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Micro Magnum, Team Magnum, NSW
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Eidg



Joined: 27 Nov 2015
Posts: 3
Location: Switzerland


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Well it is indeed really important to set tolerances, if they are important for the function.
I alway send a .step or .dxf dataset with the parts.
And of course your drawing shuold be, as clear as possible, especially if you send the parts to china or so.
The machinists at a proper company, in west europe, normally nows the norm tolerances, which for example are used for holes etc...

Post Thu Jan 11, 2018 7:59 am 
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MoonSet416



Joined: 25 Sep 2016
Posts: 375
Location: Sydney


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Thanks for the info.

I do have a question though... Is tolerance a part of the .step or .dxf files, or do you have to specify it somewhere else?

Post Thu Jan 11, 2018 9:31 pm 
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Glen
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Joined: 16 Jun 2004
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Hey, Eidg has good advice.

Depends on the shop and the part. Simple parts only need a drawing generally as they are done using conversational programming at the machine.

Complex parts need the step or iges file so they can use CAM to generate the toolpaths.

If the part has tapped holes you need to specify the thread and how deep it goes (not the hole depth usually), a bearing bore needs a tolerance, h7 for example and shafts too (H7 lets say). Keep it simple - having too many dimensions and tolerances everywhere makes it very confusing.

For 99.999% of robot wars stuff just specify h7 on your bearing bores and the tapped hole thread and depth and the parts should be okay. I think this machine shop you use is no good! very very rare to get parts so dramatically poor from an actual machine shop. At worst maybe some sizes are a bit off or they forget to tap one or two holes or something..
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Post Thu Jan 11, 2018 10:13 pm 
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Eidg



Joined: 27 Nov 2015
Posts: 3
Location: Switzerland


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Normally i draw the model centerline, for example 10 +/- 0.05,
except from special tolerances as h7/H7 like Glen said before.
Depending on which cad you use, there are several options.
I ALWAYS sending a drawing, including all measuremets and descriptions like materials, aditional instructions and general tolerances, at pdf format.
The reason why i do this is really simple....
If you have problems with the parts, and there are out of tolerance, you can verify with the drawings whats wrong.

Post Thu Jan 11, 2018 10:56 pm 
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Nick
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Joined: 16 Jun 2004
Posts: 11757
Location: Sydney, NSW


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I agree - a PDF drawing with dimensions and notes is a cheap insurance policy.
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Post Thu Jan 11, 2018 11:44 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 375
Location: Sydney


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Thanks guys.

EDIT: Another question I forgot to ask: what kind of thread depth (and the relation between thread depth and actual hole depth) do you usually specify? I noticed in Solidworks that it will tend to automatically have a very deep hole but relatively shallow threads. Does it have anything to do with ease of machining or something like that?

Post Thu Jan 11, 2018 11:50 pm 
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maddox



Joined: 21 Dec 2006
Posts: 769
Location: Belgium


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Yes, it all has to do with ease of machining and strength of thread.
M6 needs X*6mm thickness to get full strenght.

Guideline for thread-depth.
Steel, titanium, stainless 1 time diameter strenght
Aluminium, good grade, 2 times
Brass and bronze 1.5 times
PA6 (glassfibre filled nylon) 3 times.

Post Fri Jan 12, 2018 12:29 am 
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Nick
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Joined: 16 Jun 2004
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Location: Sydney, NSW


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Having a deep hole compared to the depth of the thread makes threading slightly easier and more reliable as there is somewhere for the chips to go as the tap cuts the thread. That's more important for a gun point tap (my favourite type) and let's the machinist cut the thread in one go without clogging up the tap's flutes and possibly snapping it.

Another possible reason is so that a starter tap (which has 7 to 10 tapered threads at the tip) can be used to cut the entire thread. That means there will be up to 10 partially cut threads at the bottom of the hole.
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Post Fri Jan 12, 2018 7:42 am 
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MoonSet416



Joined: 25 Sep 2016
Posts: 375
Location: Sydney


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Ah I see... Thx.

Post Fri Jan 12, 2018 3:06 pm 
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