The purpose of this page is to offer an explanation of various factors that affect the surface quality, strength, and precision of 3d prints parts. Many of the files available for print on commercial3dfiles.com such as RC Toys and Gearblocs STEM Building System components require a higher level of precision due to components that must fit and function together. We hope the information offered here will help you go from printing objects for visual display to printing functional devices with intermeshing parts that fit and function as intended.

Before you can manufacture anything with precision,  you must have ways to precisely measure what you make. Figure 1 below is a selection of tools we recommend for printing parts that need to fit together and function in a specific way. 

A digital caliper is a must have for precision 3d printing. There are many brands available that run from complete junk at the cheap end to a fine precision tool at the more expensive end. This is one area we recommend not going cheap. A cheap caliper will be inaccurate and inconsistent. Our favorite brands that we have first hand experience with are Mitutoyo and Starrett. There are many cheap brands that are really bad quality, but one that we’ve seen used with some success is Fowler. We strongly advise you not buy one any cheaper than Fowler. Digital calipers should be kept clean. You must select the units you want to measure in, and then set them to zero before use. Clean the faces of the jaws by wiping with your clean finger or a tissue, then close the jaws completely, and press the zero. See figure 2 below.

One of the most often unknown or overlooked adjustments on calipers is the gibs. A gib is a wedge device inside the caliper that adjusts the tension of the sliding jaw of the caliper to the fixed rail of the caliper. If the gibs are too loose, the sliding jaw can rock a little (feel slightly loose) on the fixed rail and cause the jaws to not stay parallel. If the gib is too tight, the sliding jaw won't slide anymore. Adjust the screws to the most tension you can for the jaw to still be able to slide smoothly on the fixed rail. You will find this to be a very small amount of tightening tension on the gib adjustment screws.

Digital Calipers can be used for measuring outside dimensions, inside dimensions, and also depth by utilizing the rod that protrudes from the center of the rail. Care must be taken not to use too much force in taking measurements. It’s possible to squeeze calipers hard enough to flex the steel and cause measurement errors. Use only enough force to make the jaws fully touch the surfaces to be measured. See figure 3 below.

It's important to orient the parts such that the jaws of the calipers are square to the surfaces of the parts. Failure to do so will result in measurement errors.

In the example below where the part isn't square with the jaws, the calipers would give a measurement larger than the true value.

In the example below, failure to get the internal measurement jaws of the calipers on the centerline of the ring will result in a measurement smaller than the true value. Take time to wiggle the parts around and find the centerline by feel. The centerline of an internal curved ring, or a hole, will be where the calipers read the largest value. NOTE: Calipers are not as accurate at measuring a concave curve, or hole, as they are at measuring inside flat surfaces or measuring outside surfaces. This is due to the geometry to the calipers themselves.

Steel dowel pins are a very precise tool for measuring holes to verify fit. When you buy a 6mm diameter steel dowel pin, for example, it will normally have a tolerance of +.002mm to +.009mm (that's +2 microns to +9 microns). You can use dowel pins to check the size of 3d printed holes with much more accuracy than using calipers. There are 3 categories of fit when talking about a round shaft (such as a dowel pin) fitting inside a round hole. If the dowel pin will pass through the hole without much force, then it is a "slip fit". When you insert a dowel pin inside a hole, try to move it around and see if you can feel any movement. If there is movement, that is reflective of the hole size being larger than the pin, a value that we call "clearance". As you reduce the size of the hole closer to the size of the pin, you reach a point where there is no more clearance, the pin can still be inserted into the hole with the force of just your fingers, but the pin fits snug and cannot be wiggled at all inside the hole. We call this a "bearing fit". A bearing fit is right on the line between a slip fit and a press fit. It means the hole is very close to the dowel pin size, normally within a few microns of the pin size. There is a point where the pin is slightly larger than the hole, and it requries more force to push the pin into the hole. The amount of force is relative to exactly how much bigger the pin is than the hole. We call this a "press fit" because in metal working, it requires an actual press (mechanical or hydrualic) to force the pin into the hole. A press fit is also known as an "interference fit". It means that material is having to compress, or move, in order to allow the pin to pass through the hole.

