How to determine acceptable clearance between parts?

Hi,

I don't know if this forum is the right one to post this question, but as the parameters seem very machine dependent I suppose it's not so bad.

I just downloaded a peristaltic pump openscad model, so basically a gears set enclosed and printed in place. My first 2 prints gave me rather disappointing results: the model came out OK, but gears were entirely stuck, melted together. I suppose I have to increase the clearance between the gears, but what value would you consider a "normal" one?

By default the model is set with a 0.15mm clearance, apparently too small for my setup (0.5mm nozzle). With 0.3mm it seems better but still a little fused (I stopped the print as soon as I had enough height to test it and the motor mounting hole, also too small compared to the model dimensions).

I have not found a rule of thumb as how I can determine reliably a proper clearance for my printer, and I suppose that would highly depend on the nozzle diameter as well. What clearance do you use in such a case? Is 0.35mm really off or is it a normal value for the Ormerod2?

Matthieu

Edited 1 time(s). Last edit at 09/27/2016 05:22AM by mat_fr.

There are a few factors that can affect how well two close objects remain detached. First, the extrusion factor affects the perimeter dimension, so it may be an idea to do an extrusion test using the filament you use for the part. Note that the perimeter extrusion shape is semi-circular on the outside, and the part size will be the average of distance between the outer "bulges" of each layer and the inner "grooves". The difference decreases with layer height, so you get better accuracy with a small layer height than a bigger layer height. Then there is blobbing that occurs - you might try changing the seam position (start & end position) in your slicing software. In "Slic3r" this can be found under "Print Settings" > "Layers and Perimeters" Setting it to "aligned" will probably cause the parts to fuse at the seam position, but at least it will be a single place to cut or break away. Slowing the perimeter speed to something very slow might help. You can also try changing the perimeter extrusion width - this is usually left as automatic in Slic3r, but may be changed manually. You could try making it smaller than your nozzle diameter, but if you make it too small the quality will suffer.

If the clearance is needed on layer 1, there is also the issue of the first layer height (Z home position). If the Z home position is slightly low, the first layer will have thicker extrusions and so the parts will fuse at the base but may be OK for all other layers. So you need to err on the side of having Z home too high rather than too low - though if it is a lot too high the first layer will not stick to the bed. You could also try using a raft.

After you have minimized the above factors, I suggest just increasing the gap in OpenScad in stages until the parts can be broken apart with a bit of force. I've usually found that a gap of 0.15mm works for most things with my 0.5mm nozzle, though if there are fine details or sharp angles on the perimeters (such as small gear teeth), this may need to be increased. Nozzle size is irrelevant on straight sections, it only becomes an issue on sharp corners.

Dave

Well, that's a thorough answer.
I went through the extrusion calibration process according to the method of printing a single walled structure containing lines, straight angles and curves and measure the wall size at the top. It seemed good with a extrusion multiplier of 0.9.
Seam position is set to aligned in slic3r.
There is a clearance needed on layer 1, and maybe I've set the Z home too low, the first layer is squished a little.

I've never tried using a raft, I'll give it a try.

I suppose the temperature can also have an effect on the squishing, if the temperature is too high the extruded plastic will be too soft?

So all in all, 0.15mm should be a usable value if other parameters are correct.
Thanks.

Matthieu

Quote
mat_fr
I suppose the temperature can also have an effect on the squishing, if the temperature is too high the extruded plastic will be too soft?

No. The width of the extrusion is governed only by the volume extruded per unit length and the height of the extrusion. The volume is fixed by the slicing software (which calculates volume per unit length from desired width and layer height). The height of the extrusion is fixed by the distance between the nozzle and the previous layer - or the bed in the case of the first layer. It's geometry ... if V = volume, W = width, L = length and H = height, then:

V = W x L x H
therefore
W = V / (L x H)

If volume and length are fixed, then width is proportional to 1 / Height. Temperature does not come into it. The viscosity of the plastic will never become so low that it slumps down below the gap under the nozzle and so flows out sideways on its own.

Dave

That all makes sense.

