form error

[heeler]

Could someone tell me and maybe refer me to something that will tell me what form error is?

 

We just had some parts that our CMM rejected because of a form error of .009. The DIA. of the cylinder was perfect at 1.5630 and the location was perfect. They then took measurements of 3 circles in the cylinder and had measurements of 1.5631 top, 1.5630 in the middle and 1.5630 and the bottom. the center points were the same. Our CMM inspecters claim that the hole is out of round by .009. They even told me where the hole was short and long, so I showed them with a pair of calipers that the hole was not different by .009. I know that calipers are not the best way, but I should be able to see .009.

 

Thanks

[Scott Bond]

Hi Heeler

 

Could you shine the the part up and give it back to the same inspector, and don't publisize to anyone that it is the same part?

 

quote:

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Positive attude leads to positive results.

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Smile

[Lucky]

Maybe the CMM report is metric?

[McRae]

Untill Ernie reports in with a definitive answer, lets look at what they are telling you. The diameter is perfect within .0001" and the form is out .009 - 90 times worse that it really is...

 

Where are the projection references set to? These machines will project everything back onto a datum plane and this could be some of the problem. Ask them to check the feature manually and when they can't do that, show them how. Get a copy of the latest GDT manual and then hold a training session. It is obvious that the result from the CMM is not being interpreted/applied correctly.

 

I had the displeasure of working with some JackA$$ that had one year's experience 25 times over telling me that my parts were wrong... When in fact, he didn't know what he was looking at!

[chris m]

Turn a plug .0005" undersize, stick it all the way to the bottom of the hole and extend your middle finger toward the CMM

 

That is my patented method

[MACHINEMASTERG]

chris m if this is a patented method i think i owe you some royaltie money

[chris m]

Please send your check to:

 

Harmonic Drive Technologies

247 Lynnfield Street

Peabody, MA 01960

 

Thanks in advance

[MACHINEMASTERG]

check is in the mail

[Scott Bond]

Today I have to compress the delivery of an aircraft part . It has no print ,and no part number. Only the zip with thre words " Create part to Iges file" written on it with a sharpie.

The polisher brought thr part to inspection ,and because they can't even open a file in there, The inspector paged me (sumond me into inspection)and while holding the now existing big dollar part in his hand ,,,asks "what is this for". I said "it,s for money" and I will check it out from my end and get back to you.

 

Do any of you guys have inspectors that will not even look at a cad file,,,This is like 1978

 

I am havring a hard time putting the middle finger away today.

[McRae]

Chris' method is just the right number of "Significant Digits"...

[heeler]

Still looking for solutions, I like some of them but being QA has to buy all my parts, I keep my fingers in my pocket.

[Guest CNC Apps Guy 1]

quote:

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... I keep my fingers in my pocket.

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Pocket pool..... I see!

 

j/k - Couldn't resist that one.

[McRae]

Your CMM technician should be the one to tell you what Form Error is if they are saying that it is not correct. What are the drawing callouts for this Form?? Explain to me how an error in diameter of .0001" and perfect position be out of tolerance by .009" (unless the hole is not being interpreted as perpendicular at the origin plane).

 

Give more detail - CMM Make Model and Software +version and we may be able to put the wheels back on this part so you can ship it by year end in 3 months...

[camguy]

Heeler, was the part held in a 3 jaw chuck when it was bored? If this is the case, then the part could be out of round by.009. checking with calipers or a 2 point bore gage will not show this. the only way is to check it with a pin, or a 3 point hole micrometer. I ran into this same problem many years ago. when the part is removed from the chuck, the contact areas of the part with the jaws will move out about .0045" (in this case), and the areas opposite those 3 points will collapse in by .0045". this will always give you a "correct" measurement with calipers.if you can clamp the part with less pressure, you will see the out of round will decrease. hope this helps.

[Verisurf - Ernie Husted]

Camguy, made a very good point.

 

You can not check roundness with a pair of calipers.

 

I'm betting on the inspector. in these days they are mostly right.

 

How do you feel when that finger gets stuck up your xxxx.

