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Ajohn

Anyone have view on the Velleman K8200 3D

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On 2/4/2016 at 18:14, Ajohn said:

:grin: I'm bleary eyed from reading the thread.

A

For some reason I can get rid of the quote. :smile:Took 3 days to read the long one.

Interesting again. From this and the other my vision looks like it will involve a sturdy frame. Not sure which aluminium section to use yet. Comparing prices may as well be the V guide type but not sure yet. Expensive option but I had a weird Ebay refund recently and they didn't want the item returned and I managed to sell it. Be good to spend it on something useful. So it's looking like a 500mm cubic frame with space for bits at the bottom and a 300x200 build area and what happens to be left or less for Z.

X at the top and Y on the table as I suspect long runs of braid or belt may be a problem. I'm still inclined to feed the Z from the X axis which might balance up the masses on x and y and use nema 23's for those. I noticed the need for gearing on the extruder so not sure about 17's.

Any thoughts welcome.  :icon_mrgreen: I wont be able to print anything until it nearly finished.

One question Gina. How do the bought and the home made compare now ? Also I think I noticed one with the usual linear guides around still?

I'm surprised no one has come up with an engraver based around ramps.

John

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None of my printers use linear bearings now - they all use wheels on rails, including the little UP Plus 2.

Nema 17 stepper motors are plenty strong enough though I've gone for 60mm ones for my Giant printer and the Z axis will use one with a gearbox.  The GinaRep Titan uses 40mm Nema 17s for X and Y but a 60mm with printed gearing for the Z which involves lifting the heated 300mm square print bed.  I originally had a Nema 17 with built-in gearbox but found it too slow so I printed a big spur gear on the Titan then swapped the geared Nema 17 for the 60mm not geared with my own printed gears.

The X and Y axes on the Titan both use about 1.5m of fishing line with no problem - just occasional re-tensioning by a mm or so. 

I'm still working on the upgrade for the GinaRep Pilot but I'm thinking of swapping the 1.2mm nozzle on the Titan for a 0.4mm one and seeing how well that does for small, accurate parts.

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I beg your pardon - my Pilot printer is still using linear bearings for the print bed and Y axis ATM.  I have an 80mm wide V-slot extrusion I plan to use instead.

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Thank's Gina

The reason for thinking 23's one X and Y is that the X will be carrying 2 steppers and the guide and bearings for the X axis. Also in my view the same unit should from a reasonable basis for an engraver /  super light router/miller. When ever this is mentioned on the web people  compare it with cnc millers driven by mach 3 which in my view constitutes missing the point. They also mention missing G codes. At the moment there is a lot of work going on in various places using Arduino for cnc on lathes so why not mills. Initially I thought drive current to the steppers might be a problem but then found out how the steppers seem to be spec'd - 80C temperature rise. Often the case with electronic items, use the spec max's and things will let the smoke out. I need to check that area a bit more thoroughly.

I wondered about 12v steppers as from memory they generally have more power but loose out on speed unless the drive voltage is much higher and in practice will probable never match the usual 2 or 3v parts. Not at all sure if this matters.

Some one on another forum has a commercial unit that uses V rails. Initially they used an eccentric mounting on the wheels for adjustment. It seems that they caused problems and have switched to a rather stiff flat spring mounting which according to the owner is a lot better.  In the same area something in the dim and distant past rings bells. I'll use 4 wheels not 3. I'm sure I have come across something similar where 3 didn't work out - down to very slight varying curvature on the rails.

The rails for the frame work are a bit expensive but the fittings, corner blocks etc are stupidly so. Like £6 each for top corner 3 way joints. T nuts 60p a piece. My £100 ebay freebee looked like it would easily cover the lot till those  cropped up - excluding electronics.

:icon_mrgreen: Looks like we may have cross posted again  - or maybe the site doesn't like my browser as there doesn't seem to be anothe post no I have submitted it.

John

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Edited by Ajohn

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I'm using 12v steppers in the Titan.  I found the 3v ones tended to overload the A4988 drivers to get enough power - 12v ones take less current for the same power.  RAMPS uses a 12v supply for the steppers.

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That's interesting Gina. When I looked at steppers the 12v ones didn't show up.

I need more details on the electronics but my impression is that the stepper drivers are plugged into ramps so it's possible to use drivers such as these which allow the drive voltage to be increased which will also increase the speed the motors can switch at.

http://ooznest.co.uk/3D-Printer-Electronic-Parts/Boards/DRV8825-Stepper-Motor-Driver

These could be used for instance to switch 36 or 40v into the 12v parts which in principle will speed up the switching. I tried to find an online calculator that includes the effect of the series resistance as well as the inductance. No joy but 36v should cause them to switch state in under 600uSec. That's to a full 0.4A on a 12v part with a resistance of 30ohms and an inductance of 37mh. The time constant of the inductance and resistance 1.23mSec, twice as long. That's to a current level of 1/4 amp, it's = about 66% of V/R.  A 1/4 amp  with 36v would be faster than 350uSec. The times are to when full torque would be reached.