Thank you again for all the time you spend responding to all my questions. I've learned much since hanging around.

Just as a follow up, once the z home set a little higher than usual I ended up with a pretty decent print, at least on the bottom side (this is with 0.20mm clearance I think)


But this model has gears with a little hangover, like this one [www.thingiverse.com], and on the upper part the gears teeth shrink and go up, then ruin the print, ending up like this :



I tried with PLA and PETG (the pictures are PETG), and both prints ended up the same way. I can't figure out how to limit the shrinkage on such sharp shapes, it usually comes out OK when there is a little overhang but on a regular and massive wall...

Edited 3 time(s). Last edit at 09/30/2016 02:51AM by mat_fr.

Overhang issues require the extrusion to be cooled as soon as possible after being laid down to prevent curl-up, so needs a good nozzle-cooling arrangement - a second cooling fan being the normal method. I have successfully babied a print with an overhang by using a drinking straw to blow air at the base of the nozzle on every layer as it got to the overhang region. Boring and time-consuming, but it seems to work well.

Dave

Blowing through a straw... good old low tech solution indeed
As I previously said in another post I ended up printing the acrylic spacer for the hot end, so I should be able to add a second fan eventually (there was only one on the first Ormerod2) but from what I've seen the second fan mount ends up being lower than the nozzle so maybe something is wrong with the print, or the model, or my nozzle setup. I'll work on it.
Thanks

Matthieu

Quote
mat_fr
As I previously said in another post I ended up printing the acrylic spacer for the hot end, so I should be able to add a second fan eventually (there was only one on the first Ormerod2) but from what I've seen the second fan mount ends up being lower than the nozzle so maybe something is wrong with the print, or the model, or my nozzle setup. I'll work on it.
Thanks

Matthieu

There are two pips inside the cooling fan duct slot that need to locate into the holes on the fan spacer. I couldn't get the duct high enough till I realised some brute force was needed to get the duct on all the way. With a quickset nozzle you need to have the neck of the nozzle just on the edge of the cooling block to get the right height, but I think with the one piece nozzle if it's screwed in correctly it should be the right height anyway.

One thing that may or may not be an issue is the distance from the duct outlet to the nozzle. The (single) quickset nozzle is normally located in the cooling block position nearest the duct making it quite close. The one piece nozzle sits in the centre of the cooling block making it further away from the fan duct outlet. As it is, with the quickset nozzle and cooling fan on, the bridging quality is impacted by the direction of travel.

I have found the original Fan duct to be too noisy becouse of too thin airway which the air comes out and it didnt cool anaugh till i really rameped up the speed on the fan. I have modified the Fan Spacer to a new hight and added a fan duct designed by another forummember which i dont remember who(sorry).



I also glued a peice of Metal sheet( part of Shielding which came with the printer and i never installed it becouse it didnt seem to fit) so the air wouldnt blow on the Heater which couse temperatures to drop when a lot of cooling is required.

I will attach the STLs for the duct and fan spacer in this post if anyone wants it.

Quote
trevmas
There are two pips inside the cooling fan duct slot that need to locate into the holes on the fan spacer. I couldn't get the duct high enough till I realised some brute force was needed to get the duct on all the way. With a quickset nozzle you need to have the neck of the nozzle just on the edge of the cooling block to get the right height, but I think with the one piece nozzle if it's screwed in correctly it should be the right height anyway.

One thing that may or may not be an issue is the distance from the duct outlet to the nozzle. The (single) quickset nozzle is normally located in the cooling block position nearest the duct making it quite close. The one piece nozzle sits in the centre of the cooling block making it further away from the fan duct outlet. As it is, with the quickset nozzle and cooling fan on, the bridging quality is impacted by the direction of travel.

I noticed, but I couldn't make it slip high enough until now, even with brute force. I just managed to put the duct high enough (barely, it seems...), the X carriage was not absolutely horizontal (on the Y dimension) due to the bearing screw not being tight enough. That wasn't a big deal with only the nozzle brushing past the bed, but it becomes critical with the part fan duct surface. I hope it won't get loose too quickly.

