Accuracy

[dirk]

Jeff
I checked the steps per inch calculation on the spreadsheet and I couldn’t find any errors. Maybe someone else could run a calculation to confirm the results. If I remember right The PR handled unit values for the Z axis differently than x and Y so that may be the difference.
Dirk

[mikejohn]

Now I am amongst the worst 'thread drifters' on the Forum, and as I said to Dick above, thread drift often gives a lot more information than the original question intended, which has certainly happened here.
Now all above is explaining how and why the tolerances evolve, but no one has yet answered my original question. If I accept ShopBots specifications "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?"

I will repeat that it is the theoretical possibility I am asking about.

(and if any one has "The Dummies Guide to All above" I would love a copy
)

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

[rhfurniture]

My guess is that if you issue the same instruction twice the result is electronically identical.
I havn't tested it though.

R.

[gerald_d]

Jeff, I deleted it because I realised that I didn't know what you meant by "recommended" or what model you had, as I hadn't really been following the thread. It was just luck that I said something that helped.

Why wasn't I following this thread.....because I have enough real problems without looking for hypothetical ones!

[mikejohn]

Gerald
Don't you see that , if you are chasing down an accuracy problem, unless you know what can happen in a 'perfect' set up as far as inaccuracy goes, you may in fact not be chasing a 1mm inaccuracy, but in fact a .1mm inaccuracy?
It all comes down to knowing when you should stop tweaking.
I do not understand why no one can answer my question?

rh
That is my point.
If I say
J3, 10,20,-5
MY,50
then send the bot to 20 different positions around the table with M3 or J3 commands, then go
J3, 10,20,5
J3, 10,20,-5
MY,50

can the second MY move be inaccurate in any of the 3 axis by the maximum published tolerances of ShopBot?
If so, do I add position and cutting tolerances together?
As it is published + and -, is the possible 'error' double the combination of both, in the worst possible case?
............Mike

[gerald_d]

Mike, if your budget doesn't balance at the end of the month, do you start analysing the accuracy of your calculator?

I live under the assumption that the PC's accuracy of calculation is a couple of orders better than the flexing and lashing blue beast with joined racks on double-sided tape, grubscrew-held pinions, turnbuckles, bedsprings, soft rails, etc. Why argue about little numbers your screen......because they are so numerical? Don't let the digits lull you into a sense of accuracy. You and I can measure exactly the same groove with the same digital caliper and get different readings - my thumb may push harder than yours.


Having said that, I take the published accuracy figures with a huge dose of salt. Too many factors influencing the accuracy are not defined, and you are not going to get the SB company to tighten up on their definition because that will close convenient loopholes.

[mikejohn]

I guess what it comes down to is this.
Inaccuracy occurs for a number of reasons.
Often one inaccuracy may be reduced by an opposing inaccuracy.
And somewhere within this, dwells the theoretical accuracy, which is, in fact, unmeasurable and indeterminable amongst the general inaccuracy.
So it doesn't matter any way.

I think I will forget the NASA contract



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

[rhfurniture]

My guess is that provided you repeated the step before the one you want to measure to take out any mechanical slop variations, it would be damn near, provided you havn't lost steps.

PS: Absolute exists only in Black holes.

R.

[mikejohn]

I'm thinking of using Black hole dust as a means of hold-down.
Just a bit worried where the ShopBot might end up if I hit it with the bit!

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

[rhfurniture]

Actually if I remember right from when I used to browse cnc specs, some makers quoted repeatability factors (shopbot don't) which was usually some stupidly small number, say 1/100th of the accuracy figures.
Of course cutting pull will make a monkey of it all anyway. (sorry monkeys, no offence meant)

PS: Absolute also polutes the mind (unless you are a mathematician)

R.