Decimal Precision

[rink]

What decimal precision does VCarve Desktop use?

For example...If I move or copy something 39.37007874015748 inches, will it actually move/copy it EXACTLY that distance? Or will it round to, say, 4 decimal places and move/copy it only 39.3701 inches?

Is the precision "displayed" different than the precision actually used in the math?

I hope I explained that question well.

Thx, rink.

[Rcnewcomb]

What decimal precision does VCarve Desktop use?

What problem are you trying to solve? That will help use give you a better answer.

To the best of my knowledge Vectric products use floating point arithmetic. Float values have between 6 and 9 digits of precision, with most float values having at least 7 significant digits.

Finally, when you output the toolpath the number of decimal places output is controlled by the post-processor settings. The settings below show four decimal places of precision.

Code: Select all

VAR X_POSITION = [X|C|X|1.4]
VAR Y_POSITION = [Y|C|Y|1.4]
VAR Z_POSITION = [Z|C|Z|1.4]
VAR ARC_CENTRE_I_INC_POSITION = [I|A|I|1.4]
VAR ARC_CENTRE_J_INC_POSITION = [J|A|J|1.4]
VAR X_HOME_POSITION = [XH|A|X|1.4]
VAR Y_HOME_POSITION = [YH|A|Y|1.4]
VAR Z_HOME_POSITION = [ZH|A|Z|1.4]

[rink]

Randall, thanks a lot for the quick response and info.

First, what I’m trying to solve: I’d like to make my own scale to use on the wings of my mitre station. The stick-on tapes I’ve found all seem to have an unacceptable degree of inaccuracy. Even the ones from reputable sources. When I built the mitre station, I used Kreg tapes, and they’re accurate but they’re imperial only. I want both imperial and metric markings. So I thought maybe I’d just engrave them myself in thin aluminum strip. But a VCarve project is limited to either inches or metric. I can’t mix both in a single design. Same for the post processor…it wants one or the other, not both.

So my plan is to lay out inches on one edge, no problem there. Then on the other edge, lay out the metric marks by using a conversion factor. But if the calculations in the conversion get rounded, even a small inaccuracy could accumulate over 60 inches to an unacceptable error.

I did look at my post processor, and it shows “1.4” in all the key places, as you pasted in your response. Is it ok for me to change that to another number? Like “1.6” or something like that to achieve greater accuracy? Of course, I suppose that’d be limited by the precision of the hardware (stepper motors, etc.), right?

Thx, rink.

[Rcnewcomb]

I suppose that’d be limited by the precision of the hardware (stepper motors, etc.), right?

You are correct. Because of that there is no benefit to increasing the decimal precision. It would just make the files larger with no real benefit. Resolution of 1/1000th of an inch (3 decimals) is more than enough.

If you were machining parts for NASA then 1/10,000th of an inch accuracy matters, but you would need a substantially more precise, and expensive, machine to achieve those tolerances.

[TReischl]

You proceed from a false assumption.

If you layout something say 60 inches long in a CAD program and then divide it into equal units the error is NOT cumulative. Your concern was that the error would increase with each step. That is not the case.

And yes you can mix units in the software. Just use the conversion factor. If I am working in inches and I want to make something exactly 25mm long then all I need to do is enter 25/25.4 . That is not a rounded conversion factor.

[garylmast]

If you were machining parts for NASA then 1/10,000th of an inch accuracy matters, but you would need a substantially more precise, and expensive, machine to achieve those tolerances.

You mean that rocket ship I'm making probably won't work?

Gary

[TReischl]

I dunno, I just put a DRO on my radial arm saw a few years ago. Press a button, inch, press again, metric.

Is MUCH more accurate than EYEBALLING a scale.

[rink]

Well, unless they need a new space shuttle made from walnut and cherry, I don’t suppose NASA will be calling me anytime soon!

Trieschl: I was originally thinking that if I made an index line…then made another copy 0.0394 inches over…then another one…and so on using the array copy feature…that I would end up with an accumulated error since 1mm is actually slightly smaller than that at 0.03937 (0.03937007874015748, or so the internet says).

