How do you determine tolerance??

I think you already have your answer but may not realize it. I've found allowing for a 1  mm tolerance works pretty good. If two pieces need to fit together, design to spec and then subtract 1 mm on any edges that need to fit together. Personally, the hardest part for me was shifting from the mindset of fine machinery tolerances, to 3D printing tolerances. A thirty second of an inch just seems awful sloppy and I have a terrible time forcing myself to design stuff that way. You can tighten that up a bit using the best print quality settings, but using an average print quality and speed, 1  mm should get you where you need to be on most things.

Really 1mm? You may want to check the tuning on your machine. I can easily allow for .3mm maybe .5mm at most on all my prints from paper to print. Also the issue is not so much the machine but the shrinkage of the material as it cools. ABS shrinks by about 2% once cooled so you need to oversize all your dimensions by 2% when making a design. PLA is usually around 1% shrinkage. This % can very to a small degree even within the same vendor and different batches as well.

What I did was made a model that is 50mm long, 20mm wide, 10mm tall, with two 15mm vertical bores through it with 20mm between the bores. I print this with every filament change and measure the shrinkage vs the actual dimensions and then I know the shrinkage % for that roll and can adjust my designs accordingly. I even started a spread sheet with various vendors and their colors and the shrinkage so that I can use it for reference. Again though I have seen some variance which is still acceptable with the same vendor and same color but different batch.

It was not my intention to sound abrasive. I was just trying to point out that tolerance issue are most often not the machine fault.

I was actually wrong about the shrink rate of ABS. It is 8% and a simple Google search will confirm this.

http://www.fabbaloo.com/blog/2014/5/8/s … easurement

When ABS cools after printing it shrinks approximately 8%, although this will vary somewhat depending on the particular ABS in use. PLA has far less shrinkage.

This means (aside from the warping issue) that objects printed in ABS to specific sizes  will not actually be that size due to shrinkage. A 50mm print might actually be 46mm instead.

https://groups.google.com/forum/#!topic … gT-JfnEXt8

PLA for the record is only about 2% but does not hold up well under harsh environments.

Can I just point out that 8% of .48mm is .03~.04mm OR ABOUT 4/100 of a millimeter.

I have my specs down very exact I calibrate my X,Y,Z so my calibration box is exactly 10X10X10.

It's true I get warping issues and I so love using acetone with an injection needle so it creates an ABS slurry at the first layer, my designs are solidly cemented to the base. very little warping if any at all.

Thanks for all the help!

A blogger from Canada and an Internet message board as a source for the 8% figure? I'd take that one with a grain of salt.

Doing a quick search, this site quotes 4-6 thousandths for shrinkage on injection molding:

http://www.injectionmoldtips.com/shrinkage.html

This site gives shrinkage ranges for different flavors of ABS from 4-7 thousandths of an inch, per inch, which would be .4% to .7%:

http://ides.typepad.com/all_about_plast … of-ab.html

I wouldn't question a ballpark figure of a half percent, but 8%? That be an eighth of an inch on a 1.5 inch part and a half inch on a 6" part. Casual observation alone should be enough to question the source on that one. Yes?

I've been using 3D printing mainly for prototyping smaller parts under 2", with 100% infill, and have been regularly checking the results with calipers. To the extent the parts vary from spec, it's always on the large side, with slow and low settings producing the smallest tolerance issues. As near as I can tell, it's the inevitable banding effect of 3D printing that causes most of it. You're basically trying to fill a square hole with a round peg. The filament comes out round, gets smooshed flat, and the distance it gets smooshed depends on how much material is being flattened. If the flow rate is too high, it squishes out farther, making the part slightly larger than intended. If there's any shrinkage taking place, I haven't found it to be significant, but the flow rate can make enough of a difference in the quality of a print to make it worth adjusting. In playing around with it, if the top layer is overly rough and getting too much of a kind of waffle pattern, the flow rate needs to be tweeked down a bit. In any case, pardon me if I over reacted to your earlier post and came off as too much of a grouch. Hopefully the resulting discussions were at least a little bit productive.

A good guess for plastic shrinkage is to multiply the difference between the room temp and glass transition ("melting") temp by the thermal expansion coefficient of the material: e.g. for ABS (105 - 20) * 7.38e-5 = 0.63%