How to Measure Straightness of Small Bars or Tubes (Long Length)

[janedoe]

Most shops use a CMM to measure straightness. Depending on the feature of interest, a CMM uses a different approach than an open set up would.

Straightness is a feature that is checked against itself; deviation of a straight line from a theoretical perfect straight line. (It is sometimes confused with flatness, but the principle is the same).

A CMM will have a more difficult time checking axial straightness than surface straightness, because the variation in the diameters will go along for a ride when the former is the objective. Surface will only tell you that it is straight along the line checked....not at place about the entire diameter.

If an open setup is used, a precision collet could be employed to check the axis, yet size lengths will limit your ability. If the surface is in question, flatness is checked with an indicator running through the table, from underneath, and the item is then dragged across the nib.

It sounds to me that a designer may want to controll a feature other than straightness, but is calling this feature as a control item. For example, if the designer desires to maintain a constant force application to limit buckling, then it may be of more benefit to control axial runout at each end and several places along the lenght especially if the item is not supported in the mid sections.

 

[cabdriver]

Can anyone give some advise how to measure straightness on small bars or tubes?

The bars range from diameter 10 - 20 mm (0.394" - 0.625") and a length of app. 3660 mm (12').
We have to guarantee straightness of 1 mm / m on the full length. These bars are machined by our customers in automated CNC machines.

According to the standards (for example EN 754-3) we have to place the bars on a table in such manner that the weight of the bar reduces the deviation in the straightness (this means placing the bend in the bar upwards on the table). In this position we have to measure the height of the remaining bend (should be less than 3.66 mm).

At this time we only use a visual inspection by the operators while packaging, but this is not acceptable for some customers. We would prefer an inline automated or semi-automated measuring device.

Next week we will do a test to manually measure the straightness on a table of some visually rejected bars. I'm quite sure that some of the visually rejected bars will turn out to be within specification if we actually measure according to the standard (because the weight will reduce the bend).

What methods or devices are used by machine shops to measure straightness on long length of small bars?

Hy,

I have similar issue like collegue above.
In mine case situation is following.
We cut the stell tubes dia. 20 mm on lenght 660,5 (origin lenght is 6000 mm) and sell them to our customer.
They found one part where staightnes was 2,7 mm (in drawing is 1mm/660,5 mm).
They requested 8D and control of the straightness for all tubes.
Since I know my customer (quite difficult one) they will request in-line control (during sawing or after).
Whta do you think:
- should I just prescribe control per bundle (now is one time per production)
- or should I try to found some in process measuring devices for this mistake.
- some other idea????

Thanks for help

 

[tamale]

We used a Faro arm and workrd well. Check them out at:

www.faro.com

Tamale

 

[ScottK]

Hy,

I have similar issue like collegue above.
In mine case situation is following.
We cut the stell tubes dia. 20 mm on lenght 660,5 (origin lenght is 6000 mm) and sell them to our customer.
They found one part where staightnes was 2,7 mm (in drawing is 1mm/660,5 mm).
They requested 8D and control of the straightness for all tubes.
Since I know my customer (quite difficult one) they will request in-line control (during sawing or after).
Whta do you think:
- should I just prescribe control per bundle (now is one time per production)
- or should I try to found some in process measuring devices for this mistake.
- some other idea????

Thanks for help

1mm in 660.5?

what is the specification for straightness on the full length that you purchase?

 

[Ninja]

Also, you could have the same machine shop fab a Go/ No-Go Sleeve that would permit a very quick pass through type inspection. You would have your inspectors insert your tubes into the pass through gauge with a known Go and a No-Go I.D.

Not sure if this is appropriate here or not, given the 12 foot length, but maybe it is...my industry measures in micrometers/mm so the scaleing may be a challenge

In the ceramic substrate industry, camber guages are common. The typical test is an 'envelope slot test' where the substrate is passed between two inclined parallel plates as a Go-NoGo.
The top plate is adjustable to account for substrate lenght/width and median thickness. A "pass" is when the substrate goes through the slot under it's own weight.

