lashmaze instructions low rez

7/28/2019 Lashmaze Instructions Low Rez

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Backlash in DetailCompensating for backlash through software

Original article appeared on www.woodsmokeworkshop.com Aug 2011

Appendix A: Reader comments

Appendix B: Larger imagery

Appendix C: Forum Discussion
http://web.archive.org/web/20111008021021/http://www.woodsmokeworkshop.com/wordpress/?p=359http://web.archive.org/web/20111008021021/http://www.woodsmokeworkshop.com/wordpress/?p=359


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Backlash in DetailCompensating for backlash through software

Having made all the physical corrections that you can to your 3D printer, you may find thatbacklash is still evident. The only remaining solution is to compensate for the shortcomingsof the hardware with work-arounds in your software. Skeinforge has a module forachieving this.

To configure for your backlash correctly we will explore the Lash module in conjunctionwith this Thingiverse thing:10524. The inspiration for the lashmaze developed from Erikde-Brujins research. First, print the part without the Lash module activated. 0.35mm layersand 1.4 width over thickness should serve admirably. Print the part as close as you can tothe centre of your build plate.
http://web.archive.org/web/20111008021021/http://www.woodsmokeworkshop.com/wordpress/?p=359http://web.archive.org/web/20111008021021/http://www.woodsmokeworkshop.com/wordpress/?p=340http://web.archive.org/web/20111008021021/http://www.thingiverse.com/thing:10524http://web.archive.org/web/20111008021021/http://builders.reprap.org/2009/08/identifying-backlash.htmlhttp://web.archive.org/web/20111008021021/http://www.woodsmokeworkshop.com/wordpress/?p=340http://web.archive.org/web/20111008021021/http://www.thingiverse.com/thing:10524http://web.archive.org/web/20111008021021/http://builders.reprap.org/2009/08/identifying-backlash.htmlhttp://web.archive.org/web/20111008021021/http://www.woodsmokeworkshop.com/wordpress/?p=359


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Above is a screen shot of the original CAD model, Thingiverse:10524 and below thecorresponding print from woodsmoke at 25mm/s.
http://web.archive.org/web/20111008021021/http://www.thingiverse.com/thing:10524http://web.archive.org/web/20111008021021/http://www.thingiverse.com/thing:10524


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Notice the slight misalignment betweenthe central vertical channel in the upperand lower halves of the part. This is theresult of backlash on the X axis. Verysmall, but noticeable. In addition,although the vertical and horizontalchannels in the CAD model are of equalwidth, you can tell that they differ in theprinted part. The horizontal channel isnarrower than the vertical channel, whenthey should be the same. To measure the

amount of backlash on your own printeraxes accurately you will need a digitalcaliper.


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Having printed your own lashmaze:

1. Measure and record all the distances labelled A.

2. Now discard the highest and lowest values.

3. Take an average of the remaining 10.

4. Next record all the distances marked B, discard the highest and lowest and averagethe remaining ten. The two values you end up with are used to calculate X axisbacklash:

5. Take the difference between your two values and half it. This is your X axis lashvalue.

Now repeat the same steps above for C and D to provide you with a Lash value for your Y

axis.


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0.0mm backlash correction 0.1mm 0.2mm

My Reprap 3D printer's Woodsmokes A and B averages are 5.54mm and 5.77mm. Theirdifference is 0.23mm and so the appropriate value for Lash on woodsmokes X axis is0.12mm odd. Above are photos of lashmazes printed by woodsmoke with X axis Lash

values of 0mm, 0.1mm and 0.2mm respectively. Hopefully you can make out the properrealignment of the vertical channel when Lash X is 0.1mm (middle). While overcompensating with a lash value of 0.2mm produces prints which look a little wacky (on theright). The images are repeated at a larger scale in the appendix.

I recommend printing your lashmaze at a slow speed to minimise vibration of the printhead as it alters course. Be gentle when taking caliper measurements, its very easy todeform the single filament walls and generate an inaccurate reading. Walls near corners inthe maze are stronger. You can also read more about compensating for backlash inSkeinforge over at [Reprap.org].

woodsmoke
http://web.archive.org/web/20111008021021/http://objects.reprap.org/wiki/3D-to-5D-Gcode.phphttp://web.archive.org/web/20111008021021/http://objects.reprap.org/wiki/3D-to-5D-Gcode.php


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Appendix A: Responses to Backlash in detail #2

1. hb Says:

August 5th, 2011 at 8:27 am

this is very useful, ive been fighting some small amount of backlash on a reprap for a while.however, your example is a bit subtle. can you perhaps highlight what were looking for between thestarting point image, the corrected image and the exaggerated image?

