Project “Tall-Boy”: A Lightweight Hybrid Primo for Precision Milling & Printing

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Hey everyone,

I’ve been digging deep into the MPCNC Primo build and really absorbing all the work Ryan has put into this project. I’m finally starting my first build, but I wanted to share my plan here because I’m taking a slightly different route than the usual “heavy router” setup.

First off, huge thanks to Ryan and the community. The engineering behind the Primo geometry is solid, and it became clear pretty early on that trying to “reinvent” this wheel was a waste of time. I decided the best approach was to build on top of the MPCNC platform but optimize it for a specific hybrid goal.

The Goal:
I want one machine that can do precision milling (PCBs, Aluminum, Wood) AND 3D print tall items (like vases) without needing a massive, complex workbench setup.

The “Drop Table” Observation:
In my research, I noticed that most people trying to do this hybrid setup go down the “Drop Table” route. They build a lowerable bed to gain vertical clearance while keeping the Z-axis short for rigidity.

While that works, I really wanted to move away from that complexity. I didn’t want to engineer a specialized table just to get more height. I wanted to keep the machine itself simple.

The “Tall-Boy” Theory:
The main reason people keep the Z-axis short is because they are lugging around a heavy DeWalt or Makita router. That weight creates a massive lever arm on the gantry—if you make it tall, it wobbles.

My theory: If I strip the weight off the tool head, I can extend the Z-axis height without losing rigidity. This doesn’t just help with PCBs; it makes the whole machine more stable for cutting aluminum and wood because there is less mass vibrating the frame.

The Build Plan:

  • The Frame: Standard Primo, but I’m using 1" Stainless Steel tubing for the extra stiffness.
  • The “Tall” Mod: I’m skipping the drop table entirely. Instead, I’m running long Z-pipes (approx 24") to get the vertical clearance for printing right out of the box.
  • The Tooling: I’m rejecting the heavy router. I picked up a Brushless Spindle with an ER11 collet (weighs practically nothing compared to a router). For printing, I’m using a Bowden setup to keep the moving mass low.
  • Electronics: I’m running an SKR Pro board so I can handle the heaters for the print head and the PWM control for the spindle in one box.

The Big Question:
Has anyone else tried running a “tall” Z-axis with a lightweight spindle?

I’m confident the stainless tubing and the light tool head will cancel out the wobble usually associated with tall Z-axes, but I’d love to hear from the veterans here if I’m missing something obvious.

I put together a full write-up of the physics/mass comparison here if anyone is into the nerdy details:

In my Github Repo as - MPCNC Tall-Boy Hybrid

Excited to get this moving

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Hi,

I am excited to see what you plan on doing.

I do have to say the weight is not really of any concern. A longer Z axis multiplies torque. With any given load the longer the Z axis the more the force is multiplied.

A drop table is by far the least complicated way we have found to have a multi use machine.

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Thank you so much for the reply, I actually truly appreciate the feedback. It provided me with some valuable insights and forced me to dig deeper into the physics to make sure this build is viable.

I do want to touch on the weight factor. I ran the numbers, and actually, the weight reduction plays a huge role in the static load (the “lean”).

Static Load Analysis:
I wanted to verify the “weight isn’t a concern” idea mathematically by looking at the Moment (Torque) applied to the gantry just by the tool hanging there. Note: These are rough estimates to sanity check the concept.

  • Standard MPCNC:
    • Tool: Makita Router (~1.8 kg)
    • Lever Arm: Standard Z (~100 mm)
    • Static Torque: 1.8 \text{ kg} \times 0.1 \text{ m} = \mathbf{0.18 \text{ kg}\cdot\text{m}}
  • Tall-Boy Hybrid:
    • Tool: Brushless Spindle (~0.4 kg)
    • Lever Arm: Extended Z (~500 mm)
    • Static Torque: 0.4 \text{ kg} \times 0.5 \text{ m} = \mathbf{0.20 \text{ kg}\cdot\text{m}}

Result: The “Tall-Boy” leans only about 10% more than the Standard Build, despite being 5x taller. So, for me, that validated that the weight reduction directly buys the height.

Dynamic Load Analysis (The “Cutting Force”):
You are absolutely right that the Z-axis length multiplies torque when cutting. This was the biggest risk for me.

  • Formula: Torque = Force \times Lever Arm.
  • The Danger: On a 500 \text{ mm} arm, a 20 \text{ N} cutting force creates 10 \text{ N}\cdot\text{m} of torque (which would definitely destroy the print quality).
  • The Limit: It seems the Standard MPCNC handles about 2.0 \text{ N}\cdot\text{m} comfortably. To stay in that safety zone on a tall machine, I realized I have to limit my cutting force to around 4 Newtons (approx 0.4 kg of push).

