The very compact pushbutton switch challenge

[studiovap]

I'd like some advice on push button switch design, its all about tactile switch actuation force, surface area of buttons, piston and bore materials, friction on slider and slid against materials, wear and corrosion, moisture and expansion of materials, ratio of diameter of piston to length of contacting bore or stroke.
Is any of this making sense to anyone?
Let me put it another way, I want your feedback experience and expertise applied to a problem to give me some ideas for a upcoming design I have in mind.

I have drawn up five or six designs of the switch/button in sketckup, but I'm not 100% happy with any, and before I commit to a design I would really appreciate some advice and ideas that don't come from my tired brain

I have googled my posterior off over this and haven't found anything useful, so I am asking you wise and gifted folks at the pointy end.

Ok here goes:
Say you had a stabilized wood box and you wanted to incorporate a nice pushbutton switch for the fire button, the wood is only 7mm thick and you want a brass button exiting the box around 10mm in diameter, you have the option of 150-250gf tactiles ranging from 1.5mm to 3.2mm high and 3x3mm to 7x7mm footprint. you want the button action to be nice and linear without rocking or binding by having a piston in bore type actuator for the tactile underneath.

The tactile switch under the brass piston needs to be contained by probably at least 1.2-1.5mm of wood at the back to stop the pusher pushing the piston and the tactile into the box cavity.
You also want the machining /milling of parts to be kept to a minimum to control time and cost involved.
So some questions are:

1.Is stabilized wood stable enough to be used as the bore for the piston to run in, or will it swell trapping the piston or rapidly wear causing piston slop and binding?

2. Will the brass corrode with time and juice spillage causing binding and premature wear of the wood bore?

3.Is a brass sleeve inserted into the wood bore a satisfactory solution, or will corrosion again cause a demise of fluid motion.

4. Would a delrin or other engineering plastic sleeve handle the brass piston in a better fashion, or would wear be an issue again.

5. What is a workable ratio of the piston travel/bore/stroke ect to the diameter of the piston to avoid slop/binding, considering well thought out interference tolerances etc.

All ideas and thoughts will be lapped up like a thirsty puppy

 

[asdaq]

Studio, I've had good experience with running brass actuators through wood, finished with linseed oil and beeswax. Probably the key here is the beeswax, as it protects and lubricates. Also the more polished the brass, the less of a problem you will have with corrosion. The friction of pressing the button helps keep it clean too.

I'm using 1.5mm and 3.5mm actuators in curly maple and koa, but with stabilized wood it is even harder. For tolerances, I would recommend as tight as possible, and take off a little brass from there. Slop just isn't nice with buttons.

 

[studiovap]

Here are a few screen grabs from sketchup,please excuse my lack of experience with the program, still learning it's many unintuitive ways the materials are not so much the point of the drawings, but the poston/bore ratio and the use of available space. I have gone with an extended bush/bore on these top two to maximise bore to piston diameter ratio without increasing the thickness of the stabilised wood wall. the 3mm or so under the bush is for a CK tactile with 160-240gf.
the grey image is a more recent Idea to have the piston lot right over the tactile to again improve piston dia to bore ratio. I keep thinking I must be missing something elegantly simple as a solution.
I wanted to keep the button looking brass in appearance, although I have just though of torching the piston to achieve a golden colour if I had to go with a brass/SS combination, still churning ideas over so thanks in advance for any more insights.

[/URL]Button design by Thomas Studios, on Flickr[/IMG]

 

[studiovap]

Studio, I've had good experience with running brass actuators through wood, finished with linseed oil and beeswax. Probably the key here is the beeswax, as it protects and lubricates. Also the more polished the brass, the less of a problem you will have with corrosion. The friction of pressing the button helps keep it clean too.

I'm using 1.5mm and 3.5mm actuators in curly maple and koa, but with stabilized wood it is even harder. For tolerances, I would recommend as tight as possible, and take off a little brass from there. Slop just isn't nice with buttons.

Thanks for your feedback asdaq, I am looking at considerably larger dia actuators, although you will see from the above pics that one design uses a stepped actuator with a 4mm dia in the smaller shaft.

 

[asdaq]

I see you got many more replies in the other thread, I like the dedicated topic here though. With diameters like you have, there is really no need to have the button stand proud of the housing, the travel is minute on a tact and your finger will certainly find it.

 

[Tbev]

i would think that if you run a brass or copper button it is going to get corrodedm thats just hoe that material works. especially if you run it in wood. i dontknow how much th ewood is going to swell\contract... if you were to go with an aluminum button for example, you would get rid of half of your troubles. if you went with a tube insert that had the button sliding inside of it you would be heading in the right direction imo. . .
your also going to have an issue, over time, of the metal button wearing against the wood housing. ..again not my area of expertise, i dont deal with wood personally but i wood imagine, haha, that to be an issue.

if you would like I would be happy to whip up a couple simple buttons for you to try out. take a look at my mods and see how my buttons work, if i were you i would do a button like mine that rested on top of a tactile switch or somethingm whatever your gonna wire up. you could get enough return from the button to be fine. i have half in brass copper and aluminum that i can turn down on a lathe to any lenght and diameter youd like. good luck bud.

 

[turbocad6]

I'm not so sure why you need a precise bore and any kind of serious stroke here because the tact switches have a tiny fraction of a mm travel so it's not like there is any kind of significant stroke, plus the amount of pressure needed is also minimal. this is how I do my tacts and my buttons have almost no rocking when trying to rock it, no real losness and def no binding, the amount of travel is maybe the thickness of a hair. the trick to having the buttons not loose is a fairly precise fit to the bore ( I do leave it slightly less than snug though) and having the tact rest right up against the actuator button so there is no play... I use this thing in the shower every day and wet it often and never have any binding or losness issues nor sticking buttons, no issues at all. tacts are held in the bore with a bit of epoxy, doesn't take much because again, doesn't take much pressure to actuate the switch really

 

[turbocad6]

here's a way to do this without a separate switch cover but would need to fabricate the threaded collar instead, woud give a more conventional switch type look

 

[studiovap]

Hi turbo, thanks for your thoughts. The main reason the length of bore is important to me is not due to the need for lots of travel it's to allow the use of a large diameter button. As we know the further you get from the equal bore to diameter ration and move to a shorter bore the more risk of binding and or slop. To take it to an extreem, an 18mm button face with a 4mm bore is going to be a recipe for disaster. I am merely trying to see how large a button I can go for and still maintain a silky action over the 0.3-0.5mm of travel. My MOSFET wood box posted a week or so ago used the exact same setup as your second drawing, but without the tread and just a glued in collar, but the button is only about 6mm diameter, I have considered a threaded insert collar, but it would have to be spanner drilled to be removable (which is a desirable quality) and along with the button retainer lip, the threading of the collar and the spanner drilling all makes for a very work intensive design for each button unit. I will take onboard all your comments though and add them to my own meandering ideas. Thanks again

 

[studiovap]

i would think that if you run a brass or copper button it is going to get corrodedm thats just hoe that material works. especially if you run it in wood. i dontknow how much th ewood is going to swell\contract... if you were to go with an aluminum button for example, you would get rid of half of your troubles. if you went with a tube insert that had the button sliding inside of it you would be heading in the right direction imo. . .
your also going to have an issue, over time, of the metal button wearing against the wood housing. ..again not my area of expertise, i dont deal with wood personally but i wood imagine, haha, that to be an issue.

if you would like I would be happy to whip up a couple simple buttons for you to try out. take a look at my mods and see how my buttons work, if i were you i would do a button like mine that rested on top of a tactile switch or somethingm whatever your gonna wire up. you could get enough return from the button to be fine. i have half in brass copper and aluminum that i can turn down on a lathe to any lenght and diameter youd like. good luck bud.

Thanks for the suggestions and the offer mate, I will keep it in mind. I have great respect for you bombproof squokers