Surface plates - Questioning the need for periodic calibration of surface plates

M

mhess

#1
:thanx:
I appreciate everybody's help in responding to my questions.
This is the first time I post here, and I couldn't find any posts that bring light to my issue below.

Some people at my organization are questioning the need for periodic calibration of surface plates ($ issue), or \the use of surface plates at all during inspection of the layout of duct assemblies we produce.

Inspections are currently performed on calibrated surface plates using all the usual gages (height, etc.).

The assembly's tolerances range between 0.020" to 0.030".
Questions:
1) Could we eliminate the need for performing this inspection on a surface plate?

2) Could we do some checks on the surface before inspection instead of sending the plate for calibration?

3) A vendor of a portable CMM (arm) is telling us that we won't need a surface plate to inspect the assembly if we use one of these arms, but I found some conflicting information elsewhere that indicates that inspections using portable CMMs do require a surface plate. What is your experience on this?
 
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Jen Kirley

Quality and Auditing Expert
Staff member
Admin
#2
Surface plates are expected to be accurate, and are very often included in calibration system audits because of their function of establishing a plane of basis for other measurements. When their use involves direct contact with jigs, parts and so on, wear can occur. When wear occurs, it usually does so in the form of losing flatness.

Calibration is done by running a sensitive gage along a known straight track that rests on the edges, and the measurement instrument's point touching the surface plate. It's done in a grid fashion, so as to note isolated wear patterns. As it is run along the track, the gage moves while finding low spots. Checks can be done in-house if you have the right equipment, and frequency can be set in accordance with the amount and type of use, as well as the tolerances, which you have described as hundredths.

Did you buy the plate new? If so, it should have come with a calibration certificate. If not, its flatness should be established with a lab's calibration (some make house calls) so you can show the evidence of calibration. Then you can set up your calibration method and frequency in a way that assures parts are being measured correctly, and the results are accurate.
 
#3
It could well be that, based on the tolerances that you mention, calibration of the surface plate is not gong to be needed very frequently..........

As Jennifer correctly states, you need to know the condition of the surface you're working from - with height gauges, this is the surface plate.

The cmm arm (could it be a Pharo?) supplier is somewhat correct, in that the use of a cmm usually means means defining 'planes' for a datum and the operator then works from that, so the condition of the surface and any supporting (jig) blocks etc. have negligible influence.

A quick check might be to use a known good (calibrated) straight edge which can be carefully laid on the table in various positions. A thin feeler gauge (0.0015 in) can then be used to 'probe' any gap there may be. Is rough and ready, but could be used for the type of duct work you are measuring.
 

howste

Thaumaturge
Super Moderator
#4
I do some work with an aerospace company that overhauls aircraft ducts using Faro Arms of various accuracies. Depending on the accuracy required for the surface plate (and the accuracy of the Faro Arm), the Faro Arm itself should be able to be used to calibrate/verify it.
 
M

mhess

#5
Thank you very much to all for the information...

Howste:

Could the company you mentioned AVOID altogether the use of a surface plate if they are using a Faro arm?

The information I got from the arm supplier seems to imply that I can assembly on the top of ANY table, and then probe the assembly with the arm without having a need for a surface plate AT ALL. Is this true?

I am reluctant to accept this, because regular CMM machines come with a built-in flat surface, so I am not sure why a portable CMM would eliminate the need for a flat plate. :frust:
 

howste

Thaumaturge
Super Moderator
#6
Howste:

Could the company you mentioned AVOID altogether the use of a surface plate if they are using a Faro arm?
In some applications they aren't using a surface plate. They probe a datum surface on the part itself, then the measurements are aligned to it. However in most applications they have a fixture replicating the mating surface of the duct, which I believe they calibrate with the Faro Arm.
 
B

brobinson

#7
Having used a Faro-Arm for the past 2-3 years, I may be able to help you some.

I have taken our arm to numerous locations within our facility and accurately measured parts without having a granite surface plate.

If you use a granite surface plate to place your duct work flat on it and choose to use the granite as a representation of the duct, then yes, your surface plate should be certified. With your duct resting flat on a certified granite, you can use a shim and determine the workpieces flatness. Our crankcases have a flatness tolerance of .003". Some crankcases are relatively small while our largest is 15' long.

One way to avoid the surface plate certification($$) and still use the surface plate with the Faro-Arm will be to support the workpiece using basically anything to elevate the part so that access can be gained to the bottom. Keep in mind the work piece needs to remain in a single location during the entire measuring process.

I have used two 4"x36" parallels to place my workpiece (approx, 4'x3'x3';3500#) on top of the granite. This allows me to measure all surfaces without using the granite as a datum. I have used the tripod method many times with gage blocks when measuring smaller pieces.

Not supporting Faro, but we have a full line of their equipment. Laser tracker, Faro-Arm and Faro-Gages. I prefer the Arm and Laser over the Gages but each device serves its purpose.
 
S

Sorin

#8
Just throwing in my :2cents: b/c it seems no1 stated the obvious (or I passed too fast on the posts and in this case mea culpa).

In theory you won't need a surface plate as long as during the whole measurement/inspection your part will be perfectly still - in a fixture or something along the lines of a fixture.
 

gpainter

Quite Involved in Discussions
#9
We had ours calibrated once every five years. On the second calibration it needed to be lapped. Used it for plastic parts, so not a lot of abuse.
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#10
:thanx:
I appreciate everybody's help in responding to my questions.
This is the first time I post here, and I couldn't find any posts that bring light to my issue below.

Some people at my organization are questioning the need for periodic calibration of surface plates ($ issue), or \the use of surface plates at all during inspection of the layout of duct assemblies we produce.

Inspections are currently performed on calibrated surface plates using all the usual gages (height, etc.).

The assembly's tolerances range between 0.020" to 0.030".
Questions:
1) Could we eliminate the need for performing this inspection on a surface plate?

2) Could we do some checks on the surface before inspection instead of sending the plate for calibration?

3) A vendor of a portable CMM (arm) is telling us that we won't need a surface plate to inspect the assembly if we use one of these arms, but I found some conflicting information elsewhere that indicates that inspections using portable CMMs do require a surface plate. What is your experience on this?
If you are inspecting ducting, similar to A/C or oven ducting, chances are you can go to a "B" grade plate, as that is machine shop kind of grade.

Periodic calibration is necessary because the plates wear. At times they may need to be "lapped", that is resurfaced.....ground down if you will.....

The real criteria for your inspection is, how flat does the surface need to be?

Hope this helps.
 
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