Post Processing Guide

Even on a well-calibrated 3D printer, 3D printed parts are usually in need of some post-processing. At a minimum, the skirt and brim surrounding the first layer need to be removed. Parts that have holes for shafting or fastening screws usually need to be drilled out. Too, there is often some ‘drool’ where the plastic didn’t stop extruding at the precise location it was intended to. Or sagging, where a bridge crossed a void. For functional as well as aesthetic reasons, these require post-processing.

For post-processing, not a lot a workspace is needed. It should be a space where you can leave a small number parts and tools for a day or two without disrupting other things needing to go on.

A small vise for holding the parts while they’re being worked on is recommended. It’s also handy to have a work-light—a desk lamp will do—and a magnifying glass for viewing small features.

If your workspace is fairly large, a roll-around chair is handy to be able to move about.

A very few basic tools are needed for post-processing tasks. Some others are recommended if the budget exists.

TIP: A word on buying tools: unlike cellphones (is a pet peeve showing here?), tools can last a lifetime when reasonably cared-for. It makes little sense to buy a cheap tool that will frustrate you while you’re using it, and further frustrate you when you have to replace it when it fails to get the job done. Neither does it make sense for the hobbiest to invest in high-end professional tools, unless really, really serious about the avocation. Various well-known brands are known to be cheaply made—designed to be sold only at the lowest price-point. My advice: Stay away for them! Other brands are known to give good value: they do a good job at what they’re intended for at a good purchase price. These are the ones to own.

Basic tools:

  • cutting board
  • sharp knife to cut away the skirt/brim, hole entrances and inadvertent ‘drool’
  • sharpening stone to keep the knife in working shape
  • machinist’s oil (light) for the sharpening stone
  • set of drill bits, either in metric, or in inch (aka imperial, SAE)
  • hand-held variable-speed power drill
  • container of water to cool down the drill bit after use
  • your noggin. As always, when dealing with things mechanical, a good sense of improvisation is very handy—and will be further developed as you come across little problems for which you don’t have the perfect tool. While not the best use of the tool, I’ve used tiny drill-bits to remove plastic debris from hard-to-get places and pieces of cut-up clothes-hanger for various things.

 

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Basic tool-set for post-processing (A wall-powered drill would work just as well).

Recommended tools:

  • set of wood-carving knives (These work great for plastic, too)
  • sets of both inch and metric drills.
  • battery-powered variable-speed portable drill (Makita, or similar)—one with two batteries is preferred, so as not to interrupt the workflow.

 

 

 

 

 

 

 

 

 

 

The recommended tools simply make the task of removing plastic easier and with better results.

Sets of wood-carving knives come with different blade shapes which are helpful in removing plastic in difficult places. In my experience, the most useful knives are: one with a 45 degree angle blade; one with a chisel-like blade; one with a round-edged blade; and one with a slim, straight-edge (as shown in both photos above). To keep these sharp, a sharpening stone and machinist’s oil should be kept within arm’s reach of the work area.

A set of metric as well as a set of inch drills allows drilling “in-between” those sizes available with either set. This can be the difference between a “loose fit,” a “slip-fit,” and a “force-fit” of a shaft, for example. Below is a table comparing the sizes of inch and metric drill bits. I keep this table handy in a text editor during post-processing and refer to it frequently so as not to ruin a fit due to drilling out a part oversize.

TIP: When drilling out holes in plastic that will be used as a 3mm (M3) “locknut” for example, you can use a 7/64” drill bit to keep the hole just a bit under 3mm. (You’ll drill out a lot of holes for M3 screws). If #4-40 screws are easier or cheaper for you to obtain, a 3mm or 1/8” drill bit will give similar results—a just-under-size hole that will act as a lock-nut. similarly, a 5/16” drill bit is just a tad under-size for an 8mm hole.

