DIY inexpensive Double elastic joint.
Here is a little DIY Double elastic joint, intended to be used with small motors (I used it for my home-made cnc mill, which has low-power step motors saved from an old slide projector).
This kind of joint act like a 2-stages cardan joint, which tolerate all misalignment of the 2 axes: angle error, parallel shift and overall length.
Here Plexiglas® pieces and plastic screws were used, in order to keep it light and because I already had this material off-the-shelf (so, really inexpensive).
The joint is composed by a series of 3 Plexiglas pieces, separated by some sheets of elastic material (here was Teflon®, for the same reasons than the Plexiglas); in this way every piece can move respected to the others.
This Instructable is for a 5mm axis Motor, turning a M5 screw (the well-known 'screw-and-nut' mechanism).
Notice: To do this double joint, only 'old style' tools were used: no Laser cutter nor 3D printer (too long for me to create the models and to find such machines near me).
So, you will need:
* Some Plexiglas (or other lightweight material), thick enough to cut off some 10mm thick pieces;
* Some flexible plastic sheets (I used Teflon);
* 2 M4x15 plastic Bolt-and-nut with washers;
* 4 M4 plastic screws (short – you can cut some other M4x15 screws…);
* Threading tools for M4 holes - of course, you can use screws of a different size, and you just need the threading tools of the same size;
* a long M5 screw (I used an Headless M5 x 100mm one), useful as an aligning tool;
* Saw, files, drill, to shape and pierce the Plexiglas;
* Scissors (and punching tool, if you have) to cut and pierce the flexible sheet;
* Screwdriver and suitable tools for screws and bolt-and-nuts.
Step 1: The Central Piece.
Cut a Plexiglas bloc of about 10x10x40mm, and than pierce with 3 aligned holes:
* the central one is 5mm diameter (for the centering tool - see later)
* the outer two are 4mm diameter (for the M4x15mm plastic bolt-and-nuts).
The 3 holes are about 12mm spaced;
the overall dimensions are not critical, but the distance between the holes must be the same for the 3 Plexiglas pieces, for mounting reasons.
Step 2: The Two End Pieces.
At the beginning they are just like the central piece, with just a difference: the 2 outer holes are 3,2 mm diameter because later they will be threaded as M4.
morning-after edit: one of the 2 pieces has the central hole threaded as M5, because it will be fixed to a long screw of the same size (see later on).
With saw and file, cut in a ‘T’ shape the two end pieces, as in the picture (please forget the plastic sheet, for the moment);
Thread to M4 the outer holes (so, 4 threaded holes totally).
Step 4: Flexible Sheets.
Cut two 40x40mm (roughly) pieces in the flexible sheet;
(I used 4 pieces, fixed 2 by 2 in sandwich, because my Teflon sheet was too thin).
Pierce every flexible piece with 5 holes of about 5mm diameter, like in the picture; by the way, the distance between the central hole and each outer hole is 12mm, the same as on the Plexiglas pieces.
Step 5: Mounting the Joint
During all this assembling work, You can use a M5
screw (a long one, I used a 10cm one) to keep all the pieces aligned.
* fix one outer T-shaped Plexi with one flexible sheet, using two short M4 plastic screws;
* place this composed piece on the centering tool, just like a barbecue skewer (see 1st picture). If one of the two T-shaped Plexi has the central hole threaded, it's maybe easier to place this Plexi first (the other one will be placed at last).
* place the central Plexi piece on the centering tool; prepare the two M4x15 screws inserted, but without the nuts for the moment (see 2nd picture);
* prepare the second flexible sheet fixed on the other outer T-shaped Plexi (here again use two short M4 screws);
* place this last composed piece on the centering tool; fix its flexible sheet to the central Plexi with the 2 plastic nuts (see 3rd picture). Tighten the two bolt+nuts;
* remove the centering tool;
Step 6: Rapid Test...
Every piece can move slightly respect to the others: flex, push, pull, parallel shift…)
Step 7: The Joint in Its Final Place (exemple)
Here for the final use; in this case:
* on the right is a Stepper motor with a 5mm axe (fixed only by insertion! I was lucky…);
* on the left is a 50cm long, M5 screw: to fix it, I had to thread as M5 the corresponding T-shaped Plexi.