SMTX Primo - New Build

This is REALLY cool! I’m just now at the phase where I have all the tubes cut and I can start building my MPCNC, but I’ve been holding off because I can’t think of a good table design.
Your design is well thought out and it looks easy to build first, and add a sound dampening cover after the fact.

I hope you don’t mind if I do a similar table. The uncreative part of me will ask for your plans with dimensions that I can modify, but I totally understand that you might not want to share them after putting this much work into it.

Looking forward to your results after wiring! :slight_smile:

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@crtprf Thanks for the kind words. Sorry for the late reply. We were hammered down here in Texas by the last winter storm system and we’re just getting back to normal.

My design is completely unproven since I am in the process of building so you’d want to use it at your own risk but PM me and I can send you some info.

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Weather finally cleared up and my garage was inhabitable again so I was able to do some preliminary wiring to test function.

Here’s my first crown. Woohoo!


I also created some calibration patterns to check dimensions. Overall looked really good. The image below is two runs on top of each other to check repeatability.

Next up, permanent wiring.


Hard core Awesome Build!!! :heart_eyes:
Way better than 25 not round fidget spinners! :sunglasses:

I haven’t seen a drop table before. I love the idea and yours is cheap and awesome! Also, I am totally stoked about the tape measure trick!

@Dytoractor ha, ha. thanks! Ryan recommends that you locate the cabling above the steppers to allow you to have the maximum working openings to place larger workpieces that can hang outside of the working volume. My initial design had the cabling below so I have made a few modifications to bring it over the top.

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I finished my custom wiring harnesses.

It took a while to solder all the wires to the different barrel connectors and wire splices.

Here’s my X and Z tape run.

Here’s my Y & Z runs. Foreground shows the Y and background shows the Z across the aluminum angle.

Here’s a view of my Z assembly.

I verified function at each stage of wiring and have broken it up into individual sections to hopefully make it easier to diagnose and replace any future wiring issues.

The braided sleeve material was stiffer than I expected. I wish it was more flexible because it seems to impact the tape measure motion. The ends also frayed a bit. I hit them with a lighter but it would have been better if I had cut them with a hot knife.

I still need to make an enclosure to finalize my electronics but I couldn’t wait to make some chips. So I thought I would start by cutting the table threaded insert holes. You can see them in the background of the wiring photos. I originally ran my router cable adjacent to the braided sleeves but it jammed up during my initial cutting operation and pulled my z-axis out of position for a couple rows (until I noticed the issue). Further proof of the amplification effect when things are pulling on the Z tubes. So I’m going to have to recut the holes after I confirm that I didn’t mess anything else up. Fortunately I started with a spot drilling operation so they are not through holes.

Next steps: Print an enclosure and complete the wiring/electronics placement


It’s been a while since my last post. I’ve been trying to fine tune my machine but have been getting some strange results when drawing calibration patterns. I was getting non-square corners and a slight deviation from the designed size.

Everything seemed to check out on my machine so I believed that it could be related to my pen holder design. I started to investigate multiple designs to try to solve the issue and I iterated a couple with limited improvement.

Some designs worked better than others but I was still getting a few non-square corners. It turned out that my attachment method was not rigid enough. I had built an intermediate attachment part that would allow for “quick” attach/removal however there was enough play in the stack-up to allow the pen to move slightly when changing directions. This resulted in a slight arc at the corners (when changing Y direction) and the size difference from design.

I finally removed the intermediate piece and attached my pen holder directly to my tool mount. That seemed to take out the majority of the play and resulted in pretty square corners all around. I then ran the calibration patterns again and notice that my size was still a little off. I decided to attach a fine pick to the end of my pen holder aligning it over the top of a steel ruler w/ 0.5mm increments (aligned along each axis direction). I played a bit with the steps per mm setting and was able to dial in the size. We’ll see if it makes a difference with tool deflection when I start cutting.

I was planning on dialing in my dual endstops next however I noticed that one of my stop blocks was broken so I need to reprint that part. I’ve had similar problem with this filament on a few parts however this sheared across the print layers so I must have hit it with something.

Next steps: reprint a stop block and dial in my dual endstops.


It has been said on the forums before, but before making fine adjustments to the steps per mm, I strongly suggest that you ensure there are no sources of mechanical “slop” in the build. The steps per revolution of the motors is very precise, as are the steps per mm on the belt and the teeth per revolution of the pulleys. Steps per mm should not vary if these components are “on dimension.” Changing steps per mm may end up masking other issues, making them more difficult to iron out later.

Are you certain the ends of your belts are well fixed, and that the set screws in the pulleys are tight enough to prevent “backlash” when the motor changes direction?


And triple check those grub screws!

