Do digital linear scales stretch?

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Discussion Overview

The discussion revolves around the effects of temperature changes on digital linear scales, specifically focusing on whether such scales can be damaged when subjected to thermal expansion in different materials, such as aluminum and granite. Participants explore the implications of thermal expansion, material properties, and mounting techniques in the context of precision measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions if a digital linear scale attached to aluminum and subjected to temperature changes will be damaged, particularly when calibrated with a laser interferometer.
  • Another participant suggests calculating the compressive force from heating to determine if it exceeds the elastic limit of the materials involved.
  • Thermal expansion differences between aluminum and granite are noted as potentially significant in the mounting of the scale.
  • A participant emphasizes the importance of defining what is rigid in the experiment and the effects of thermal expansion on connected materials.
  • Concerns are raised about the assumption of linear expansion based on a specific temperature change, with a participant questioning whether this assumption holds for different lengths of aluminum samples.
  • Discussion includes the need to consider the properties of aluminum alloys, including changes in crystal structure and the effects of thermal cycling.
  • Participants inquire about the type of scale and its mounting method, with specific mention of Renishaw metal tape scales.
  • Concerns are expressed about the implications of bolting stainless steel to aluminum, particularly regarding their differing coefficients of thermal expansion.
  • One participant argues that the mounting instructions should ensure the scale's proper functioning, suggesting that it should only expand relative to its zero point.
  • Questions arise about the main concerns regarding the mounting and thermal expansion effects on measurement accuracy.

Areas of Agreement / Disagreement

Participants express differing views on the implications of thermal expansion and mounting techniques, with no consensus reached on the potential for damage to the scales or the adequacy of the mounting methods. The discussion remains unresolved regarding the best practices for ensuring measurement accuracy under thermal changes.

Contextual Notes

Participants highlight the need for careful consideration of material properties, mounting techniques, and thermal expansion coefficients, but do not resolve the complexities involved in these interactions.

Ranger Mike
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Hello friends, i hope this is correct forum.
If digital linear scale with a resolution of 0.1 micron is attached to a very flat piece of aluminum, at 68 degrees F, and is calibrated with a laser interferometer, and is then exposed to a slow increase of temperature to 100 degrees F, then permitted to cool to the 68 degrees F temperature, will the scale itself be damaged?
If this process was repeated with a piece of hand lapped granite , what would be the result?
what long term effects will the heat process have done?
 
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The way to solve the problem is to compute whether the compressive force that results from the heating compresses the material beyond its elastic limit.
 
i was thinking thermal expansion would effect the scale mounting. aluminum vs. granite thermal expansion.
 
You need to define what is really rigid in the experiment and what is allowed to thermally expand.

In thermal expansion "damage" problems it's the difference in expansion between things that are rigidly connected that is important.

You cannot damage anything that is only connected at one end (as your problem defines it). You need the scale to be squeezed or pulled between two plates. What is connecting the two plates and are the connections subject to thermal expansion also.
 
If we have a 100 mm long aluminum sample and change the temperature 5 °C (9 °F), it is observed to grow 25 micron in length. Can it be assumed if we had a 1000 mm long sample it would grow 250 micron or is this a dangerous false assumption?
 
You can assume things are linear as a first approximation. Some aluminium alloys change crystal structure and hence dimensions when thermally cycled.

The problem is that many metals need to be age cycled over the operating temperature range before they will give consistent results. Study the annealing and work hardening characteristics of the alloy. It is also important to look at a phase diagram for the alloy and determine if you are in the vicinity of, or crossing a transition.
 
What kind of scale is it? glass? PCB? Metal tape? How is it mounted to the aluminum or granite?
 
renishaw metal tape scale mounted directly on granite. in next case , renishaw metal tape scales on renishaw fast track stainless steel mounts then bolted to aluminum
 
Ranger Mike said:
renishaw metal tape scales on renishaw fast track stainless steel mounts then bolted to aluminum
When you bolt stainless steel to aluminium, what coefficient of expansion do you expect would apply ?
 
  • #10
baluncore..that is my concern..stainless steel has a low expansion rate and the scale is anchored at one location only so it can expand and the aluminum machine is referenced to these scales for positioning...i think it is a poor engineering idea. two dissimilar thermal expansion materials and growths
 
  • #11
As long as you follow the mounting instructions for mounting from Renishaw, you should be fine. It sounds like you are using them for your intended purpose.

The scale is your length reference, it should only expand about one point, that is your zero point. The stainless scale helps compensate for growth of the aluminum frame. The scale should not expand with the machine, it should be allowed to float relative to its zero point. Then you can calculate temperature based position error based on the stainless CTE alone.

What is your main concern?
 

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