Size limitations

I just ordered a mpcnc kit for 3/4 conduit.
I plan to do shallow 99 degree v bit cuts of differing depths to achieve different line weights in some simple sign making projects. The router will be the HF drillmaster trim router or the dewalt.
Speed is not important… even if the wood (not sure what type of wood yet) burns a bit.
I am planning on going for a 24x48 print area.

1 - is that reasonable? I see the lowrider is recommended for that size.

2 - any reason i couldn’t just go for it and if i find it doesn’t work just cut down the rails?

3 - would filling the conduit with resin, or lead (or anything for that matter) have any positive effects?

4 - any other advice for my application?

Thanks in advance! I look forward to becoming a contributing member.

  1. Too big, I think. That allows a lot of chatter in the middle of the span.

  2. You could try it, I built mine for 25x37, and found it too big (with 1" steel tubes.) I planned to cut it down, but looks like maybe I might start a new one.

  3. We’ve looked at this. It might help, but cost/benefit doesn’t work out.

  4. At that size, go LowRider. Particularly for the v carves, I think.


Ditto on this one!!! You can cut large things, but need many, many passes with tiny DOC.
Definitely should go with a lowrider for that size.

Here’s some chatter on a 24x48 primo with 1 inch DOM.
Sorry to everyone who’s already seen me post this a dozen times, lol.

You can do it, and it will work, and you can cut a lot of things on it…but it’s going to be a slow, frustrating, unpleasant experience.

If we just got you back onto the fence, I’d say get ahold of Ryan asap (tag him here is probably fastest) and see if he can put a hold on your order until you get decided. If you want to go with the primo anyway, maybe you can knock the size down to 24x30 or less? You can still tile larger jobs and have a much better machine for smaller ones.

Slo mo

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The usual “company line” is that 48" is way, way too big for an MPCNC. Yes, it can be done, but the window for tolerances in build, CAM, and feeds & speeds is incredibly tight, to the point of being an expert-level exercise. If you must have a 48" dimension, build a smallish LR3. Or a 24"x24" MPCNC, and be prepared to jig and set/reset your workpiece(s) to accommodate your stock.

Regardless, whichever you build, get some loctite for your grub screws. Trust us.

Also, as for filling the conduit, my personal opinion (and I am not an engineer of any relevant flavor) is that whatever you fill it with needs to adhere to the conduit completely. My theory being that in order to increase stiffness, you’re effectively making a torsion tube. If you could get crossed ribs down the center of the conduit, but welded to the conduit along the entire length, you might see an appreciable increase in stiffness. But barring that, you’re just increasing the weight and inertia of your moving parts for very little, if any, gain. But again, this is armchair engineering, and every real-world attempt has shown that the cost and effort has not been rewarded with any significant benefit, and usually results in unintended consequences (the MPCNC is designed under the premise that control cabling is run through the conduit). It’s usually easier/cheaper to just get thicker-walled steel tubing.

We have a whole thread on filling tubes with stuff.

Tubes are stronger than solid bar of the same weight.

Ok, thanks for all of the replies so far.
Im still unclear as to how big of a commitment going large is? Other than shortening all rails… and trimming the belts… what other considerations need to be made for downsizing a bigger one?

Does aspect ratio matter? I.e. would 1 x 4 work better than 2 x 2? I would think the longest axis is the weak point and the max is the max in both directions. If true… what do you all consider the max for the mpcnc for hard wood with a v bit in a trim router doing shallow cuts 1/8" to 1/4" max.

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I’m just about to shrink mine to 2x2 overall. Need to disassemble the rails from the feet leaving core assembled. Relocate and mount the feet, and drop the rails back on top sticking out on one side. Cut conduit that sticks out with a dremel. Tighen down and square, tram etc. Reattach and trim belts to length and give it a run. No need to overcomplicate it.

