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DIY inexpensive Double elastic joint.

Good morning

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).

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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.

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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).

Step 3:

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.

In sequence:

* 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.

Thanks! Great idea. Is there any backlash and if so is there a way to reduce or eliminate it?
<p>I didn't notice any backlash; I imagine it may depend upon which material is used for the 'flexible sheet': a too soft material maybe could give this kind of problem.</p><p>Anyway, in my case I don't need such a precision: a backlash would be an error of 1 step (let's say), that is 1/48 of turn (48 steps motor) that means, with an M5 screw, a 0,8x1/48mm (= 0,017mm) linear error - no problem to me, the mechanical error between the screw and the nut is much more than this, in my toy-like cnc mill.</p>
<p>Brilliant. Just what I need for my CNC (hack from a laser engraver) where rhe motor support and the Z stepper are not perfectly align)</p>
<p>Thanx. Happy that it may be useful.</p>

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