Also, I assume youre intending to tell us to take the difference and divide by two to enter into theSkeinforges Lash components?

2. Ali G Says:

August 12th, 2011 at 1:40 pm

If you drew peoples attention to the green and then the red axis, they may see the differences moreeasily.

3. Ron Says:

August 18th, 2011 at 10:10 am

For myself anyway, taking the difference and dividing by two produced the wrong result measuringthe first print produced a result of .244 for the X axis printing a second lashmaze with that setting

produced a result of .122, indicating that I had only corrected for half of the lash.

Just entering the difference (without dividing by two) resulted in a fairly even print (although with thecorners a bit rounded, as you would expect when attempting to correct that much backlash).

4. woodsmoke Says:

August 18th, 2011 at 7:19 pm

Thanks for taking the time to write me some feedack Raldrich. Admittedly I rather took the theory forgranted and didnt follow through by measuring the wacky looking print. Having experimented a bitfurther today I think the issue deserves some forum discussion. Ill put a post together.

5. Declan ShanaghySays:

September 30th, 2011 at 9:21 am

Is it (A-B)/2 or |(A-B)|/2

I used the former and ended up with a negative value for X.Im just wondering if a negative value is valid.

6. woodsmoke Says:

October 5th, 2011 at 8:10 pm

Hi Declan, not sure what your second equation means Im afraid. A negative value indicates that yourprint head is travelling in the opposite direction to everybody elses I think. In the picture of the biglashmaze above, the upper central vertical channel is slightly left of the lower central verticalchannel. Is that the same for you?

My only other suggestion is inaccurate measurements. When I print the maze I repeat each

measurement with the calipers until I get one repeated 3 times. Sounds laborious I know. It will helpavoid all the easy ways you can measure incorrectly. Things like catching the caliper tips on filamenton the base of the maze, measuring too tightly, or on a diagonal etc.

A should be greater than B, if your printer prints perimeters anticlockwise. Having said that yournegative value as a positive might work. All you can do is reprint and remeasure!

Good luck with it.

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Appendix B: Lashmaze Images

0.0mm (no) correction


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0.1mm correction


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0.2mm correction


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Appendix C: Reprap.org Forum Discussion

SF Lash modulePosted by WoodsmokeAugust 25, 2011 03:47AM

Hello,

Perhaps you can help me with this:

Recently Woodsmoke, my printer, struggles to replicate stls involving circular edgesaccurately. Attempting to print overtly circular components results in scruffy, egglikenonsense. The symptoms are already discussed under another thread:[ forums.reprap.org ]

Some alteration made to Woodsmoke's mechanics has introduced backlash. Prints of the

Wades Extruder body no longer accept the 608 bearings without excessive postproduction. This seemed like an opportunity to learn about the Lash module in SF. Thenext step was to design a calibration piece: [ thingiverse.com.]

Intuitively I understand how Erik's original gcode path identifys the problem, but struggle todescribe the process with words. Essentially the lashmaze allows you to measure the printhead's positional inaccuracy at a point, when it is traveling in one direction along an axiscompared with when it is traveling in the other direction? Half the difference is the amountof backlash on that axis. Great. Fine. I took the theory for granted and entered the lashvalues measured. Circles seemed better.

Raldrich was thorough enough to reprint the maze with his corrected settings and goodenough to post me his findings. He had found that there was still measurable backlashafter software correction. More specifically He reasoned that the lash value should be thefull difference between positions, rather than half. I believe his exact halving of the lashvalue with a corrected print to be a fluke and I get the impression that he didn't remeasurehis second set of corrections. Raldrich can confirm. But clearly I should have been thatthorough. If the lash module (and the lashmaze) works, one should be able to print a mazewith no measurable backlash.

The trouble is I can't.

I was going to delay this post until I had time to print a proper array of tests and write upmy best guess at what is going on. But it seems several people are having problems withthe method. Has anyone else had positive or negative experiences attempting to measuretheir backlash with the lashmaze? For my own part I have found measuring with a digitalcaliper particularly finickity, with care needed to get repeatable results. How accurate couldone expect their final lash results to be?

>0.1mm ?

Now hopefully the topic is open to discussion. I will follow this up with some testing when Ican.

woodsmoke
http://forums.reprap.org/profile.php?154,9529http://forums.reprap.org/read.php?151,71788,page=1http://www.thingiverse.com/thing:10524http://forums.reprap.org/profile.php?154,9529http://forums.reprap.org/read.php?151,71788,page=1http://www.thingiverse.com/thing:10524


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Lanthan[PM]Re: SF Lash moduleAugust 25, 201106:49AM

Recently did your lashmaze test with my pirated cupcake.

