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D_Harris
11-30-2010, 01:50 AM
I need to make some electrical contacts out of either .008"(32 gauge) to .032"(20 gauge) Phosphor-Bronze.(Or perhaps Beryllium-Copper instead).

Since I need thin strips I was looking for ideas on the best way to cut it. I was originally thinking along the lines of making some sort of jig and using something similar to a slitting saw, but without teeth, so I could score the metal and then break it off.

But this seemed too complicated and I figure that perhaps I should just use a sharpened HSS parting tool and simple jig for this.

If anyone happens to have any ideas/suggestions on a better way to go about this I'd appreciate it.

Thanks.

Darren Harris
Staten Island, New York.

winchman
11-30-2010, 02:33 AM
Assuming you need narrow strips of thin material, I'd make two clamps of the required length. Bevel the edges of the clamps so you can use them to bend the material back and forth with just a narrow gap between them. After the strip breaks off, dress the edges with a file.

darryl
11-30-2010, 03:09 AM
Maybe you don't have a shear- something like that is easily parted, though you have to have a sharp blade and back up the material so it stays flush to the table and doesn't try to fold instead of cut. The 008 you could probably cut with sharp scissors- that would be the first thing I'd try.

For the thicker material, you might be ok with sandwiching the material between sheets of hardwood, then bandsawing it. At least that would give you a way to position the material for an equal width strip each time. The edge is probably going to be a little messed up though- maybe the bottom piece of the sandwich could be steel. The top piece might have to be renewed for each cut because it acts as a speed moderator as you push it through the bandsaw. Without that action, you will probably get a fairly ragged cut edge on the pb. You will need a sharp blade to get the best results, even though you would find that it's easy enough to push the 'sandwich' through a duller blade. I think that any operation using a bandsaw is going to require that you subsequently grind or power sand the edges afterwards. Shearing will leave the best edge, but it will be very sharp so you would probably want to smooth it somehow anyway.

gmatov
12-01-2010, 12:30 AM
Why are you making them out of Phosphor Bronze or Beryllium Copper? They are both high resistance metals. Arcing will occur, and possible welding.

Are they very low current?

George

randyc
12-01-2010, 12:58 AM
I'm guessing that he needs "spring" characteristics. Those are both pretty good material choices for electrical conductors that require high yield strength.

Cheers,
Randy C

D_Harris
12-01-2010, 10:12 AM
winchman: I'm not sure what you mean by bevel the edge of the clamps, but whatever I come up with will be rounded at the edges.

darryl: Band-sawing or using shears will not work. The strips need to be too thin.

gmatov: Phosphor Bronze or Beryllium Copper are not considered "high resistance metals". Phosphor-Bronze is a standard electrical contact material.(I considered Beryllium-copper because of it's durability). And there will be no arcing arcing, because all of the connections will be break-before-make. Power is cut before the connectors pull apart or come together.

And outside of data the current through these will be along the lines of +5, +1, +12, +24, & -5V.

randyc: Yes, I'll need the contacts to be able to take some mechanical stress, and I've been trying to find out where to get spring temper: (C50700(Grade E) Phosphor-Bronze because I'll have to also solder 18g leads to each strip.

Thanks.

Darren Harris
Staten Island, New York.

Stepside
12-01-2010, 11:07 AM
I have cut Beryllium-Copper with a jewelers saw. I used the jewelers V shaped cutting support and a real fine blade and it was quicker and more accurate than trying to shear something that small. Sawing allows you to produce neat rounded ends on your parts as well.

derekm
12-01-2010, 01:26 PM
I have cut Beryllium-Copper with a jewelers saw. I used the jewelers V shaped cutting support and a real fine blade and it was quicker and more accurate than trying to shear something that small. Sawing allows you to produce neat rounded ends on your parts as well.
dremel or proxxon have thin circular saw blades. The one I have in my hand is only 0.005" thick 0.6" dia. Make a mount for a dremel on your lathe or mill

RTPBurnsville
12-01-2010, 02:04 PM
Remember to take safety precautions if grinding or being exposed to the arcing of Beryllium-Copper. Airborn particles of this stuff is not good for you!

Toolguy
12-01-2010, 02:09 PM
I've had good luck using a paper cutter shear for nonferrous shim stock and stainless steel heat treat wrap. It doesn't seem to dull the blades at all. You have a straight edge to keep things square and it's a very clean straight cut.

AlleyCat
12-01-2010, 03:00 PM
I've made electrical parts from thin brass, beryllium copper and similar materials by photo-etching the sheet material. Thicker materials are more difficult because you have to adjust the tooling films to compensate for the "etch factor".

Etch factor is the ratio of undercut to the etching depth. If everything is perfect you can get close to a 1:1 ratio on thin materials but not always on thicker materials. The best part about this process is the parts don't distort from mechanical processing like shearing and punching etc. There are no burrs to deal with either.

