Mounting Hole in Footprint (BoB)

[tarik]

Hi,

I’m doing some footprints for display break out boards.
The display boards have drill holes to get them fixed (In addition to connectors).
How do I include e.g. “MountingHole_3.2mm_M3” from the"MountingHole" library footprint to the new footprint?
I’m aware that in PCB Layout there is an icon “Add footprints”, but in footprint editor it seems not.
I of course could read out the properties of “MountingHole_3.2mm_M3” and create it in the display footprint, but I hope there is a smarter method - ?

Thanks for your ideas!

Tarik

[eelik]

Copy and paste works across footprints. Open one, copy, open another, paste.

[BlackCoffee]

This may be a more useful approach…

• Edit your library footprint
• Add the Hole shape (just draw a Circle with desired Diam/Radius) on another layer, such as Eco1.User. Save the footprint (it’s an editable text .mod file)
• Open the .mod file in any text editor (best to have font at UTF-8)
• Change the layer from (example) Eco1.User to Edge.Cuts. Use the names shown in your Kicad main panel ( some folks like to customize them). You can do a ReplaceAll (depending on your text editor). Save the footprint

• Now, use it

[tarik]

@eelik,

My first tries to copy between footprints failed. I always gave up at “Select Reference point” what I considered as very local and handling it wrongly, it really fails. Encouraged by your mail, I tried a tiny bit differently - it now works.
All things what confuses could be fixed, in footprint editor or sometimes more easy in .mod file, as BlackCoffee mentioned.

Thanks to you Two!
Tarik

[Russ]

Is there a rule of thumb for how close such an M3-sized mounting hole should be placed away from the board edge ?

[BlackCoffee]

Generally, the minimum is, 1.5 x Diameter

Thus, for M3: 1.5x3 =

EDIT: Just to complete the story… screenshot… M3 PanHead x 6

[paulvdh]

@Russ

Is there a rule of thumb for how close such an M3-sized mounting hole should be placed away from the board edge ?

1.5x the Diamater seems like a reasonable rule of thumb. Another way is to take the diameter of the head of your screws into account. Especially with bolts and the tool goes around the head.

A significant deviation of such a “rule of thumb” would be the M.2 cards for SSD and WiFi. For those the center of the screw is even placed outside the board.

But other factors are more important for mounting hole placement. Plugging stuff into connectors may exert high bending forces to a PCB (especially when (bad fitting) screwdrivers are used) and SMT resistors and capacitors are brittle and may break when the PCB is bent.

[Rene_Poschl]

@paulvdh

1.5x the Diamater seems like a reasonable rule of thumb.

That seems large to me. I mean for M3 the typical through hole size is 3.2mm and your suggestion would result in 4.5mm which is the size normally used for M4 (so one size up)

The largest i ever used for M3 is 3.5mm (mostly because i don’t always have a 3.2mm drill available)

Update it seems to be part of EN 20273:1992 / ISO 273:1979.
According to it there are three standard sizes for M3. It defines 3.2mm, 3.4mm and 3.6mm*. German source, German wikipedia article (I sadly could not find a similar source in english)

Note: the details of when to use which of the 3 suggested sizes is surely in the full version of the standard but i am not going to pay for that just for a comment here on the forum

[Russ]

I thought that an M3 was shorthand for a machine screw that fits into a 3 mm diameter hole.
EDIT:
No, I was wrong. Couldn’t be that simple. An M3 screw takes a 3.2mm hole on one and a 3.5 mm hole on the other end.

[BlackCoffee]

This is Not about tooting my horn - we all have great experiences… I hesitate posting this and wasn’t going to but, between Covid down-time and comments about screw/hole sizes, well, lucky you… Criticize me for posting it if you like, I’ll ignore it.

Yes, there are Industry and Military standards. Standards for nearly every material, loading, geometry, environmental conditions, etc and, many Phd’s spending countless hours working on this stuff and dissertation’s.

I have many books and standards to look at but, always reflect on this story…

I remember sitting in a Joint Military meeting 30 yrs ago (regarding a Nuclear Warhead) in a room with 4-feet thick concrete walls, reviewing a failure. Fifteen seasoned Phd’s sitting across from me and uniforms with stars & stripes sitting on my side of the table.

As the review progressed, my anxiety increased as I struggled with ‘How to tell all these really smart folks what was wrong’. For sure, all of them (plus many others) did reviews prior to the meeting. I couldn’t take it anymore so, I said, “I have a question about something back on page 1.”

I said what I had to say (it took 10 seconds), and the meeting was immediately called to an end and I was told to stay. Fifteen minutes later I was driving a rented car, following a Jeep with MP’s, sirens blasting and red-lights blinking to an air field to get on a Delta Airlines plane that had been re-routed just to pick me up. Why? Because I thought it best to not say ‘No’ when rewarded for saving 120 Million Dollar program.

When I say 1.5x, I say it with 50yrs of Engineering experience, knowing that every specific application/usage will require commonsense, good judgement and knowledge that One-Size does Not fit all…

Lastly, and don’t mean to offend… Perhaps, if anything, to encourage others, the most important words whispered into my young ear, when deciding what field of Engineering/Physics to pursue, was this; “Get a degree in Mechanical Engineering because you’ll learn enough about all other Engineering disciplines to enable becoming an expert.

[chschlue]

Funny, I was told something along the lines of “get a degree in EE, you can always pick up the easy stuff later as you go”.

Back on topic: I believe @paulvdh was referring to edge distance, not hole diameter.
Edit: @BlackCoffee, too.

[tarik]

I’m dealing with 2.8" displays for Arduino delivered from China.
The mounting holes were as much as possible moved to the corners,
distance to border is in a way that the screw head does not exceed PCB.
A good design I think takes into account expected mechanical forces.
To close to the border does not allow forces a screw could deal with, but reduces the force allowed to the limit the remaining small part of PCB can resist.
Although I think the Chinese design will resists all expected forces, the 1.5 rule of thumbs provided by BlackCoffee makes sense to me for general purpose.

Cheers!
Tarik

[Rene_Poschl]

@chschlue

Back on topic: I believe @paulvdh was referring to edge distance, not hole diameter.

Ah … That makes sense. I completely missed that.

[SembazuruCDE]

@BlackCoffee

Yes, there are Industry and Military standards. Standards for nearly every material, loading, geometry, environmental conditions, etc and, many Phd’s spending countless hours working on this stuff and dissertation’s.

Back off topic… This reminds me of the following XKCD comic:

Standards

My apologies if you have seen it before and have gotten tired of it.