Friday, October 25, 2013

Tightening the frame and installing Y axis smooth rods (Videos 5 and 6)

Videos 5 and 6 deal with precisely positioning the nuts along the threaded rods and tightening them.

Contrasted with videos 1-4, these two videos are tedious to complete.  You use your digital caliper to precisely measure distances between the vertices so that everything lines up.

Measuring with digital calipers can be tricky.  My digital calipers are Pittsburg 12" model 47261.  This model lets you make a measurement and zero the display at any time with a press of a button.  All measurements after that are an offset to your original measurement.  It measures to within .01 millimeter or .005 inch.

You can measure the distance between the inside of two objects:

or you can measure the distance between the outside of two objects:

You can also make a depth measurement:

It's an "auto-on" caliper which means that it turns on automatically when you slide the jaws.

It also has a locking knob.  When the locking knob is tightened, it prevents the jaws from sliding which is useful if you want to set the calipers to some pre-determined size and susbsequently adjust something until the distance matches the caliper setting.

There is an important pitfall to avoid when measuring for this project: do not measure on any printed part or and nut or washer that is clamping a plastic part.  This doesn't give a correct measurement because the printed part compresses from the tension applied to it by the nut.  Only measure from the actual rods.

A few days passed since I completed video number 4 and one of the interesting things I noticed is that the nuts are not as tight as they were before.  I suspect this is because the vertices have deformed slightly as a result of the pressure applied on them by the nut+washer.  Since the vertices are printed using PLA with a 70% fill there are voids inside them that got compacted.

Around 2:30 into video #6 I also take some issue with the way that bearing for the Y belt is positioned.  The video states that it's important to get the bearing exactly halfway between the verticed.  In the video, a ruler is used to make a rough measurement of the distance between the vertices (234mm).  That number is divided by 2 (117mm) to find the half-way point.  The bearing is adjusted using a ruler also.  I feel the right way is to use the calipers to adjust the bearing until the distance from either side of the bearing to the neighboring vertex is equal.

Around 11:00 into video 6, the four bar clamps for the smooth rods for the Y axis are installed.  Two bar clamps are installed in the front and two in the back.  The distance between the two front clamps and the two rear clamps must be identical to ensure that the rods are perfectly parallel.  Having completed a later assembly video at the time of writing this blog entry and having experienced an issue, I have to state that I disagree with the advice given on the positioning of the Y-axis smooth rods.  The advice in the video is to measure 40 mm from one side to position the first clamp and then measure 139mm from the first clamp to position the second clamp.  What I found in a later video is that the result is that the center of the MDF board did not line up with the bearing for the Y belt.  It was very satisfying to see that the Prusa being assembled in the beginning of video #9 suffers from the same issue.  So my advice is to measure 20mm from either side to position front clamps.  This will center the clamps, the rods and the MDF board.  Then do the same in the back.  Then measure the distance between the two front clamps and compare it to the distance between the two rear clamps.  If they are different, adjust one of the rear clamps until the distance between the two rear clamps matches the distance between the two front clamps.

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