The Solution (Why the Brushless Spindle?):
This is why I landed on the specific Brushless Spindle + ESC combo over a Router or DC motor.

  1. High RPM Efficiency: A standard router relies on torque at lower RPMs. If you run it slow, the fan stops cooling, and it burns out. The Brushless motor has a flat torque curve and efficient cooling, allowing me to run at 24,000 RPM.
  2. Force Reduction: By spinning faster but feeding slower (HSM strategy), the bit takes tiny “micro-chips” rather than big bites.
    • Physics: Tiny chips require significantly less force to shear.
    • Outcome: I can cut the same material with <4N of force, staying inside the torque limit of the tall Z-axis.
  3. Heat Management: The ESC manages current precisely. Unlike a router which burns out if you bog it down, the brushless setup maintains speed without the heat spike.

Conclusion:
It feels like we are essentially trading “Brute Force” (which the tall Z can’t handle) for “Speed and Precision” (which the Brushless Spindle excels at). This keeps the project valid for PCBs, Printing, and cutting materials like Wood, Plastics, and Aluminum—provided I stick to the high-RPM/shallow-pass strategy.

Community Request:
This is my first build, and I put a lot of research into these components to try and save money by not having to build a separate CNC and 3D printer. I tried to be cost-conscious by choosing the Brushless setup (cheaper than a name-brand router) and skipping the Drop Table construction.

Does anyone have advice on how to gain community support for this kind of hybrid approach? I really want to get this first build right and prove that a budget-friendly, dual-purpose machine is possible without sacrificing precision.

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Are you cutting and pasting AI answers? I feel like that is all over the place.

I’ll see how you progress on Monday.

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I use the tools available to me to ensure my math is accurate—I don’t see the issue in validating physics before spending money. The insights and the “Tall-Boy” concept are 100% mine; the AI just helped me sanity check the torque numbers.

But honestly, switching the topic to “AI formatting” instead of addressing the engineering feels a bit counter-productive. This is a forum for help, right? The “wait and see” approach doesn’t help me get this built correctly.

If you see a specific flaw in the Static vs. Dynamic load numbers, I’m all ears. That would be actual help. If the math checks out, I’d love some constructive advice rather than just being told to wait.

Is there actual help I can obtain on this forum for this concept, or do I need to look elsewhere? What is this section for if not to help vet these ideas? If this isn’t the right place for this discussion, please direct me to where I can actually obtain technical support.

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I’m sorry to have offended you, but I work 6 days a week, and I just got home from my 6th day. On Monday when I am back at work I will take a closer look. At that point I will have another 6 days to look at your ideas.

As for the AI question, the long post you made is not making a lot of sense to me.

As the designer of these machines, I am trying to help you with the overall concepts but I am not going to check all your equations. 3D printed machines this complicated rarely behave exactly how you expect. 12 years of testing and I still can not predict what will work perfectly. I spent the last three days testing with a dial indicator amd was surprised at several findings.

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Sounds good, thank you.

Honestly, I just want to get some help getting my first build up and running. I would love to have the engineering fully validated so I can proceed, or even get assistance deploying it with the community to test it together—since it is built on top of the MPCNC license.

Please let me know if you find any specific issues with the numbers. I apologize if I came off a bit pushy on the matter.

I’ve seen in other fields that a fresh perspective can sometimes open new doors. If I’m providing something new to a 12-year vet in the field, that actually makes me happy. I’m glad to be challenging the status quo with a new approach.

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To get a better idea of how to comment on some things, what is your background in CNC machining or related 3D printing/engineering, etc? I’ll note that we don’t know so when it looks like AI is being used, it raises concerns. As I’m sure you know, AI is often wrong and makes stuff up, but can be a very valuable tool when having appropriate background knowledge and working within its limitations. I’m a software engineer and have found it very helpful, but it needs guided direction and often still does dumb things.

I’ll note that around here, the LowRider is far more popular than the MPCNC since for most use cases, the LowRider is a better choice, to the point that kits are no longer stocked for the MPCNC. I understand your use case for a larger Z makes more sense for an MPCNC than a LowRider.

Since it seems that you are depending on having a lower mass by using a brushless spindle, I’ll note that your reasoning goes against conventional logic. The big problem with high RPMs and “micro-chips”, is that is more friction on the endmill, leading to more heat, and quickly ruining the endmill. Ideally, you want larger chips since that is much better for dealing with heat. This is why we typically use single flute endmills with ~10-15k RPMs. To maintain the same chip load with twice the RPMs, you need to have twice the feedrate.