At times I've used a plating rectifier to "etch" the parts by making the sheet of material the anode (+) and plating to the stainless steel tank which became the cathode (-). Works pretty good with thin brass and copper. Either process may be something to look into.

Evan
12-01-2010, 06:12 PM
If you need precise dimensions and repeatability then etching is the way to go.

This is brass that was coated with very thin paint and then scored using my CNC mill. It was then etched in ferric chloride. Ferric chloride will etch copper, brass, bronze, aluminum and stainless steel.

http://metalshopborealis.ca/pics5/encoder4.jpg

KEJR
12-01-2010, 09:16 PM
FYI, you will want to [ideally] gold plate the contacts for low voltages. I try to use gold plated contacts for anything under 24V but that is probably a bit conservative. Nickel seems to work pretty good too.

darryl
12-01-2010, 10:01 PM
Sounds like you're making up a strip of contacts side by side, possibly .1 spacing. If so, your strips would be about half that- an assumption. Using .008 material, I'd have lots of strips cut already with scissors in the time it's taking me to type this reply.

More elegant is Evans method- etching. The thicker the material, the more it will undercut as the etchant works, but you should be ok with less than 10 thou thickness. The uniformity of your pieces will be directly dependent on your skill in scratching away the resist material to expose the metal to be eaten, leaving your parts behind.

If you go the etchant method, you are of course free to design your contact fingers and connection pads any way that suits, ala regular pc board design. You can epoxy some fiberglass cloth to one side of the phosphor bronze sheet, and that will be your 'custom' pc board. You can leave some of the metal uncoated and 'draw' your fingers there. Once etched, the fingers will be free of any backing, while the rest of the traces are fixed in place via the glass/epoxy. That's one way to get a nice neat row of contacts, individually flexible and spaced equally apart, etc.

You'll probably have to duplicate typical manufacturers techniques of using multiple contacts in a row to carry heavier currents. Phosphor bronze has in the past at least been chosen as a contact making material, but of course you can make a strip of it act like a fuse, like any metal. Personally I like the concept of multiple contact spots per 'tab'- each spot you add reduces the total contact resistance up to a point.

Hopefully I haven't veered too far away from what it is you're doing-

D_Harris
12-01-2010, 11:30 PM
Stepside: Those Jewelers saws look flimsy, and cutting accurately and consistently with it may not be possible. I'll need to make almost 800 contacts for just one of the itmes I'm making. It this works out then I may have to make quite a lot more.

derekm: I have a Dremel, but I need to just score the material, so slitting saws that have teeth will not work for me. And designing and building a holder and some sort of jig that allows repeatable results is just too much.

RTPBurnsville: Yes, I've read about the health issue that can result if one inhales Beryllium oxide or similar.

Toolguy: The contact width will not allow for me to use a shear. A jig for that would take some thinking anyway.

The fingerboards will have traces that are .156 wide, so that is the contact width I'm trying to get close to. A paper cutter definitely wouldn't work for me.

AlleyCat: It seems as though you are talking about pc boards. I'll be working with Phosphor-Bronze stock. And etching is at this moment outside of my abilities. (Unless I have to learn to make my own fingerboards, which I may have to).

KEJR: I don't see a need to plate because when the contacts make or break there will be no current. The reason for choosing Phosphor-Bronze is a good conductor, corrosion resistant, and flexible. Gold or Nickel plating would makes things way to complicated and out of my ability to do.

darryl: The end of the contacts will have to be soldered to fingerboards similar to this: http://www.therealbobroberts.net/splicer.jpg

The fingerboard will be seated in nylon, which the contacts will wrap around. The nylon will have grooves in the sides that will secure each contact.(I'm still figurng out how to create a "gang-saw" to create the grooves in the nylon).

And I still need to decide on a thin, flexible backing material(rubber?) to lay the contacts across.

Thanks.

Darren Harris
Staten Island, New York.

Evan
12-02-2010, 12:27 AM
I'll need to make almost 800 contacts for just one of the itmes I'm making. It this works out then I may have to make quite a lot more.

And designing and building a holder and some sort of jig that allows repeatable results is just too much.

Gold or Nickel plating would makes things way to complicated and out of my ability to do.


You either need to learn a few skills to do this or, if you aren't willing to do that then have somebody make them for you. You need at least 800 of these parts yet it isn't worth your time to make a jig that allows you to make them? what are you expecting for an answer? Throw the brass into the air and shoot it with bird shot, then collect the finished parts?

whitis
12-02-2010, 01:42 AM
You have not adequately described how these contacts are to be used but what you have said sounds an awful lot like a PCB edge connector. In that case, it would be far cheaper, easier, and more reliable to use or modify an off the shelf edge connector.

gmatov
12-02-2010, 01:55 AM
You know what you know.