But after fiddling with it options for a while using both VCarve and Excel, I arrived pretty much where you went. Also, in the process I learned how to use VCarve’s “Copy Along Vectors” function and using a specific number of copies to get them all evenly spaced.
So, problem solved. And, as usual, thanks to helpful people here!

I really appreciate quality software like VCarve and great support like this forum.

BTW...anybody put an aftermarket DRO on a planer? How accurate are those?

Thx, rink.

[adze_cnc]

... I want to make something exactly 25mm long then all I need to do is enter 25/25.4.

Or: 25*i=

and original poster can use: 1000*i= and he’ll get all the precision that the software can muster.

I expect if I tried I could come up with a dozen things that are needed ensure that the imagined 5 decimal place accuracy is realized. bit sharpness, software floating library, no dirt on mitre saw blade, placement of etched scale, etc.

[Rcnewcomb]

Keep in mind that aluminum will change size based on temperature. If I did the math correctly a 1 meter piece of aluminum will increase length by 0.236mm for a 10 degree change in temperature.

For people who geek-out on accuracy, the Kern can produce and hold tolerances less than 2 micron or 0.00007" (70 MILLIONTHS) of an inch.

[Wayne Locke]

That sounds like an interesting project but it would seem that you are searching for dimensional accuracy beyond what a miter saw is capable of. I have been a designer/builder of very high end furniture for almost 45 years and if you can stay within a 1/32” accuracy with standard small shop power tools, you are in the game. Put three tape measures together and they are unlikely to all show the same 36”. I had a digital readout on my industrial table saw for less than a day because I found when I tried to set it to 18 and 1/4” , or whatever, I found myself wasting time trying to get it to 18.25 and kept ending up with 18.265 or some other ridiculous number which mattered not a whit. I don’t even have an intuitive grasp of what 25/32 is. To me it is 3/4+. That’s a number I can use. Three decimal points is overkill on standard machines but the CNC is a different animal which, in spire of everything it does well, has no grasp of “a little bit more than 3/4.”

I don’t mean to be negative or critical but if I put 18.2638 anywhere near my miter saw it wouldn’t stop laughing for several hours.

[TReischl]

The reason I put DRO's on both the table saw and the radial arm was to be able to repeat a cut later on if I needed to. If I mess up a piece down stream it is not a big deal to make another piece that is the same size as the first.

[rink]

Good morning.

...it would seem that you are searching for dimensional accuracy beyond what a miter saw is capable of...I don’t mean to be negative or critical but if I put 18.2638 anywhere near my miter saw it wouldn’t stop laughing for several hours.

No worries. As happens sometimes, this thread has gone in a bit different direction than I expected And all the replies have been helpful.

I've been woodworking for many years, and I understand well the accuracy limitations of all my tools. I really wasn't looking for mitre saw precision to several decimal places. My question was really simply about how VCarve does math and if it would result in an accumulating error when converting from imperial to metric or vice versa. If that were to happen, of course it could result in variation that would be material and unacceptable. But that doesn't happen, so all good.

Fortunately, my original question was answered very quickly. Also, I kinda resolved it myself anyway with some trial and error shortly after posting my original question. I should have just done that before posting. But good learning in this thread for me, and hopefully for others as well.

And...through the various responses, I learned a couple other things. Like the "[mm]*i=" trick. I had read that before but forgot. And again...VCarve turns out to be just as great software as I've come to expect.

Thx, rink.

[ger21]

My approach when I need to work in metric is to draw everything in metric, and then scale the entire drawing at one time to convert to inches. Then you don't have to worry about rounding errors between entities.
I work in a cabinet shop, where we work in inches, but most of our hardware machining is metric (32mm system). So I do this almost daily.

So, in your case, I'd draw the metric side of the scale first, then scale it down to inches (/25.4), then add the inch scale. No rounding errors over the entire length of the scale.

[Tailmaker]

Not directly answering your software question, but I used a flatback tape measure for my miter saw: https://www.amazon.com/FastCap-PMS-FLAT-16-Flatback-Tape-Measure/dp/B005O3AWZG.

Has inches and mm and is not too expensive to cut up. I attached it with double sided adhesive tape.