Backlash on a 3018 pro CNC. How to reduce it, so that circles are really circles?

Hello,

I've recently bought a low-cost 3018 CNC (here is the ebay link: https://www.ebay.fr/itm/124282612756).

The set-up has worked surprisingly well: with no prior CNC experience (but a lot of
embedded programming engineering experience) I was rapidly able to set it up
to carve my first projects.

To start, I wanted to be able to carve circles and disks.
And the result is not satisfactory. On a small circle of 16mm in diameter,
I get diameter measures (in the carved result) from 15mm to 17.5mm.

I understand this is due to the famous "backlash" phenomenon but I don't know:
- how to reduce it (by using different SW or improved nuts?)
- how I can work around it (what projects are less sensitive to it? Maybe photo carving?)
This is where I need your advise.

My software controller is based on GRBL, so I assume there is no backlash control setting
(maybe I am wrong). I am willing to invest a bit in new nuts, and maybe change the SW.

Best regards, and a happy new year,
Dopo

For such a lightweight machine,there may be other factors than backlash.You may well be pushing it harder than it's rather light construction likes and a few things could be flexing-which is a different situation.Have you tried cutting those discs with an initial diameter of 18mm and an onion skin layer at the bottom,prior to going back and reducing them to the final size with several cuts at modest depths?

To be blunt,those machines are of limited use if you intend to cut anything very substantial as the round rails flex and the gantries are less than rigid.The backlash in the screws is fairly insignificant by comparison.You can get things made but only if you take it very gently,it will lead to disappointment if you try to use depth of cut and feed rates that apply to industrial level machines.

Photo carving may be a way of gaining experience but it will take a long time to finish a cut.My only attempts were done using the inbuilt program generation capability of LinuxCNC and worked out OK.I don't have experience of any other photocarving software.You may be able to create some files from bitmaps using F-engrave-if it has a post processor for your control.

Thanks for your reply.

Quote:

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Originally Posted by routalot

For such a lightweight machine,there may be other factors than backlash.You may well be pushing it harder than it's rather light construction likes and a few things could be flexing-which is a different situation.Have you tried cutting those discs with an initial diameter of 18mm and an onion skin layer at the bottom,prior to going back and reducing them to the final size with several cuts at modest depths?

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The carving depth was 1 mm.
The drill bit went through the (very) soft wood like through butter, and it was not moving fast.

I'm pretty sure it was backlash, because I tested turning the lead screw by hand.
Doing this, I have witnessed that for something like 20-25 degrees of turn there is no move on the axis.
The nut also has quite some place to move, thus absorbing movement that should go on the axis...

The problem is that I don't know what is the good way of addressing this problem.
New nuts? New lead screw? Shims to block the surious movement of the nut?
Harder anti-backlash springs?

Quote:

---

Originally Posted by routalot

To be blunt,those machines are of limited use if you intend to cut anything very substantial as the round rails flex and the gantries are less than rigid.The backlash in the screws is fairly insignificant by comparison.You can get things made but only if you take it very gently,it will lead to disappointment if you try to use depth of cut and feed rates that apply to industrial level machines.

---

I don't have big expectations, this is why I'm using very soft wood (I can mark it with my nail),
small drilling depths, and I'm looking for easy projects.

What I'm looking for with this cheap CNC is to understand the basic issues and
see that at least some things can be done. I'm also looking for a predictable path
of improvement.

Quote:

---

Originally Posted by routalot

Photo carving may be a way of gaining experience but it will take a long time to finish a cut.My only attempts were done using the inbuilt program generation capability of LinuxCNC and worked out OK.I don't have experience of any other photocarving software.You may be able to create some files from bitmaps using F-engrave-if it has a post processor for your control.

---

Speed is not an issue, since this is really for testing purposes.
Not being able to make a real circle or a nice V letter is an issue, though.

Also, for photo carving, I foresee a problem when the drill bit turns. Ideally, to
avoid backlash problems, all carving runs should be done in the same direction, whereas
usual carving paths go both ways, so I expect pretty nasty "interlacing" effects. :)

Two questions:
* GRBL does not have backlash control settings. Is there a way to move from GRBL to some other (free) CNC software on my 3018?
* Is there a way to add feedback control to the 3018? In other words, to buy a sensor kit and a different controller allowing to measure the actual move on the X/Y axes?

Best,
Dopo

You seem to have a disturbing amount of freedom from movement if you can rotate the leadscrew 20-25 degrees without anything happening.First thing to check would be the couplings just to make sure there isn't any lost movement there.The fact that the nut on the leadscrew can move is clearly not helping and eliminating the freedom for it to do so would have to bring about an improvement.I have to admit to not knowing what anti-backlash springs are contributing to the machine as they have to be introducing some freedom of movement,which isn't helping you to achieve accuracy.

One operation that might help identify which component is the greatest problem would be to cut two discs that use different machining strategies.For your example of 16mm diameter discs you might try cutting one with a climb cut and one with a conventional cut.Mark them in some way so that you can identify them later and remember the orientation in which they were cut.Examine them closely for similarities and differences in both the X and Y directions and you should have some guidance regarding which areas need close examination.Good luck.

Hi,
if your leadscrews have that much backlash you will really struggle to get an interpolated circle round.

Some leadscrews have two nuts with spring washer between them to 'absorb' the backlash. If the leadscrew is accurately made, which is questionable, then
there will be issues like tight spots and rapid wear requiring readjustment on a frequent basis.

There are some CNC software solutions that with a compatible controller offer backlash compensation, Mach3 for example. The results are often disappointing, there is no way that
software can truly fix a hardware fault.

If the machine has very limited rigidity then it may not warrant too much modification. If however you happy enough with the rigidity and speed then upgrading
leadscrews to ballscrews is worthwhile. They range from cheap rolled C7 grade (50um in 300mm accuracy, 28um cyclic) for less than $100 to ground C3 grade (8um in 300, 4um cyclic) for $1500 depending on size.

Craig

As others said, your problem is not backlash. It is the overall rigidity of the machine.

As I can see from the photos, the machine uses T8 trapezoidal screws with brass nuts. While not perfect, those screws work pretty well in a lightweight desktop machine. They DEFINITELY do not have 1mm backlash - this is physically impossible. The circle diameter deviations you see are caused by the flexing of the machine itself.

I have a CNC machine quite similar to yours - plastic PVC frame and T8 trapezoidal screws. This is how precise it can be:

https://i.ibb.co/jwGvWfh/IMG-20200729-124337.jpg

What can you do about the lack of rigidity? Not much. But you can learn to work around your machine's limitations. Take lighter and shallower cuts. Find the best feed/speed/depth combinations for each material and each tool you use. Use separate roughing and finishing passes.

I would also think about replacing the "spindle" with something a bit better. The motor you have now will not work very well or last very long.