Just thinking out loud...it seems in this case that rod rotation must be ensured to measure all axes. The advantage of parallel plates over a diameter is that it allows adjument.

 

[Barahir]

Hy,

I have similar issue like collegue above.
In mine case situation is following.
We cut the stell tubes dia. 20 mm on lenght 660,5 (origin lenght is 6000 mm) and sell them to our customer.
They found one part where staightnes was 2,7 mm (in drawing is 1mm/660,5 mm).
They requested 8D and control of the straightness for all tubes.
Since I know my customer (quite difficult one) they will request in-line control (during sawing or after).
Whta do you think:
- should I just prescribe control per bundle (now is one time per production)
- or should I try to found some in process measuring devices for this mistake.
- some other idea????

Thanks for help

I would say that your solution depends on what is justifiable in this instance. The first thought would be to look at prevention. Is the bow condition caused by your saw operation, handling methods, or can it arrive like this from your supplier? What kind of straightness allowance are you giving to your supplier for the long length part (is it in line with what your customer is requiring from you), and are you verifying that at receiving inspection?

Do you use PFMEAs? If so I'd take a look at that and see what your Detection number is and how far down you can lower it that makes sense for your process and keeps your customer happy. Severity can be determined by what will happen at your customer's location when that part is out of specification. That's kind of static, you can't really do much to change the severity ranking of a bowed tube in the customer's process. So the numbers to focus on would be occurrence and detection. From what you listed above, your detection would be a 9 I think ("control is achieved with indirect or random checks only."). If this is the only part to have been bowed out of tolerance in 100,000 or greater quantity of parts supplied, that's a low occurrence so going all out on your detection would probably not be the wisest way to spend your resources. The higher the frequency of failure the greater the effort to be spent on detection.

I've seen three different methods here that all work:

Surface table & feeler gauges
v-blocks and dial indicator
custom go/no-go gauge (table with a machined slot custom for the specific part of family of parts)

Looks like others have more technical stuff as well like the faro arm.

So far as detection goes, even keeping the same test method you have now but requiring it at a higher rate should improve your detection rating. Perhaps requiring it multiple times per production run instead of once, or one check per bundle of tube like you mentioned. Doing a first piece check, one per bundle and a last piece check and charting those would not only give you a higher level of detection control, but data collected over several production runs might give you insight into better prevention controls.

Bottom line is whatever you do it needs to make sense for your process and deliver the results you (and your customer) are looking for.

 

[bobdoering]

Another method might be to roll the bar on a surface plate with the bow up in the middle and use a feeler gage to measure the gap between the bar and the plate.

One question is do you have a 13' surface plate?

Also, is the callout straightness per unit length? Sometimes a gage that is a tube - maybe a foot long, depending on the spec - that the part must go through if it is straight enough has been used. If it duplicates the results of the callout on the spec, or if the spec technique is used as a "referee method", you may find it more practical.

 

[cabdriver]

Dont ask, it is 1mm/m.

 

[bobdoering]

Dont ask, it is 1mm/m.

Wow. 1 mm/m for aluminum? It must ship in velvet lined wood boxes! You pretty much need a 13' surface plate and feeler gages.

 

[cabdriver]

Wow. 1 mm/m for aluminum? It must ship in velvet lined wood boxes! You pretty much need a 13' surface plate and feeler gages.

Sorry, probably I didn't explain in 1 post:
Tubes are welded.

Anyway, we had talked with our tehnician and we foud some kind of solution:
since we have 5 product which goes to our customer in straight form(some drilling but this is not important) we decided that we will build on the end of sawing line two plates with max. tolerance of straightnes (so for this tube is max. 20mm + 0,15mm for diametter tube tolerance + 1mm=21.15 mm distance betwen this paltes).
Plates should be on minimum angle so that they need to roll in the box with final products.

Some other idea??

P.S.:
I'm thinking that we adjust this plates according real diameter of tube (for each batch)