Best practise for tolerances?

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SBS_Pilot
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Joined: Sat Apr 22, 2017 1:00 pm

Best practise for tolerances?

Post by SBS_Pilot »

First I want to say Thank you to all who have contributed to this excellent program.

I'm not a very advanced user and am (slowly) learning 3D-cad so there are many concepts that I still don't grasp. At the moment I'm making frames/holders for servos to my radio controlled airplanes. I can measure the servo measurements to well within +/- 0.1mm, and I can produce a crude but working freecad-model including paths for my CNC-router, but sometimes I need to have a little bit of space to allow for "wiggle room" (as opposite to press fit) when mounting the servo to the frame. What is considered best practice for that? Add (or subtract) the wiggle room to the measurements of the servo frame? Or is that where the "Extra Offset" and "Pass Extension" parameters are used?

Please excuse if this question has been asked before, but my English is too limited to find the correct search terms to find them.




OS: MX 15
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.17.9758 (Git)
Build type: Unknown
Branch: master
Hash: f3f7b01202646d7103961ba24063658a9f72e650
Python version: 2.7.9
Qt version: 4.8.6
Coin version: 4.0.0a
OCC version: 6.7.0


Link to what the servo looks like: https://www.hoelleinshop.com/RC-compone ... =187&p=187
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sliptonic
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Re: Best practise for tolerances?

Post by sliptonic »

I don't know if there are 'best practices'. I'm not a professional engineer nor do I have a formal education in CAD/Design. So others might have a better answer or point us to more formal conventions.

I prefer for the design to reflect the ideal part that I would like to make. I use the extra-offset to account for real-world effects of manufacturing and to fine-tune the result.

For example, when cutting wood, the cutter tends to compress the wood fibers slightly before cutting them. The cut fibers then absorb moisture from the air and swell. Getting an accurate and precise cut in wood is challenging so trial and error is often needed to get the desired result.
chrisb
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Re: Best practise for tolerances?

Post by chrisb »

sliptonic wrote:I don't know if there are 'best practices'. I'm not a professional engineer nor do I have a formal education in CAD/Design. So others might have a better answer or point us to more formal conventions.

I prefer for the design to reflect the ideal part that I would like to make. I use the extra-offset to account for real-world effects of manufacturing and to fine-tune the result.

For example, when cutting wood, the cutter tends to compress the wood fibers slightly before cutting them. The cut fibers then absorb moisture from the air and swell. Getting an accurate and precise cut in wood is challenging so trial and error is often needed to get the desired result.
I would second that. When I started with CNC I got the hint from an unbelievable experienced machinist to write my GCode programs for the part as it should be, and add or subtract the clearings by fooling the machine with pretending to have different tool diameters.
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SBS_Pilot
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Joined: Sat Apr 22, 2017 1:00 pm

Re: Best practise for tolerances?

Post by SBS_Pilot »

Thank you, I'll try that.
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