Bearings on Z Axis

Mostly Printed CNC - MPCNC Troubleshooting
1

Hi, I did several jobs before practically without issues, mostly plastic, some aluminum. I recently put my MPCNC to a new table and did some test runs and found several problems:

Some bearings connecting the gantry and Z axis have only a very light touch, how important are they? Should they all make a good contact? I think now it has some unwanted slack and the Z is not as rigid as it should be.

This leads to a quite severe chatter during the cut. When I apply pressure to the Z axis with hand the chatter is mostly eliminated.

The test cut was in soft plastic, with a 6 mm single flute bit at 9500 rpm.

I tried feeds from 100 to 1000 mm/min with no improvement.

Also the rollers do not have a perfect fit, some of the bearings are touching the stainless steel tubes rather gently which again adds to some slack. I have the original ones with aluminum motor mounts.

 

The table should be OK, I measured all the distances between legs and it is within 1 mm difference.

Some info about the build:

Working area 460 x 340 mm so on the small side.

Z travel 70 - 100 mm depending on the bit length.

Electronics - Arduino CNC shield with GRBL and bCNC, with endstops - this works great BTW.

All parts sourced/printed locally.

Also what is the max bit diameter you are using? I worked with 2-4 mm before and now I tried a 6 mm single flute and the machine went crazy… Is it too much for it to handle?

I will try to reassemble the whole thing and maybe reprint some parts when I have some time, just wanted an opinion on what to look for and avoid possible problems.

Thanks for any suggestions, I can upload some pics later if it would help.

Andrej

 

 

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The bearings can be pulled together by tightening the bolts in the “backbone”. Don’t over tighten them, but all the bearings should touch at once.

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I am trying bolt after bolt to get it just right.

I seem to keep having trouble with the two bearings in yellow/green color on attached picture. The inside bearings in the big gantry parts are OK.

If I keep the gantry bolts loose, as per assembly instructions, then the bearings make barely any contact…

I tightened the bolts circled in blue to get better fit.

By backbone you mean the long screw connecting all the parts together? The question mark on attached picture?

I printed one of the new carriages and it seems to fit tighter than the old ones. I have to print another one to try the whole axis together, but it looks promising.

 

mpcnc_problem.jpg
mpcnc_problem.jpg1500Ă—2000 437 KB

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The best thing to do, and I do it myself once and a while, is take the entire center section apart. Then put it back together, watching the whole time for square and tight bearings. use the big XY pieces on the other axis, swap them. For you I would pay particular attention to the Z axis and make sure it is square and not twisted, use one of the z mounts to make sure the tubes are spaced correctly and put the tool mount back on, or perhaps even flip the z tubes and drill new holes.

It takes a bit of time but once you get it right the tension bolts usually do not even need tension and all the bearings should have a firm grip.

I know it is kind of a bummer but when it is put together really well it makes it so much easier to use.

 

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Thank for the tips Ryan,

So far I have checked the Z axis and it was indeed a bit twisted. I had 5 mm holes and re-drilled them to 7 mm to give the tool mount more wiggle room, now it is nice and straight.

I will try to rebuild the whole middle assembly and hopefully get it running.

What about the 6 mm bit, do you think it is too much to handle or should it work OK (for wood / plastic) when the machine is built correctly?

 

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I have a couple quarter inch bits that I use on mine for hogging out material for 3d shapes or for flattening the top of a piece. They work okay, but you really have to take it easy with them.

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I have been using a 1/4" bit quite a lot…it does act totally different than a 1/8" bit, so your settings will definitely be different. Make sure the bit is chucked up as short as you can. You want a bit with short flutes so you can do this. I don’t usually ever “cut out” with a 1/4" bit. There’s rarely an advantage to it. When cutting out your using the full width of the bit. I have done it some, but it does have a tendency to chatter. I use it mainly for pocketing at 45% step over . So after the initial cut it only cuts at part of the width of the bit, and is much easier on things. Also, I use peel on big pockets when possible at a pretty aggressive rate. Peel by nature just cuts with a lot less chatter in my experience. It is a slower method, so I compensate with the depth and feed rate. Seems to work for me. The number one thing though is spindle speed. I run a 500w Chinese spindle with a max speed of 12,000 rpm. If I run it at 3/4 to full speed it will chatter like a banshee every time. There is a sweet spot, but I think it would shock a lot of people how slow I’m running a 1/4" bit. It’s really quite slow and nearly eliminates the chatter. It just sets up harmonics when running at a high speed. You want that bit to look like it’s taking chips off almost like you want a plane or chisel to do. You want it cutting and not rubbing. I know in the manual milling I do, the bigger the bit the slower the speed. I don’t run into rigidity and chatter issues with a big mill obviously but I can instantly tell if I chuck up a big bit and haven’t turned down the spindle speed. The bit rubs and cuts like total garbage until you get it slow enough. Hope this helps.

Thanks,

Andy

2 Likes
8

Huh, weird. I have the same issue with mine even though it cuts fine. It could even be the exact same bearings not making contact, I would have to check.

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Just a brief update.

Got it working after complete rebuild of the gantry, Z axis and printing new carriages.

All the bearing are touching firmly now, all is squared as it should be.

Did some test cuts in PVC.

Used 1F 4mm endmill at 9000 RPM.

Tried to push it a little, max 5 mm DOC, 0.75 mm WOC with trochoidal (2D adaptive in fusion) at 1000 mm/min.

Cuts great and the results are more than acceptable.

Time to get some real spidle :slight_smile:

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Those are some good looking cuts, you should keep that speed and feed written down!

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