Alternative X-Z Axis Carriage Post-processing & Assembly

An alternative X-Z axis carriage was designed to be more easily aligned, give more Z-axis travel, be stiffer, reduce complexity for easier assembly, and reduce parts count when compared to the original.

dsc00313Alternative X-Z carriage

While it succeeds in these goals, it requires more attention in its post-processing than the original design.

dsc00314Alternative (L) and original (R) X-Z carriages together.

Easier alignment is achieved by having a monolithic structure encompassing the sleeves for all six Z-axis bearings and its three shafts.

The new design yields approximately 1 inch (~25mm) more Z-travel. The parts count was reduced from thirteen to four and the number of fasteners, from 28 to 10.

However, post-processing the new design is more demanding. In that the bearing-capture sleeves are fixed, the main component of the z_carriage—the bearing-sleeves—must be printed such that the three bearings closest to the x_carriage must be in near-perfect axial alignment with their correspondent bearings above, which share the same shafts. This means there can be no “shifting” of layers during the print—a common problem with printers set up to be “close-enough” in operation.

The other demand the alternative carriage makes is that the bearing sleeves must be carefully drilled out to a fairly tight tolerance. While this can be marginally done with careful hand-held drilling, a drill-press is a better, surer, arrangement.

Additional tools are also needed. The progressive drilling out of the bearing sleeves requires three drills, two of them in inch sizes and one in metric. The drills—in progressive-drilling order—are 9/16”, 15mm, and 19/32.” With the exception of the 9/16” drill-bit (in countries using the inch—imperial—system), the others can be a challenge to find. The reverse may be true in countries using the metric system. Ebay, Amazon, and AliExpress are your friends. If your drill accepts a maximum diameter drill-bit of 1/2” (12.7mm), pay attention to this fact when you are buying.

 

Post-processing

1. The x_carriage remains the same part as in the original design—although with three additional bosses and holes for the 6mm bolts. The x_carriage parts supplied will accommodate either original or alternate z_carriages.

NOTE: The alternate Z-carriage consists of two parts and three 6mm (or 1/4”) bolts and locknuts to fasten them together and to the x_carriage. The two parts are termed “z_carriage_alt” and “z_carriage_cap_alt.” The bolts used are M6x135mm (or 1/4” x 5 1/2” – 1/4” x 5 1/4” if you can get it) in length.

The z_carriage_alt part has three tunnels. Each tunnel has two diameters. At the tunnel ends the diameter accommodates the press-fitting of the bearings. Between the ends, the diameter is slightly smaller in order to constrain the bearings from moving axially, in case the press-fit was more of a slip-fit.

2. As in normal post-processing, get the two parts free from their respective brims, eliminate “drool” or other cruft left over from the printing, and drill out the holes.

3. The holes for the Z-axis shafts can be drilled out to between 9mm and 10mm (11/32” and 13/32”). these dimensions aren’t critical. They just need to be wide enough to allow the shafts to pass freely. (And narrow enough to discourage debris from falling in over time).

4. Holes for the 6mm (or 1/4”) bolts should be drilled out as “pass-through” holes—that is slightly larger than the bolt’s diameter. For 1/4” bolts, drill the holes out to 17/64”. For 6mm bolts use a 6.5mm drill.

5. Holes on the z_carriage_cap_alt for the plastic anti-backlash lead-nut are meant to secure the nut and should be drilled out to either 2.5mm or 7/64”—that is, as “locknuts.”

6. Now from the hard part. Start drilling each of the six bearing sleeve holes with a 9/16” drill. These larger—compared to what we have been using—have a more serious “bite” into the plastic, and therefore a stronger torquing effect. If hand-held, the drill is also more apt to go out of control and dig deeply into the tunnel, voiding the rationale for having a smaller diameter central tunnel.

TIP: You can wrap some masking, or other, tape around the drill at a distance from the beginning of the cutting portion of the drill, set by a bit more than the length of the bearing (24mm for the LM8UU linear bearing). The tape will give you a good indication of the depth of penetration of the drill. As you’re about to do progressive drilling, do this with each of the drills you’ll be using.

The drilled hole needs to allow the bearing that will be inserted to be inserted completely, so the hole needs to be slightly deeper than the bearing length.

7. If you are drilling by hand, set the drill to a slow rotational speed and be ready to apply some back-pressure as the drill enters the cavity of the tunnel.

8. After you’ve used the 9/16” drill, widen the six holes with a 15mm bit. Follow that with the 19/32” bit. Now test the hole by fitting a bearing into it. You are testing for a nice “press-fit.” I found that a bit more time spent with the 19/32” bit rotating in the hole improved the fit from a “force-fit” to a smooth “press-fit,” bordering on a “slip-fit.” Test all the holes in the same way, but don’t fit the bearings just yet.

When all the bearings will fit into their respective holes, it’s time for assembly.

 

Assembly

1. Inspect each of the three tunnels for any crud left over from earlier operations. Obviously, if any remains, remove it.

2. Fit a bearing into its sleeve. Carefully and slowly, insert an 8mm shaft into this bearing and through the tunnel until it protrudes from the other end of the part.

dsc00297Shaft and bearing fit to z_bearing_capture part

NOTE: The shaft doesn’t have to be the shaft you’ll ultimately use. I’ve used cut-up remnants, as I’ll set the assembly aside until I’m ready to use it in a build.

4. Now fit a bearing onto the protruding end of the shaft.

dsc00298Bearing about to be slipped onto shaft

dsc00299

5. Press the bearing into its sleeve.

dsc00300Bearings semi-seated in their tunnels

6. Do the same with the other tunnels.

7. When all bearings are inserted, press one end of the z_carriage_alt part and then the other against a flat surface to seat them.

dsc00304Bottom bearings seated

dsc00305Seating the top bearings

Assembling the X-carriage

8. With the x_carriage_bottom resting on a flat surface, place four 10mm linear bearings into the post-processed bearing sleeves.

dsc0030110mm bearings seated in x_carriage_bottom

9. Place the x_carriage_top part above the bearing and secure it with four M3x25mm screws.

NOTE: For more detailed instructions, see the instructions in the Visible Robot Assembly Guide.

dsc00303Completed x_carriage

Tying X and Z carriages together

10. Place the z_carriage_alt part atop the x_carriage such that the boltholes of both parts line up.

dsc00306z_bearing_capture mounted to x_carriage

11. Fit the z_carriage_cap_alt to the top of the z_carriage_alt.

12. Slide the three 6mm (or 1/4”) bolts through the x_carriage and the z_carriage_cap_alt.

TIP: If you slide the bolts up through the bottom, with the threaded ends at the top (opposite to what’s shown in the pictures), you’ll have slightly more Z-axis travel.

13. Fasten the assembly together with locknuts. The fastening should be tight, but not too-tight. You don’t want to damage the plastic parts.

dsc00313Finished alternate X & Z axis carriage assembly

14. Done. Go watch a movie, take a walk or a nap, or give your pet some love.

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