 

If this was a drilled hole it is probably out of round. Here is why;

 

You can model the working end of a drill bit as a single straight line of finite length. If you fix one end and try to rotate it, the opposite end of the line sweeps out an arc. (the drill flexes) After about 1/3 revolution, the stuck end breaks free and sweeps out another arc while the formerly free end sticks. With alternate ends sticking, then breaking free, the arcs will form a kind of polygon with arcs of radius equal to the drill's diameter. After the first cuts, the "corners" of the polygon tend to stop the sweeping cut for each drill flute. The most common I've seen is the triangular hole, but other polygons are definitely possible. I suspect that this occurrence is related to some sort of resonance in the drilling setup.

 

The suggestions that followed may be useful to anyone trying to drill holes. Some of them may qualify as 'obvious' but they're still worth bearing in mind...

 

Ensure the drill is sharp.

Make sure the work is firmly clamped

Don't try and run the bit too fast for the drill size and work material.

Don't force the feed rate; as with *any* cutting process, let the cutter do the cutting.

Keep as much of the drill in the chuck as possible. The more flexibility there is in the drill, the more likely you are to have problems.

When drilling thin material, it is often useful to provide some form of backing clamped to the work. This has the added advantage of keeping the burrs to a minimum.

The drill tip may need to be ground to a different angle, depending on the material being worked.

An undersize pilot hole is often a good idea. If you are drilling using a mark made with a centre punch and the tip of the drill is larger than the mark, you are unlikely to get accurate placement.

Don't forget to use a cutting lubricant

The quality of the hole is only going to be as good as the machine you are using will allow. If the drill spindle is sloppy, there may be nothing you can do about it.

As a final comment, if you really want a round, accurately sized hole, you are unlikely to get it with a twist drill. Drill undersize and use a reamer if it's important.

 

Hardway, your shop needs Verisurf

 

Ernie

 

[ 10-05-2002, 12:16 PM: Message edited by: tri-tech5-axis ]

[Verisurf - Ernie Husted]

Back to the original question.

quote:

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Could someone tell me and maybe refer me to something that will tell me what form error is?

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Form error is the deviation from the true shape of a single feature.

 

Form tolerances control straightness, flatness, roundness and cyindicity.

 

 

Ernie

Verisurf Software

[heeler]

The hole was made with a boring bar. We were trying to interpolate the hole and they were telling us the same thing. When they showed me on the CMM the hole measurments, they were saying that the hole out of roundness was straight across from each other. That is why I think that I should be able to see the difference with calipers. At first I thought that we were clamping the block to tight and sqeezing the hole, but we started being more careful and it is in a block that is 8" X 8" X 3" thick. As far as the form error, I was trying to understand if it is an actual length or measurement error, or just a devation or mathmatical error.

[McRae]

Does the hole indicate round in the machine??

[heeler]

We had checked the holes with a digital bore guage for size and removed them, then cut our second setup. So we can't put it back in the same setup. An engineer has bought the tag that our Q.A. wrote that allowed them to install the bushing that go in the hole and to check for gaps. There were none. So I'm still confused about what the form error is, but I still don't think that the hole is out of round by .009". Maybe if it goes back to a datum somewhere on the CMM?

[chris m]

(my question was answered as I was typing)

 

[ 10-07-2002, 10:14 AM: Message edited by: chris m ]

[Scott Bond]

quote:

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Does the hole indicate round in the machine??

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Would be possible to personaly re-inspect an existing part in the machine (or a machine)in the approximate area of the inspection's three circles.

 

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They then took measurements of 3 circles in the cylinder

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They even told me where the hole was short and long,

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Has this been resovled with inspection ?

[Verisurf - Ernie Husted]

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I was trying to understand if it is an actual length or measurement error, or just a devation or mathmatical error.

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If your just looking for roundness then the suggestion from Andrew will work, the total indicator reading when you spin 360 deg. on the machine should give you the idea of a roundness error. Make sure you have a good true running spindle.

 

Ernie

[Verisurf - Ernie Husted]

You may need to Locktite the bushing to fill in the gaps.

 

Ernie

 

[ 10-07-2002, 10:25 AM: Message edited by: Tri-Tech 5-axis ]

[McRae]

What is the material condition of the top surface - As Rolled, Rough Cast, Machined to a flatness of .0001"?? The bore mic is a static measurement and the Form Error is both a dynamic measurement. The CMM is approximating the Form Error thru Mathematical Calculations. By knowing where the datum planes and points are taken, one can then analyse his/her setup to find the source of error.

[Eric S.]

Just a thought but the hole could be correct. It may be the CMM's lobing error causing it to measure out of round .