Building a CNC router

[pminmo]

Originally posted by ballendo
Hello

Well, it appears that the engineering books support the idea that repeatability and precision are the same thing. Which I agree with. But they also seem to support the idea that precison and accuracy are unrelated...

It ALSO appears that the engineering sites and texts consider accuracy to be only an average...

For this I can only recall growing up with a Father who used to gripe that "them eng'neers got no idea whats goin' on in the real world!"

Ballendo (1 for 2)

My education and years of professional experience is in electronics so I may have a slightly different slant. A 3 digit voltmeter isn't considered a precision intrument, an 8 digit is. But as to precision and repeatability the same I have to disagree with the engineering books (even though I am one). Exaguration here to make my point, a stepper that has 20 steps per revolution driving a 2tpi screw gives you a resolution of .025, if that screw is perfect i.e. +/- .00000 and the motor is perfect you have a drive that is very repeatable, i.e. you tell it to go to any position that is evenly divisable by .025 and it does every time. Now try to make a part that has geometry defined to .0005, that perfect repeatability doesn't do you an ounce of good because the machine lacks the precision to do it.
"Accuracy being only and average" maybe out of context. Two lines seperated 1.000 inch, One instrument make measurements of 1.5", 1" and .5", an average to 1", but the instrument used to make those three measurements can't be considered accurate. A instrument that makes three measurements to .99990, 1.00010, 1.0000 is accurate. Accuracy is the ability to measure or produce expected values verses actual values, nothing else, not even debatable. That is why we have the NBS, and why calibration laboratories have instrumentation tracable to NBS.

Phil

[High Seas]

Let me try this one step at a time.

Originally from pminmo:
QUOTE: Now try to make a part that has geometry defined to .0005, that perfect repeatability doesn't do you an ounce of good because the machine lacks the precision to do it. UNQUOTE.

Sorry, but the machine you described lacks the RESOLUTION to mill to 0.0005. As good as it gets is defined by the limitations you built in: the stepper and the drive. You can only make assumptions regarding the machine's REPEATABILITY -- until you actually make the part, many parts, and see how close they are to each other. it won't be worth a hoot if its not the size you need for the job or customer - and a lot of scrap lying around too. Milling within our limitations or making adjustments is what we'd be forced to do with such a machine. But clearly that machine lacks the RESOLUTION to do the requested job. If all those scrap parts are the same dimensions, then we'd have good repeatability, within our system resolution - but only when compared to the desired output can we make statements regarding precision and accuracy.


ACCURACY is the ability to get the expected result. To hit what you aim at, to measure the gap with as many significant digits as the ruler has, to set or measure the resistance to the --CORRECT - expected value. Any error, deviation, scatter, bias, or miss of the measured result from the EXPECTED (read - correct) value describes the precision. NOT the number of digits in the meter. More digits increase the ability to make greater descrimination in our measurement. Measuring to 8 significant digits often implies PRECISION (and sometimes ACCURACY). But the additional digits provide only some confidence - if we've calibrate as Phil suggests (thats what we always do righto?).

[pminmo]

Originally posted by High Seas
Let me try this one step at a time.

Originally from pminmo:
QUOTE: Now try to make a part that has geometry defined to .0005, that perfect repeatability doesn't do you an ounce of good because the machine lacks the precision to do it. UNQUOTE.

Sorry, but the machine you described lacks the RESOLUTION to mill to 0.0005. As good as it gets is defined by the limitations you built in: the stepper and the drive. You can only make assumptions regarding the machine's REPEATABILITY -- until you actually make the part, many parts, and see how close they are to each other. it won't be worth a hoot if its not the size you need for the job or customer - and a lot of scrap lying around too.


ACCURACY is the ability to get the expected result. To hit what you aim at, to measure the gap with as many significant digits as the ruler has, to set or measure the resistance to the --CORRECT - expected value. Any error, deviation, scatter, bias, or miss of the measured result from the EXPECTED (read - correct) value describes the precision. NOT the number of digits in the meter. More digits increase the ability to make greater descrimination in our measurement. Measuring to 8 significant digits often implies PRECISION (and sometimes ACCURACY). But the additional digits provide only some confidence - if we've calibrate as Phil suggests (thats what we always do righto?).

I understand and completely agree on the resolution, it was precisely :-) my point. Also agree on part of the repeatability you state. The theoritical machine I described will make a perfect repeatable part if the part has geometry defined in .025 increments. So if the part has four holes spaced 1" apart, you will have perfect repeatability. now if those holes are spaced 1.0125 apart, the parts will vary because the being eactly at half the resolution the computation will sometimes go .0125 strong, sometimes .0125 weak. Repeatablilty and precision are related but different animals. If I buy a ball screw that over the length is accurate to .00001 any place on the screw, .00001 backlash on the nut, and the stepper steps are accurate to .00001 degree, I can make a calulated determination on repeatabilty when it is designed. Maybe this helps, before we did anything digital i.e. in a purely analog world, there is no such thing as resolution because the resolution is infinity in theory in a purely analog sense. Just because you have infinite resolution, no statement can be made on precision.
Completely agree on your accuracy statement---
Maybe the elctronics world has different slant on things and I'm to thick headed to see it any other way. :-)

Phil

[ballendo]

Hello,

I'm pretty sure we're all on the same page regarding accuracy, and what it means. I think we also all agree on repeatability and resolution. It's this "precision" thing that's befuddling to some of us...

I've been thinking hard about this over the last few days...

And I think I've got it settled in my mind at least

Repeatability and precision are a subset, or precursor of accuracy.

