Introduction: Bolting 3-d Printed Plastics Together (w/o Nuts)

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In the course of 3-d printing there comes a time when multiple prints need to be fastened together toward some greater goal. In 4 steps, the following instructable will outline how I have determined to be a good way to easily bolt your projects together without needing to tap each hole or use nuts.

Steps:

  1. Determine CAD and print settings
  2. Determine drill sizes needed
  3. Ream holes to size (one to pass a bolt, the other to take threads)
  4. Assemble parts

Materials:

  • 3-d printed parts to join
  • bolts/fasteners (see note in Step 2: CAD: Length: for recommended bolt length)
  • vsr electric drill
  • appropriately sized drills
  • safety glasses
  • vise/clamp
  • file
  • utility knife

On one hand, an ideal CAD file plus ideal printer setting will result in perfectly sized holes; sized big enough to pass a bolt through the part or small enough that threads can be tapped in the plastic. This would be a best case scenario.

However, this ideal situation doesn't exist in my workshop. My Prusa printer currently sports a 1.0 mm nozzle and prints 0.4 mm height layers. It would be safe to assume that my printer is always over extruding to an extent and that I print with a little hotter extruder temperatures than some might. My prints might look a little sloppy here and there and therefore requires some additional light machining (filing, reaming) to get them to where I need them to be.

Disclaimer:

This method of threading 3-d printed parts is the quickest, easiest, and most fail-safe way I have come up with. It is not the only way or even the right way per-say. It can also be mildly dangerous if you don't have the part fastened adequately because when you begin to drill/ream the part can start revolving around the drill if not secured. Wear safety glass, secure your work piece and proceed with caution.

Pictures:

Above Picture: The picture with the weights has a main beam that is 100 * 30 * 10 mm with 3 holes sized for 3, 4 and 5 mm bolts. The cross members are 50 *15 * 6 mm, each cross member has one 3, 4 or 5 mm * 20 mm long bolt thru it and has no nut (see assembled part in step 4). Hanging from each cross member is a five pound weight being entirely supported by the bolt threaded into the 10 mm thick main beam following my methodology.

Below Picture: The 3-d parts have 2 bottom layers, 3 top, and 3 perimeter walls. The 4 parts printed in 45 minutes according to my software.

Step 1: CAD Dimensions and Print Settings

CAD:

For an example, I'm going to be using a 4mm bolt.

Diameter:

  • I choose a 4 mm diameter cylinder for a hole I want the bolt to slip thru (thru hole). It might be necessary to resize this hole slightly so that your drill bit cuts ever so slightly in step 3. It could be sized exactly, but I personally like to ream the hole to the perfect size with the drill.
  • I choose approximately a 3.8 mm diameter cylinder for the hole I want the bolt to thread into (thread hole). Again, It might be necessary to resize it slightly so that your drill bit cuts properly in step 3. I personally feel like it is very important to ream this hole to the perfect size with the drill versus printing it the correct size.

Length:

  • Remember to make sure that if your thread hole dead ends in your part (versus passing through like mine did), make sure that your cylinder in CAD is of adequate height (h) to accept the length bolt you are going to use. EX: 15 mm bolt - 6 mm thru part < h... h = 9mm+0.5 mm(extra space)... h = 9.5mm cylinder height.
  • In designing parts and sizing the length of bolt needed it would be good allow for having between approximately 5 and 10 mm of threads on the bolt be self tapped in the thread hole. Too few (<4) or too many (>12) might result in ultimate/premature failure in the joining the parts.

Also, depending on your printer you might need to use slightly different sized holes in your model depending on if a given hole is vertically vs. horizontally oriented while printing.

You are ultimately going to have to tinker with these dimensions so they match your printer, settings and size of bolt.

Print settings:

  • For instance, maybe you might need to increase the amount/thickness of perimeter walls in order to ensure there is enough thickness of plastic to ream and then to thread.
  • Or else you might need to increase bottom and top layers to reinforce the top and bottom of horizontal holes.

Adequate layer/wall adhesion and thickness of plastic around the holes is necessary for threads in 3-d printed plastics. This is no problem for me... with a 1.0 mm nozzle that prints 3 perimeter walls there is plenty of plastic to cut some away and still have way more than needed for threads.


Step 2: Sizing Your Drills

Choosing the right drill sizes to accommodate your desired bolt is very important. If we go back to my example in the last step (a 4 mm or M4 bolt) it might be logical to think that a 4 mm drill and 3.5 mm drill would be used to enlarge the holes to the right sizes. However, if we go this route we might likely need to actually tap the threads in order for the bolt to drive reliably.

I did mention this is a quick and easy version of threading 3-d plastics. I'm also American, so don't have a metric drill set. The bottom line is that if you arranged both an American and metric drill set by nominal sizes they would alternate metric/U.S./metric/U.S. drills(give or take). What this eventually boils down to is that if we select one nominal size up and one size down in the opposite (metric-U.S.) size we get a bolt that slips through the thru hole easily with zero resistance and a hole that is perfectly sized to thread a bolt directly into without having to tap the hole.

