G-Wizard’s New Thermal Expansion Calculator

I was paging through new posts on Practical Machinist and came across a discussion of thermal expansion. Since it looked like something useful, and something I wanted to experiment with, I whipped up a Thermal Expansion Calculator for G-Wizard:

G-Wizard’s Thermal Expansion Calculator…

It’s pretty simple to use:

Select a material, and it will enter the Coefficient of Thermal Expansion for that material. Keep in mind the note at the bottom–it uses the average across many alloys. For more precision, look up the exact coefficient for your material and enter it. I’d wait to see what the average reveals and if its within a factor of 2x or so of worrying you, look up your exact material’s coefficient.

Next, pick a temperature unit system, Fahrenheit or Celsius. Enter your Reference and Target temperatures. The standard temperature for measurements is 20 degrees C, and if you push the “Std Ref Temp” button that is what you’ll get.

Lastly, enter a length and/or a radius.

Press Calculate and G-Wizard will tell you how much the temperature differential between the Reference and Target will move the material in length or radius.

Let’s consider a simple problem you might use the calculator for. Too much bearing preload drives up temperatures, and in the worst case, it leads to thermal runaway which will destroy the bearings in a hurry. They get hot, they expand, that increases the preload, which raises the friction, which makes them hotter, so they expand more, yada, yada.

We can use G-Wizard to see how much expansion we’re talking about. The screen shot shows the scenario. Imagine we want to run our spindle bearings up to the point where they’re at 140 degrees. I’ve come across a number of references calling for this as a good goal or maximum for bearing temperature. Less and you don’t have as much preload as you could. More, and you may have the thermal runaway situation or break down your bearing grease.

We can see that for a spindle 12″ long and 2″ in diameter, if we assume we’re starting from the reference temp and running up to 140 degrees, we will see that spindle grow 0.006″ longer and 0.001″ larger in diameter. That’s pretty significant when bearing tolerances are measured in tenths!

Or, consider a CNC machine’s leadscrew. Let’s say it is 30″ long. It is precisely calibrated at the reference temperature, but we’re running the machine on a hot day and we’re spinning the heck out of that lead screw. So it creeps up to 98 degrees or so. How much longer did it get?

Turns out it grew about 0.0063″. Heck, even if you’re only machining a part that requires you to use 6″ of travel, that’s a difference of 0.0013″ in length that goes against the accuracy of your handwheels or of your calibrated CNC servos or steppers. That’s a lot of error!

Hence manual machines benefit from DRO’s that tell how far the axis really moved and CNC machines benefit from scales that are essentially DRO’s telling the controller the same thing. In some cases the CNC may rely on a temperature sensor to estimate, but the scale is a better solution because it tells how far the axis really moved.

 

Related posts:

1. G-Wizard Calculator Video Tour
2. Last Week of G-Code Calculator Beta Testing

4 Comments

• A show of hands..... - Page 4 - CNCzone.com-The Largest Machinist Community on the net! 2011/02/17 at 6:06 pm

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  Reply
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    Shriram 2011/07/03 at 8:18 pm

    Hi…. I would like to measure the temperature of the CNC using sensors for thermal expansion… I would like to know the best spots for placing the sensors…

    Reply
• A show of hands..... - Page 4 - CNCzone.com-The Largest Machinist Community on the net! 2011/02/17 at 6:06 pm

  [...] [...]

  Reply
  • 
    Shriram 2011/07/03 at 8:18 pm

    Hi…. I would like to measure the temperature of the CNC using sensors for thermal expansion… I would like to know the best spots for placing the sensors…

    Reply