Size    inches   mm

1mm = .0394″ 1.0

1.5mm = .0591″ 1.5

1/16″ = .0625″ 1.59

5/64″ = .0781″ 1.98

2mm = .0787″ 2.0

3/32 = .0938″ 2.38

2.5mm = .0984″ 2.5

7/64″ = .1094″ 2.77

3mm = .1181″ 3.0

1/8″ = .1250″ 3.18

3.5mm = .1378″ 3.5

9/64″ = .1406″ 3.57

5/32 = .1563″ 3.96

4mm = .1575″ 4.0

11/64″ = .1719″4.37

4.5mm = .1772″ 4.5

3/16″ = .1875″ 4.76

5mm = .1969” 5.0

13/64″ = .2031″ 5.16

5.5mm = .2165″ 5.5

7/32″ = .2188″ 5.56

15/64″ = .2344″ 5.95

6mm = .2362″ 6.0

1/4″ = .2500″ 6.35

6.5mm = .2559″ 6.5

17/64″ = .2656″ 6.75

7mm = .2756″ 7.0

9/32″ = .2813″ 7.14

7.5mm = .2953″ 7.5

19/64 = .2969″ 7.54

5/16″ = .3125″ 7.94

8mm = .3149″ 8.0

21/64″ = .3281″ 8.33

8.5mm = .3346″ 8.5

11/32″ = .3438″ 8.73

9mm = .3543″ 9.0

23/64″ = .3593″ 9.13

9.5mm = .3740″ 9.5

3/8″ = .3750″ 9.53

25/64″ = .3906″ 9.92

10mm = .3937″ 10.0

13/32 = .4063″ 10.32

10.5mm = .4134″ 10.5

27/64 = .4219″ 10.72

11mm = .4330″ 11.0

11.5mm = .4528” 11.5

29/64″ = .4531″ 11.51

15/32″ = .4688″ 11.91

12mm = .4724″ 12.0

31/64″ = .4843″ 12.3

12.5mm = .4921” 12.5

1/2″ = .5000″ 12.7

7/16″ = .4375″ 11.11

13mm = .5118″ 13.0

17/32″ = .5313″ 13.49

13.5mm = .5315 13.5

14mm = .5512″ 14.0

9/16″ = .5625″ 14.291

4.5mm = .5709” 14.5

15mm = .5906″ 15.0

19/32 = .5938″ 15.08

15.5mm = .6102” 15.5

5/8″ = .6250″ 15.88

16mm = .6299″ 16.0

16.5mm = .6496” 16.5

17mm = .6693″ 17.0

11/16″ = .6875″ 17.46

17.5mm = .6889” 17.5

18mm = .7087″ 18.0

23/32 = .7188″ 18.26

18.5mm = .7283” 18.5

19mm = .7480″ 19.0

3/4″ = .7500″ 19.05

19.5mm = .7677” 19.5

25/32 = .7813″ 19.84

20mm = .7874″ 20.0

13/16″ = .8125″ 20.64

21mm = .8268″ 21.0

27/32″ = .8438″ 21.43

22mm = .8661″ 22.0

7/8″ = .8750″ 22.23

23mm = .9055″ 23.0

29/32″ = .9063″ 23.02

15/16″ = .9375″ 23.81

24mm = .9449″ 24.0

31/32″ = .9688″ 24.61

25mm = .9843″ 25.0

1″ = 1.000″ 25.4

When post-processing a number of parts at one time—the way I prefer to work—a portable drill with two batteries—one in use, while the other is charging is very handy and avoids long breaks in the work-flow, waiting for a battery to charge.

Safety:

The thing about sharp knives is: well, they’re sharp! Being sharp, they’ll cut you if you don’t take care. An admission right off: if you use sharp things, sooner or later, you will get cut. An alternative—though a bad one—is to use duller tools. The problem with using duller tools is that they will certainly cut you more often! They are far more likely to slip off the workpiece while being used.

Not to alarm, but in addition to the tools mentioned above, it’s a smart practice to have a roll of paper towels and at least a couple band-aids—just in case.  😉

Two rules worth adopting:

1) Always keep your tools sharp.

2) Always pay close attention to what you’re doing. If you’re distracted, put the knife down and do something else until you can better concentrate.

What follows is not at all to be considered to be “best practices.” I’m not an expert in the field. I’m certainly not an expert on safety. My scars will attest to that. I’ll simply show how I get parts post-processed and ready for assembly.

The following is something I have to say to make the lawyers happy:

I take no responsibility for injury you may incur while following this demonstration of how I do things. Do so at your own risk. If you know of better or safer ways of doing things, by all means, use them.

