Printer Calibration

[skyline1]

Getting Z spot on can be a little tricky as it must be a multiple of the layer height. So the actual Z dimension will always be up to + or - 0.5 times the layer height.

The printer will will always print one layer less or more than the required dimension whichever is closest to the wanted size if you see what I mean.

For example, assume we are using a 0.15mm layer height and require a part 15mm high. the height printed will be almost bang on (less shrinkage) as 15mm is exactly 100 layers. But suppose we wanted 15.1mm We would actually get 15.15mm as this is the closest number of layers (101)

If we wanted 15.2mm We would also get 15.15mm printed as again this is the closest number of whole layers but this time from the opposite direction.

This, unfortunately, is one of the limitations of 3d printing with it's discrete layers method of producing things.

Adaptive slicing can help where the layer height is not fixed but can vary between limits depending on the complexity of the part at that layer and the exact height of the part and certain features on it. The slicer will vary the layer heights to reduce these height discrepancies as much as possible. It will also try to reduce the "staircase" effect on surfaces that have very gentle vertical slopes.

Both Slic3r and It's Prusa variant have this feature but the settings are a little difficult to find.

On Slic3r it's under Print Settings - Layers and Perimeters - Use adaptive slicing And on Prusa You can edit it in the model itself when slicing. I'm not sure about Cura as the version built in to Repetier host is rather old and doesn't seem to have it but i'm sure later versions do.

Adaptive slicing is a fairly new technique but up to date versions of most common slicers can do it. It's well worth a try for parts needing high Z dimension accuracy. I've used it myself on difficult parts and it seems to work very well.



Best Regards Mark

 

[CFLBob]

To scale X, Y, and Z individually click on the "padlock" icon to the left in the picture I posted, under the label "scale factors". Also note that there are three levels you can select for more or less variables/settings to change: simple, advanced and expert.

Ah! Stumbling around the interface as a complete newbie, I saw and used the padlock icon while looking for that display of XYZ in the printer settings tabs, and didn't find it until I gave up and went back to the main screen. And then I didn't notice the padlock.

There is a Prusa user group that I have found to be very helpful on topics like this.... some people out there are real experts and using this software and equipment. Other owners besides Prusa owners do post questions there.

Are you referring to the one in their "Support" menu, or another?
https://forum.prusaprinters.org/
Since I've tried the deliberate elephant's foot compensation in both Cura and Prusa slicers and neither had any effect, is there something else I should look for? Taper the bottom mm in all of my models, or print an extra mm on the bottom that I can break off?

 

[Technical Ted]

Yes, that's the user group I belong to and it is very helpful. It also looks like we have some pretty knowledgeable people on this group as well! I'll be looking into that adaptive slicing setting!

If you are designing the part yourself in a CAD program you can add a small chamfer on the bottom of the part. I haven't done too much yet as I'm a newbie, but when I have I chamfer all edges and fillet sharp corners and it makes the parts turn out much nicer. I use Fusion 360.

Ted

 

[Technical Ted]

Now the interface for adaptive layering is a little different than in the video. You have to click the tool bar menu item in the attached picture (top tool bar) and the adaptive layering menu comes up. It's the icon with the horizontal lines next to the return arrow.

Ted

 

[CFLBob]

My test standoffs were disappointing. I used the same scaling as the cube yesterday, and they still came out a little too small, by about .010" diameter. The threaded holes were too small as well. If I scale the 100.5 scale factor by that .010 out of 0.375", I get 2%, 1.033. I will re-do those.

I also had an issue with the layers not adhering as I got farther up the pieces. I note the Prusa slicer drops the temperature of the extruder after the first layer while Cura doesn't, and since I didn't have that problem before, I'll disable that and leave it the same temperature for the whole print (210C - it dropped it to 200).

More experiments needed.

 

[Technical Ted]

I'm not much help here with those problems. The prints I've done so far and measured, with PLA only, have been spot on expect for Z which was just a little off. When I say spot on, I mean within 0.001". The edges did budge out a few thousandths, but the wall dimensions were very good. That's where I think the chamfers will help some.