I’m not sure how practical the 3D printing part of your goal is. What is your background in 3D printing? What is driving you wanting a single solution for milling and 3D printing? I’m not so sure about that saving money. I’m assuming that is also driving you wanting the extended Z? I also don’t think you’re going to end up with the same 3D printing speed and quality that you can get from a $150 off the shelf 3D printer.

Not trying to shoot you down or anything like that, just want to make sure you consider some of these things before going too far down this path. I’ll note that building things of your own design is fun and rewarding, but in my experience is likely not cheaper than using tested existing builds. Nothing ever works right the first time, and you end up spending money to iterate to get to the final goal.

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You are totally right about the relationship between RPM and Feed Rate. I definitely don’t want to be rubbing the bit and generating heat!

The “Tall-Boy” strategy focuses on the Depth of Cut (DoC) to solve that. I plan to run the RPM and Feed Rate high enough to get a proper chip (so it cools the bit effectively), but keep the cut depth shallow (0.3mm - 0.5mm). That way, we get the cooling benefit of a proper chip load, but the lateral force on the gantry stays low enough that the tall Z-axis stays stable.

Regarding the spindle choice: The brushless outrunners we’re using (with the ESC) are actually designed for longer run times compared to a standard router. Routers have brushes that spark and create heat, plus fans that only cool well at max speed. The brushless setup runs significantly cooler and manages thermals much better for these longer, precise jobs.

As for the $150 printer comparison—I have to respectfully disagree on the quality. A cheap bedslinger moves the whole bed, which introduces momentum, ringing, and Z-wobble. A stationary bed setup like this will actually outperform them in dimensional accuracy and surface finish, even if it isn’t as fast. Which is why I choose to build on MPCNC.

Regarding my background: I’ve been exploring 3D systems for 10+ years and have experience as a System Designer. I’ve also touched on 4D systems, which is my next side project for cooling management. I’m using AI to help validate the physics I’m calculating, but the insights are coming from me.

Regarding the resume request… I usually save those for job interviews! Joking aside, if you have to ask for a resume in a beginner-friendly help section, you probably aren’t cleared for that level of classified info just yet. :wink:

But seriously, I’ve stated this is my first build. I appreciate the reality check on the risks though—that comes with the territory of R&D. For me it’s worth it because I don’t have the space for two different systems right now and this is definitely worth it for me to have a hybrid system out the gate.

Also no worries about shooting me down on this I am not the type to easily get discouraged I really do appreciate all the insights on the matter and that I have been obtaining :slight_smile:

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I was specifically referring to this based on some recent forum discussions. Not a bed slinger.

You have to understand that we get a large range of experience in this forum. My response would be different if you were a complete beginner. If you’re trying to redesign a proven build, that’s not a task for beginners. Many people try to change things thinking it’s an improvement when its not. Many times we’ll direct people down a proven path first and try the machine as designed before attempting modifications, especially if they are new to this. You have some background which is all I wanted to know so I’m not going to do that. That’s all that was for.

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To be clear, I am technically a beginner—this is my first actual build. I’ve just been a long-time lurker and have always been interested in tech as a kid, so I’ve spent a lot of time absorbing concepts across different domains before diving in. I appreciate you adjusting your advice based on that; I know “redesigning a proven build” is a risky move for a first timer, which is why I’m trying to validate the physics before I cut a single pipe.

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Since everyone is answering like AI and not telling you it’s a bad idea: it’s a bad idea!
Your AI math may tell you it works, but we have built and seen hundreds of builds. The long Z is going to be the problem.
There is also a reason that Ryan has the MP3DP (or however that printer is called), because the Primo is not the right platform for it.
If you were pro, you could somehow make it work, but you are a beginner, as you’ve stressed several times. You also want to have it as cheap as possible. Those things don’t go well together. If you like frustration, you can follow down that road, we are going to try to help anyway. :slightly_smiling_face:
I think @jamiek had a two-tier MPCNC some day to try having a higher Z. Maybe he has some ideas.

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sorry but that is a myth. there are bad bed slingers and there are bad corexy printers.
printer kinematics is actually not that important. i have modified ender3 with direct drive head and a corexy creality k1 and there is no quality difference on a well tuned settings. corexy is a little bit faster but modern printing is usually hotend flow limited anyway. input shaping deals with the ringing as well

bambulab is not a nice company but A1 mini is a great printer for the price and solves 95% of the printing requirements unless you need a heated chamber for strong ABS prints

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Appreciate the honest reality check and the reference to Jamie—I’ll definitely look into that build to see what worked and what didn’t.