In particular: one technique you may want to do differently is to use clamps on parts you’re drilling rather than using your hands to hold the parts while drilling, as I typically do.

That said, let’s get to it!

It’s probably best to start out with parts that have a fairly simple geometry and can’t be easily ruined.

This first part is a group of six 8mm linear bearing sleeves that I printed together.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A closer view, shows the fastening holes and rings used to capture the bearings

Looking at the part, I’ve decided to drill out the holes before removing the brim. The holes already printed into the part will serve as “pilot holes” for the drills used. The assembly these parts are used on is the X-Z-axis carriage. The reason for mentioning this is that four of the parts are drilled out to a different size than the other two due to the way they are used in the assembly.

These four, mounted to the inside of the assembly, receive 3mm screws in all three holes in the part. That is, all their holes act as lock-nuts. Therefore, they’ll only be drilled out to 7/64” (to 2.5mm if you have only metric drills).

In the other two sleeves the screw shafts will pass-through. These will be drilled out to 1/8” (or 3.5mm).

I use what’s called “progressive drilling.” I’ll start with a smaller drill bit—in this case, 2.5mm—and then move to one that’s larger—a 7/64” bit. The reason is to avoid, as much as possible, the “torquing” reaction of the part when the bit takes too big a “bite.” The torque reaction can quickly take the part out of the clamp you’re using and even throw it across the room and break it. If you’re holding the part in your hand to work on it, it can easily jerk the part out of your grip. (Ask me how I know this 😉

Depending on the amount of work the drill bit is doing, it should be cooled down, lest it lose its edge. I dunk the bit (still in the drill’s chuck), and only the bit, in a nearby cup of water to cool it. (If you are using HSS drill bits, you may prefer to use oil to cool the bits to avoid rust).

All holes in all the sleeves are first drilled out to 2.5mm and then to 7/64”.

Now, on two of the parts, I’ll drill them first to 3mm and then to 1/8”. On these two parts, the center hole isn’t used, so I won’t bother with drilling.

While I have the drill going, I might as well get after the mates to the sleeves I just drilled.


 

 

 

 

 

 

 

 

Mating parts to the linear bearing sleeves

These, being mates and all, the attentive reader might guess are also to be drilled out differently. The two parts mating with the sleeves having “lock-nuts” are to be pass-throughs. The other one, mating with the two we drilled as pass-throughs, will become lock-nuts.

The center holes of this last one are also used as locknuts.

Well, that was a bit more complicated than I intended! The complication arose from the parts coming off the 3D printer being used differently.

On to simpler things!

Like taking off the brim and drool!

Piece-a-cake!

Using a thumbnail against the side of the part and the brim, twist the brim away from the part.

 

 

 

 

 

 

 

Now the parts are separated, but there’s still some trimming to do around the part where maybe the brim hasn’t been completely removed. Get after this with a good, sharp knife—always cutting away from you.

The bearing sleeves and their mates are both simple geometries, yet there’s a need to make sure that the insides—the section that is in contact with the bearing—really is in contact with the bearing. A small bit of crud between the bearing and its sleeve can put the bearing out of alignment.

Good alignment is critical to remove friction. When it comes to motion, friction is the devil in the build. To the extent practical, we need to reduce and remove it.

To assure good alignment, any bits of hardened ‘drool’ that may be on the inside should be taken out with a knife. The best tool I’ve got for doing this is a flat-bladed knife with a slightly curved shaft, that allows the blade to go in at a better angle than a straight shaft would. There are photos of both, below.

TIP: I find that it helpful to try to always work parts systematically. When post-processing like-or similar parts, try to start from the same spot and work either clockwise around the part or counterclockwise. Likewise, you can choose to work downward, or upward, fairly consistently. This will help you avoid “holidays” in the work, and also help avoid places where you redo what you’ve already done. When there are multiple parts to do, you might choose to finish one before starting the next, or to do a particular operation on all of them before the next sequential operation. It’s up to you.

TIP: As you finish with an operation or a part, it’s a good idea to separate the parts that have had the operation done to them and also separate the parts completed. I’ve, at times, not done this and found when it came time for assembly there were parts still needing post-processing. It doesn’t ruin your day, but it’s a distraction to go back and do them, nevertheless.