For a 4 mm or M4 bolt... use a 5/32" drill for the thru hole and a 9/64" drill bit for the threaded hole; if we stick to this pattern we can easily and securely thread regular bolts into 3-d plastics reliably without needing to tap the hole.

  • M2 bolt= 5/64" thru hole, 1/16" thread hole
  • M3 bolt= 1/8" thru hole, 7/64" thread hole
  • M4 bolt= 5/32" thru hole, 9/64" thread hole
  • M5 bolt= 13/64" thru hole, 3/16" thread hole

Notes:

  • I have not tried this using U.S. bolts and metric drills, but the same logic would apply (at least within the same nominal ranges). Additionally, I have not tried sizes greater than 5 mm, I would speculate at certain size the thread hole would need to decrease in size slightly to account for the increasingly sparse and deep screw threads (could be wrong). Or else at that point, maybe larger bolts need to be legitimately tapped.
  • Also, my method could be used in such cases as a M4 bolt with a 4 mm thru hole and 3.5 mm thread hole or a 1/4" bolt with a 1/4" thru hole and a 15/64" thread hole. These cases may not work quite as well, the same or perhaps better... my drill sizes have largely evolved because I have sets of metric bolts and American drills. (see Step 4: Discussion: for details of what may happen with too tight of thread hole)
  • The above sizes of bolts and drills have worked very successfully for me.

Step 3: Drilling/Reaming Your Holes

  • the printed part needs to be secured properly while drilling so you don't get hurt.
  • safety glasses should be worn.
  • sharp drills should be used.
  • practice makes perfect.

I have used the term reaming rather than drilling. I want to draw this distinction. We want to ream the holes to size, not drill them. We are actually going to be running the drill in reverse in reaming the holes. I have found that while drilling in a forward rotation that the drills' flutes have a tendency to dig too deeply and sometimes bind, enlarging your hole too much.

  1. Before using the final drill sizes established in step 2. We want to take the drill one size below our finished hole size and see if it slips through or if we need to first use it to ream the hole once prior the actual final reaming. Remember we are running the drill in reverse.
  2. Now that the hole is already pre-reamed or already bigger than the one size below drill, it is time to ream our thru hole or thread hole to the proper diameter.

It is important to find a balance between applying the right amount of pressure, number of rpms (pretty slow), and maintaining plumb with respect to the hole. It is important not to ream more than is necessary or to quickly, because it might heat the plastic, perchance depositing it on your drill ruining your hole, breaking your drill or both. At some point, it might be necessary to run the drill in a forward direction in order to get loose or actually ream the hole, but primarily the reverse direction should be used to ensure reliable reaming of the hole.

After the holes have been enlarged there maybe some burrs around a reamed holes top and/or bottom edges. This needs to be cleaned up so that it is a clean and level surface. Sometimes a quick once through with the drill maybe warranted during cleanup... at this stage the drill bit can be used by hand.

This process of reaming could prove tricky, so it might be worth practising some before you ruin a part that took 3+ hours to print.

Step 4: Assembling Your Parts/ Further Discussion

  1. Take the part with the thru hole and put the bolts thru it.
  2. Align the parts, making sure that the tip of the bolts are resting on/in the thread hole.
  3. Tighten the bolts by applying enough pressure to help/ensure the bolt threads itself as it goes.

Important:

  • It is very important to tighten yet not over-tighten the bolt, if this happens your threads are gone. Stripping a practice piece by over tightening might be helpful.
  • Also, if you take your parts apart and then reassemble them make sure you don't cross-thread the holes by starting the bolts by hand providing very little inward force.

Discussion:

An ideal CAD file plus ideal printer setting will result in perfectly sized holes; sized big enough to pass a bolt through the part or small enough that threads can be tapped in the plastic. This would be a best case scenario. If this can be achieved reliably it could be the quickest way.

If you are going this route be warning that if the thread hole is loose or tight it may fail. If it is a bit tight the bolt may start, but after multiple threads build resistance the first 3-7ish threads may strip out rather than the bolt going deeper and fully threading itself. I imagine this might happen regularly with a M4 bolt with a 4 mm thru hole and 3.5 mm thread hole. This tighter thread hole could be legitimately tapped rather than reamed as I have done in this method or self tapping fasteners could be utilised. Too loose of hole and the bolt won't thread adequately. Too tight of hole and it may not thread at all.

Also, my method could be used in such cases as a M4 bolt with a 4 mm thru hole and 3.5 mm thread hole or a 1/4" bolt with a 1/4" thru hole and a 15/64" thread hole. These cases may not work quite as well(see above paragraph about tight or loose thread holes), the same or perhaps better... my drill size selection has largely evolved because I have sets of metric bolts and American drills. Necessity is the mother of innovation and so is trial and error. You may need to figure out exact combinations of sizes that work in your individual circumstance.

This method of threading 3-d printed parts is the quickest, easiest, and most fail-safe way I have come up with. It is not the only way or even the right way it. It's just the way I do things; and I have bolted quite a lot of 3-d printed plastics together both successfully and unsuccessfully.