"gmatov: Phosphor Bronze or Beryllium Copper are not considered "high resistance metals". Phosphor-Bronze is a standard electrical contact material."

Phosphor bronze has 18% the conductivity of Copper. Berylium is 9.32% the conductivity of Copper. Phosphorus, itself, is an insulator, to wit, 10 to the minus 17, for conductance. Silicon, an insulator, also, is only 10 to the minus 4.

Do as you wish, I am not smart enough to argue with you. I'd use something with silver in it, but it would cost a bit more.

Cheers,

George

whitis
12-02-2010, 03:16 PM
Phosphor bronze has 18% the conductivity of Copper. Berylium is 9.32% the conductivity of Copper. Phosphorus, itself, is an insulator, to wit, 10 to the minus 17, for conductance. Silicon, an insulator, also, is only 10 to the minus 4.


If it was 100ft long, that could be an issue. If it is an inch long, it probably doesn't amount to squat unless you are pushing so much current through it that it can't get rid of the locally generated heat. Lack of gold plating on the contact, insufficient contact force, contact geometry, surface finish, maleability, parallel contact redundancy, and other aspects of the contact design are likely to be more of an issue than the conductivity of the material.

Another sources put the conductivity of phosphor bronze at 25% to 50% of copper.
http://www.keytometals.com/Article79.htm
While another puts it even lower than your number:
http://www.copper.org/applications/industrial/designguide/conductbronze02.html

The resistance of a strip of copper 8mils thick, 100mils wide, and 1 inch long is 0.8 milliohm. If phosphor bronze is 10 times worse, that gives a resistance of 8milliohm. A current of 1 amp flowing through it will generate 8milliwatts of heat and produce a voltage drop of 8 millivolts. The contact resistance of a professionally made edge connector with gold plating is around 30milliohm. Without the gold plating and proper contact pressure, a home brew contact could be much worse, especially after it ages.

The electrical conductivity of the metal does affect the contact interface itself. Since the metal atoms of the two surfaces are only actually in contact for a small portion of the nominal contact area, the resistivity of the material contributes to constriction resistance. But other factors outweigh it.

A contact without noble metal plating requires ten times as much contact force to pierce the oxide layer and make good contact - about 1kg of force on each contact.


If you have a system that depends on 800 amateur made unplated contacts, the probability of all 800 actually working at any given time is probably not very good. Entire books have been written just on electrical contacts: big, heavy, expensive books.

ckelloug
12-02-2010, 05:01 PM
I remember sitting next to a contact engineer on an airplane once and he pointed out the importance of the contacts "wiping" or moving past one another by a few thousandths in order to ensure a good connection. I'm sure that this helps break the oxide layer.

Stepside
12-02-2010, 10:48 PM
Yes they look flimsy, but for a couple of parts even in 16 gauge steel they are amazing. We had a kid in a college art class saw out the outline of the eagle on a quarter with one and then it was still good to go for his class work.

A different issue when it becomes 800 parts. I think I would explore a punch and die especially if there were another 800 in the offing. Having punched parts should make them uniform which would make the next step in the process easier.

A gang saw or a stop on the mitre gauge of a band saw or table saw this too depends upon quantity and accuracy desired as well as the material.

D_Harris
12-02-2010, 11:37 PM
You either need to learn a few skills to do this or, if you aren't willing to do that then have somebody make them for you. You need at least 800 of these parts yet it isn't worth your time to make a jig that allows you to make them? what are you expecting for an answer? Throw the brass into the air and shoot it with bird shot, then collect the finished parts?

I never said that it wasn't worth my time to make a jig. But why would I go through all the trouble of designing and making a holder for a Dremel tool, and a jig for the work after deciding of course if it is the work or the tool that should move during the cut. And then figure out a way to attach them to my lathe or mill, and also figure out how to get/make a slitting saw that has no teeth so I can score the work?

Why would I attempt to do all that when I can just make a simple work holder and score it with a HHS parting tool?


You have not adequately described how these contacts are to be used but what you have said sounds an awful lot like a PCB edge connector. In that case, it would be far cheaper, easier, and more reliable to use or modify an off the shelf edge connector.

?!? I am modifying "off the shelf" connectors... (Unless I decide to make the fingerboards myself).

As I said, one end of each contact strip will have to be soldered to fingerboards.

And you say that I have not adequately described how these contacts will be used, but their use was not supposed to be the issue. This thread was about getting them made.


You know what you know.

"gmatov: Phosphor Bronze or Beryllium Copper are not considered "high resistance metals". Phosphor-Bronze is a standard electrical contact material."