I'm not sure if this matters on a 3d printer but it generally does on cnc which usually means use the highest possible drive voltage. The drivers regulate the current in the steppers.

I'm a bit bugged by the ratings of the steppers. I looked at some industrial spec's and they mention numbers associated with an 80C temperature rise which would burn them out unless they were in a freezer. A 1/4 amp in a 0.4amp part would cause the temperature to rise by 0.25*80/0.4=50C and the torque figures will reduce by the same amount. Only problem is that there used to be these 1/2w resistors at work that they couldn't use so help yourself. Later I bought some of ebay and wondered why they got hot. :icon_mrgreen: I did think they looked a bit small but later found industrial ratings are roughly 1/2 the commercial ones so can be used at the ratings on the can. No more smoke now. I'd hope they didn't use a similar factor on steppers.

Hope them quick sums are correct but the principle is valid.

John

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Edited by Ajohn

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I'm not sure those driver modules are compatible with RAMPS.

One thing I can say is that the standard RepRap type A4988 stepper drivers by Pololu give plenty of speed with 12v steppers.  The printing speed is not limited by the XY stepper speeds but by how fast you can deposit filament.  There is a limit on how fast you can melt the filament even with high tepmeratures.  Too high a temperature causes to filament to degrade to carbon.  I even once had the filament catch fire when the temperature control failed :eek:

Edited by Gina
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One thing I would keep in mind, and it depends on where you locate your printer, but ventillation is important. An enclosed printer lends itself well to having an extractor fan fitted. My printer is setup in a small study room, in a shelved recess next to a window. I thought it would be sufficient to provide fresh air and get rid of nasty stuff just by leaving the window open. My mistake - it isn't. Right now my sinuses are burning and I have a sore throat which I reckon is due to sitting in the room with the printer running, so just today I fitted an extractor fan which vents everything outside. I hope that resolves my issue. With an un-enclosed printer you want to locate it well away from where you spend time - outside preferably!

ChrisH


IMG_0823_zpskyvnguej.jpg

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Yes, I use fume extraction too.  But one point, the ventillation doesn't want to be too harsh as to cause a draught across the print as this can cause printing problems.

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I'm pretty sure that the stepper drivers are completely compatible with ramps. They are offered as an alternative. The spec might be a bit misleading as the part they use is rated at 1.8 amps rms. The 2.5 amps is peak and they might best be limited to 24v even though they will go higher.

I managed to find a little bit more on the stepper motor ratings, from an RS data sheet. The 80C temperature rise is correct and it seems they are ok with an ambient temperature of -20 to 50C, :icon_mrgreen: I wouldn't want to touch one running at 130C though and would probably pass out if the ambient was 50C.

I've been nosing about on what power the extruder needs. Only comment I have seen so far is 5kg cm / 0.5nm. That's nearly as much as the longer 60mm nema17 gives that I noticed on ebay. Just a bit more than a kit which I think is made up in the UK for reprap  prusa. That uses 4kg cm 1.7 amps on everything. This is for 1.75mm and I assume the usual nozzle size. Not sure about 3mm and larger but have seen comments that the 1.75mm can be sent through quicker. Think that was from a Chinese retailer.

John

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Yes, the smaller size filament is easier to drive and direct drive is fine but 3mm filament takes more force and most extruders for this size use gearing to increase the torque.  In fact I've found 3mm filament is pretty difficult to drive and ABS particularly (I have less/little experience of PLA) tends to shred off and form a notch and then, of course, the hobbed pulley or bolt just turns and the filament doesn't move.  To prevent this from happening when the hotend produces too much load on the filament drive I have deliberately turned down the stepper motor drive current (preset pot on the driver module) so that the stepper just cogs rather than turning and chewing up the filament.

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I thought the volcano site might give more info on extruders but apart from offering up to 1.2mm even for 1.75mm filament little else. They do a 5:1 geared stepper but is seems to have rather low power for that sort of ratio.

I wondered if it might be possible to use a screw feed to get more contact area. Length of bolt pressing on the filament but it would need a seriously long drive shaft to avoid having a sharp  bend in the filament. Might be an idea for a bowden type as might a rubber wheel.

I looked at the data on the other driver I mentioned. It's 2.5a peak, 1.8a rms but a lot depends on the heatsink. I'd guess 24v and a lower current than 1.8 amps would be a good idea. Some of the heatsink may be built into the pcb. The link I posted includes a stick on one as well but I wouldn't assume the full 2.5a could be used.

The kit I mentioned that uses 4kg/cm stepper uses a gear on the extruder (Gregs but for 1.75mm). Reading the manual they suggest setting up the axis about 1/4 turn on the pot past the point where the steppers get stable after a move/move smoothly and that during prolonged printing they should get rather warm but not hot. Looks like it's supplied in the UK, £279 and includes everything including loaded firmware. They seem to have sold 4, good feed back and at the moment it's not relisted. Good build manual too.

