation with say a 0.5mm finishing pass it should be much better. You can try shallower or slower or both. I don’t like the straight flutes. I almost exclusively use the single flute up cut and vbits. In the 1/8 diameter I don’t think it gets any better. A larger straight flute might be okay do to larger flutes but a single flute is going to make a world of difference in the cuts you are doing…don’t believe the hype on those straight flutes.

I think most would do a majority of the job with an upcut and if the wood has a veneer or a delicate surface they were trying to keep a sharp corner on (rare) you would then do your finishing pass with a straight flute, never use it for slotting.

Now you know why I repeatedly asked what bit you were using.