Arc/Bend in X rail

Good idea, I repeated my 3 drilled holes in the ends to check that and came to the same .5mm arc in the middle.

As someone who chases microns for my aluminum machining, I can only imagine how much this is bothersome, especially not knowing what is causing it. I am running out of sound (based on the mechanics and hardware) suggestions as to the cause and what tests to try. I can think of things that would result in linear issues, not a deflection in the middle of the x gantry travel.

Hey Brenden, hey Tom, hey Sid, hey all,

okay, a theory.

  1. You start at the left end of X travel where the rails are fixed in the foot of the gantry.

  2. In the middle the mechanical load of the milling bit at the end of the Z assembly/spindle/collet/bit lever allows more deflection, which means maybe a minimal bowing of the single rail and of course some play in the linear bearings, as well as twist between the two rails (see diagram), because the rails are not supported in the middle:

  3. At the right end of the X travel, again, the rails are again fixed in the foot of the gantry so can not be bowed or twisted.

    In this case it would be deflection of the chrome-plated tubular rails in their middle portion, plus play in the linear bearings. But the resulting total deflection at the end of the bit would be much bigger than the deflection of a tubular rail or the play of the linear bearings, because, as I tried to show with the diagram, it’s a twist of two rails in two different direction with a long lever.

Thanks Bill @Machinist for making the diagram!


I like your theory, few condsiderations, I accounted for the mechanical (dynamic if that’s what you mean) load, with zero load I still get the arc. This is also an elite machine, so it has the third rail, which in the previous poster mentioned solved his problem with this issue in his previous model.

Im going to reassemble the thing as a last resort, its a 1/32 off square and my table legs could be levelled another 1mm on one end, perhaps this all feeds into it, I’m not sure. Throwing everything at it…

Hey Brenden,

with “load” I meant the force the cutting edges of the rotating cutter have to apply against the resistance of the workpiece material when cutting. This force should not be underestimated (it’s why you measure your spindle power in kilowatts and not in watts)

The method to falsify my theory would be to have a bit with the spiral in the other direction and let the spindle run counterclockwise. The bow would have to be in the other direction then.

That diagram was meant to relate to the tramming screws that are present on the gantry rail carrier blocks.

Viewing one end of the gantry, the green circles would represent the near end, while the red circles represent the far end (or vice-versa, it really doesn’t matter). There is a point of convergence where you could have your theoretically perfect front-to-back tram, but you would lose that tram as the X axis travels from side-to-side.

I believe the original conversation was to demonstrate that tramming must take place at both limits of travel, and not just in the center.

Hey Bill,

good morning! Yes I know I diverted it from its intended use, I would not have needed both red and green circles, but it serves well! It shows the movement of the rails very well, a very dynamic image that you have accomplished with few means! And everyone recognizes the ball screw in the middle I believe :slight_smile:

Yes it was from Seeking Advice For Restoring Front To Back Tilt thread

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No worries my friend, I was glad to see you could put it to use to help explain dynamic deflection. :slightly_smiling_face:

Have yet to have the time to take a deeper dive, still TBD on what’s causing it. But without the spindle running it arcs, why would you think the way to falsify it is run the spindle counter clockwise as opposed to not running and still seeing it? I don’t follow, to my mind that showed it not coming from the load of the spindle.

Hey Brenden,

you showed a milled line that has a bow, so I assume your spindle was running.

I don’t understand how you measured without spindle. You get a bow when you measure with a dial gauge? If so, what is your reference? What is the guranteed tolerance of your straight edge that you measure against, when measuring with no spindle? Your festool rail is no straight edge with specified tolerance.

Not without a spindle but without the spindle running. I put a straight edge on the line that was cut in the spoil board and jog the spindle (not running manually across and it’s the same. Also use a straight edge along the Y and a framing square against that on the x and jog. Same.

The rail tolerance is moot since the measurement I’m dealing with is .05mm over 2’ as stated above.

Thanks for your time, but I’ll update this thread when I get to the bottom of it when work calms.

Hey Brenden,

I only own the 60 cm straight edge (24"/2’) but its tolerance is 0.03810 mm (0.0015"), I would not consider a festool rail as a straight edge.

I would use a dial gauge to measure such things.

You are helpful, but also tend to clog up threads with noise and drown out useful info with voluminous replies that meander off course. Like I said, I appreciate your time. The rail is straight, my woodpecker square is straight. My Stabila levels are straight. They all read the same. Enough, please.

Hey Brenden,

0.05 mm is the half of the thickness of a hair :slight_smile: .

I am reading .68mmm with feeler gauges. The photo above shows 1mm with the ruler. No one is talking about hair.

Hey Brenden,

I just wanted to point you to the fact that the relevant thing is what is your reference when measuring. Neither is a woodpecker square usually long enough, nor are a Stabila Level (made in Annweiler Trifels, Germany) or a festool rail a straight edge.

And if a $72 expensive straight edge is only 0.03810 mm accurate…

Respectfully, may I ask if you are on the spectrum?

Hey Brenden,

if you measured with a dial gauge in your collet against a straight edge of your machine’s axes length which has a specified tolerance, I will take this serious. Otherwise you are chasing phantoms at the moment.

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You did, you said 0.05 mm

Oops. Like the several posts above and the photo and video showing the actual measurement We were never talking about a hair. Apologies for the typo. Good night, I won’t be replying to keep this thread from bloating any farther until I have more to say about the cause. Please do the same.