Robo3d - Z Ribbing Causes and Cures (June 2014)

A Number of Robo users are seeing very significant Z artifacts on their printsand are disappointed with their print quality. In many cases these artifactsappear to be Z Ribbing although many of the proposed cures/mods are actuallyfocussed on reducing Z Wobble (eg Z Axis stabilizers).

The purpose of this note is to look only at the root causes of Z Ribbing on theRobo and describe fixes to eliminate (or at least reduce) them.

Z Artifacts

Anyone who is seeing Z artifacts on their Robo prints first needs to checkexactly what is happening – Z Ribbing or Z Wobble (or maybe something elseentirely eg over extrusion).

There is an excellent article describing the taxonomy of Z Artifacts on 3Dprints. In my opinion, some of the reasons cited for the artifacts are not entirelycorrect or complete, but it is still very useful information.

https://www.evernote.com/shard/s211/sh/701c36c4-ddd5-4669-a482-953d8924c71d/1ef992988295487c98c268dcdd2d687e

To summarise the differences between Z Ribbing and Z Wobble :

Z Wobble is HORIZONTAL movement of the Z axis. On a print, Z Wobble canbe seen as a convex wave on one side of the print with a corresponding concavewave on the other side at the same z height.

Z Ribbing may be caused by variation in the VERTICAL Z step size (in otherwords, the layer height of each step). Z Ribbing can be seen as a convex waveon one side of the print with a corresponding convex wave on the other side atthe same Z height.

The root cause(s) of Z Wobble and Z Ribbing are very different.

To see whether Z artifacts in a particular print are Z Wobble or Z Ribbing,obviously you need to carefully check the wave pattern on both sides of theprint at exactly the same Z height.

Of course it would also be possible (but probably unlikely) that both wobbleand ribbing are present in a particular print.

Again this note only addresses Z Ribbing.

Z Ribbing Artifacts

Z Ribbing is caused by tiny variations in layer height of the nozzle during theprint. The variation itself is just a small fraction of the full layer height (typicallybetween 0.1 to 0.3mm) so the variations can be in the order of hundredths of amillimeter.

Generally the variation in height is not random and the ribbing is noticeablebecause the layer height varies in a regular pattern. In many cases thefrequency of the ribbing pattern closely aligns with the pitch of the threadedrod.

Possible causes of Z Ribbing on the Robo

As the Robo Z Axis uses a vertical threaded rod/nut, coupler and stepperdesign lifting an extruder nozzle at a fixed height in the X Axis, variations in Zlayer height can only be caused by:

- Variations in threaded rod or nut pitch geometry.- Variations in the way the coupler either transmits the rotation from

stepper to rod or variations in length (ie height) of the coupler itself as itturns (and hence threaded rod)

- Variation in the way the stepper motor rotates in response tomicrostepping pulses from Marlin/Ramps.

- Or possibly (but unlikely), some variation in the height of the bed underthe nozzle.

Extent and Severity of Z Ribbing on the Robo

The extent and severity of the Z Ribbing appears to vary widely betweenmachines in no identified pattern. But as not all Robos suffer from the ZRibbing it is unlikely there is any fundamental design issue.

Of the Robos that do suffer from Z Ribbing, the severity of the ribbingsometimes varies between prints and sometimes comes and goes over time.To add to the complexity, a number of users modify their machines (oftenextensively) so it is difficult to pin down any significant difference between thebasic Robo design and the configuration of the user’s Robo.

Identification of Possible Causes of Z Ribbing on the Robo

In an attempt to find a systematic way of testing and pinning down thecause(s) on any particular machine, a systematic analysis of the possible causesfrom firmware to Z drive mechanics was done.

A number of fixes were identified and tested successfully (on one machine sofar). The fixes are targeted at a number of identified root causes of Z Ribbingon a particular machine. But each machine may not have all the root causespresent.

Z Ribbing Cures and Fixes (June 2014)

Before starting any fixes

Go over your Robo thoroughly checking for loose screws, belts, rods, couplersetc etc and make sure some other problem is not going to make the fixesappear to not work or cause even more problems.

Make sure your rods are straight (eg roll them on glass)

Make sure your 5mm to 8mm helical couplers are in good shape. They mustnot be bent or damaged. A good coupler should be near to a perfect cylinderand compress slightly with just finger pressure.

Make sure your Robo is on a flat, stable surface which does not shake or moveduring a print. Also the base must not be twisted or bent eg because someobject is caught under an edge.

Finally, print the test.stl (attached) in PLA at 0.2mm layer height. It only takesabout 15mins and will be a reference against the fixes. The test is round withvertical surfaces to show up any Z Ribbing clearly.

