Screw comp on master/slave axis

[cnckeith]

I'm buying directly from HiWin, so there's good chance the two Y screws are from the same lot. Which means they should be close (fingers crossed). I'm using the R1 rotating nut in 20/20 2650 mm length. Buying ground screws and scales will blow my budget.

Ideally, there should be individual screw comp for paired axis (Centroid Support, pretty please). I'm also a software engineer, so if Centroid will let me have a look at the source code, I can probably figure out how to implement it.

what is the application? the methods we describe above provide more than enough accuracy for even the most finely tuned cnc machines.


are you ordering double nut screws?

[ekbiker]

CNCKeith, I'm using a rotating nut, there is no double nut for that. The methods describe above doesn't actually work for gantry machines. As I explained, volumetric compensation does not work because it needs a perpendicular reference. But for the case of a traveling gantry with paired screws, the perpendicularity changes as you travel down the Y axis because the two screw pitch errors are not the same. So we are back at trying to get two screws that match. I'm not the only one asking for this feature, will you please consider it.

[cnckeith]

VC comp would work.
XYZ position is measured from the XYZ home position. what ever the machine is out at a commanded XYZ point you simply tell the control to move the amount needed for X and/or Y and/or Z to put the tool in the corrent position in space.

[ekbiker]

Yes, but you need scales to tell the machine where the spindle is at vs. what the controller thinks it's at. The problem is on a gantry machine, the X scale is mounted on the gantry itself, so it is moving with the gantry. As I travel down the Y, X scale will absolutely not move. And vice versa, as I travel along the X, the Y scale will absolutely not move. There is no connected reference between X and Y. To make VC work on a gantry type machine, you can't rely on scales - you need a real time CMM system to measure spindle movement.

Let's use an example, starting from home and we assume it starts square - the gantry (X) to the Y. Let's move the gantry 1 inch in Y, so now due to pitch error variation between the two screws, the gantry is no-longer perpendicular to Y. But the X scale is useless in telling me that I'm not square and I cannot use the X scale. I can use a straight edge placed on the table and keep moving it down the axis, each time using gauge blocks to space by 1 inch. But really, for a 4' x 8' large format router, this is so error prone and would take days. And we are also assuming the table is absolutely flat. The other alternative is to use a CMM, but I don't have a CMM in my garage.

Individual screw comp will correct both the perpendicularity and the pitch error using linear scales - a rather fast and graceful solution to a thorny problem. Large CNC routers are very popular, there will be more and more people like me who would want to build something more precise than what's commonly available.

[cnckeith]

you don't need scales to run Volumetric Compensation.

it doesn't matter that either side of the gantry is ahead or behind the commanded position, all that matters is the commanded tool position and correction value for any given point in space.

[tblough]

As an example of VC, you command your machine to go to X34 Y56 Z12. The machine moves to what it thinks is that position. Now, using a laser interferometer or the measurement method of your choice, you determine the point is actually X33.020 Y55.992 Z12.003.

You dutifully measure many more places in your work envelope and enter those values into VC. It does not matter if the axis are not square because you are not measuring relative to any of those axis.

[ekbiker]

To cover the entire x-y area of a large format gantry CNC router, it would take days even if you have a laser or CMM. This makes VC impractical and unsuitable.

Individual screw comp is a simple and elegant solution using just scales.

[ekbiker]

I'm sorry, I think i've hit on a sacred cow. I'm just making a feature request and explaining why it would be needed for my particular situation.

[suntravel]

To cover the entire x-y area of a large format gantry CNC router, it would take days even if you have a laser or CMM. This makes VC impractical and unsuitable.

Individual screw comp is a simple and elegant solution using just scales.

You can not correct a bend in the gantry or a wrap of the table with screw compensation or scales...

You can also build some kind of thermo compensation with VC.

But I also would appreciate screw compensation and scales on both Y axes.

Uwe

[cnckeith]

laser VC correction takes an afternoon to do. yes you need a $50k laser to do it. there are some guys online that built there own.

.. paired axis with different screw comp tables is on the list. but imho its a way over the top for a hobby machine. these kinds of features are found on controls that charge big bucks for them.

What are you plans for the machine accuracy and repeatability goals?

its on our list but, what is more important to the Acorn majority of the user base?

We can take a survey. Here is my list off the top of my head.

v5.08 release
Rotary Table setup page in Wizard
C axis setup page in Wizard
Exclusion zones setup menu with teach in
step and direction output for spindle motors
G64 for a lathe
Improved g code backplot graphical features
VC released, off beta with setup menu
Fiber laser UI, Oxy flame UI
Improved TT/Tool lenght setup menu
build new CNC12 installer
build new versions of Mill and Lather intercon with long list of requested features such as big improvements to dxf in.
rebuild digitizing menus and add export to dxf
CNC12 screen set builder
add the Wizard to Hickory
Port backup along g code from Plasma to Mill, Router and Lathe
Wizard "import report" for easy updates.
Relay Auto squaring in Wizard.

and about 100 little items to numerous to list here. see feature request threads on the forum.

when all that gets done, then i'll work on different screw compensation tables for different gantry axis screws.