Phosphor bronze has 18% the conductivity of Copper. Berylium is 9.32% the conductivity of Copper. Phosphorus, itself, is an insulator, to wit, 10 to the minus 17, for conductance. Silicon, an insulator, also, is only 10 to the minus 4.

Do as you wish, I am not smart enough to argue with you. I'd use something with silver in it, but it would cost a bit more.

"Something with Silver in it"? As I mentioned the flexibility, corrosion resistance, and electrical conductivity were all factors in deciding on Phosphor-Bronze, which I also mentioned is a common contact material.

Those numbers you mentioned mean nothing without taking into consideration all factors involving current and surface area of the contacts.(Both of which will be very small).


A contact without noble metal plating requires ten times as much contact force to pierce the oxide layer and make good contact - about 1kg of force on each contact.

If you have a system that depends on 800 amateur made unplated contacts, the probability of all 800 actually working at any given time is probably not very good. Entire books have been written just on electrical contacts: big, heavy, expensive books.

Well, all 800 will not have to be working together. At most, two rows of 28(56) will be in contact at any one time. And most of those will be for data.

Also, the only real unknown is how corrosion resistant the "corrosion resistant" Phosphor-Bronze really is.


I remember sitting next to a contact engineer on an airplane once and he pointed out the importance of the contacts "wiping" or moving past one another by a few thousandths in order to ensure a good connection. I'm sure that this helps break the oxide layer.

I had been thinking about that, and I'm still wondering if I definitely need to design a little wiping into the project. (The only other options are plating with either Gold or "Electroless" Nickel). :(

Thanks.

Darren Harris
Staten Island, New York.

Optics Curmudgeon
12-02-2010, 11:39 PM
Google "finger stock". Many styles are available and can be cut up to make contacts.

Joe

whitis
12-03-2010, 03:35 AM
am[/b] modifying "off the shelf" connectors... (Unless I decide to make the fingerboards myself).

Those aren't real female connectors with spring contacts built in like this:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=S3358-ND
Or this, if you want solder tabs for attaching wires:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=S3356-ND
Or this:
http://www.mikesarcade.com/cgi-bin/store.pl?sku=CECM56

It seems like those are the wheel you are trying to reinvent.



And you say that I have not adequately described how these contacts will be used, but their use was not supposed to be the issue. This thread was about getting them made.

What they do affects how they are made. Details matter. Especially those regarding the actual configuration of the contacts. 2 rows of 28 opposing contacts which are engaged with a PCB card edge fingers by sliding parallel to the direction of the contacts? That can be done with off the shelf stuff. Individual wires coming out the other side or are you trying to connect a PCB to another PCB? That can easily be done with off the shelf connectors by bending the solder tails or wire wrap pins inward slightly (and you can obtain connectors which have been already bent like this from the manufacturer). See contact options 555 and 556 on this data sheet:
http://www.edac.net/file_library/series/44_305-315-355%20series%20(new)
Or this one which is pre-bent ($4.68 each qty 10):
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=EBM28DCTN-S288-ND

That is a big difference from constructing, say, a hollerith card reader (80 rows of 12 contacts) or a bed of nails tester, where you have a high density matrix of contacts and connect end on. Or contacts for sliding/rotating parts like a position encoder disk or slip rings.

Other details like how often they are connected/disconnected, service life, environmental conditions (corrosive? wide temperature changes? etc.), reliability requirements, etc.

Recommendations that are appropriate for one application can be inappropriate for another. There are a very wide variety of potential applications for electrical contacts; you need to narrow the problem domain.



Well, all 800 will not have to be working together. At most, two rows of 28(56) will be in contact at any one time. And most of those will be for data.

You said: "I'll need to make almost 800 contacts for just one of the itmes I'm making. It this works out then I may have to make quite a lot more."

Data contacts need to be make a reliable connection as well, or why bother connecting them in the first place? Sometimes the low power circuits can be the most problematic. Connector pins carrying low current can deteriorate faster:
http://www.semikron.com/skcompub/en/CIPS06-27.pdf

D_Harris
12-04-2010, 04:18 PM
Those aren't real female connectors with spring contacts built in like this:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=S3358-ND
Or this, if you want solder tabs for attaching wires:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=S3356-ND
Or this:
http://www.mikesarcade.com/cgi-bin/store.pl?sku=CECM56

It seems like those are the wheel you are trying to reinvent.


What they do affects how they are made. Details matter. Especially those regarding the actual configuration of the contacts. 2 rows of 28 opposing contacts which are engaged with a PCB card edge fingers by sliding parallel to the direction of the contacts? That can be done with off the shelf stuff. Individual wires coming out the other side or are you trying to connect a PCB to another PCB? That can easily be done with off the shelf connectors by bending the solder tails or wire wrap pins inward slightly (and you can obtain connectors which have been already bent like this from the manufacturer). See contact options 555 and 556 on this data sheet:
http://www.edac.net/file_library/series/44_305-315-355%20series%20(new)
Or this one which is pre-bent ($4.68 each qty 10):
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=EBM28DCTN-S288-ND

That is a big difference from constructing, say, a hollerith card reader (80 rows of 12 contacts) or a bed of nails tester, where you have a high density matrix of contacts and connect end on. Or contacts for sliding/rotating parts like a position encoder disk or slip rings.

