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

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?

:thanx:

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

:thanx:

What is the material of the tubes/bars?

Stijloor.

What is the material of the tubes/bars?

Stijloor.

Material is aluminum.

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


A search revealed a description of a newly invented device (http://www.patentstorm.us/patents/7165681-description.html).....

Stijloor.

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

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

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?

:thanx:

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

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-products/products/bar-and-tube-straightness-meter-4.html

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.

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-products/products/bar-and-tube-straightness-meter-4.html

Thanks for the info. I was already aware of this application, but this device is only for a diamter range of 80 -130 mm.

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

Tha standard does not require that we test each bar using this method. We have to guarantee the straightness if measured with this method. Up to us to proof that our process is capable to guarantee this straightness and that the inspection method we use, sorts out the nonconforming bars.

I will set up some trails and perform a kind of msa on some of the methods suggested in the reply's. For the trails se will need a value of straightness , because we
we will also perform a DOE to investigate some parameters influencing straightness.

1. test on a slope --> look at difference in speed or time going down the slope (possible, but will it pass msa?)
2. test on table, bends upwards and using feeling gages --> possible, but very time consuming
3. V blocks and dial indicator: we sometimes use this set-up, but the highest point of the bend is not always in the middle of the bar

Here is one link for an Straightness measuring instrument..

http://www.mitutoyo.co.jp/eng/products/gaugeblock/kijunki_01.html


If you have CMM, You can check it there..

Sathish

Tha standard does not require that we test each bar using this method. We have to guarantee the straightness if measured with this method. Up to us to proof that our process is capable to guarantee this straightness and that the inspection method we use, sorts out the nonconforming bars.

You'll have to use the standard method as a "referee." It will be helpful if you can demonstrate that an alternate method is reliable in predicting that questionable bars will be accepted/rejected using the method in the standard.

We measure straightness on our steel tubes (diameters as low as 0.500") using a table we made with v-blocks to support the tubes and a dial indicator to measure the variation.

In the past we also used the surface table and feeler gages, but this method we found to be more time consuming and far less reliable than the dial indiactor.

We measure straightness on our steel tubes (diameters as low as 0.500") using a table we made with v-blocks to support the tubes and a dial indicator to measure the variation.

In the past we also used the surface table and feeler gages, but this method we found to be more time consuming and far less reliable than the dial indiactor.

Does your customer utilize the same inspection method/measurement system?
If not, there could be trouble as a result of "receiving inspection" or other verification method. Many problems between customer and supplier exist because of differences in interpretation of standards, methods, etc.

Stijloor.

Does your customer utilize the same inspection method/measurement system?
If not, there could be trouble as a result of "receiving inspection" or other verification method. Many problems between customer and supplier exist because of differences in interpretation of standards, methods, etc.

The ones who actually care enough about the straightness to measure it at receiving inspection (and I'm not sure there are any) use the same methods we use. Honestly for most it is simply an afterthought and the only time straightness becomes an issue is if the tube won't fit all the way over the mandrel when they go to fabricate it. That happens to be for one customer where the tube isn't round so we have a jig I designed to check longer pieces of tube and the customer has provided us with two of their older mandrels to use as a "for reference only" go/no-go check for tube straightness. If we were to need actual numbers, we'd have to use the surface table and feeler gages for this product, but it is not required by the customer.

Just a thought but you could technically verify visually the straightness by rolling the bars/tubes on a surface plate.

Much like when you pick up a Pool stick before playing billiards, you always roll the stick on the table to find the sticks that aren't straight.

Just ta thought in trying to keep it simple.:bonk:

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.

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.

Fabricating such an attribute-type inspection device would be an enormous technical/metrological challenge considering the OP's tube/bar specifications. Look at Post #1.

Stijloor.

Fabricating such an attribute-type inspection device would be an enormous technical/metrological challenge considering the OP's tube/bar specifications. Look at Post #1.

Stijloor.

Oops, Just re-read the length @ 12 feet. Youre right Stijloor:bonk:

In the case of the length being that long and diameters <1 inch with aluminum type material there will be some inherant deflection within the parts.

I would ask your customer what their expectations are in regard to the tolerances, or even the end use/purpose of the tubes. Often when designing equipment the design data looks good on a model, but in application it's often redesigned to allow for improvements discovered somewhere in the overall process.

There really isn't enough information to provide a good answer. Is the straightness of the axis desired, or does the straightness apply to the surface? If it is the axis, then V-blocks are a poor choice. Use a precision collet.

If this applies to the surface then precision V's would be acceptable. Consult your customer. To eliminate the variation is size, take several readings along the surface of the underside (the side closest the inspection table) of the rod. The maximum deviation from the lowest point is its straightness.