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@vicious1 & @ttraband, Thanks for the feedback. Grubs loctited and belts were taught.

I tried to make sure all belts had the same tension by using the following test method. I moved the gantry to the center of the table so that each length of exposed belt was the same. I then deflected the belts a fixed distance with a force gauge placed at the midpoint of each belt. Finally I adjusted the tension so that the readings were all similar. I think this should get them pretty close but let me know if I’m missing something.

One reason I adjusted the steps was that both axis were off by the same amount. I thought it could be related to component tolerances so step calibration/adjustment could be done to account for that.

My change was actually pretty small. I imagine most would have left it at the default value but the corner problem annoyed me enough that I changed the default values out of spite. :grin:

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If you really want to adjust your steps per mm, make sure you test it at an absolute maximum travel for all axis. A 10mm test is nowhere near as accurate as a 1000mm test. While we are at it is the error is the same at 10mm as it is for 1000mm then it is not a step per mm error.

Yes, I took my measurements at 550mm in X & Y which is around the max travel with my pen holder attached. I verified at 100mm increments after I made the change and it looked really good. But I am calibrating my machine to an uncalibrated machinist ruler so I won’t know if it’s worth it until I start making chips. :man_shrugging:t2:

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You might want to check out Jamie’s test pattern generator. It makes a bunch of lines to compare to a ruler, and it also goes back along the same lines, which is very helpful for seeing backlash.


@jeffeb3 Thanks. I forgot I had bookmarked that a long time ago. :man_facepalming:t2: I went with a no touch option (pick hovering slightly above a machinist ruler) to rule out any deflection in my holder but I think I’ll run Jamie’s test pattern to double check my changes. It won’t hurt and it’ll give me a little more debugging time for my pen holder. :grin:

Well the Texas heat and my sauna of a garage have been slowly killing my Primo. So far I’ve had to replace two Endstops and two Core Clamps and I’ve had to re-square/calibrate with each time.

I’ve also noticed large cracks in the other two Core Clamps and the Z Motor & Coupler parts so I’ll need to replace them soon as well.

Now I know that the first thought will be that It was due to over tightening but that was not the main reason. It was due to loose grub screws. :stuck_out_tongue_winking_eye:

Actually, I did hear a pop on the first couple screw installations during the initial build but I lightened up afterwards so the majority of screws are on the loose side. So I believe the root cause is mainly related to bad filament. I had adhesion issues with my first roll of filament so I changed to a different brand. However I printed most of the smaller parts with the bad filament while waiting for the new filament to arrive.

I also used pretty coarse print settings to reduce print time by going with a larger nozzle and layer height. That saved me a few days at the front end but I’m paying for it now.

So the moral of the story is don’t cut corners. Take your time and do it right!

Slow is smooth, smooth is fast.

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Typically, thicker layers are stronger in the way we use the parts. My guess at those cracks if it wasn’t overtightening is layer adhesion, either too fast or not hot enough when extruding.

We had an 111 degree day and a lot more over the 100 mark around here, and luckily I have not found a popped part yet in any of my builds. I print hot and slow though. Actually now that I think of it that is not true. One of the bearing holders on 1 of the 8 MP3DP’s popped. It is still usable as is but I am pretty sure I overtightened it. I can tell it is hot because I have to readjust the Z offset on my printers, and then back down in the winter.

I agree with that assessment on the majority of the parts because they failed at a layer however the Stop Blocks have all been failing across the layers. So I’m scratching my head a little.

A third one failed this afternoon. They have been at different locations on the machine and on different axis so the only thing I can think is crappy filament.

I may try using a zip tie instead of screw & nut to generate more uniform pressure around the diameter. So even if the printed part fails the zip tie will hold it in place.

I’ll also try to check my garage temp the next time we have a really hot day for reference.

I think the stop blocks would actually work without a screw and nut. Those breaks look like over tightening breaks, and with high heat the metal expands a tiny bit and that happens.
The micro switch has very little trigger pressure. I would just print another set and use very little tension. This has not really been a common issue. So hopefully your new filament is in better shape.

Be careful, and M5 screw can exert 5680 N of pressure. So snug…is at least 100x too tight.

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The non-obvious thing is that the bolts are chosen to be too big. This is to make the total part count smaller, and to make the plastic have more to “grab onto”. But the downside is that the amount of force you would usually use to tighten a M5 bolt is too much for this application. And the nylock nuts fight you so much that your muscles are already primed. But you don’t have to go any farther than the right position.


Back up and running. I just finished machining the counterbores for the threaded inserts in my lower elevator board.

I planned for a good amount of screw points outside of the machining area so I will have to drill those by hand. But I am planning on using the upper elevator board as a drill template so that should speed things up considerably. Then I’ll spend the next week or so installing the inserts. :grin: