Mechanical Arithmetic giving me a headache

[ruffrecords]

I am trying to position some ALPS push button switches on a PCB so the key cap protrudes the right amount through the front panel but the mecahnical drawings are some convoluted it is driving me bonkers trying to work it out. Here's the switch:

http://uk.farnell.com/alps/spuj190900/switch-2pole-latching/dp/1123857

and its data sheet:

http://www.farnell.com/datasheets/85579.pdf

So I know the distance from the front most pin (my zero reference for the PCB footprint) to the front of the switch is 7mm and from there to the front of the plunger is 9.8mm, a total of 16.8mm. From there you need to go back 4mm I think to get to the face of the knob (key cap) (which is actually the back of the cap not the front face). So the back side of the key cap is 12.8mm from pin 1.

Now it looks like the shaft size is 2.8mm (that's the width of the plunger that the key cap fits onto) so I need to use the mini key caps like this one:

http://uk.farnell.com/alps/pb0151000g/cap-mini-grey/dp/1123891

and its data sheet is here:

http://www.farnell.com/datasheets/301064.pdf

where is says it is for shaft size 2.8mm. This key cap is 11.2mm high so the font/top of the key cap is 11.2 + 12.8 = 24mm from pin 1. The switch data sheet says the difference between the switch latched and unlatched is 2mm but it goes in 3mm before returning so I need to allow for 3mm of travel. So lets make the key cap stick out 5mm when unlatched. The front panel is 2mm thick and there is a 2mm gap between the front face of the PCB and the rear face of the front panel so the PCB front is 2 + 2 + 5mm = 9mm back from the front of the key cap. Since pin1 is 24mm from the front of the key cap then pin 1 must be set at 24 - 9mm = 15mm back from the edge of the PCB.

It has taken me all day staring at data sheets and drawing sketches to work that out. Can someone please check my figures before I commit this to a PCB.

Going for a lie down now.

Cheers

Ian

 

[gemini86]

Did this exact thing with the 9K2 pcbs... let me find my notes.

 

[ruffrecords]

Did this exact thing with the 9K2 pcbs... let me find my notes.

Brilliant. I was hoping someone had already done this.

Cheers in advance,

Ian

 

[gemini86]

Well, the math seems correct...

I tried measuring my board, and the dimensions are spot on, but I used a 9.06mm key cap, so it's shorter than yours, also my board goes directly to the back of the face plate (I'm assuming you're using your eurocards which have a bit of a gap).

What made this whole process easier for me, was using google sketchup. Drew it all up based off data sheets and laid it all out, that let me double check measurements and stuff.

Also, does your pcb layout software not have a dimensions tool? I often use that for locating critical components like this...

 

[ruffrecords]

Well, the math seems correct...

I tried measuring my board, and the dimensions are spot on, but I used a 9.06mm key cap, so it's shorter than yours, also my board goes directly to the back of the face plate (I'm assuming you're using your eurocards which have a bit of a gap).

Great! Thanks for confirming my arithmetic.

What made this whole process easier for me, was using google sketchup. Drew it all up based off data sheets and laid it all out, that let me double check measurements and stuff.

Also, does your pcb layout software not have a dimensions tool? I often use that for locating critical components like this...

Placing the part is no problem. Not sure what you mean by a dimensions tool but the package I use does allow you place the part simply by entering its coordinates. It is knowing where to place it that gives me the headache.

Thanks for the help.

Cheers

Ian

 

[Andy Peters]

Placing the part is no problem. Not sure what you mean by a dimensions tool but the package I use does allow you place the part simply by entering its coordinates.

Does your tool not have a measurement or dimension tool, where you can draw the usual arrows and dimensions on a mechanical or other layer?

Anyways, last time I ran into the problem you've described, I figured, "Oh, I guess I should try drawing this in a mechanical CAD package."

So I found a couple of demo 3D CAD packages, imported the STEP or IGES or whatever file it was (for a Neutrik connector) and then realized that I have no idea how to use any mechanical CAD package, and gave up. My layout tool lets me place dimensions (on a separate mechanical or assembly-info layer) and that seemed sufficient. My PCB layout footprint for the connectors also included some mechanical-layer info, like "edge of PCB" and "back of front-panel metal" so as long as the mechanical design was correct.

-a

 

[CJ]

make sure the switch action will not damage the board,

seen a lot of failures where something mechancal is attached directly,

mostly pots,


and tube sockets,

anybody who designs something with tube sockets on a pc board is either lookin to make a quick buck or lacking in engineering  intuition,

too much heat, expansion and contraction, long term failure, guy is out of town before it breaks,

maybe run some flexible wires to the push button switches so the constant flexing of the pc board does not cause the traces to develop hairline cracks that become a nightmare to troubleshoot,

if you are the only person to use the equipment, then no problems,

some of these places they let the musician into the control room which is always trouble,

unless you  are hendrix and want all along the watchtower to sound...




different,

 

[gemini86]

Not sure I see things your way there, cj...

Pcbs are pretty robust and well placed stand offs and other supports take care of breakage. A good design will make sure the traces are not the only thing supporting the sockets or switches, making sure the hole sizes are correct...etc. More specifically to these switches, I thinks the mechanics of the switch would break well before the pcb could suffer mechanical failure, so long as the hole sizes are correct, not too large to allow the solder to be the only support.

 

[ruffrecords]

Placing the part is no problem. Not sure what you mean by a dimensions tool but the package I use does allow you place the part simply by entering its coordinates.

Does your tool not have a measurement or dimension tool, where you can draw the usual arrows and dimensions on a mechanical or other layer?

Pass. I have never had a need to add mechanical dimensions to a PCB layout. I'll find out if my package (FreePCB) has such a feature.