Ball Bearings are a mechanical component that has a very precision outer diameter ( O.D. ) and also a very precision inner diameter ( I.D. ) This makes them equally suitable for checking holes much in the same way dowel pins are used, and also for checking 3d printed pins or other round protrusions by adjusting the feature's size until it fits to the inner diameter ( ID ) of the ball bearing. Here, the same principles of fit apply as with the dowel pin described above.

The image below shows an example of using both a dowel pin for checking the size of a hole, and a ball bearing for checking the size of a pin or shaft. In both cases the 3d printed objects require a light force for the dowel pin to be inserted in the hole or the pin to be inserted into the ball bearing, indicating a very precision fit.

There is a big difference between static tools like dowel pins, bearings, etc, and dynamic tools like calipers or micrometers. A static tool is a fixed metallic object that doesn't change size or shape unless subjected to great force. A dynamic tool has moving parts, and in the case of digital calipers, it has electronic parts that can be damaged or malfunction. Something that is often unknown to new or casual users of calipers is that they need to be calibrated, or checked against a gage, a gage being a static object of precise and known dimensions. There are gages that are built to be gages, such as in the image below, that are specifically for calibrating and verifying tools. Then there are other objects that can be used as gages, such as precision steel dowel pins and ball bearings mentioned above. You can use these precision object to verify if your calipers are measuring correctly. It is wise to keep a few gages around for various common sizes that you must print with precision, and use these gages to check your calipers for accuracy frequently. I once had the tip of my caliper jaw become damaged very slightly from getting dropped. It bent the tip an amount too small to be seen, but enough to prevent the jaws from closing all the way due to the microscopic beak shaped bend right at the tip. I discovered this problem because the calipers were reading a value that was off by 0.1mm of known gages when I would check the gage at a spot on the jaws OTHER than where that tiny beak shaped bend was. After some investigate I could feel the tiny bend right at the end when I would slide my finger along the caliper jaw. I removed this defect very carefully using a fine precision stone. This issue caused a string of measurement errors that cost me a week of work due to measurement errors that were too great for the parts I was building.

In the image below, a precision ball bearing is used to verify the accuracy of digital calipers. The bearing is a 12mm OD and 8mm ID. You can see the caliper is within 0.01 mm of the true size of the OD of the bearing. This is sufficient for calipers, as they are not capable of more precise measurements. The ID measurement is off by 0.05mm, but this is partly because calipers are less accurate at measuring internal round diameters. This phenomenon is due to the shape of the internal measuring jaws of the calipers and the way they must slide past each other. Using this information from the image, you would know therefore that your calipers will show a value that is 0.05mm smaller than the true value when measuring an 8mm inner diameter ( ID ). This amount of error changes with different diameters as the radius of the diameter changes. This is one reason I mostly use precision dowel pins, gage pins, or bearings to function as static gages, to check the fit of 3d printed holes, rather than calipers. I only use the calipers on small holes when an approximate value sufficient. The accuracy of the caliper in measuring internal holes will improve as the hole size gets larger. But it will never be as precise as using a gage. However, sometimes it just isn't practical to have a gage for every hole size, and it's expensive for large cylindrical gages like 50mm diameter. For these the calipers will have to due.

A deburring tool is very helpful in smoothly removing the sharp edges around a part, or for removing a brim. See image below.

In the example below, using magnification would greatly enhance the ability to see fine details like over-extrusion in the case of this image vs. perfect wall layers, but it also assists with seeing blobs, layer separation, seams, or bulging corners that need to be corrected with linear advance settings. We recommend magnification with lights. The one pictured here isn't ideal because it's very small, but it works. A larger magnifying glass with lights built in is more convenient to use.

A hands free magnification device like the one pictured below would be much more user friendly.