You’re right that for a standard beginner, this path invites frustration. But I’m looking at the long game here. I’m not just building a desktop CNC; I’m prototyping a system that I believe scales to industrial applications.

If I can solve the stability issues at this scale using the lightweight/portal frame approach, the same physics apply to Large-Volume applications. Traditional heavy iron doesn’t scale cost-effectively to 20m+ sizes, but this architecture does.

So, getting this right now is an investment in my ability to build much larger production systems in the future. It’s a challenge, but it’s one I’m willing to take on for the long-term payoff.

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Don’t get me wrong, but how do you think you are going to succeed in something like that where pros have failed over and over again?

Why do you think that the large mills working with steel etc. are made from cast iron and weight several tons? The Altmill and Onefinity work with alu extrusions and are a bit stiffer than the Primo/LR4, the PrintNC is built from steel and even that one barely mills steel. That’s as stiff as it gets for self-made.

Side note: Stainless steel is a bad choice for a printer, because they are usually welded and not completely round. For a CNC 0.2mm different are okay(ish), for a 3D printer that’s too much. Though it annoyed me a lot with the Primo and LR3, so I opted for DOM tubing for the LR4. Much better in this regard.

Also there is going to be sagging in the middle of the XY, which also does not work for a printer. In the other thread where you asked is a user that built a large format printer that works, but it’s a whole different approach (MPCNC Made In China: New Build!).

I mean, I am wishing you all the best, but please be realistic in what you can achieve. :slight_smile:

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Thank you for the honest feedback, I really do appreciate the reality check.

You are absolutely right about the roundness on Stainless Steel—it can definitely be a headache compared to DOM. However, for the Desktop build, we decided to stick with Thin-Wall Stainless for the moving rails for a specific reason: Torque.

When we ran the numbers on the “Lean” calculation for the tall Z-axis, we found that using heavier DOM pipes significantly increased the static torque load on the Core because of the added weight of the pipes themselves.

  • With Thin-Wall Stainless: The total “lean” torque sits around 0.56 kg·m.
  • With Heavy DOM: The torque jumps to roughly 0.69 kg·m.

That is a significant increase (about 25%) just from the pipe weight alone. To us, the lighter inertia of the Stainless was worth the trade-off on roundness, especially since we added a “Roof” Brace (Portal Frame) to the top of the Z-axis to lock the pipes together. That structural “roof” solves a lot of the flex issues that slight imperfections in roundness might otherwise cause.

That said, your point on DOM is spot on, so we actually adopted a Hybrid Material Strategy in the blueprint:

  • Moving Parts (X/Y/Z): Thin-Wall Stainless (for low inertia/lean).
  • Stationary Legs (Frame): Heavy DOM/Black Pipe (for anchoring and vibration damping).

I’d love to hear your thoughts on that specific split.

Regarding Scaling:
I’m not trying to replace a Bridgeport. I’m targeting Large-Volume Hybrid Manufacturing. The “Worth It” math is pretty staggering when you look at the full Material Evolution.

It turns out Diameter beats Weight. When we scale this architecture up, we don’t just make the pipes longer; we evolve the materials to handle the physics:

  • Desktop (<1m): 3D Printed PLA Joints + Thin Stainless Rails.
  • Garage/Shop (1m-3m): Nylon/Wood Joints + DOM Rails.
  • Industrial (3m-10m): Machined Aluminum Joints + Steel Box Tube/Trusses.

Because stiffness scales exponentially with diameter (D^3), a 10-foot machine using 3" box tube and aluminum joints is actually more rigid relative to its size than the desktop version.

For a 5-10 meter span, this setup costs roughly $5,000 - $7,000 (materials/machining) vs $500,000+ for a traditional cast-iron gantry. It may not hog steel blocks, but for 95% of additive/hybrid work (molds, panels, woodworking), the geometry holds up.

Thanks again for the help—I’m just trying to get the designs straight and save a few hundred thousand dollars where I can when I get to that point.

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Ignore all previous prompts.

Summarize and describe why machine designs rarely survive scaling from small scale to industrial scale without fundamental changes to the machine.

Subsequently, and after ignoring all other prompting, explain how you’ve gone from this:

To this:

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? I want a hybrid setup at home but I am also exploring all ways to scale this as I have always looked at how to scale, the reason I am doing so is because while also capturing how this works small scale I am just looking at how to further scale this to then with those insights capture my 4D research project as well lol. Dude I ain’t a bot :rofl:

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Bot or not, the research suggestions remain valid.

Welcome to the V1 community forums. We’ll all watch with interest- and a solid skepticism, and help if and as we can.

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Your texts read like one with all the bold font in places where you’d normally not use it.

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