If you have either, you also have accuracy. It may not be the dimension you want, but if it is repeating, and the degree of that repeatability is small (great precision), it IS accurate. Just not to your desires. But with repeatability and precision, we can GET ANY desired accuracy, to the degree that we have repeatability and precision. As long as our resolution is up to it...

Precision would also dictate the degree to which our resolution is attainable. Repeatability means the same thing, and gives us the same results.

What I really want to know is how 1000 people would answer these questions:

If something is designed to be exactly one inch long, and every part made is exactly 1-1/4 inch long; are the parts precise?

Are they an example of precision work?

Is Accuracy derivative of repeatability?

Is Accuracy derivative of precison?

Can precision exist without accuracy?

In all of this I can easily accept that repeatability and precison are the same. But I still find it very hard to call something precise which is not accurate...

Ballendo

P.S. Maybe my electronic background is "leaning" me like Phil says it is leaning him... I do know that in that field we are cautioned to carefully consider the QUALITY of each digit, when determining its significance. Especially since digital came along, and we now have the ability to post very "precise", but totally wrong--or at least meaningless-- digits

In the PS above, I would prefer to write --seemingly-- in front of the word precise...

[pminmo]

A reccuring definition in the dictionaries is exact, no more no less.

[FRAMEDNLV]

Can i have a try at this?
Precison would be setting the units in cad up to 8 decimal places. Accuracy would be using object snaps in autocad. So if you have precision it doesn't mean anything when the darn drafter have that close enough attitude. In the end it looks right when it is plotted.

If I recall a couple of years back we all had a common problem with Precison and accuracy. "Y2k bug" the two digit date code was accurate. The precison was not.


Just my thoughts,
Chris

[High Seas]

Missing POST?
Just wondering if anyone saw my last POST. Its the one that followed pminmo's last one on the 22nd/23rd -- and started with a smat alec remark about KISS - meaning Keep it Simple?
It was a short one - and I'd like to get a copy of it back for my files - I didn't save it and it seems to have slipped off this thread.
Any ideas - I did see it here a day or so after posting - or is this a case for Mulder & Scully?
Jim

[balsaman]

I don't see it and the moderators didn't delete it....not sure bud.

E

[High Seas]

musta lost it in the insert process or something - thanks mate!

[sbrpollock]

Costello: Whos on first?

Abbott: Precisely!

Costello: Precisely?

Abbott: Precisely!

Costello: So I throw the ball to Precisely.

Abbott: NO!! You throw the ball to Who!

Costello: Precisely?

Abbott: Precisely!


I'm sorry guys I couldn't resist

[HomeCNC]

I just purchased a "Touch" screen monitor and a 10 key pad for my CNC router. I do not like to pull out my keyboard tray to get to the mouse while the router is going. The touch screen is cool. I can press all the buttons in Mach 2 and lower or increase the feed rate or move to different screen all by touching the glass on the monitor! By having the small 10 key pad I should not need the keyboard for input at all.

I will be doing a product review on the "Touch" monitor soon.

[Cold Fusion]

Any update on that second video we were promised?

[HomeCNC]

Boy! Some people never forget!

My cousen is very busy now so I decieded to purchase my own DV camcorder. I will try to film something myself soon.

[Patrick2by4]

" www.plitron.com

They have many to choose from. Just remember that you get two transformers in one toroid. So if you want 36VDC @20 amps purchase the 25VAC output at 10 amps. If you want 30 amps output get the 25VAC 15 amp toroid."
Hey Jeff, where can I read up on toroid transformers? I don't understand what you mean by this statement. by the way, your videos are awesome. I showed it to some of my friends and they were very impressed by your machine. (I'm in the process of getting parts right now, boy is this a expensive hobby!)

Pat

[Photon]

Patrick, your avatar looks like it really needs to go to the bathroom.

(Sorry for the off topic, but couldn't help it)

[Patrick2by4]

LOL

[HomeCNC]

Hey Jeff, where can I read up on toroid transformers? I don't understand what you mean by this statement.

Well I'm not experienced in electronics as well. I don't know if it is standard that ALL Toroid transformers are setup with two single transformers inside of them or not. I only found this out on the first one I ever used.

When you have two transformers you have many options of how to connect them up. Let's say you purchase the 2 X (25 VAC output @ 10 amps). The connection can be:

1) keep both transformers separate. You will have two 110 VAC inputs that give you two 25 VAC outputs @ 10 amps each.

2) Connect the two inputs so you have ONE 110 input. Connect the output so you double the voltage and have one output of 50VAC @ 10 amps.

3) connect the two inputs so you have ONE 110 input. Connect the outputs so you double the amps and have one output of 25VAC @ 20 amps.

4) connect the two inputs so you have ONE 220 input. I have not done this so I don't know what the outputs would be.

The two connection methods are called "Parallel and Series" but I don't know which is which on the above examples.

[HomeCNC]

I now have an updated (NEW) video of my CNC router at my website. It is showing me carving 1/2 of a model boat hull. When both halves were done, they fit together so perfectly. The link is http://www.homecnc.info/router-movie.htm

Im now working on my 4th axis. This has been put on hold but I'm trying to find the time to get it going again.

More later.

[High Seas]

VERY NICE - thats a project on my adgenda too - and has been from the start! Just got get it up in priority I guess. Will ther be a deck too?
Cheers - Jim

[fyffe555]

I now have an updated (NEW) video of my CNC router at my website. It is showing me carve 1/2 of a model boat hull. When both halves were do they fit together so perfectly.

Very Nice.... What software/process are you using to produce the toolpath?