I don't have the quickset nozzle, mine is at the center. And I just can't test it yet since, having the first version of the ormerod, I only have one fan and no screw matching the part (as I just noticed, I need M2.5x10).

Quote
Darathy
I have found the original Fan duct to be too noisy becouse of too thin airway which the air comes out and it didnt cool anaugh till i really rameped up the speed on the fan. I have modified the Fan Spacer to a new hight and added a fan duct designed by another forummember which i dont remember who(sorry).

I also glued a peice of Metal sheet( part of Shielding which came with the printer and i never installed it becouse it didnt seem to fit) so the air wouldnt blow on the Heater which couse temperatures to drop when a lot of cooling is required.

I will attach the STLs for the duct and fan spacer in this post if anyone wants it.

Thanks for the part. I'm not sure I can print such a part, as there is much overhang and support material would end up in the duct, but I can try this one.

you can print the duct with no supports. But little cooling would be good

I've finally managed to put a fan on the stock duct this week-end. As I changed the main hot end fan I was able to put the former one as a part fan (and hear its marvelous noise again, oh joy).
I've tested this morning a print, it's really efficient on the part fans'side (X end side), a lot less on the other side indeed.

I was able to print your fan duct also, so I'm now printing the matching spacer.

It's incredible how we can tweak this printer... Alluminum arm, spring loader extruder, helical gears for extruder and Z axis, Y rod adjuster, extra fan. And all that does make a big difference.
Seems that I only miss DC42 differencial IR sensor and a cable chain (cables are a mess on my printer).

Quote
mat_fr
I've finally managed to put a fan on the stock duct this week-end. As I changed the main hot end fan I was able to put the former one as a part fan (and hear its marvelous noise again, oh joy).
I've tested this morning a print, it's really efficient on the part fans'side (X end side), a lot less on the other side indeed.

I was able to print your fan duct also, so I'm now printing the matching spacer.

It's incredible how we can tweak this printer... Alluminum arm, spring loader extruder, helical gears for extruder and Z axis, Y rod adjuster, extra fan. And all that does make a big difference.
Seems that I only miss DC42 differencial IR sensor and a cable chain (cables are a mess on my printer).

That is the joy in RepRap

The metal part i used to deflect airflow over the heat block is this:


It came with the printer for CE certificate or something. I couldnt figure out how to apply it to the motors and other parts.

I broke away the peices where red line is and Cut with a dremel where the blue line is after and Hot gun glued it to the Fand duct i use to hold DC42s sensor in place(http://www.thingiverse.com/thing:992528) then just bend it so its just above the nozzle tip.

Edited 1 time(s). Last edit at 10/05/2016 11:14AM by Darathy.

I couldn't place this one either, and as I learned recently that it was only a EMI certification issue I removed all the shielding that was getting in the way.

I was wondering how you glued it in place, but hot glue seems a good way, I'll go for it (the firmware complains if the fan is on and the temperature doesn't rise fast enough).

I glued it where the yellow blob is:


in the end it looks like this:


Well if u wana glue it like this you will have to have the duct for the hot end fan i linked erlier.

Edited 1 time(s). Last edit at 10/05/2016 11:37AM by Darathy.

I printed it and installed it last week-end, I'm good to go

Quote
mat_fr

It's incredible how we can tweak this printer... Alluminum arm, spring loader extruder, helical gears for extruder and Z axis, Y rod adjuster, extra fan. And all that does make a big difference.
Seems that I only miss DC42 differencial IR sensor and a cable chain (cables are a mess on my printer).

I've just upgraded my Ormerod 2 bed support to the aluminium version. [www.3dprintandstuff.co.uk] Well worth it, it came really quickly and was a doddle to install.

I have a box section for my Y axis [www.thingiverse.com] and it was let down by the plywood bits, especially as they started to crack and were squished at the bed adjuster / mounts. Now everything seems much more rigid and so much easier to get level.

If you've got an Ormerod 2 I would thoroughly recommend it.