Fix 1 – Firmware configuration change

Why

There is absolutely no doubt that disabling the Z steppers between movescause inaccurate microstepping. Robo need to fix this configuration parameterin future Marlin releases.

How

Make the firmware configuration change and reflash the firmware as describedhere.

http://forums.robo3dprinter.com/index.php?threads/z-axis-artifacts-known-issues-and-fixes.2576/#post-20406

Check that the configuration.h steps/mm is correct for the pitch of the rod youare using. For example M8 rod should be set at 2560 steps/mm.

Note: If you have made other firmware changes, check that none of thesemight change the Z Axis behaviour from the stock Robo settings (egacceleration or jerk settings).

Fix 2 – Making sure the threaded rod/nut stiction andfriction is as low as possible.

Why

This fix is required so the stepper can do its job with low torque and noslippage on all of the connections between the Z motor shaft and X carriagenut.

The forces of acceleration and torque are especially high at the connectionbetween the 5mm motor shaft and coupler. The aim is to take as much load offthe motor shaft/coupler connection as possible and minimse the possibility ofslippage.

To minimse stiction and friction (and hence torque), the CNC world generallyrecommend using a plastic(nylon) nut and dry PTFE lube if you are usingstainless threaded rods.

How

First you need to remove and thoroughly clean your rods. Hot water,dishwashing liquid and a nail brush works well. Afterwards look at the rodsunder a magnifying glass to check they are completely clean. I foundcompressed air was good for a final clean.

Source clean, good quality nylon nuts and check they rotate freely on the rod.(The nylon nuts will be used to replace the metal nuts that sit in the X Carriagenut traps.)

With the nuts removed from the rods, spray the rods and nuts with the dryPTFE lube. Let the rods and nuts dry and from then on make sure they remainclean of contamination.

This type of dry PTFE lube appears to bewidely available

Fix 3 – Setting the motor shaft in the coupler so it does notslip

Why

One of the major causes of Z height variation on the Robo is slippage at theconnection between the motor shaft and the coupler. As the shaft is small(5mm) the surface area in contact with the coupler has to be as great aspossible to prevent slippage. Even if the slippage is as small as 1-2 degrees,that is enough to cause variation and ribbing (and it did on my Robo).

So the motor shaft needs to be held fixed tight in the coupler with no slippage.And the shaft needs to be vertical in the coupler.

How

First you need to remove the Z motors from the Robo.

Make sure the disassembled couplers are within spec and not bent ordamaged.

To maximise the contact area between shaft and coupler, the motor shaftneeds to be positioned in the coupler so the end of the shaft just sits justproud of the hole drilled in the coupler. There seem to be a number ofdifferent coupler styles so you need to look inside the coupler to check theshaft is in the right place as shown below.

To properly align the shaft/coupler it’s important to get the 2 flats on themotor shaft orientated correctly so the clamp screws can be tightened and theshaft stays vertical. I have no idea why there are two flats on the shaftbecause it certainly does not make the alignment very easy.

I found the only way to get the shaft and rod to stay in alignment was toposition the two flats like in the diagram with the round part of the shaftacross the squeeze gap with the clamp screw.

And to make sure the whole assembly stays tight with no slippage, use liquidblue Loctite (or blue Threadlocker) on the motor shaft and inside the couplerhole before pushing the rod into the coupler.

Liquid Blue RemovableThreadlocker (equivalent to Blue Removable Loctite)

Drop or two of Loctite is enough

Then tighten the clamp screw as tight as possible without damaging thecoupler. Must be as tight as possible to stop any slippage. It is not necessary toLoctite the clamp screw itself, only the motor shaft.

Finally, spin the coupler by hand to make sure it is aligned properly with themotor shaft. Wipe any excess Locite off, especially inside the coupler orbetween the helical spring cuts.

Notes :

The Loctite MUST be the liquid blue “removable” type which can bedisassembled later using hand tools and acetone. If you do use the rednon removable/permanent type you will need a blow torch todisassemble it later!

It is possible to disassemble the blue removable Loctite joints later byhand. Then soak the coupler in acetone to loosen the remaining Loctiteresidue. I know this works as I had to do it during testing.

Fix 4 – Stopping any height variation due to the couplergeometry

Why

a) If the end of the threaded rod is not flat and square, and the rod isseated all the way down in the coupler, the length of the coupler willvary slightly as it turns.

Note this diagram could definitely be improved to better show the bend in the coupler

This diagram is exaggerated to show the effect of an off square end ofthe threaded rod and the helical coupler bending to accommodate theinevitable misalignment between motor shaft and threaded rod/Xcarriage. As the coupler turns, the length of the coupler will vary(compress/expand slightly) in a regular pattern depending on thedegree of slant of the rod and the precise geometry of the cut at the endof the threaded rod.