Showed good alignment of the ridges, but sensibly reduced gaps in the Y axis ascompared to the X axis. Will write down the values.And yes, looking at printed circles, they come out slightly un-round. (sigh!)

Have not yet played with the settings in skeinforge, that will be the last resort. First I amgoing to check :- Y belt tension (notoriously difficult to access in my setup), feels rather normal and similarX belt tension to me, but maybe a bit in the low? BTW what would be the optimal belttension?- Y idler adjustment on its axis (hadn't thought of that, but yes, of course, it does matter), Ithink there is some play there

- Y motor current (could it be too low? also need to check that motor's temperature - it ishidden and out of reach under the carriage)...any other sources of backlash one could control?The rods are outstandingly smooth, friction is at a minimum, well oiled/greased...

Lash is a fact of life when you do any kind of CNC, as well as dopey ballscrews with +/-0.15mm of positional error per 300 mm, let's face it: few things more common.But people get emotional quickly when it is question of 'lash (cf. other thread).

nophead[ PM ]Re: SF Lash moduleAugust 25, 201108:45AM

Other things that cause it are: loose pulley set screws allowing a little slip in each directionfrom the flat, broken wires in the belt, loose bushings allowing the carriage to rotate slightlyif it pulled from one side, pulleys with teeth that are narrower than the gaps in the belt.

I think belt tension should be high as possible to reduce lash on Mendel, but on Prusa it islimited. As discussed elsewhere too much on Y bends the motor bracket. Too much on Xbends the X rods and makes the top Z bushings come away from the bars. The SellsMendel is a lot better in that respect.

I think one side effect of belt tension is I think it can change the resonant frequency of theaxis, which limits the maximum zig-zag rate when doing small infills.

[www.hydraraptor.blogspot.com]
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nophead[ PM ]Re: SF Lash moduleAugust 25, 201103:24PM

Another side effect of belt tension is it must affect the XY calibration a little as it must

stretch slightly, even with steel or kevlar reinforcing.


nophead[ PM ]Re: SF Lash moduleAugust 25, 2011

04:04PM

AdminRegistered: 6 yearsago

Posts: 5,853

I wonder what tension the belt pitch is quoted at?


Lanthan[PM]Re: SF Lash moduleAugust 25, 201107:52PM

Registered: 2 yearsagoPosts: 228

nophead Wrote:-------------------------------------------------------> I wonder what tension the belt pitch is quoted at?

I have been foraging for similar information, but nothing definitive yet.

An interesting document for modelling the Mendel and similar:

[www.gatesmectrol.com]

As for the Prusa I think I got the Y motor bracket problem solved with a modified mount.Not yet tried.

Should help, the Y axis gets to push quite some mass if doing the heated bed plus glasssetup, so it needs more tension.

As for the X axis: have been thinking of doubling the smooth X rods with a pair of M8 rods,15 mm outside, coplanar.
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Not sure it is the best idea.

Not yet arrived to fine measurement of bending in the bars, dealing with the gross andmost obvious stuff first...

Buback[ PM ]Re: SF Lash moduleAugust 26, 201109:32AM

The tension on the belt is like the tension on a guitar string. A tighter belt should move thenatural resonance to highter frequencies. But can't we also move the resonance frequency

up by forcing the belt into a harmonic mode?

I'm thinking a ball bearing held at the right point on the belt should damp out most of thelower resonant modes.

Lanthan[PM]Re: SF Lash moduleAugust 28, 201104:27AM

Menawhile here is a spreadsheet with the measured distances and calculated values.Woodsmoke, given the discussion in your blog, should I enter the (averaged A - averaged

and (averaged C - averaged D) values in skeinforge, or halve them as you recommend?

Confirms what could be oogled: X lash is 0.067, Y lash is 0.184

@all: What values of lash are you getting in your machines?

Attachments:open | download - woodsmoke_labyringh.ods (14.6 KB)
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Justblair[ PM ]Re: SF Lash moduleApril 15, 201203:15PM

Registered: 1 yearagoPosts: 60

Holy Thread Revival Batman!!

Just did the backlash test.

My results...

backlash x: 0.046mmbacklash y: 0.107mm

This is on a Prusa V2 (pretty much) with Syncromesh cable rather than belts.

Ok.

I tried putting the above measurements into the lash module and ran the test again.

On the Y axis I saw an improvement, 0.019mm. That's quite a difference.

The X axis got worse though 0.098mm. I am wondering if there is a natural limit to whatyou can improve on in software?
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