Other details like how often they are connected/disconnected, service life, environmental conditions (corrosive? wide temperature changes? etc.), reliability requirements, etc.

Recommendations that are appropriate for one application can be inappropriate for another. There are a very wide variety of potential applications for electrical contacts; you need to narrow the problem domain.


You said: "I'll need to make almost 800 contacts for just one of the itmes I'm making. It this works out then I may have to make quite a lot more."

Data contacts need to be make a reliable connection as well, or why bother connecting them in the first place? Sometimes the low power circuits can be the most problematic. Connector pins carrying low current can deteriorate faster:
http://www.semikron.com/skcompub/en/CIPS06-27.pdf

You're still way off track.

You are not aware of the compartment space limitations, switching format, the number and specific connections made.

I know these things and therefore know that there are no "off the shelf" options, which is why I am doing this. So it is not reinventing the wheel.

To again address what I am doing, the best I can do is use a link you referenced above: (http://www.mikesarcade.com/cgi-bin/store.pl?sku=CECM56) and ask you how would you, with the turn of a knob, switch the data/current coming through each of those 56 pins toward any one of a dozen similar 56 pin connectors?(And do it within an area that comes out to less than 64 square inches).

Darren Harris
Staten Island, New York.

D_Harris
12-04-2010, 04:20 PM
Google "finger stock". Many styles are available and can be cut up to make contacts.

Joe

Thanks.

I'll look into that, but I'd think that it would be easier to just make my own from whatever raw stock I settle on, because I'd have to cut to my specs anyway.

Darren Harris
Staten Island, New York.

darryl
12-04-2010, 04:57 PM
You might also look into a contact lube. This is different from a contact cleaner, although some spray cans have both. Cramolin used to make such a thing. MG Chemicals probably has an appropriate one as well. I would be tempted to wipe them down with a lint free cloth with that solution, just prior to assembly. It would be good if you could include a few holes in strategic spots so you can insert the tube from the spray can for periodic treatment, or at least ensure that there is a way to spray the contact lube in after assembly.

To me, it's a bit of a toss-up- is it better to have the contact assembly fully enclosed, or not. Any switch or control that I've ever had anything to do with has needed the contacts cleaned at some point, whether the part is sealed or not. If it's sealed, it's often difficult to get the cleaner/lube to where it's actually needed. Parts that slide or hinge need lube, and don't respond well to just a cleaner- they need lube also. Obviously it will be good to keep contaminants out of the contacts.

ckelloug
12-04-2010, 05:19 PM
Darren,

If that's your problem, it might be possible to use digital crossbar switches which are an IC switch see http://www.analog.com/en/switchesmultiplexers/digital-crosspoint-switches/products/index.html?gclid=CIfg-PXC06UCFSpj7AodMRUmkw

This has the advantage of no contacts at all if you do it right.


--Cameron

Evan
12-04-2010, 06:27 PM
Why would I attempt to do all that when I can just make a simple work holder and score it with a HHS parting tool?


Then why are you asking how to make them????


What Cameron suggests is by far the best way to go.

D_Harris
12-04-2010, 07:17 PM
darry, It will be enclosed, but removing a 1/2 dozen screws will allow the main assembly to be slide out. If there was/is a contact cleaner I could just spray inside and wasn't detrimental to the Nylon housing or other metal parts then it might be a possibility depending on how often this would have to be done.

I do however have an idea to incorporate "wiping" instead and need to find out what materials can be used for this.

ckelloug, Nope. This definitely has to be completely mechanical.

Evan, If you read the first post I was asking for a better idea than what I came up with, but it doesn't seem like there is.

Darren Harris
Staten Island, New York.

TGriffin
12-04-2010, 09:41 PM
Phosphor bronze shears nicely. If you don't have a shear, sandwich the stuff between a couple pieces of aluminum for support and mill, or saw and belt sand it to size. For the contacts themselves, use a spot of silver solder.

Stay away from beryllium, it's not healthy to work with.

Tom

whitis
12-04-2010, 10:17 PM
You're still way off track.

You are not aware of the compartment space limitations, switching format, the number and specific connections made.

Gee, wasn't that MY point? That you haven't provided enough information?



I know these things and therefore know that there are no "off the shelf" options, which is why I am doing this. So it is not reinventing the wheel.

