AIRFOIL PRINTS

Thank you. Yes, most commercial printers are garbage in this respect. The ones at retail copy stores are among the worst, as they get so much usage. Always check your prints in both X and Y axes. If you need a really accurate print, try and find a commercial plotting service for engineering firms. They're getting harder to find, and are usually co-located with what's left of the commercial blueprint shops, but they'll have the most-accurate plots. You'll need a CAD file format, though. Pretty sure most of these don't take PDF.

 

You can draw them by hand; it was done that way for years before computers. Get the table of airfoil ordinates, lay out the points, and connect them using french curves and ships curves.

 

You shouldn't. PDF is a super-set of the Postscript page-description language. When you create a PDF from a vector drawing program, including CAD programs, the interpreter is just translating the native vectors to Postscript vectors and wrapping that all in a PDF shell for portability. Adobe gives away the interpreters for free (or at least they used to - I'm not sure if that's still the case) for companies to include in their software for "PDF export", which is why you see that option everywhere. If the type (text) in the source file being sent to PDF is composed of system-installed fonts, the interpreter copies the font file (or a subset of it, depending on settings) into the PDF file as a Postscript library, and font references in the PDF point internally to that particular library.

Raster images are pretty much just included into a PDF file as-is, with the appropriate headers in Postscript for page placement, scaling, etc., and then the PDF "wrapper" is put around that. Creating a PDF from a raster file does exactly zero to improve the quality of the image, because of this. If you see any "improvement" in the output of a raster image printed "raw" or first made into a PDF (a claim I hear from time to time), it's that the latter is forcing your printer to use a Postscript interpreter driver instead of whatever native raster image processor driver is built into the printer. For most consumer-grade printers, the native RIP driver is set up to make photographs be smooth and bright color - exactly the opposite of what you want for engineering drawings.

 

1) Presuming their machine was capable of that kind of accuracy in the first place, which it isn't. Most laser and ink-jet devices are capable of ±1-2% dimensional accuracy brand-new out of the box. Large-format machine are generally much, much worse than that. I worked at a national top-ten prepress house for eight years. During the transition from traditional bluelines to digital ink-jet "bluelines," we had a top-of-the-line Epson large-format ink-jet printer set up with specialized software for printing "digital bluelines" and then overlays to show die-cuts. The best we were able to "calibrate" it was within about 0.0625" across a 48" print area. Changing humidity, changing paper stretch because of the amount of paper on the roll decreasing over time, changing paper stretch because of the amount of ink being put down across different areas of an individual print, and so on, all made "calibarating" it virtually impossible. The solution was easy: We discovered that the customers just didn't care. Most people who aren't trying to make parts from the drawing as a template, don't care at all.

And...

2) Presuming they "calibrate" their hardware. Most consumer- and even "prosumer"-grade large-format printers don't have discrete X-, Y-axis scaling "calibration". At best, it's in the print-driving software, and even that is uncommon for anything but top-flight prosumer-grade equipment. Most have, at best, a simple size-scaling adjustment. And this even presumes that the copy center involved actually does any "calibration" of the dimensional accuracy of their equipment. In college, I worked at a commercial copy center and, having used others later in my life (generally FedEX Office, not Staples), I can virtually guarantee that, unless you ask for it specifically, they never calibrate their equipment. It just gets used, and service is called when it breaks down. If you ask them to calibrate it dimensionally, you're going to get blank looks, then be told that it can't be done and, if you press them, someone will go and try to find a manual that's been getting dusty on a back shelf for years. They'll ask you to come back on Thursday night because "the guy who's really good on that machine" will be on-shift then, and when you come Thursday night, you'll run through the same cycle until he finally finds the manual and sits down to read it.

In theory, yes, you should be able to get reasonably accurate prints. But it's not going to be true in practice, unless that particular location just happens to have customers who need and request dimensional accuracy on a regular basis. And, if they do, the center probably has an engineering plotter for that work.

If you go ahead, a better test than a grid is to print a large-diameter circle and have them make a print. Go back the next day and get a second, identical print. Measure the diameter to see if the basic scaling is correct. Then lay them on top of each other, with light shining through, and see if the circles align all the way around their circumference. Then rotate one print 90°. See if they still line up. Any X-Y errors - and inconsistencies between print runs on different days - will jump right out at you.

In the end, it really depends on what you really need out of these printers. If their accuracy is better than your shop accuracy on the same part, it's "good enough." If you can - and need to - make things more accurately than those printers can print, then they're not "good enough." Whether or not you "need to" is subject enough for an entire thread on its own. Unless you're building something with a highly laminar-flow shape, and are capable of achieving the needed accuracy in your own shop, you can probably get away with less printing accuracy than you might think.