How to get REPEATABLE straight cuts

I have been building and rebuilding my unit for a while. Been using it for past 2 months, but, not really for precision cuts yet. I have been tweaking it here and there. Finally got it working good, Except One BIG issue. My cutting area is 4ft by 3ft. That is fine, it is stable enough, BUT, the Gantry doesn’t always track in the same straight line. The problem is the X or Y center pipes can get Cocked… With the steppers turned off, you can manually cock the X or Y pipes. By that I mean you can move one side of, say the X axis, and not the other for a slight distance. For example, let say that with the gantry all the way to one side, and just dealing with 1 axis at a time, say that both sides of the X axis are 10 mm from the Corner post. You can manually move just one side, slightly, to where it maybe 15mm from the corner, and the other end of the pipe could still be 10mm from it’s corner. If you start cutting that way, then the cutter will move slightly off angle, because once the motors are energized they will move in sync to each other. What is happening is that I move the gantry around manually to assure that my work is laid down on the table nice and ‘straight’ where i can run the cutter from one end to the other perfectly along the edge. BUT, then I manually move the gantry and again, and the pipes cock slightly differently, and now the cutter goes off by 4 or 5 mm across the 4 foot span. It all depends on how the axis is cocked, but, I don’t know how to make it the SAME each time. I don’t see any way of making sure that the X or Y axis are always ‘square’, as soon as I turn off stepper power and move it manually, it is different each time.

Am I explaining this correctly, or do i have to make a video to show what I am talking about?

You are very close.

Set all 4 rollers to the distance you want nice and equal/square. Now instead of moving it by hand to check your positioning, use the controls in repetier or the lcd screen. That will energize the steppers and lock them, and it will even return back to the initial position for you. The steppers should stay energized the whole time holing your rollers equal position.

This is assuming you are trying to square it then run it don the length of your work piece to ensure correct positioning.

Yeah, I see that once it is set, it stays in sync, but, my problem is how to get it to set to the same exact point each time. I have made jigs to hold the work in place on the table and I want that i can place the jig down and know that it will cut the work exactly each time. But, it looks like with this set up i will have to try squaring it up each time. That doesn’t seem a good way to do it, and it isn’t going to be totally exact each time.
My son is trying to get me to wire up each stepper on it’s own independent axis with their own separate Zero Limit Switches so that they will home to same angle each time, then run them in sync, but, that is a whole re-wire and custom programming.

Your sons suggestion is possible but there is an easier way. Use something like my endstop holders on thingiverse and use them as a positioning bracket. same as an end stop without having to wire it up. just move them into place and touch your rollers too them before you start each time. What are your tolerances? How “perfect” do you need this to be? ± 5mm, 1mm, .1mm?

Another thing, like you said, don’t move the gantry around manually, use the controls to do it. Yea, that is another thing. I have been trying to figure out a hand held way of moving and nudging the x-y-z using the knob on the Ramps / Marlin LCD is a PAIN, and not very portable with my setup. And using Repetier or MatterControl from the PC is also annoying, since my PC is 5 foot away from my table. I have used MicroSoft’s Android Remote, putting the PC screen onto my cell phone and trying to move the gantry around that way, but Repetier is totally Touch Screen UNFriendly… Useless is another word. I have been using Matter Control on my 3D printer, and have used it for the CNC. It is much easier to work with using a touch screen, but, not really that good. The problem is, i also send manual Gcode to move to an exact location, and I don’t like mattercontrol’s terminal. I would really like to see a phone app that can easily control the gantry, it shouldn’t be too hard to add blue tooth to the Ramps setup.

As far as tolerance, i am hoping for at minimum .5mm across the 4ft span, but, .2 would be preferred. I have printed a jig that holds some work on an 18 deg angle, so i can rout out an edge at that angle. It needs to be fairly precise as there will be 20 pieces that have to fit just right to make a cylinder in the final work. How deep the cut is, how for it cuts in and the angle all have to be just right (at least ± .5mm on this cut) or the whole thing won’t fit together correctly. But, that is just one thing i am working on right now. Otherwise, overall i am so far very happy with the machine, and very thankful for your work.

0.019" over 4’ that is extremely tight. I assume you aren’t using wood right, humidity can change it much much more than this.

As for the screen I don’t really use the machine in that way. You might want to looking octopi or some other web based options. Ramps and bluetooth suck, don’t waste your time, that is why there aren’t any apps.

You maybe right about the tolerances, I was just going off the top of my head - because I am cutting wood, and you’re right - it probably isn’t so exact. I am printing some spacers right now to use to align the axis and see how that works -

as far as the hand held jogger for the gantry, I think using the phone or tablet and remoting to the pc may be the way for me to go, and then write a small app with touch screen gui in windows and it can send gcode to the machine I just want to get the x y z numbers that i can plug into work gcode - but i don’t have spare time right now to work on it or i also have asmall touch screen LCD that I could may rig up as a hand held to and use with Ramps instead of the stock LCD - latter though

Hey Ted, the Vicious1 (so far, you really don’t strike me as being vicious at all… :wink:
Here is my solution, at least for now.

I do use Zero Limit Switches, and I’ll explain how shortly.
But, what i did is printed 2 ‘Bumpers’ - mounted them to the corner posts opposite where i have the zero limit switches. When I send the X or Y to home, i have the bumpers just where if the pipe is straight it will just bump the bumper as it hits the Zero sw. After the axis hits the Limit Sw I check the other end and as long as the center roller is just touching my bumper all is good. Now I am getting very good repeatable results.

To test it I Homes it with the Bumpers, then moved the gantry so the bit just barely touches the edge of my work piece. and aligned the piece to the bit just brushes it along it’s edge, from 0 to 1219mm
then i turned off the steppers and manually moved the gantry around, cocking the pipes a little, and hit Home - making sure the bumpers touched - and ran the cutter back to the edge, and it tracked it perfectly from 0 to 1219mm…
I repeated the process several times, and each time it was perfect - enough for what i need. Not sure if it is .2mm, but, actually, it was less that a sheet of paper from touching the work originally, less than a hair. By line of site I could just see a thin space between the bit and the work, and it repeats now, every time. Just what i needed. - Using a Zero Sw on one end and a ‘bumper’ on the other to make the pipe align up the same each time i home it.

Obviously, the stepper that hits the bumper skips a few times when it hits the bumper, but, only the one time to align it. Once it is aligned and i don’t manually move it, when i home it again after that it just touches it without skipping. All i have to do is make sure it does touch it (if it is cocked too far away, it will hit the zero limit before it hits the bumper and there will be a space between the roller and bumper. If that happens then i have to manually uncock the pipe (or slightly cock it the other way) and home it again, and it resets itself.
Now it is working the way i want it to

Here’s how I am using the Zero Switches.
in GCode

;Project 18Deg_Bevel_Cut
;Created by Estlcam version 9 build 9.007
;Machining time about 12:07:57 AM hours
G90 ; use absolute coordinates
M03 S24000 ; start spindle motor 24000 means to use speed set for Tool

g28 Z ; home to limit sw < Z first, to move Bit up out of the way
g28 Y ; home to limit sw
g28 X ; home to limit sw

; Move spindle to desired Origin - Start Pos (Zero for this File, NOT the Zero Limit Sw Home)
G00 X20 Y137.9 F8000
g00 z83.4 f1800 ; NOTE I am using the 8mm Lead Screw with x16 stepping, and can get a much faster Z speed

G92 X0.0000 Y0.0000 Z0.0000 ; reset ZERO all axis

;;; now resume the GCode for mill work
G00 Z-5.0000