To again address what I am doing, the best I can do is use a link you referenced above: (http://www.mikesarcade.com/cgi-bin/store.pl?sku=CECM56) and ask you how would you, with the turn of a knob, switch the data/current coming through each of those 56 pins toward any one of a dozen similar 56 pin connectors?(And do it within an area that comes out to less than 64 square inches).

Darren Harris
Staten Island, New York.

64 square inches is an area, not a volume.

That is possible to do with all the critical contacts coming from off the shelf and/or semicustom components.

For example, rotary wafer switches like these
http://lgrws01.grayhill.com/web1/images/ProductImages/Rotary_71.pdf
can be disassembled and more wafers stacked on. Those particular switches would be an inconvenient 12+ inches long with 56 poles switched if you put them all on a single shaft. It also isn't cheap as the switches will cost a couple hundred bucks. But if mass produced switches cost that much, that should give you some idea of the true cost of rolling your own, especially if you are going to have any degree of reliability.

Me? Depending on the exact requirements and quantity, there are a number of different approaches. In low quantity, just solder wires from the contacts to off the shelf switch wafers. In higher quantity, I might might make 14 copies of a gold plated PCB to serve as 4x12 contact wafers, and stack them on 0.312" centers, with socket pins on the edge that wire wrap leads on the actual connectors plug into. Just outside the bore hole, both sides, is a ring of copper split at 6 and 12 o'clock. Adjacent to that are contacts spaced 15 degrees apart. A sliding gold plated shorting contact with multiple fingers connects 4 of these outer contacts to one of the inner rings. You only need 56 "special" contacts total and these probably already exist if you look hard enough. Or, I might construct a rectangular crossbar like switch matrix in which contacts slide between two parallel printed circuit boards with connectors surface mounted on one side. If the requirements were a little different, I might mount analog switch ICs on the back of each connector and tie them to a common backplane, use cross bar switch ICs, etc. In some configurations, I might be sneaky and have some 1/2oz double sided printed circuit board stock tin plated (as is done in making plated through holes) up to 2oz and gold plated in such a way that three sides of the board (top, bottom, and 1 edge) are plated and then mill away stripes on the edge (stripes on the top and bottom faces can be either etched or milled). It is also possible to photoetch the edge, if milling disturbs the plated on copper, but this isn't something you can sneak through a dead standard PCB fabrication process.

Take one of those off the shelf connectors, saw it in half, and glue the halves back together the wrong way (original outside edges back to back) and bend and solder the opposing pins together. Take another and remove each contact so you have a pile of lose contacts. Those two exercises might give you some ideas as to how to use readily available off the shelf contacts that are already cut, already gold plated, already have redundant contact fingers, and already are bent into a spring shape to provide the proper contact force.

There are times, though, when you really need a custom contact. I have seen enough products rendered unreliable by as few as one poorly designed roll your own contact. Proper contacts made from sheet stock, incidentally, are typically stamped (or sometimes etched) out of a sheet and left connected together while processing with the strips connecting them together cut off at the end, rather than being handled loose. If you were going to solder 28 contacts to one side of a fingerboard, for example, they could be kept as one assembly while cutting, bending, plating, and soldering and then the excess trimmed off after soldered to the board. This maintains alignment between operations and also provides a common connection needed during electroplating.

rustamd
12-04-2010, 11:24 PM
If you need a quick way to mount dremel on lathe make/buy something like this: http://cgi.ebay.com/Dremel-tool-holder-frame-1-4-in-Shank-/360208501637?pt=LH_DefaultDomain_0&hash=item53de198b85#ht_1159wt_905

Evan
12-05-2010, 12:33 AM
I am in agreement with Whitis as well as Cameron. There is no way that 800 contacts are going to be reliable for even a short length of time even if they are commercially produced. Back in the 70's I worked on machines that were controlled by relay logic and microswitches operated by cycle cams. They were primitive electromechanical sequencers that produced timed operations and measurement of switch closures. Even the good quality gold plated sealed relays from the top names gave trouble. If there isn't enough current the contact resistance goes up from lack of self cleaning and if there is too much current the contact resistance goes up from contact burning.

Mechanical contacts are good when the fail safe mode needs to be open circuit since they do that naturally. Welded contacts are a sign of overload. Mechanical contacts are especially unreliable at voltages under around 30 volts which is a major reason that the phone systems run on about 50 volts.

For logic level voltages there are only a few mechanical designs that work well and the best is magnetic reed switches followed by mercury tilt switches. Open contacts such as you are considering are extremely subject to air pollution.


To again address what I am doing, the best I can do is use a link you referenced above: (http://www.mikesarcade.com/cgi-bin/store.pl?sku=CECM56) and ask you how would you, with the turn of a knob, switch the data/current coming through each of those 56 pins toward any one of a dozen similar 56 pin connectors?(And do it within an area that comes out to less than 64 square inches).



