Adjustable X? Is this possible? Needed?

So I know Y length doesn’t really matter or effect the rigidity of the machine. But the smaller X the more rigid the machine.
I need to be able to cut a 4x8 sheet of 3/4 mdf (I know it will be slow), but most of my work will be 32"x2-5’. So like 1 week out of 6 I need a 4x8 sheet.
Am I asking a lot? I was almost thinking there has to be a way to attach something that can ride in a track to give extra support at the shorter width?
The smaller size will be hard woods, carbon fiber, and probably aluminium.

That is a great idea…absolutely awesome idea. It would not take much, 2 wheels. The killer on the length is really the twist more than anything else. Hmmm, two wheels and a printed part or two. Dang!!!

I have been thinking of making the X rails out of the larger conduit, but that means the belts should either be double up or at least increased to 10mm. Then if I go through all that work I really want to move the belt closer to the table. I have snagged my belt so many times it feels like a rubber band now. Vicious circle of improvements, complete redesign on incremental?


Two wheels riding in a shallow track that could be cut by the router… Should be easy, right? Would resist sag and torsion to an extent.

You can get steel reinforced belt instead of fiber, should prevent any stretch.

Why not go square tubing? Would be significantly more rigid.


You sure about that?


I don’t think anything special is needed, just clamp on a trolley anywhere with two more skate wheels. The only issue I see is twist, clamp on a trolley and the twist is gone (half way means 4 times more rigid).


Small Diameter pulleys. I think the stretch I have is from everyone snagging stuff on the belts. My lowrider is in a cramped space and if anyone comes in, including me, the belt is sure to get plucked like a guitar. I think I have damaged the fibers directly after swapping belts almost every single time. I also think using 2 6mm belts on each side would be equivalent to a 10mm belt but I think two gives different tensions on each so they would cancel out any tiny oscillation that might happen better than one large belt. I was working on a redesign two weekends ago and nothing groundbreaking came to me. So I put it aside for a bit and started the PID instead.

Yes I am sure about it. (for how the cnc sees load). It is heavier though.
Strength (in bending and buckling) comes from the moment of inertia (I)
I (round) = .12 in^4
I (square) = .21 in^4

Here is a C&P example as I am an Electrician, not structural engineer.
As far as resistance to bending forces, I.E. Axial Compressive resistance, square tube is approx. 50-60% stronger as the same size and wall thickness round section, but it is also heavier as well. I will give you two examples: Using 350MPa steel, the Mr (Kn-m) for 4"x4"x.188" is 20.3, with a mass of 9.45# per foot, while the round section of 4", .188" wall has a Mr of 14.1 and a mass of 7.67# per foot. 3.5"x3.5"x.188" has a Mr of 15.2 and a mass of 8.17# per foot, while the round section 3.5", .188" wall has a Mr of 10.7 and a mass of 6.66# per foot. Basically a 4"x.188" round section is going to be as strong, and have a mass close to a 3.5"x3.5"x.188" wall square section.

I like the idea of doubling up the 6mm belts, although I am not sure I understand what you mean by each side without adding more steppers?

There is no sense in comparing a 4" square tube to a 4" round tube the only thing they have in common is the 4" name. Maybe weight is a better way, but only if you take into account the rest of the machines hardware and cost as well.

A round tube has symmetrical properties and takes 3/4ths the hardware a square one does. A square is really strong on its diagonals, but not on it’s flats. The forces on the rails are all over the place, it isn’t just along the Z axis. $7 for 10’ stick of conduit…square, not so much, also every company seems to make square in whatever dims they want conduit has a standard. Weight/mass is a huge factor in acceleration. Cost of conduit, 1/4th less hardware vs square tube and 1/4th more hardware?

Now how are you going to balance the force of all of those bearings against all of those face and maintain perpedicularity of every one of them and the correct tension? On a round tube you have 7mm of face to hit on a single point, a much larger target than a perfectly parallel set of faces on four sides if a tube that may or may not be uniform. Wrong tension on a square tubes and the bearings will drive themselves off the face.

The rail is only the first part, how you mount and tension it is actually a little more important.

A perfect machine doesn’t mean squat if I am the only one that can source the correctly sized rail. The MPCNC can be built anywhere in the world, I have gotten emails of people “importing” conduit by motor boat to there shop in some places I have never heard of. If the LowRider gains popularity I will make the other two common sizes available. Three sizes of plastic parts is all it takes to make a machine easily accessible around the world. No fancy extrusions and eccentric hardware, I have no idea of any square tubing standard worldwide.


What would it take to adjust the size of the X? If you were willing to have extra pipe hanging out, you could move the far carriage on the rails, shorten the X belt, move the one Y belt. The big trouble would be the table. You could build it as two pieces, and have a funny gap in the support under the table to allow you to open the table enough for the carriage to travel through.

I’m not understanding how the support piece will attach, and how it will adjust with height, since the X pipes move up and down with the Z. It would have to have a Z motor, right? And the cart wouldn’t increase rigidity in Y unless you also had a Y motor on the cart.

And there goes the simplicity…dang it Heffe. Kinda forgot about the Z axis.


So Jeff’s diner table leaf idea is probably the most valid.

Oh, right. Shoot.

What about putting a CF shaft in the center of the SS tube and use an epoxy or something to fill the gap between the tubes? Would this add enough rigidity that keeping the extra span wouldn’t be an issue?

Much easier and cheaper just to go up a size in the rails. After MRRF I think I will test that out.

Easier for you than me :stuck_out_tongue:

The cost isn’t that much. I’d go just CF rails if I was worried the bearings would destroy them in short order. It is significantly more rigid than the SS and significantly lighter too.


Would going high torque 92oz/in nema 17 for the Y and then just filling the stainless with a granite epoxy to add rigidity be best? The larger steppers to deal with the extra weight?

CF won’t work. A tiny bit of extra outer diameter is worth and entire filled tube. Both of these ideas come up very often around here. It was actually a sticky post for a few months.

Has anyone tried rebar reinforced tube? Wasn’t going to try before I first built mine to see if stiffer tubes were even needed, but steel tubes with internal rebar suspended in foamcrete seemed like an interesting experiment to try.

Will add extra weight. Weight is bad.

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A large majority of the stress is on the skin, compression or tension, the portion in the center actually has zero load on it at all. For it to help it would have to be more rigid that the steel, and as close to the surface as possible. the closer to center the less effect the filler has.

The one thing that can effect the surface from the center is tensioning the tube, as far as I know it isn’t even in the realm of possibility for us with steel. I super thin steel tube around a thick CF rod might work but the price would not be worth it.

Just going up a tiny bit in diameter greatly increases it’s rigidity. have a look at the calcs here,

Going from 1" to 1.25 halves the deflection…but doubling the wall thickness has a much smaller effect.

I have larger conduit I am ready to design for, I just need to find out what is available around the world…and the larger stuff is less accurate/uniform. So might have to stick with larger stainless if we want to push this further.


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I’m doing a modification of the lowrider and not too concerned about the weight of the tubes… Yet

Maybe 2" EMT? It seems to be the largest diameter EMT that Home Depot keeps in stock for same day pickup. $18 for 10 feet.

Josh, as far as I understand, the problem with the weight of the tube is not with regard to the table, but the flex it can add to the tube itself. If you increase the weight more than rigidity, the non supported horizontal tubes will flex, without any consideration of the rest of the equation.

For the lowrider, where you can’t add support on the “tubed” axis (don’t remember if it’s X or Y), that can lead to difference in the Z height depending on if you’re at the center or on the border.

2 inch emt flexes more than the three quarter.