All it takes is a few hundredths of a millimetre variation to cause ribbingon the print.

The coupler length variation (and hence Z height variation) can also beexpected to follow the degree of turn of the threaded rod. Andconsequently, it follows that the pattern of ribbing on the model willclosely match the pitch of the rod. If the rod is slanted in the coupler,every turn of the rod will generate the same pattern of Z heightvariation.

b) Alternatively, if the rod is not seated all the way down in the coupler, thecoupler acts as a vertical spring (see https://vimeo.com/97023189 ) andthe height can vary very significantly (potentially far more than asdescribed above). For example, as the X Carriage moves from side toside, the couplers will compress slightly in turn to support the weightchange.

Either way, the coupler is a major potential source of Z height variation. On myRobo I had both coupler height variation mainly due to the rod not being cutsquare as well as motor shaft slippage in the coupler (fix 3).

How

You will need two 3mm ball bearings for this fix. The size is not absolutelycritical but they do need to be close to 3mm. The ball bearings can be sourcedfrom most bicycle shops because the balls are used widely in bike axles.

Make sure the end of the threaded rod is cut absolutely flat and square. One ofmine had been cut at an angle like this.

My rod was not cut square

The end of the rod needs to look like this and be flat. Grind/file as required.

Next take the motor and coupler and place a 3mm ball bearing in the hole onthe top of the motor shaft so that when the threaded rod is inserted it will sitbetween the rod and motor shaft. A little lithium grease on the ball helps aswell.

Ball just sits in the hole in the end of themotor shaft

The ball bearing changes the helical coupler from a "spring" (which changes itsbehaviour and length depending on how far the rod sits down in the coupler)into a more precise ball joint where the rod can not move down at all. The ballbearing on the square/flat end of the threaded rod allows the coupler to bendwithout varying the length of the coupler.

The ball bearing is a simple and very effective way of overcoming the inherentlimitations of a helical coupler when used on the Z Axis.

Fix 5 – Seating the threaded rod properly in the coupler

Why

On Robos with 5/16” rods, often the rod was wrapped in Teflon tape in aneffort to centre the rod in the 8mm coupler.

Unfortunately this procedure has two problems. First, the Teflon tape canactually encourage slipping between the rod and the coupler. Second, the rodwas not centered properly when the clamp screws were over tightened toallow for the smaller diameter rod.

How

Best long term solution is to use M8 rods (pitch 1.25mm) which fit properly inthe 8mm coupler.

Put a few drops of blue Loctite on the rod and inside the coupler where it willbe clamped into the coupler. Then insert the rod (the square, flat end) whilethe motor/coupler assembly is standing vertically and the ball bearing is inplace on the motor shaft. The rod needs to be inserted all the way down to siton the bearing as shown in the diagram above.

Finally tighten up the clamp screw as tight as possible. Again it is not necessaryto Loctite the clamp screw.

Set this motor/coupler/rod assembly aside standing vertically for at least 24hours to ensure the Loctite cures completely.

You do need to be a little careful the ball doesn’t fall out of its spot if you pullon and stretch the coupler too much. Once it’s in the machine there will be noproblem.

It is easy to check the ball is in place by pushing gently down on the threadedrod. There should be no vertical compression of the coupler at all.

Note

If you only have 5/16” rods, then I suggest you wrap the end of the rodin enough aluminium foil strip to centre the 5/16 rod in the 8m coupler.Use Loctite to keep the foil in place as well.

A better solution might be to wrap some copper wire into the 5/16”thread – but what gauge? (MAYBE someone can come up with a bettersolution??)

Finally check the rod and coupler are in alignment and centered as far aspossible before allowing the Loctite to cure.

Last Steps

After waiting for the Loctite to cure, the motor/coupler/rod assemblies withthe nylon nuts for the X Carriage can be reinstalled in the machine. It is veryimportant to give the Loctite enough time to cure.

Check that there is only a slight wobble (maybe 2-3 mm) at the top of thethreaded rod when the Z Motor turns.

Slight wobble is good and shows the coupler is doing its job. Any moresignificant wobble suggests that either the coupler is not sitting vertically onthe motor shaft or the rod is not centered properly in the coupler.

Check that you cannot push the threaded rod down by hand and compress thecoupler. This confirms the ball bearing is doing its job.

Check and adjust your Z Axis so it is horizontal and the height above the bed iscalibrated as you normally would before starting a print.

Note that the coupler sits a lot further down on the motor shaft than the Roboguys suggest. But it is very important that the coupler sits down on the motorshaft like this to maximise contact area with the coupler and avoid any

slippage.

Finally print the test.stl model again and hopefully you willget the same great results I did.

Actually my Robo is printing better now than it ever did.