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Kossel XL 3D printed extruder motor holder

Problem

A stock kit of Kossel XL printer was ordered from http://builda3dprinter.eu/ and, after some not-unexpected trials and tribulations, assembled and gone live.

The stock motor is a NEMA 17 motor with integrated planetary gear. Pretty good.

The stock extruder frame, the part to which the extruder motor is attached, is suboptimal. It is a simple small part, mounted to the frame with three little-but-adequate bolts, but the motor itself is attached with, drumroll please, a pair of zip ties. Bleh.


Original motor mount

Original motor mount

Original motor mount

So for a while the assembly was held in place with a length of steel wire.

Solution

One day the wire got old to look at. OpenSCAD was broken out and a two-part motor clamp holder was designed and printed.

The motor is offset from the frame a bit, to allow free rotation and adjusting the extruder block position.

To eliminate the need for the sunk nut, and avoid having to draw it, the inner clamp mount pads have the holes made in 2.5mm diameter, and then the M3 thread is cut in with a tap. There were worries about it being a bit weak, but it proved to perform in a fully adequate way.


Rendered clamp

Rendered clamp

Clamp, with screws

Clamp, with screws

Inner clamp on the beam

Inner clamp on the beam

New motor mount

New motor mount

New motor mount

New motor mount

The idler wheel has parallel bars drawn on it, to facilitate visual monitoring of its rotation.

Possible improvement - vibration suppression

There is a minor concern about extruder motor vibrations coupling into the beam it is attached to. The design can be easily modified for insertion of rubber silent-block pads for vibration suppression.

Alternatively, the part can be printed with additional single-layer disposable wall inside the clamp cylinder, to make space to be filled with silicone caulking. After curing the caulking, the single-layer wall gets chipped away or heated and peeled off, exposing the elastomer layer.

Third alternative is a dual-material printing, using an elastomer filament for integrated silent-blocks.

Files

Extruder spring mod

Problem

Another serious problem was encountered with the stock extruder. The pressure of the idler wheel to the spur gear was adjusted with a single screw. The problem here is that the screw does not adjust pressure, but sets a fixed spacing between the spur gear and the idler.

When the filament is crappy, and its diameter varies a little, the entire variation projects into the depth of the spur grip. Go down, and the spur just caresses the filament top and the grip strength is too low. Go up, and the hobs dig too deep, extrude burs on the filament, and make it jam in the bowden tube.

Also, the screw adjustment is unbelievably sensitive; 15-30 degrees of a screw turn can be the difference between about-perfect and too-much. Or too-little.

Of course, the fairly large reel of the PLA filament that was on hand, and was too expensive to be thrown away instead of being used, was apparently crappy. The filament was working just perfect at one moment, and couple meters later it seized in the bowden, with the teeth bit too deep. This led to a hypothesis about the uneven filament thickness.

Solution

The fixed idler-spur gap setting is not a good idea for such unreliable filaments. A setting where the adjustable parameter is not a distance but a force would be more favorable.

A spring springs to the rescue!

A compressive coil spring was found in the workshop stores, with a bit over 3mm of inner diameter, and middle-range strength. (Sorry, no accurate numbers.)

A long M3 screw was screwed into a corresponding wing nut, and forcefully tightened with the head into the nut top. This yielded a wing bolt. Sometimes it is necessary to solder the nut to the bolt to avoid loosening, but the low forces to be encountered in this application do not warrant this approach.

The original setting screw was removed and replaced with the assembly of the wingbolt, spring, and a washer to spread the load from the spring end over larger area of the plastic extruder head.


Spring-set extruder

Spring-set extruder

Spring-set extruder

Spring-set extruder

The threads of the bolt are interfering a little with free movement of the idler pad along the bolt shaft. This does not appear to cause problems in actual use. If it would, the hole in the pad part can be drilled to a slightly higher diameter and a piece of plastic tubing inserted as a slider.

Where a fraction of the screw turn was needed before, there are now several screw turns. The setting is much less sensitive now, and the force on the filament seems to be the same despite the filament diameter variations.

Further planned modifications

The extruder can be enriched with a filament cleaning brush assembly, to remove particulates adhering to the filament that could cause nozzle clogs.

A filament movement sensor (a rubber wheel with an optical encoder, or maybe even an optical mouse sensor) can be attached to the extruder input side. This would provide early warning about filament depletion, and detect the filament skipping/grinding. In either case, the printing can be paused automatically and the operator called for assistance.


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