why does my cnc cut like this

[iuliancalin22]

hy, i have a Genmitshu 4030 and i noticed on some cuts it does some weird shananigans 

i marked the picture

any ideas?

[engineguy]

@iuliancalin22 

Could be a case of trying to run too hard/too fast and you may be getting loss of Stepps if your machine has Stepper Motors and not Servos or possibly/more likely Tool Deflection

[jscott6SWZG]

Yup i got the same machine

it is not a strong stiff machine.

i have found that when you see evidence of bit chatter.  Like your picture.  There is probably a loose screw some where. 
i am cutting aluminium. But pretty sure pushing hard wood fast will cause the same issues

[jscott6SWZG]

Typically. Gantry, xmotor and z bolts. 

[jscott6SWZG]

I like fast feeds with smaller stepdowns and higher rpm.  This tends to reduce bit loads.  

recently i went to a 3 flute rather than 2 flute bit.  Even lighter bit load.

[iuliancalin22]

Thank you all for replying. I will post some pictures later because i did some modification to my machine, like spindle upgrade and gantry. I do have 2 screws that are lose from over tighten. Not sure if that is a real problem but will see. Also, the y axis left side is acting kinda weird. I'll post later more details like i said. 

[iuliancalin22]

ok, so an update to what i have found today, aparently all screws were damaged, if not lose, the thread was almost gone, and so i ugraded to 6mm ( see black screws in the picture)

also one screw on the spindle plate that screws to the z axis was lose

what i did so far

1 - those 8 screws made it way more stable then before with 5mm, it sounds better at least

2 - i had one nut and replaced the one damaged for the spindle plate

now, is there and other way to fasten the spindle to the z-axis? i really dont like those t-nuts, or maybe i can use more, instead of 4 maybe 6 or 8? yes i can do that 

[engineguy]

@iuliancalin22 

One thing I didn`t mention is that your machine is likely fitted with Lead Screws rather than Precision Ground Ball Screws, also likely that your Axis run on some lengths of round bar with Nylon bushes so there will be a real possibility of those bending under high loads, not what you wanted to hear but a real thing with small machines.

Botton line with small machines like yours is reduce load as much as possible, use small stepdowns/stepovers and as slow a feedrate as possible without burning your way through the material, the expression used here is "take a load off", let the machine "chill out"  🙂 🙂 🙂

One question, if you get a small area as in your first photo that is "missed" if you were to just re-program and re-run that one edge does it clear the material second time through ???

[iuliancalin22]

i'm not sure but i belive it has brass bushings , again, im not sure

now, i did some test runs todays after changing those screws and i have to say, particularly that as in the first picture didn't happen anymore but it does happen something similar, if you look at the pucture below the second one top left (middle) the top right corner it looks like it slipps for some reason, theres also in the first one top left but not that much

the bottom left is very close to perfect

now, the second cut i whent with 8mm 2 flute flat end mill 10mm deep 7mm stepover 600mm feed rate, i know it is way too much but i had to try it. i also did a rerun on that with no load and it didn't "fall in", actually very weird cause load thends to push out (it happened on top right bottom edge), right? might be wrond

if i go with 5mm deep 6mm stepover 500mm feed it does cut ok but it still "falls in" but very little (see bottom left)

on the other hand tho, it doesn't happen on aluminium, but on alu i go slow like 400 feed 1mm deep and half of the bit stepover

[iuliancalin22]

oh, i searched for "Precision Ground Ball Screws" and now i see what you mean, so no, it does not have that for sure

but good point

[engineguy]

@iuliancalin22 

That looks the tool has "grabbed" and been dragged out, is that the start of a "Feed Optimization" point where the Feedrate suddenly slows down ?? That would mean that the load suddenly reduced and if the tool is a 2 flute that could be the cause, going to multi flute with small diameter in Wood can be risking clogging of the flutes unless it is a specific wood cutting tool spits the chips downwards, sometimes it is better to use Conventional Milling in Wood, have you tried that ?? Do you have a Single flute cutter that you can test in Conventional cutting direction ??

[engineguy]

@iuliancalin22 

One other small point, if you don`t mind me pointing it out is that from the photo I would say that your workholding is also a little suspect, your clamps appear to be only touching the edge of the stock and are in effect pushing the stock away from the clamp, really if the clamps are offset as in the photo they should have the back end higher than the front, that way the clamp is fully holding the stock down, the pushing method as in your photo would only be OK if the stock block was up against a "stop" on the bed on the opposite side 🙂 🙂

[iuliancalin22]

on wood i always cut right with a upcut bit

i'm not sure if in the settings i have feed optimization, i have to look into that (edit, i never used feed optimisation, should i try ?)

i do have a 4mm one flute, i'll have to try it to see if it's the case 

also the clamps, usually i use 4 of them, today i just used 2 cause i was a bit lazzy 🤣 and i didn't had a proper wood stock. i can confirm that the stock didn't move during operation

[jscott6SWZG]

Just found more loose screws on z backlash unit. 

[jscott6SWZG]

Turns out.  It was an original design error and an assembly error

the backlash plate had the long screw in the short screw hole, so the plate couldn’t be properly tightened. The design error was the lead screw is ground round at each end.  The thrust bearing gradually wore the shoulder down.  So i found spacer that worked.  No play at all.

the design 

[iuliancalin22]

Could you post a picture please with the screws you found lose?

[jscott6SWZG]

Yup first is looking up the z axis from bottom brass plate with loose screws

second is looking at z-axis top thrust bearing with new spacer. It took a little filing to nail the bearing preload

[jscott6SWZG]

And random bolts on gantry and frame.  Used up lots of thread locker

[iuliancalin22 · Accepted solution]

I found 4 lose screws on the z-axis assembly. I did check before and there wasn't a significant play in the z axis, but after a few tries of a aluminium plate those screws lose even more to a point i was unable to continue. So i took the assembly apart and used some thread locker. Now it doesn't do that anymore. 

I guess thanks for the tips.