Squaring the machine

I read this discussion: https://shapeokoenthusiasts.gitbook.io/shapeoko-cnc-a-to-z/squaring

Reading it, it implies to me that to check the square of the machine, I push the gantry all the way to the back and check for contact on both y axes with the back of the machine. I found that in my case, y2 was contacting before y1 and so I shimmed using aluminum foil for a result of almost perfect contact simultaneously.

Somewhere I read or saw a video that measured the gap with the machine in the home position. After shimming as discussed above, I put the machine in the home position and measured about 2mm difference between the y1 and y2 gaps.

This leads to my questions:
Which position should I measure squareness from? Back of the machine? or homed?

Why would it matter? Shouldn’t the gap be consistent no matter which position the machine is in?


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With the greatest respect, I think the question is the wrong question. The squareness of the X axis rail should be the same for the front and rear endpoints on Y1 and Y2. The position of the Z carriage (home) cannot be usefully judged until the Y1 and Y2 rails are parallel with each other and measure the same height from the base frame boundary.

Once the two Y rails meet parallel and height parameters, the X axis rail must be precisely at 90 degrees to both rails. As it is, if you are only shimming the out of true error at rear of one end (Y2 from your description) then the front of Y1 will also be out of true. Shimming the apparent fault at the back of the Y2 end of the machine will not correct the identical but mirror error at the front of Y1.

Put simply, if the rear of Y2 is out of square and needs correcting, then the front of Y1 where the X axis butts against the Y1 rail must also be out of square.The conundrum to be addressed is this: How to keep everything square while joining components together. I assembled my own standard SO3 wrongly because I had not appreciated how much rotational movement of the X axis rail nor how much up and down play there was at each end stop where it held the screws for Y1 and Y2. I also had a 2mm gap at the front left and the rear right Y rails so that the X axis rail did not meet the endstops evenly.

I resolved my issues with out of square rails by loosening all of the cap head screws in the end plates. I then screwed each of the screws in an opposite corner working inwards pattern, to exert even tensions, until they stopped turning. I then used a large engineers square and held sections against each other while nipping up the screws by 1/4 turn to hold the component in place. Having done that all around, I then tightened the screws in the same pattern and added 1/4 turn and then checked for square. All of my gaps had resolved. A very thin 0.001 piece of metal was inserted behind the screw end of one rail and all was well.

The X axis rail was rotated forward from the top and I loosened those endplate screws while exerting force with a long arm clamp until it was perpendicular and tightened the screws. The final test was resurfacing the spoilboard. It demonstrated some tramming was needed and after that was done, and the belts correctly tensioned, I have a machine that feels correct and works well.

My mistake was in thinking that approximately square (cheap non certified square) was good enough. It was not and the certified engineers square made a world of difference. I also had developed a much better idea of what had to be done to achieve squareness.

Your y-carriage v-wheels could be making the machine be not square in home position, they have precision washers, but can still be off a little as a whole…

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Hi Jeff. No offense taken. I probably took too many shortcuts in my description of what I saw and did. I did shim the front of Y2 and the back of Y1 to get the x axis to contact the back at the same instant. After that, I read or saw where someone checked for squareness by measuring the gap from the homed position. If “homed” only refers to the position of the z-motor then I did misspeak. I meant to describe the position of where the router bit is when the shapeoko is connected to carbide motion and the homing sequence is performed.

Hopefully that describes the situation both in how I first checked for out of square (at then back end) and then when “homed” the measured difference in gaps between y1 and y2 to the back stop was 2mm.

I can’t wrap my head around why a fraction of an inch of travel (from ‘homed’ to back stop) would create a two mm difference and it made me wonder if there was a correct location where I should be measuring for square. That said, I had intended to follow the link I posted above and after squaring at the back end, slide the gantry forward and check squareness at the front end.

Anyway, I’m all ears looking for wisdom on how to do this. And again, the link seemed to indicate the place to check was at the complete rear of the machine and when I saw the video (which I cannot find now) I got to wondering and figured here was the place to get more info.

Hi Al, No worries, I just had not read between the lines. :grin:

OK. got that. In Carbide Motion, if you mouse over the word Position you will see it is a hyperlinked word on the splash screen. Under the word are the values for X, Y and Z. I may have this wrong but I believe they are the last known/programmed values. I am sure someone will pick that up if it is incorrect.

If you click on the word, Position, you will see it change to the legend, Machine Position and my understanding is that this set of values is inviolate and accurately reflects the position of the spindle carriage (or the axial position of the spindle?). The implication is that when you are homing, you should see the same numbers appear under the Machine Position heading. This may help you to iron out any positioning anomalies.

I have noticed that when rapid positioning the carriage to the North East position, the carriage does not sit quite a snugly as it does when homing is initialised, so i would expect the Machine Position numbers to be different from the homing position numbers. All of the foregoing may be nonsense but it may also help you to focus on your issues.

Stating the obvious and thinking aloud… Everywhere one axis rail meets another rail there should be a 90 degree joint. The 3 sided rectangle described by Y1 and Y2 and a X axis rail, that can sit anywhere along the path of both Y rails, should always comprise right angles. That completes how the rails sit in relation to each other if you look at them from above. You will see an ‘H’ shaped construction with the short bar across the centre of the ‘H’ shape being moveable and capable of being positioned at any point along the Y rails.