I have never, so far anyways, tweaked any of the temperature settings, or other settings for that matter and have had good results, but typically I'm not shooting for exact dimensions, just a decent finish and appearance.

I made this Impossible Dovetail puzzle which I drew up in Fusion 360 and after giving the dovetails 0.002" clearance they slide very nicely. I made them out of brass and aluminum on my shaper, but these are MUCH quicker and easier!!! X and Y on them is very within ~0.001". Z was just a touch shy maybe 0.004-0.005". Hole size was 0.001" smaller than design.

I know filament type/brand can have a major impact, but I've just use the default setting in Prusaslicer and everything has come out fine so far. The printer can have a major impact as well. My printer is the new model Prusa Mk3S+. For filaments, I've used the Prusa PLA that came with the printer, Sunlu and Jayo (same filament, different brand name) and Labists. They all work very well for my use. Only done a few things with PETG, but never had any issues so far. I've also got MatchBox, Paramount3D and Overature, but haven't printed anything with these yet, just building up my color selections.

Maybe others can help, or you can always post in the Prusa site and get some very good advice. Testing is always fun as long as you can maintain your patience!

Good luck and please keep us informed with your progress!
Ted

 

[CFLBob]

I think you just helped me a lot.

I think my problem with these standoffs has been that I created them wrong in CAD. What I did was draw a cylinder 3/8 diameter by 1/2" tall, then imported a CAD drawing of a screw from McMaster Carr. My mistake was that I then did a Boolean subtraction to remove the screw, leaving screw threads in hole. You said "after giving the dovetails 0.002" clearance they slide very nicely " and that made me think that the way I drew it, there's no clearance at all. The holes are threaded, but too tight. I think I need to enlarge them by a few mils.

I made two in the drawing, one was 1/4-20 and the other 6-32. It doesn't take long to print them, and they use about 1 gram of filament, so it's a cheap and easy experiment.

 

[skyline1]

Are you printing the screw threads in ?

I print a lot of stuff with M3 and M4 threads, A US no 6 screw from the ANSI table has a dia. of .1380" or approx 3.5mm which is between the two metric sizes

I have found that printing threads as small as this is pretty much beyond the capabilities of most FDM printers, certainly beyond mine, You might be able to do it with a resin printer though as they can work to much finer resolutions. They use an optical process rather than a mechanical one.

However you can treat the print almost like a metal component and print the holes tapping size then tap them.

In PLA, which is usually my material of choice, I have found that this technique works really well and you can get really tiny but remarkably strong threads. PLA will tap really easily (a little to easily sometimes. you need to be gentle) I have done M2 and 8BA sizes which are really tiny and well beyond what you could print in.

Whilst this does entail an extra step and the requisite tapping gear it gives really accurate tapped threads. If your tapping holes are a little under or oversize It will not matter PLA is very soft and forgiving in fact it is a good idea to make them a little under so the tap will enlarge them when it cuts the thread.

The other method often employed with small threaded holes is to print hexagonal holes, briefly pause the printer as it gets to the top of the hexagon, usually with a G Code pause at the right point in the program, but I have done it manually, then simply drop a nut in and continue printing.

This is similar to the technique sometimes used in casting where various inserts are "cast in".

Best Regards Mark

 

[Technical Ted]

They make the parts for the Prusa Mk3S+ at their factory on their own "farm" from PETG and commonly use either a slot or a hex hole and you will push nuts into these when assembling your printer. Works very well. Some also use metal threaded inserts that you can pick up from McMaster Carr and other places.

Ted

 

[CFLBob]

This whole side track of making threaded standoffs came from having seen a video where they print a crescent wrench (adjustable spanner) and it comes off the printer ready to use. But it's not FDM, it's some sort of powder bed process.

I know that the most common way to make a FDM printed part threaded is exactly what you're saying, Ted. Print a hex in the end, with a little extra lip or two to hold a nut in place once you pop it in. Or a threaded insert of some sort.