There really isn’t a “right way” or a “wrong way” to go about post-processing, as long as the parts come out as useful as they can be.

 

 

 

 

 

 

 

The tool of choice for removing crud on the inside of the part is on the left.

 

 

 

 

 

 

 

Removing tiny spots of plastic crud that don’t belong.

Regarding the bearing sleeves, avoid cutting into the printed-in rings. These are needed to restrain the bearing from axial movement. Avoid too, damaging the contact surface—as might be done with a Dremel-like rotary tool. This, just like having crud in between the contact surface, could spoil the alignment and add friction.

TIP: Pay attention to where the first layer or the last layer of the printed parts may intrude into a space that needs to be empty. Such is often the case with the sleeves. The most internal trace of the last layer bulges a bit into the space where the bearing will go and, if left there, will keep the bearing from making contact from its sleeves. This trace will need to be removed with a knife. Take care to keep fingers and other body parts out of range of the knife.

The mates to these sleeves get the same treatments.

NOTE: As you might have guessed, the critical operations are to the inside of the sleeves and sleeve-mating parts. The exterior—the part you’ll see when the machine has been built—doesn’t really need as much attention. The amount of work you want to put into getting your parts aesthetically pleasing is completely up to you and won’t affect the machine’s operation. This generally applies to all the parts in the machine.

There are other parts where you’ll need to pay attention as to whether the holes to be drilled are for the screw shafts to be passed-through or for them to be “lock-nutted” into position. For example, NEMA17 motors use 3mm (M3) screws to fix them to the motor mount. Holes for these screws in the motor-mount would be pass-through, as the motor flange is tapped to provide the “nut.” NEMA23 motors, on the other hand use 4mm (M4) screws which are passed through the motor and into the motor mount. Here, the motor mount holes are done as locknuts.

TIP: Some of the parts have holes for setscrews. These are somehow easy to overlook. After you finish each part, give it a good look over, turning it around completely so you can inspect each side. You may be glad you did!

The next part I’ll show is a complex part design, but very simple in terms of its post-processing. It’s the gantry hip.

 

 

 

 

 

 

 

Gantry hip prior to being post-processed

There’s the usual removal of the brim and the bits of plastic cruft left by printing.

Like the bearing sleeves done above, there’s also the more precise work in cleaning up the Y-axis bearing contact surfaces. These are the two flat surfaces inside each of the openings where the bearing seats.

 

 

 

 

 

 

 

Plastic debris inside the bearing-seat tunnel

There’s often some drooping of the dried plastic, due to the length of the run during extrusion on the 3D printer. Because this may keep the bearing from seating properly, Any plastic that’s drooped should be removed.

If your knife-set has a knife with a curved blade, here’s where it will come in handy.

Remove any hanging plastic that might interfere with seating the bearing. Also remove any other plastic debris in these areas, being careful, as with the sleeve parts we did earlier to leave the axial bearing-capture ring features intact.

What’s left to do is the drilling out of the holes for the gantry shafts and threaded rods (t-rods).

TIP: A drill bit, naturally wants to “dig deeper” when being turned. If you want to go only to a certain depth—which is the case with the holes for the gantry shafts—a bit of back-pressure on the drill is required. That is, don’t allow the drill to “dig” at its natural rate but hold it back by slightly pulling as it drills. Otherwise, the bit may drill right through the tunnel wall on the other side. Starting the speed of the drill at a slow setting can help with maintaining control of it.

Using a bench-vise if you have it, progressively drill out the four holes for the 5/16” threaded-rod and the 8mm gantry shaft. I started with a 9/32” drill-bit, then moved up to 7.5mm, 19/64” and 5/16” in turn, drilling from the top hole to the bottom to be assured I haven’t missed a hole.

NOTE: I haven’t used an 8mm drill to drill out the gantry shaft, because I want that shaft to “force-fit” into its hole. (In the assembly process I’ll be lightly tapping it into the hole with a small mallet).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Drilling out the tunnels for t-rods and shafts

Now, progressively drill out the two holes for the 3/8” t-rods that will connect the two hips on opposite sides of the robot. For these, I started with a 23/64” bit, then moved to 9.5mm and finally, 3/8”.

That’s it! The same or similar techniques are used on the other parts for post-processing.

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