While testing some M codes, I heard a bad noise and duh!, one arm of my Fisher delta was broken.
Looking closely, you could see that the printed ends are larger at the fork root than the printed end in operation, for tighter spring effect (it nearly doubled the load on the ball)
Anyway, mixing steel and plastic arms is not very good because the thermal expansion is quite different and with my 'light spot' heated bed, I will have temperature size change, so I decided to change all arms, and also to make them longer, for better large diameter printing (which does not work well on Fisher with original arms). I lengthen them from 160 to 170mm. I lost 11mm of height, but the maximum diameter is now workable, with no step loss. However, as arms are thinner, I did not loose maximum diameter height (150mm diameter maximum height is around 83mm, this is limited by the clearance between arms and panels)
In fact, I discover afterward that my poor calibration was due to a hole in the Buildtak at the center calibration point, so my conclusion that this was related to the new arms was quite premature.
Have a look on the BuildTak, which really needs to be retired! The small hole in the middle was the culprit for the faulty calibration, not the new arms.
How are they designed:
I initially printed end with cups but they were difficult to fit and the main problem is that it is quite difficult to measure axis to axis length and so, to adjust the arm length. Also, that needs springs.
Compared to the original arms, the holes are shaped to the ball size and the contact area is then much larger. The overall stiffness looks comparable to the original arms, and with calibration spot moved away from my Buildtak holes, I am back to less than 0,01mm difference on 7 points calibration, as before.
The arms are slightly lighter than the original (20g the pair instead of 25 g). the 170mm arm pair weight 22g.
If you decide to lengthen the arms, do not forget to modify and reload the config.g file, then shut down and restart the machine. Do not home before restart!
Threaded rod length is 118mm for 160mm arms, 128mm for 170mm arms. 14mm insertion in simple end, 19mm insertion in nut end. You shall clean well the threaded rod end. Beware not to bend the rods while cutting them if using a manual saw. Bent rods shall be replaced, it is practically impossible to re-straighten them properly.
Clean the angel hair (I have a lot of angel hair because this is printed with esun PETG which need very high temperature. Lower temperature PETG may be better).
I printed with 0.25mm layers, but you may prefer to print with 0.2mm with a standard Fisher (my Fisher Beta have a geared extruder). Slow down speed to 60mm/s. Fill in 70%, honeycomb. This is a quite long print, total may be two hours. One arm set print time is less than half an hour.
You may print a first arm set to check, then the 5 others.
I printed a calibration angle of 100mm, which is near the maximum size (that makes 145 mm on the diagonal with the peripheral loop), and longer arms worked perfectly, without the steps loss that I experienced on larger parts before (but with a travel speed reduced to 180 mm/s).
To fit the ends on balls, push them on the screw, then slide to the ball, that is easier than direct fit.
File with :
Files could also be found on Github
To measured accurately the arm length, you shall use 4mm flush rods, which fits in the end holes
If you only have a 150mm calipers, use a calibrated part to compensate the width difference (see photo).
The arm length is not very critical (you will set it in the config.g) , but all your arms shall have the SAME length, with the best precision you can obtain, and in any case, better than 0,02mm (this is better done with the vernier calipers that I have used to build my set than with a digital caliper).