I got thinking last night that simply scaling the part isn't going to work, because it scales the distances between the threads, also.

The CAD program I'm using (Rhino3D, v5) has scale commands for 1, 2, or 3 axes. Maybe scale 2D would be the equivalent of using a slightly bigger drill before tapping.

And maybe this is just teaching me one of the reasons that led everyone to using nuts or threaded inserts.

 

[awake]

Bob, I suspect you are using settings for PLA rather than for PLA+. The latter needs to be about 20° hotter than PLA, so more like 225° for the first layer and 220° for subsequent layers - or try using 220° throughout. Keep in mind that the exact temperature needed may vary some from one brand of PLA or PLA+ to another; I suspect also that the measurement of the temperature on the hot end may vary from one printer to another especially in the lower-end brands. IOW, on a Prusa, setting the hot end for 200° probably gives right at 200°, +/- 1° ... but on my clone hot end, I would not be surprised if it is off by 5 or 10°. Fortunately, this error should be fairly consistent, so it is just a matter of some trial and error to see what setting my printer needs for a given filament.

Like others, I would not recommend trying to print small usable threads on a filament printer. The smallest threads I have tried to print were 1/2" x 10-tpi acme, and even then I printed the threads more to help guide the tap than for any other reason. They did sort-of print, but I would hate to rely too much on the strength of these threads. On the other hand, if you are printing larger parts, threads of, say 1-2 tpi or 1.5mm or larger can come out quite nicely. For anything smaller, sizing the hole for tapping and then tapping them is the way to go - and it is fast; I put my tap in a cordless electric drill and can tap a dozen holes in a minute or two.

Yes, you always need to design in allowances, not just to compensate for what your printer does, but in general for any sliding fit. As you have discovered, this is best done at the design stage, not by trying to scale the print.

 

[ddmckee54]

5-6 years ago I FDM printed a 6" cresent wrench, I think I got the design off Thingiverse. It mostly worked, but it left a lot to be desired, it's opearation was not exactly smooth.

Like Andy I wouldn't recommend trying to print a small thread. Your layer thickness will tell you how many layers will fit wiithin the thread pitch. And that will tell you how closely your print will approximate the thread. Consider trying to print an M3 thread for instance. I normally print at a 0.25mm layer thickness. The surface finish is acceptable, I can get a reasonable amount of detail, and it prints relatively quickly. At that layer height, an M3 thread with a 0.5mm thread pitch gives you 2 layers to advance the thread through a full turn. Even at a 0.1mm layer thickness you've only got 5 layers to advance the thread through 360°.

I made fidget spinners for my great niece and great nephews a couple of years ago. These spinners used 1/2-13 hex nuts for weight. I printed threaded plugs to keep little fingers from getting stuck in the threads of the 0nuts. It took close to a dozen different attempts, mofifying the thread design slightly each time at the CAD level, before I got the fit on the threads that I wanted.

If I need to be able to assemble a part, and be relatively sure it will STAY assembled, I'll either design the part with nut pockets, or with locations where threaded inserts can be installed - or both if I'm feeling particularly adventurous.

Don

 

[crec]

You can print some pretty fine threads on a Prusa, but I would not necessarily depend on them for strength. The robotics team I run uses 3d printers to make a number of items that are used on our competition robots. If we are doing just a fitment test we will print the threads in as we will probably make the final item in the machine shop out of stronger materials. If tapping in a printed item I suggest increasing the wall layers to give a bit more material for the tap to work in. If printing the threads in the item a couple extra wall layers is also suggested as it gives a bit more strength and if you need to chase with a tap you will have a bit more material to work with before cutting through. If I got the pictures inserted correctly here is a test fitment using 10-32 and 14-20 bolts.

 

[CFLBob]

Just a little note to let you know that I am reading these although I haven't done any more prints since the last ones I mentioned.