:icon_mrgreen: Arggggggggggg Picking bits for these is painful but the kit suggests I can go over the top on drivers and maybe even steppers so if it doesn't work out use them on something else. Not sure what though.

John

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A question I hope some one can answer. How effective is microstepping on the extruder and axis. What sort of level is usually set?

I've got to the point where I need to tie down feed rates on all of the steppers so wonder how far microstepping can be reliably pushed?

The extruder arrived today. Some other bits have to come from far away but at least it's a start. This one could be converted to boden if needed. It appears to be the latest greatest. Doh I seem to have attached the image and can't add it to the thread.

 

 

John

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3dExtruder.JPG

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RepRap uses x16 microstepping as standard (all the time).  This is the highest that the A4988 driver chip can go though there's another that can do x32.  I have never used that chip so don't know how well that works.  One point to note though is that the higher the microstepping the lower the power.  eg. at 16x the power is reduced to 10% of that for full stepping.  Yes, just a tenth!

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As above, even using half-stepping reduces the torque by 30%. The DRV8825 will do x32, the RAPS128 would even do x128 - but for an extruder you want the torque not the speed so I would have thought it better to use a geared motor here.

ChrisH

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Yes, gearing definitely helps.  Something like 3:1 or 4:1 is ideal - I found 5:1 was a bit too much and too likely to shred the filament.

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There seems to be a number of extruder like the one I posted fitted to a number of printers, usually in Bowden mode. No gearing and the gear that drives the filament through is larger diameter than the usual ones in the geared extruders. It is shaped like a gear as well.  I'd guess it can't handle more than 1.75mm but I suspect that will suite me. It's fitted with a 1.8amp nema 17. I can give it a try anyway. A while to go yet as some bits have to come from far away to keep the prices down - angle brackets for the frame material I will be using.

I should have asked the question in another way as well. Typical steps per mm as far as the controller software is concerned.  The axis and the extruder.

John

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I had hoped some one would answer the steps per mm from the software end. I've read that too high a number can give the micro too much work to do. Looks to be dated info but leaves me wondering.

I thought stinking fish when the power drop of with micro stepping was mentioned and thought that doesn't make much sense when the motor is rotating at some speed to produce a torque  to drive something. The holding torque will drop off but that isn't what matters. The rotational torque does. 2 view on the subject

http://www.designnews.com/document.asp?doc_id=221386      a pdf to download on the page with the images

and

http://www.geckodrive.com/microstep-full-step-torque

John

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I don’t think that the statements on torque versus microstepping are being correctly interpreted here. INCREMENTAL torque per microstep decreases as the number of microsteps per step increases and as a result the positional accuracy of microstep positions increasingly deteriorates. A sine and cosine wave equating to an infinite number of microsteps and incremental torque tending to zero do drive a stepper motor.

Regardless whether microstepping is implemented, the current through the windings fluctuate between zero and some maximum value. No torque will be lost. Step the motor too fast and the inductance of the windings will affect the current flow; rob its torque causing it to stall. There are other factors as well such as motor inertia that will influence its torque, but the number of microsteps will not.

 

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I'm pretty happy about microstepping now and it's effects. Google comes up with a truth about microstepping web page at the top but I think it misses several important aspects and am more inclined to follow the comments in the links I posted.

What I am still asking about is the steps per mm set in the software. Typical 3d printers might use a 2.5mm pitch belt and either 16 or 20T pulleys. along with 16x microstepping This results in either 40 or 32 steps per mm being set into the software. They are exact numbers. 8x microstepping would result in 20 or 16 and so on. Belt stretch might result in the need to change them slightly. Lead screws and other things that could be used would result in different numbers. I read a comment about some one using a 4 start leadscrew with a pitch of 8mm. With 16x microstepping that would need 400 entering. Gina uses drums and a type of fishing line called braid. That is likely to need an entirely different number of steps per mm.

There are comments about suggesting that too high a pulse count in the software can cause the micro problems because it can't keep up so was wondering what people have used which they know works without any problem and also what type of drive it's used on - belt and pitch. screw and pitch or what ever. To few a number of steps in the software say 2 would limit the accuracy that is achievable as it could only position to 1/2mm which might ( big might, big joke really ) work out on an extruder if the build platform moved quickly enough. The software probably ties that relationship up anyway but again the numbers that can be set in the software and that will work are important.

John

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Yes, steps per mm depends on the geometry of your printer.  You can work out approximate values but the final number depends on calibration.  You make a test print of say 40mm cube and measure the actual dimensions.  From that you can calculate the change required to get the right dimensions.  AFAICT pretty well all 3D printers need calibrating.  Calibration takes care of all those little variables that are difficult to work out.  For instance, timing belts on timing pulleys don't exactly work out at the diameter of the top of the teeth - belts tend to vary a bit from make to make.  Sometimes a tiny bit of slack will alter the calibration very slightly.

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