Anyways, last time I ran into the problem you've described, I figured, "Oh, I guess I should try drawing this in a mechanical CAD package."

So I found a couple of demo 3D CAD packages, imported the STEP or IGES or whatever file it was (for a Neutrik connector) and then realized that I have no idea how to use any mechanical CAD package, and gave up.

Yes, I don't fancy having to learn a complex drawing package to do something that takes a few minutes on paper.

My layout tool lets me place dimensions (on a separate mechanical or assembly-info layer) and that seemed sufficient. My PCB layout footprint for the connectors also included some mechanical-layer info, like "edge of PCB" and "back of front-panel metal" so as long as the mechanical design was correct.

Which package do you use?

Cheers

Ian

 

[gemini86]

Sketchup has to be the simplest thing I've ever encountered...

It takes my maybe all of 30 minutes to mock up an entire 51x module, including front panel, pcb and any critical switch positions.

I use this in conjunction with the dimension tool in kicad to align objects where they need to be, that way I can double check my math at the same time.

 

[Vince@Speck]

Now it looks like the shaft size is 2.8mm (that's the width of the plunger that the key cap fits onto) so I need to use the mini key caps like this one:

http://uk.farnell.com/alps/pb0151000g/cap-mini-grey/dp/1123891

and its data sheet is here:

http://www.farnell.com/datasheets/301064.pdf

Double check the button "data sheet" -vs- "part number".  Your button shows a Alps part number but the data sheet is for Sifam buttons.  I know the switch / button combination well.... we use lots of them.

Vince Poulos
Speck Electronics 

 

[gemini86]

The part number does go with the second item down on the datasheet... 0151(000)

 

[ruffrecords]

Sketchup has to be the simplest thing I've ever encountered...

It takes my maybe all of 30 minutes to mock up an entire 51x module, including front panel, pcb and any critical switch positions.

I use this in conjunction with the dimension tool in kicad to align objects where they need to be, that way I can double check my math at the same time.

Whilst I do not doubt what you say, unless the user interface of Sketchup has changed radically, I find it the most counter intuitive tool I have ever come across. I wish it were otherwise because I would love a real easy to use 3D drawing package.

Cheers

Ian

 

[ruffrecords]

Double check the button "data sheet" -vs- "part number".  Your button shows a Alps part number but the data sheet is for Sifam buttons.  I know the switch / button combination well.... we use lots of them.

Yes, I noticed that at the time. The listing is definitely for an ALPS part but the data sheet is Sifam. I did not know Sifam did buttons. Looking on the ALPS web site, the onlu show a small round key cap for this switch so perhaps the do not make rectangular key caps.

I have ordered a few so I'll report back my findings.

Cheers

Ian

 

[gemini86]

Sketchup has to be the simplest thing I've ever encountered...

It takes my maybe all of 30 minutes to mock up an entire 51x module, including front panel, pcb and any critical switch positions.

I use this in conjunction with the dimension tool in kicad to align objects where they need to be, that way I can double check my math at the same time.

Whilst I do not doubt what you say, unless the user interface of Sketchup has changed radically, I find it the most counter intuitive tool I have ever come across. I wish it were otherwise because I would love a real easy to use 3D drawing package.

Cheers

Ian

While this is a whole other discussion, I'm struggling to see how anyone could think it's not the easiest to use cad... Click one corner of a box, move the mouse a bit, then just start typing dimensions; 5mm,5mm- that gives you a 5x5mm square... use the push/pull tool to click on the square's face and move the mouse again, then type 5mm again, and you get a 5x5xmm box. You can then draw other objects onto the square faces in this fashion to form a complex object...

But that's neither here, nor there.

I do know that all the 2.8mm caps and keys fit all the 2.8mm switches, so they're all standardized to some degree.

 

[zayance]

When i got into sketchup, it seems very easy at first glance of the videos i saw etc..., but when using it i got caught into
some weird situations, for getting answers of where i was stuck i was checking out Video tutorials on Youtube, there is a
lot of them for sketchup, and now find my way out to it, it's pretty easy, but as all programs out there, you need to get
the overall idea and practice to make full use of it...
Now for somebody coming from Solidworks, he would maybe find this program to be a little odd and vice versa...

 

[ruffrecords]

While this is a whole other discussion, I'm struggling to see how anyone could think it's not the easiest to use cad... Click one corner of a box, move the mouse a bit, then just start typing dimensions; 5mm,5mm- that gives you a 5x5mm square...

There you go you see. I can draw a rectangle in Sketchup but you cannot just type in dimensions. I fall over at the first fence. I can't even find a menu option that let's you do that.

Cheers

Ian

 

[volker]

You start drawing a rectangle in the direction you want it to be. Before you would set the end node, just type the dimensions and press enter afterwards, format is "axis1;axis2". You will see the numbers in the lower right corner before the confirmation.

Watch some of the tutorials, it's quite intuitive once you have the very few basics.

 

[gemini86]

That's the thing that I love, no menu clicks to get to the simple things...

I always enable snapping to corners and such, which makes aligning things cake.

 

[Andy Peters]

Sketchup has to be the simplest thing I've ever encountered...

It takes my maybe all of 30 minutes to mock up an entire 51x module, including front panel, pcb and any critical switch positions.

I use this in conjunction with the dimension tool in kicad to align objects where they need to be, that way I can double check my math at the same time.

Whilst I do not doubt what you say, unless the user interface of Sketchup has changed radically, I find it the most counter intuitive tool I have ever come across. I wish it were otherwise because I would love a real easy to use 3D drawing package.

Sketchup made me angry. I guess I really am not supposed to use a mechanical CAD package.

-a