When I decided to try the threaded standoffs, I had Cura set for a slice thickness of 0.2 mm, which is just under .008". I thought with the 1/4-20 threads being .050 from one peak to the next that the layers were less than 1/6 of that so it seemed like that might make little jagged edges that were small compared to the thread so it should work. Then I told myself that a 6-32 screw is .031 from peak to peak and it's still 1/4 of that. It's a cheap experiment, so why not try it?

Anything that requires post processing, like making me to run a screw or tap through it, is too much work unless this is an emergency. Let's say it's the weekend and I'm working on an old radio or something that needs a plastic standoff that's not like anything I have. That's when it's time to do this trick.

 

[ddmckee54]

Bob:

For something like the threaded standoffs you describe I would use threaded inserts, the kind you use a soldering iron or wood-burning tool to install. That way you can depend on the threads, and disasssembling/assembling the parts multiple times is no problem. When I did the threaded inserts for my dust collector airlock body it only took a few seconds to install each insert - and I used 24 of them on the airlock body. It took maybe a 2-3 minutes total, and I wanted to be able to depend on the threads since I knew it would probably be apart many times is its' working lifetime.

It took longer to pick the little bugger up off the floor when I dropped one than it did to install them.

Don

 

[CFLBob]

(I would have sworn I answered this 12 hours ago).

I went to McMaster Carr in search of threaded inserts and got lost. I couldn't tell you how many screen pages of inserts there are but there were three pages full of links to other pages to select the parts. It was overwhelming and I got nowhere.

 

[ddmckee54]

Look on Ebay or Amazon, that's what I do. McMaster Carr may have everything, but their prices are just too rich for me. I just checked on Ebay, I searched for "6-32 threaded inserts", and you can get (100) 6-32 brass inserts for 3D printing at a cost of $14.99. It's free shipping, and they ship from GA so you won't be waiting for weeks and weeks. (Ebay listing)

Don

 

[awake]

Yes, what Don said. I did a quick search on Amazon for "3d print threaded inserts" and came up with a bunch of results. Here is one as an example (no affiliation, no experience with them, etc. - just a random but illustrative result): https://www.amazon.com/Canitu-Knurl...+print+threaded+inserts&qid=1619708939&sr=8-4

As with the example above, all of the results at the top of Amazon's list featured an assortment of metric sizes. I often find that it is cheaper to go with the metric threads for something like this, sourcing the matching screws from the same source (i.e., Amazon or eBay).

And just for full disclosure - I've never used these, only seen them used in various You Tube videos; looks like the basic procedure is to print the hole to a certain size that will accept these, then put the insert over the hole and press a soldering iron to it until it melts into place. I would think these would be good to use for the smaller sizes such as 6-32, 8-32, M3, or M4, but I would still tend to go with tapping suitably sized holes for anything 10-24 or M5 or larger. Or let me say, I DO go with tapping that size. Again, with a battery powered drill and a tap, it is about one second to tap the hole, and one second to remove the tap, and on to the next!

 

[CFLBob]

My plan (saying "plan" dignifies it) was to find a 3D printing forum and see what other people are using for PLA prints. I didn't think this forum would be the one.

I don't have much hardware in metric sizes, so I'd be inclined to stick with 4-40 through maybe 10-32.

I don't need any standoffs, the whole idea behind doing the ones I did was just to see if I could do it.

 

[CraigLD]

just a random but illustrative result): https://www.amazon.com/Canitu-Knurled-Threaded-Embedment-Assortment/dp/B08B341V76/ref=sr_1_4?dchild=1&keywords=3d+print+threaded+inserts&qid=1619708939&sr=8-4

I didn't buy this particular item, but did buy a similar item:
Glarks 370Pcs M2 M3 M4 M5 Female Thread Knurled Brass Threaded Insert Embedment Nut Assortment Kit for 3D Printing

Pressing on them with a hot soldering iron cleanly sinks them into a 3D printed parts. In my case I didn't have to pre-drill a hole before inserting. I did screw in an appropriate size bolt before heating to prevent the threaded hole from clogging with the melted PLA.