Accuracy

[mikejohn]

Today I reset my z axis with a lot of trammeling, and it was a long way out.
In the end I got it pretty accurate, about .5mm over 120 cm diameter.
Then I did a number of tests including cutting two grooves 6mm deep into MDF with a brand spanking new 5mm bit.
The first groove I cut in 2 passes each 3mm deep, the second in a single 6mm pass.
I checked the groove width with the tip of my calipers,which has a precise 5mm width.
The single pass fitted very very snug, perfect even.
The one with two passes had a little play, not enough to insert a 0.1mm feeler guage in, but almost enough.
Now to my question.
In the ShopBot specs for my machine, the latest PRT96, they quote a positioning accuracy of +/- 0.127mm (my conversion from their imperial), and a cutting accuracy of +/- 0.381mm.
My 'error' in the two grooves is well inside even the first of these.
Finally my question.
Does this mean that if I continually cut the same groove, many times, it could end up 0.762mm wider than planned?
And do I need also to add the positioning error to this?
In fact, is it theoretically possible to cut just twice, and end up with +.127 and +.381 (.508mm or 0.02") error?
In all this I am assuming that there is no physical error, that in fact the accuracy of the way the Shopbot is controlled might lead to these type of errors?
Although this question is a theoretical one, and not something I have noticed whilst operating the shopbot, nevertheless I am interested in what all these specification figures mean.
My tramelling today gave me an error, at the bit, well inside the ShopBot specs.
I recognise that the set up of the ShopBot, and any bit run out can be even greater than this.
However, I am interested in the true theoretical repeatable accuracy of a perfectly set up ShopBot.

.........Mike

[gerald_d]

Are you talking of the same "errors" as in the current dovetail thread?

[mikejohn]

Gerald
No, but it does have a bearing on that thread.
Morris Dovey-tail says And no one is likely to specify a gap as large as 0.01" unless they plan to grout/spackle the joint.
My question above tries to ask what the specification accuracy of the ShopBot is.
I presume that if the file asks the shopbot to go to 100,50 it sends this information to the shopbot, but ends up at 99.83,50.18 because of the way the drivers or steppers work.

And I'm assuming there is some mechanical innacuracy with the pinions etc even in a theoretically perfectly set up 'Bot.
Practically the question is, if I did have a .01" glue gap. might this not simply dissapear within the normal shopbot working tollerances?

..........Mike

[richards]

Mike,

I think you only have a few 'unadjustable' errors that you have to worry about.

1. 1/2-step width. The stepper motors make finite moves, which are non-divisible; therefore, if your axis's destination ends up between steps, you will be, at most, 1/2-step width away from your desired position.

2. Mechanical looseness or slop. This includes such things as: V-rails that are not perfectly straight or that have tooling/cutting marks. V-rollers that are not perfectly round or have loose/worn bearings. Worn router or spindle bearings. Worn or out-of-tolerance router/spindle collets. Worn or out-of-round tooling, etc.

3. Temperature. The temperature in my shop has varied from 105-degrees F. in July to 4-degrees F. in mid-December. The metal table has to expand/contract based on the temperature. Even bringing the shop to more livable temperatures before cutting does not guarantee that the steel table has reached room temperature.

4. Material. I've had the cutter wander when cutting hardwood, especially when the feed speed is at its maximum and the tool path cuts close to a knot.

[stevem]

5. The pinion to rack ratio does not divide evenly into increments of 1”, thus causing some rounding off.

[mikejohn]

Mike
I recognise your points 2,3 and 4.
It's your 1 and Steves 5 that I am wondering about
If I am having accuracy problems, how much is outside of my control?

..............Mike

[mikejohn]

edited double post

[gerald_d]

Theoretically, a rack & pinion system has no "linearity" errors. However, manufacturing tolerances and subsequent wear & tear will make the rack & pinion less than accurate, but it will not introduce a coarse accumulative error.

[richards]

Mike,

Stepper resolution is a factor, but I believe a minor factor with absolute accuracy on a CNC router used to cut wood. The main reason that I'm saying that, is because, for particle board, I use a chip load of 0.025-inch. Even if the machine had absolute accuracy of 0.001-inch, particle board 'chunks' as it's cut so my absolute accuracy probably has a variance of +/- 0.025. (Put a straight edge across a freshly cut piece of melamine coated particle board, and you'll see what I mean. There are bumps and dips in the particle board, but the melamine is almost always perfectly smooth.)

Now, back to stepper resolution. If we assume that a basic stepper motor has full steps of 1.8-degree, or 200 steps per revolution, and that the Shopbot has a pinion gear with a pitch diameter of 1-inch, then for every step the stepper motor makes, the axis will move 0.0157 inches (3.14 inches / 200). If the stepper motor half-steps, then each step will move the axis 0.00785 inches (3.14 inches / 400). If the stepper has a 3:1 gearbox, and uses full-steps, then the movement per step will be 0.00523 (3.14 inches / 600). Other pitch diameters, gear ratios, steps-per-revolution, etc. will determine the minimum distance an axis will move per step. To find out how your machine is configured, type the UV command or look at the VU fill-in sheet. You'll be shown the number of steps per inch (steps per millimeter?) that your machine moves.

[dirk]

While trying to determine a drive for my new machine, I wrote a quick little Excel spreadsheet. It will show the effects of different size motors,gearing, cutting force and step resolution, etc.
I'll try to attach a copy.


Stepper Calculator

steppercalc.xls (20.0 k)

Dirk