I’d say try it, make it huge! In my opinion there is no real size limitation, especially if the plan is to mill wood.
Almost everything depends on your CAM skills, really. Hardware doesn’t matter as much as people think.
The sizes you mention aren’t really that big anyway

You can make it shorter later if you need to, but the real hassle is to shorten the wiring then. Nothing impossible, but it is one or two hours of tedious work that you are very likely to procrastinate on :wink:

The aspect ratio you proposed (1:2) seems pretty good, there should theoretically be slight differences in terms of accuracy between the two axis, but it will not matter at all for wood milling signs, don’t worry.

Do it!

Well, just how long CAN you get tubes? Lol.

Dui’s right. There is no actual limitation. The lowrider was born because early on people were making MPCNCs that were 4x8. I swear I saw a 5x10 on the Facebook group once, too.

For me, it’s not a question of limits…it’s a question of practicality. You will have to be much more precise with your CAM and you won’t be able to cut as fast or as deep because the rigidity suffers. If you already understand how to run a CNC, this won’t be very painful. If it’s your first CNC, no promises that won’t get frustrated, so just be prepared.

If you’ve already got the stuff, go for. I fully support the attitude “Why not? It’s YOUR machine, your time, your space”. You can cut it down later without actually buying anything new I think, and be out maybe 20 bucks vs having bought smaller stuff in the first place.

But if you haven’t bought anything, definitely consider the LR3. I think it’s a little more expensive because of the linear rails and slightly more complicated to build because you have to cut or print flat parts and partially assemble it to cut some more parts, then disassemble it to install them. But it also scales down surprisingly well.

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If you do go with an MPCNC with one dimension over 36" or so, you might consider some sort of mid-span supports for the rails. You’re not going to get away from the gantry rails sagging, but you can mitigate the axis rail sag.

edit: You can totally add this in later, if you notice a significant sag. It can be as simple as a long bolt cut to length that you wedge between the rail and your table. Just as long as it stays out of the way of the trucks…

Unfortunately, while that sounds like a good solution, it does nothing for the mid-span sag for the core rails. Trust me, i tried it. That is where you get some of the worst flexing. Especially with 3/4 conduit, which i have on mine.

I fell down the YouTube rabbit hole and saw a Tech Ingredients video on making graphene and was thinking this might be the stiffness solution for longer axes.

You could watch the hour long video (It’s pretty cool) but the upshot is that he fills 4 tubes with epoxy resin, one plain, one with 0.3" carbon black (which he uses to make the graphene) one with 0.3% graphite, one with 0.3% graphene and one with 0.6% graphene. Then he tests the bending modulus of the tubes. The epoxy is the base. Carbon black makes practically no difference. Graphite makes a significant difference, and the 0.3% graphene makes something like a 400% improvement and the 0.6% makes a 700% improvement.

Amazing stuff. Not that I’m likely to put together the equipment to make graphene myself, but maybe if good production methods become available, the price will fall into the affordable range.


A 24x48 print area is certainly doable with the MPCNC kit for 3/4 conduit. However, as you mentioned, the Lowrider is recommended for larger sizes, so you may encounter some limitations or challenges with the MPCNC at that size. You may need to take extra care with calibration and ensure that the machine is well-tuned to achieve the best results.

While it’s certainly possible to try it out and cut down the rails if needed, keep in mind that this may require additional expense and effort. It may be more efficient to start with a smaller print area and then upgrade if needed.

Filling the conduit with resin or lead may add weight and stiffness to the frame, which could potentially improve accuracy and reduce vibration. However, this may also add complexity and weight to the machine, so it’s important to consider the trade-offs and potential benefits carefully.

In terms of advice for your application, here are a few things to consider:

Choose a wood that is appropriate for your project and cutting parameters. Softwoods like pine or fir are generally easier to work with, while hardwoods like oak or maple may require slower speeds and more careful attention to prevent burning or other issues.

Experiment with different v-bit angles and cutting depths to achieve the desired line weights. Depending on the design and material, you may need to adjust these parameters to achieve the best results.

Take the time to calibrate and tune your machine properly. This will help ensure accuracy and consistency in your cuts.

Consider using a dust collection system to minimize dust and debris, which can affect the quality of your cuts and potentially damage your machine over time.