Map all the possible combinations that must be handled. I am betting that it is nowhere near the maximum possible. Then use several eproms as logic arrays to decode the address inputs of the incoming signals to the possible number of states of the control logic. For each possible state of the control
inputs a different block of memory is selected that contains the necessary state maps for the signal inputs which controls the output states of the data lines. The data lines represent both enable signals and state signals that are a function of the input control and addressing inputs.

I have other experience with machines that have a huge number of electrical contacts. I used to fix electromechanical pinball machines in my spare time many years ago. If you want any sort of reliability forget mechanical contacts.

D_Harris
12-07-2010, 11:39 PM
Phosphor bronze shears nicely. If you don't have a shear, sandwich the stuff between a couple pieces of aluminum for support and mill, or saw and belt sand it to size. For the contacts themselves, use a spot of silver solder.

Stay away from beryllium, it's not healthy to work with.

Tom

Milling, sawing, or sanding contact strips this narrow is way too wasteful and difficult. A simple jig and HSS parting tool are simpler and cheaper.

And I know all about the health hazards of breathing anything containing Beryllium. But Phosphor-Bronze is what I settled on anyway.

Darren Harris
Staten Island, New York

D_Harris
12-07-2010, 11:47 PM
Gee, wasn't that MY point? That you haven't provided enough information?


For my question I did. All of your questions concerning what I want to do is something altogether different.



64 square inches is an area, not a volume.


Then that would be 64 cubic inches.



That is possible to do with all the critical contacts coming from off the shelf and/or semicustom components.


I beg to differ. This project will entail be zero insertion force and connections that will be made when the flat surfaces of the contacts come together.



For example, rotary wafer switches like these
http://lgrws01.grayhill.com/web1/images/ProductImages/Rotary_71.pdf
can be disassembled and more wafers stacked on. Those particular switches would be an inconvenient 12+ inches long with 56 poles switched if you put them all on a single shaft. It also isn't cheap as the switches will cost a couple hundred bucks. But if mass produced switches cost that much, that should give you some idea of the true cost of rolling your own, especially if you are going to have any degree of reliability.


I don't know how you came to that conclusion. My way will be much cheaper than anything you're talking about. It will do what I need. And it will fit in the area available.(Which your rotary switches would not).



Me? Depending on the exact requirements and quantity, there are a number of different approaches. In low quantity, just solder wires from the contacts to off the shelf switch wafers. In higher quantity, I might might make 14 copies of a gold plated PCB to serve as 4x12 contact wafers, and stack them on 0.312" centers, with socket pins on the edge that wire wrap leads on the actual connectors plug into. Just outside the bore hole, both sides, is a ring of copper split at 6 and 12 o'clock. Adjacent to that are contacts spaced 15 degrees apart. A sliding gold plated shorting contact with multiple fingers connects 4 of these outer contacts to one of the inner rings. You only need 56 "special" contacts total and these probably already exist if you look hard enough. Or, I might construct a rectangular crossbar like switch matrix in which contacts slide between two parallel printed circuit boards with connectors surface mounted on one side. If the requirements were a little different, I might mount analog switch ICs on the back of each connector and tie them to a common backplane, use cross bar switch ICs, etc. In some configurations, I might be sneaky and have some 1/2oz double sided printed circuit board stock tin plated (as is done in making plated through holes) up to 2oz and gold plated in such a way that three sides of the board (top, bottom, and 1 edge) are plated and then mill away stripes on the edge (stripes on the top and bottom faces can be either etched or milled). It is also possible to photoetch the edge, if milling disturbs the plated on copper, but this isn't something you can sneak through a dead standard PCB fabrication process.


Again, you are not taking into consideration the compartment space limitations, switching format, and the number and specific connections to be made.



Take one of those off the shelf connectors, saw it in half, and glue the halves back together the wrong way (original outside edges back to back) and bend and solder the opposing pins together. Take another and remove each contact so you have a pile of lose contacts. Those two exercises might give you some ideas as to how to use readily available off the shelf contacts that are already cut, already gold plated, already have redundant contact fingers, and already are bent into a spring shape to provide the proper contact force.


I've decided to incorporate wiping, so I won't have to plate the contacts.



There are times, though, when you really need a custom contact. I have seen enough products rendered unreliable by as few as one poorly designed roll your own contact. Proper contacts made from sheet stock, incidentally, are typically stamped (or sometimes etched) out of a sheet and left connected together while processing with the strips connecting them together cut off at the end, rather than being handled loose. If you were going to solder 28 contacts to one side of a fingerboard, for example, they could be kept as one assembly while cutting, bending, plating, and soldering and then the excess trimmed off after soldered to the board. This maintains alignment between operations and also provides a common connection needed during electroplating.

