Basic scan question:

[Quality ish]

I write programs using a part that's been machined. Nothing's ever perfect, and occasionally it's not even in tolerance when it's been submitted for 1st piece...

I don't have CAD access. It's a Global Performance with 2018 R2.

Every now and then, when I have a (circular or cylindrical) scan, I'll program it in (put the move point in, located over the center of the hole, maybe .1 above the top surface, and insert the appropriate scan, plugging in the proper coordinates (according to print), it'll scan it, and what shows up on the graphic display is an arc with the two ends connected. Kind of a half moon kinda deal.

What's the deal? Is it that the hole is so far out of location that it's not scanning the entire circumference? If I adjusted my prehit/retract, might that solve the issue?

[JEFMAN]

Yes, it appears when the center / diameter is too far from theo values...

[Schrocknroll]

You could try measuring the circle with hits and align to it, then the scan should be right on the money.

[Matthew D. Hoedeman]

I get that from time to time, usually on smaller 'rounds'. The way I and found to get it to d the whole diameter is to change the number of crossings from 2 to 3, it will then do 540 degrees of scan instead of 360 degrees.

[UKCMM]

Just how far out does a hole have to be to see this problem just asking as we have had scanned holes up to 6mm out and not seen this

[AndersI]

Did you uncheck "Defined" in the "Execute mode" tab? If not, you're not following the physical surface, but running around in a predefined circle, registering the force on the probe. If the hole is off in location, you might get too big force (that's an error on most machines, I believe) or a too small force (that's "ignore those points" on most machines). Do note that many CMM/Controllers can only run in "Defined" mode, some will do it silently, whatever you tell it to do.

Schrocknroll has given the solution - first measure the hole and align to it, then the defined scan will work all the way around, or not at all, depending on the error of size of the hole.

UKCMM : The problem will show up as soon as some part of the surface of the hole is off nominal more than the available movement (max and min force) of the probe allows. If the hole is large, you're closer to the min force, if it's small you're closer to the max. Combine with position error...

[Quality ish]

Did you uncheck "Defined" in the "Execute mode" tab? If not, you're not following the physical surface, but running around in a predefined circle, registering the force on the probe. If the hole is off in location, you might get too big force (that's an error on most machines, I believe) or a too small force (that's "ignore those points" on most machines). Do note that many CMM/Controllers can only run in "Defined" mode, some will do it silently, whatever you tell it to do.

Schrocknroll has given the solution - first measure the hole and align to it, then the defined scan will work all the way around, or not at all, depending on the error of size of the hole.

UKCMM : The problem will show up as soon as some part of the surface of the hole is off nominal more than the available movement (max and min force) of the probe allows. If the hole is large, you're closer to the min force, if it's small you're closer to the max. Combine with position error...

Wow. Great answers all around. Thank you guys so much. I may have to align to the hole and then align elsewhere, as I often need a true position to the hole. If the hole was the datum, that would come back as X0,Y0, and that ain't right.

The other issue is that I'll do 1800 degrees (5 full revolutions) on threads (at the appropriate pitch) to scan threads, but if I happen to be hitting the thread at the minor diameter as opposed to the major diameter, that would have a definite affect on the diameter of the cylinder, no? Also, taking hits contradicts the whole reason for scanning threads. Taking hits inside a threaded hole gives wonky numbers for location...

I will check the "defined" in the "Execute mode" tab. Maybe I'll get lucky.

[Matthew D. Hoedeman]

Wow. Great answers all around. Thank you guys so much. I may have to align to the hole and then align elsewhere, as I often need a true position to the hole. If the hole was the datum, that would come back as X0,Y0, and that ain't right.

The other issue is that I'll do 1800 degrees (5 full revolutions) on threads (at the appropriate pitch) to scan threads, but if I happen to be hitting the thread at the minor diameter as opposed to the major diameter, that would have a definite affect on the diameter of the cylinder, no? Also, taking hits contradicts the whole reason for scanning threads. Taking hits inside a threaded hole gives wonky numbers for location...

I will check the "defined" in the "Execute mode" tab. Maybe I'll get lucky.

yes, the DIAMETER will change depending on tip or trench, but the location should still be good, and there SHOULD.... NOT.... BE.... MMC or boundary or any modifier for a threaded hole/stud position as you can NOT measure that with a CMM. This assumes that the probe you are using has enough shank clearance so that ONLY the ball of the probe is making contact. (and don't get me started on a shank probe, if you have one, throw it away!)

[Quality ish]

yes, the DIAMETER will change depending on tip or trench, but the location should still be good, and there SHOULD.... NOT.... BE.... MMC or boundary or any modifier for a threaded hole/stud position as you can NOT measure that with a CMM. This assumes that the probe you are using has enough shank clearance so that ONLY the ball of the probe is making contact. (and don't get me started on a shank probe, if you have one, throw it away!)

"shank probe"? I'm not familiar with the term, but I'm going to guess that that's pretty much just a cylinder without a bead at the end?

[Matthew D. Hoedeman]

"shank probe"? I'm not familiar, but I'm going to guess that that's pretty much just a cylinder without a bead at the end?

Imagine a ruby cylinder with a radius on the end, that is a shank probe. You can't calibrate the shank (well, Pcdmis claims to be able to calibrate it, but you can't use that calibrated data), and unless the shank is 100% perfectly perp to what you want to measure, the depth of measure will greatly skew the data collected.

[Quality ish]

Gotcha. If the backside of the material isn't perfectly parallel to the top side, and the hole is parallel to the top side, my locations are going to be skewed, based on the fact that the CMM's surface plate is perpendicular to the probe (at A0,B0).

I see how that could affect location dimensions.

[Quality ish]

Did you uncheck "Defined" in the "Execute mode" tab? If not, you're not following the physical surface, but running around in a predefined circle, registering the force on the probe. If the hole is off in location, you might get too big force (that's an error on most machines, I believe) or a too small force (that's "ignore those points" on most machines). Do note that many CMM/Controllers can only run in "Defined" mode, some will do it silently, whatever you tell it to do.

Okay. If the "Execute Mode" tab is the same as the tab accessed by clicking "Insert", "Scan", "Cylinder", it gives me two options on 2018 R2: "Feature" or "Normal". It also allows me to check (or uncheck) a box for "Use start/end delay points", which is unchecked.

Am I in the right place?