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

[pigeon]

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?

[Stijloor]

Originally Posted by pigeon

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

What is the material of the tubes/bars?

Stijloor.

[pigeon]

Originally Posted by Stijloor

What is the material of the tubes/bars?

Stijloor.

Material is aluminum.

[Stijloor]

Originally Posted by pigeon

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

A search revealed a description of a newly invented device.....

Stijloor.

[Crash Not]

I would support each end in a 'V' block and rotate the bar using a dial indicator in the middle.

[Jim Wynne]

Originally Posted by pigeon

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?

How do you propose to avoid using the method required by the standard?

[Benjamin28]

For practical measurement I would do the same as Crash suggested, seems like this would work for the most part. For an automated solution, there are a few patent-pending devices, but I'm not sure of anything very mainstream.

http://www.mefos.se/services-and-pro...s-meter-4.html

[Crash Not]

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.