Well, this is not an assembly-line project, but a prototype I'm working on. And I see no reason for any reliability problems as long as all factors involving the connection integrity is addressed(contact oxidation, contact force, and contact accuracy).

Darren Harris
Staten Island, New York.

D_Harris
12-07-2010, 11:48 PM
If you need a quick way to mount dremel on lathe make/buy something like this: http://cgi.ebay.com/Dremel-tool-holder-frame-1-4-in-Shank-/360208501637?pt=LH_DefaultDomain_0&hash=item53de198b85#ht_1159wt_905

Thanks, but I definitely won't need to mount a Dremel to my lathe.

Darren Harris
Staten Island, New York.

D_Harris
12-08-2010, 12:05 AM
I am in agreement with Whitis as well as Cameron. There is no way that 800 contacts are going to be reliable for even a short length of time even if they are commercially produced.


Why would the number of contacts make something unreliable?



Back in the 70's I worked on machines that were controlled by relay logic and microswitches operated by cycle cams. They were primitive electromechanical sequencers that produced timed operations and measurement of switch closures. Even the good quality gold plated sealed relays from the top names gave trouble. If there isn't enough current the contact resistance goes up from lack of self cleaning and if there is too much current the contact resistance goes up from contact burning.


Those machines you worked on in the 70s are nothing like what I'm doing. There will be no hot switching.



Mechanical contacts are good when the fail safe mode needs to be open circuit since they do that naturally. Welded contacts are a sign of overload. Mechanical contacts are especially unreliable at voltages under around 30 volts which is a major reason that the phone systems run on about 50 volts.


As I mentioned, this switcher will entail making and breaking connections only when there is zero current.



For logic level voltages there are only a few mechanical designs that work well and the best is magnetic reed switches followed by mercury tilt switches.


Well, I haven't finished my prototype yet.



Open contacts such as you are considering are extremely subject to air pollution.


I'm not positive what you mean by "open contacts" but it is not a concern as long as plating, or in my case, wiping is incorporated.



Map all the possible combinations that must be handled.


Combinations? I have no idea what you are referring to. I'm not building a mechanical computer. This device will have a dozen 56 pin connections to make and only one at a time.



I am betting that it is nowhere near the maximum possible. Then use several eproms as logic arrays to decode the address inputs of the incoming signals to the possible number of states of the control logic. For each possible state of the control inputs a different block of memory is selected that contains the necessary state maps for the signal inputs which controls the output states of the data lines. The data lines represent both enable signals and state signals that are a function of the input control and addressing inputs.


Forget eproms, diodes, transistors, etc. This is strictly mechanical.



I have other experience with machines that have a huge number of electrical contacts. I used to fix electromechanical pinball machines in my spare time many years ago. If you want any sort of reliability forget mechanical contacts.

Electromechanical? It is common knowledge that those pinball machines are notorious for their maintenance issues. I wouldn't ever bother getting one.(I'll stick with my 12 full size coin-operated arcade video games).

But I have to point out that the real reliability issues are with the ideas that are similar to what I'm being recommended here. Here's one example: http://www.arcademvs.com/accesseriors/A-23-1.jpg

Darren Harris
Staten Island, New York.

rdfeil
12-08-2010, 01:18 AM
Darren,

Without setting you off on another hissy fit, please explain why this assembly MUST be strictly mechanical. I have worked with some very intricate and reliable solid-state devices, some even able to handle relatively high voltages. A 56X12 mux will easily fir into your 64 cubic inches.

Barrington
12-08-2010, 07:35 AM
Darren, what are the reliability issues with the board you linked to ?

If you are making a JAMMA switcher then you really don't need to switch 56 ways. A quick look at the specs shows:-

There are 8 unused pins and 9 gnd pins, none of which need to be switched. Then there are 25 resistive pull-up inputs which just need diode isolation, not switching.

This leaves 14 other pins: 4 x +5V, 2 x -5V, 2 x +12, 2 x speaker, 4 x video.
All these could be relay switched, but the video might be better switched electronically.

Oh, wait a minute, that all appears to be exactly what's been done on that board !!!!!

http://www.arcademvs.com/accesseriors/A-23-1.jpg

http://www.arcademvs.com/accesseriors/A-23-1.jpg

Cheers

.

D_Harris
12-18-2010, 07:15 PM
I'll not bother getting into the project specifics any more, and just stick with the idea I came up with on how to make the contacts.

Thanks anyway.

Darren Harris
Staten Island, New York.

RKW
12-18-2010, 11:07 PM
On forums, that would probably be best. Some folks will redesign any and everything even though you may not have asked for it.


I'll not bother getting into the project specifics any more, and just stick with the idea I came up with on how to make the contacts.

Thanks anyway.

Darren Harris
Staten Island, New York.