RepRap Builder

July 2013

Smooth Rods Problem and Bar Clamps Problem



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July 2013

Finally the linear bearings had arrived. In a first assembling attempt it was impossible to slide them onto the smooth rods. A quick investigation with the calipers revealed that my smooth rods - made from rolled mild steel- were not round, i.e the diameter showed (oval) variations between 7.90 mm and 8.15 mm. Any diameter size over 8.00 mm will not fit into linear bearings. Measuring the diameter made it necessary to obtain digital calipers, an absolutely indispensable tool for the construction of the RepRap. For a precision instrument they can be obtained relatively cheap.

Browsing on the internet had learned me that precision grounded and hardened smooth steel rods were the best option for operating with linear bearings. These steel rods are made of tempered steel quality XC 55 with a surface hardness of 63 HRc 2 and they have a dimensional precision limit of h6. This means that the diameter of 8 mm has a variation of + 0.00 mm and - 0.06 mm, i.e. the rod will have a diameter measured between 7.94 and 8.00 mm. A high surface hardness is essential for resisting wear from the hardened steel bearing balls, present in the linear bearings. All these qualifications make these rods perfectly suitable for the job!
After ordering the rods it took three weeks for delivery and during that time the frame was just waiting with no smooth rods connected.


Empty frame
Image 1: Assembled frame without smooth rods ( FRS 2013)


Image 1 shows the assembled frame with bar clamps attached for mounting the smooth rods. Within the frame, in the horizontal plane with on each side an idler the Y-axis is realised by the sliding (heated) bed, which requires exactly parallel placed smooth rods. Therefore it is essential that during adjustment the four bar clamps can be moved over the threaded rod to which they are attached. Moving the bar clamps can only be done with a spanner and it will cause strain to the bar clamps, usually causing them to break.
Breaking will be over the round shoulder and is caused by excessively pressing the ends with the holes together, which occurs when moving the bar clamp by slowly tightening up a nut on one side of the open end, whereas the other end will be prevented from sliding caused by the threads of the 8mm diameter threaded rod.
In my opinion the bar clamps have not been designed with this effect in mind. Image 2 shows a conventional bar clamp.


bar clamp
Image 2: bar clamp (image: http://reprap.org)

The solution for this problem is either by redesigning the bar clamps with more material between the open end (up to leaving an opening of 1 mm between the open end permitting sufficient tightening of the rod hold by the clamp) or by placing a 5 mm thick ring with >8 mm inner diameter (ID) and 10-12 mm outer diameter (OD). I have used 5 mm thick rings that have been cut from 12 mm OD copper plumbing tubes.

Replacing the mild steel smooth rods by hardened steel smooth rods gave me the opportunity to use the old useless rods as jigs (guides) for making exact distances between the legs of the two triangles that form the sides of the RepRap (image 1) and between these triangles (the three squares that form the front and the back plane and the floor plane). For that purpose the obsolete rods had first to be cut to just over the required lengths and then the ends had to be ground with the bench grinder to flat surfaces and to the exact length of the guide. In a similar way I prepared guides for exact mounting of the two smooth rods used for sliding the heat bed (from one triangle bottom to a smooth rod and one for in-between the two rods).


Open Source Project

The RepRap is an open source project, i.e. it is possible to make one's own improvements as long as the experience is shared with the RepRap community.  The advantage of such an open source project is that developments proceed rapidly, but the disadvantage is that experience is not always shared in a clear and extensive way. For some of us this is experienced as a challenge, for others it can be a reason to skip the project.

An example of an improvement with (in my opinion) insufficient information is the change from printed PLA (polylactic acid) bushings to linear bearings.
In an earlier version of the RepRap printed PLA bushings were used for the Y-axis platform. These could be used on ordinary rolled smooth rods because the varying diameter of these rods was not of importance for their operations.
Image 3 shows a PLA bushing and image 4 shows the platform holder for such a PLA bushing.


PLA bearing
Image 3: PLA bearing (image: http://reprap.org)

linear bearing platform holder
Image 4: Bearing holder (image: http://reprap.org)

The PLA bushing was fixed into the holder by glueing it with super-glue (polyacrylate). A disadvantage of PLA bushings was rapid wear and therefore increasing lack of accuracy in the Y-axis movement.

Accuracy could be improved by mounting LM8UU linear bearings on precision ground smooth rods. For this linear bearing a new platform holder had to be developed. Mounting the LM8UU bearings into the holders is by snapping the bearing into the slightly warmed (by using hot air) holder.

Image 5 shows a LM8UU linear bearing and image 6 shows the platform holder.


LM8UU
Image 5: LM8UU linear bearing (image: supplier China)



bushing holder
Image 6: Linear bearing holder (image: http://reprap.org)


For the RepRap builder the difference is not only that the smooth rods need to be upgraded from cheap quality to a more expensive quality (five times more expensive) but the order of assembly needs also to be changed. The PLA bushings can be clipped over mounted smooth rods and the Y-axis platform can be pre-assembled with the (four) holders and then (after adding a dot of glue in each holder) pressed onto the bushings.
For LM8UU bearings this will not be possible. They require assembling in their holders onto the rods, prior to mounting the rods into the frame, and the platform will have to be accurately pre-drilled and fixed to the assembled bearing holders on the smooth rods.
The updated procedure has neither been described, nor a drilling template or drilling dimensions has been provided on the RepRap website(s).

In general it can be concluded that a RepRap builder needs patience, the abillity to improvise but also a fair amount of technical comprehension.




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Last Updated on: Mon Nov 10 21:31:01 2014