Each of the Y rails should be parallel to each other and set at the same height and be level. Errors can creep in if the rails are not perpendicular to the baseboard or not level. I had one Y rail that was cut straight at 90 degrees yet there was a slight angle with one part of the rail being slightly longer than the other part. Despite being cut square in the vertical plane, it was out by a small margin in the horizontal plane. It was fixed with a little shim behind the endplate.

The X axis rail is another critical component because if it is out of true, in that it is not perpendicular and plumb, whatever you cut will reflect that error and you will never be able to tram the spindle accurately. I had that particular issue as well and was able to straighten the X axis using a lever clamp to exert the rotational force while I tightened the screws.

I found screwing the rails to the endplates a little awkward because of the split in the thread. Once the screw had caught the thread it was easier. There was no end-float but there was room to slightly rotate each rail and raise or lower it before tightening the screws. Once I was happy with my reassembled SO3, I attached the router and trammed the unit until it was accurate.

FWIW if you look over the relevant bits of this linked thread, you will see what I did to straighten out my machine to fix my poor assembly.

The bit you may find helpful runs on from here: (post 52)

ok. I have re-shimmed my XL and I have almost perfect contact of Y1 and Y2 on the back stop. I then moved the x axis forward all the way to the front. Y2 contacts first and there is about a 1.4 mm gap between front of Y1 and the front end stop.

At this point, all belts are removed and the v-wheels are loosened on both sides of the x axis. (I had to loosen the v-wheels in order to be able to add shims.)

a change of 1.4 mm over about 23 1/2 inches of travel doesn’t seem great to me. But maybe I’m wrong?

(I’ve also checked the machine to be square within 1/16th of an inch and it is level)

Which is more square to the rails? Front or back? I like for the squarest endplate to be at the front, then one can pull the gantry up against the front endplate and hold it there as one powers up the machine — the rear endplate just needs to be square enough to allow the homing switch to work.

more square at the back.

It’s worth measuring your Y belt tensions and balancing them with each other.

The symptoms you’re describing are consistent with different left and right belt tension & extension.


I wish that were the case but currently the belts are removed. :man_shrugging:

I know it is a chore but maybe this warrants removing both Y beams and laying them side by side to check for matching length. You could try measuring them while still mounted on the machine, but for better accuracy, remove them.


This may sound dumb, but you did also loosen the wasteboard screws (the ones attaching the wasteboard to the frame) as you were making these shim adjustments, correct?

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Aha, then you may have less of an issue as maintaining the alignment is more of a job for the drive system than the V Wheels.

The Y endplates are quite flexible (I believe this is deliberate as the V Wheel strength is limited and this allows for some alignment errors in assembly). It is normal to be able to alter the angle of the X beam relative to the Y beams by applying small forces fore and aft on the Y plates. These forces are substantially lower than the working forces from the belt drive system.

Assuming the base is already properly squared and the Y beams are coplanar (level with a flat base).

What’s important is that in the homing (rear) position the machine sits with the X beam normal to the Y beams and parallel to the plane of the base. This allows the drive system to start up with the X beam ‘square’ and is the reason for the advice often given to gently roll your X beam into the rear stops before powering up if you need it really square.

Once the machine has started up the Y steppers are operated in step with each other and will apply corrective forces to keep the X beam at whatever angle it started up at.

If the X beam will adjust to being square to the Y beams along it’s travel with small forces (a couple of Newtons) then I would expect the belts to deal with the remaining alignment for you.


I was kind of wondering the same – could I use the belt tension to eek out that last one point something mm gap… Thanks

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I would expect so, in that video I found my machine was 0.7mm off front to back due to imbalance in the belt tension

It’s still worth checking your Y rails are parallel and of exactly equal length, I would do that before trying to make the X beam align at the front as you want it square and if the front and back steel plates aren’t parallel you’re just pulling the X beam out of square with the belts.

How much force does it take to pull the X beam into line with the front plate?

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it takes very little force to get the y1 side to contact the front after y2 contacts the front.

I checked the y rails were parallel (and they were)

I noticed that the front stop on the y2 side was slightly tilted towards the back of the machine. That made me think that the reason for contact on Y2 side first was that the front end plate was leaning towards the rear of the machine. I shimmed the heck out of it and could never get any change. Basically, no matter what I did, Y2 contacted first and the same approximate gap existed on the Y1 side.

Just out of curiosity, what would one do if the Y rail lengths were not exactly the same length?

For current production machines contact us at support@carbide3d.com and we’ll work out how to handle things.

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I appreciate that but it’s not necessary in my case. the y rails look perfect in length and at least with a tape measure, they also look square.

Once I connected all the belts and set the v-wheels, front and back travel is perfect. I also set the z axis which was out significantly in the front/back direction but spot on in the left and right direction.

Next is the spoilerboard surfacing but that may be a different day.

(I also learned the value of a good and reliable level today.)

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I gave up on mine and learned to live with the little difference in Square and just position the work piece to the x Axis. do wish i could get it to level a spoil board more consistent though.

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