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Author Topic: T5 pulley  (Read 4732 times)
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ArtF
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« on: April 26, 2013, 01:11:58 PM »

Hi Guys..

 One more. A T5 pulley made to see if I could match a belt laying about in my shop.
This took 9 minutes, was printed on low res, fast mode, and seems to fit fine. As you can see its pretty
damn small..

Art


* Pulley.jpg (327.33 KB, 800x600 - viewed 703 times.)
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Ken_Shea
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« Reply #1 on: April 26, 2013, 08:44:47 PM »

Sure look great, if time permits one will be setting on my desk.

Thanks for posting the sample prints.
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ArtF
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« Reply #2 on: April 27, 2013, 12:20:16 AM »

Hi Ken:

  Ive found the timing pulleys are the hardest to do accuratel y, the T5 seems to work great, the HTD3 however wont run in properly, but with a very small 3mm tooth profile it doesnt have to be out much not to work. I am going to try shrinking the print slightly as I can see the teeth are very slightly too large, after 8 teeth or so the belt begins to rise out of the mesh.. a sign of a small error..

 Ill let you know how the HTD's work out, but Im pretty happy with the T5's..

Art
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Ken_Shea
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« Reply #3 on: April 27, 2013, 08:46:59 AM »

Hi Art,
Does your 3D printer have any offset capabilit y, kind of like cutter comp on a CNC?


Ken
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ArtF
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« Reply #4 on: April 27, 2013, 09:03:18 AM »

Ken:

   Yes, and no. My printer, and I suspect most printers, rely on the STL triangula ted model
to do its placement of material. All the programs I have seen have scaling, which in essense is a form of "Comp light" in that while it can scale the materials to the right size, if you increase an items outside dimension you are also scaling the inside dimension . Its why mathmatic allly correct models are a necessity
if your doing any mechanica l type of work, the sizing should be proper and then the printer calibrate d to proper size.
  Even then, the issue of shrinkage and resolutio n of the printer play a strong part. The T5 pulley for
example sems to work very well, but the belt had to be run back and forth to help "run in" the gear and remove ticks and aberation s unseen by the eye. Pulleys are probably one of the lowest tolerance items,
while gears are high tolerance . Slight backlash is ually available in a gear, while a pulley has "linear growth" of error. For example, if a pulley tooth on an HTD belt is 3mm, and the printer prints
3.05mm's after 10 teeth of wrap your already .5mms out, 1/6th of a tooth and the belt will no longer sit
on the pulley. The wrapping of a pulley by the belt makes a pulley very suceptabl e to this linear error
accumulat ion. My HTD pulleys, ( the first two ) dont work after about 10 teeth, scaling the model in the
printer makes it much better but you have to print several shrinking as you go to find your sweet spot. Once you know the scaling factor to use though, you can resuse it on all pulleys since that scale is actually the variance of real world to printer world accuracy.

 So quick answer, yes and no. Smiley

Art
« Last Edit: April 27, 2013, 10:17:28 AM by ArtF » Logged
Dan
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« Reply #5 on: April 28, 2013, 12:53:32 PM »

Art,

That shrinkage you mention, is it a constant for a given printer-material combinati on that could be taken note of and implement ed on any print? Or is it merely specific per given STL file?

Dan
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ArtF
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« Reply #6 on: April 28, 2013, 01:05:47 PM »

Dan:

  I havent run enough testing to say for sure. What I suspect is its a matter of the printer itself and the current calibrati on of that printer. Since HTD's are soooo sensitive ( any small timing belt pulley really..), Im thinking the same cal factor will work on all models until you recalibra te a printer. I printed 3 to find that scale %. I suspect its good for only my machine, numbers will vary between units. Once you know it, I think you can then just use that scaling value in future. I'll know that after a few dozen more.  Smiley
   Since Im pretty sure the stl's are accurate to several decimal points, Im getting pretty confident that the scaling is the way to ensure your printer will do a good gear. Get a timing pulley to work and you'll KNOW your printer is now cal'ed for gears. That linear accumulat ion of error is a great way to test for that, if a pulley fits a belt, you have figured out your machine. Smaller the belt, the tighter your calibrati on is..

 My printer just failed me twice today.. seems noise or something as the platform just dropped to the bottom , but after about 50 models, thats not a bad failure rate. Think Ill make a servo driven one that a bit more robust.. Smiley

  Art
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Mooselake
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« Reply #7 on: April 28, 2013, 11:33:48 PM »

Art:

Calibrati on actually involves two things.  First you have to calibrate your printer so the head moves accuratel y, then you have to calibrate the width of your extruded filament, i.e. your extrusion rate, so it's actually what your slicing program assumes.

With commercia l pulleys and belts (screws, whatever) you know the real numbers and calculate what the correct step size is.  For reprap style printed parts, they're often not quite right, and you have to take out your dial gauge and accuratel y (well, as accuratel y as you can) measure the actual movement and adjust your step size.

Extrusion calibrati on takes into account how far the extruder moves the filament - usually governed by how far the hobbed bolt (or other displacem ent type filament driver) digs in.  It's easiest to just measure how far the filament moves.  You'll have to measure the diameter of your filament, which can change from roll to roll (or within a roll if you're unlucky), at several places and average it.  Since it can be oval you'll have to take that into account - usually you're told to average it around the diameter  You're really trying to determine the circular equivalen t of the oval cross sectional area, but the average seems to work.  I've forgotten how to figure an oval area, anyway.  After all this you print a single wall object and find the average thickness of the actual extrusion, and if it's not right there should be a fudge factor you can put in the slicing program.  If not, you can fudge the filament diameter.  There are some people who use cross sectional area to set their extruder and filament, but I don't use or understan d this technique .

The common practice of printing something, measuring it, and adjusting the appropria te step rate only works for that one size because you're compensat ing for two variables .  It is a good confirmat ion the other steps were right, but not a good calibrati on tool.  Good enough for Mr. Jaws, not so good for gears.

Inside holes have some other problems.  This says it better than I can.

You can fudge the scale to get it right, but if you get everythin g tweaked just right you won't have to.

Hope this is helpful.  I've spent way to much time reading about and fooling with this stuff.

Kirk
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ArtF
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« Reply #8 on: April 29, 2013, 05:46:38 AM »

Hi Kirk:

   Yes, the calibrati on can be pretty coplex. ..BUT once calibrate d I think useing a known scale factor as the final step
is a valid one. Since there are so many variables, the system is likley to be off a small amount, such as mine is with that
htd3 , but the scale factor is liek a final step in the sizing of whats going to happen based on final calibrati on results.

   I was aware of most of the links informati on, but not all, I appreciat e the link. For inside holes and such GT uses about
300 vertices to reduce the amount of vertex wrap on inside holes, and about 3000 points on outside contours for the same reason.
This is one of the reasons not all models are the same quality. Then theres the printer softwares capabilit y to do arc fitting
on contours which can fix both inside and outside poly's.

   I think its mostly software that different iates the various printers, and its getting better all the time. Im watching it morph
with interest. ....

Art
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