Punch double-D hole for 5-way binding post

[nuke]

I'm building a simple benchtop dummy load with 4x 8-ohm resistors in a small light steel enclosure. I have a bag of Keystone binding posts (7704/7005) that mount in a .328 x .250 inch double-D hole that I want to use.

But I'm trying to figure out how to punch that hole with hand tools. I don't think anyone makes a double-D knockout punch that small.

Pondering best method do that, since I have all these binding-posts on hand in my parts stash. Could just buy 4-sets of double posts, that only need two round holes.

Cutout dimensions on the data sheet attached.

[xtian]

Double D...nice in so many ways... but I'd just settle for round.

[sluckey]

Double D? That's gonna be a BIGGG hole!

[solderhead]

Greenlee comes to mind as the manufacturer for this sort of thing, but looking at their catalog I don't see a double-D that small. In cases like this I've always started with a 1/4" hole and then expanded it with a nibbler.

It may be worth contacting Keystone directly to ask them what they recommend as the proper punch to use in prepping holes for their 7004/7005 binding post. Chances are they expect you to use a nibbler, but if there's a chance that somebody makes a dedicated punch for this application Keystone should be able to guide you to it.

[jabguit]

$100 or so for special punches that you'd use once?
mount them in a round hole and call it a day...


cheers,

[nuke]

$100 or so for special punches that you'd use once?
mount them in a round hole and call it a day...


cheers,

No, the terminals need the flats to keep the post from turning when using the "binding" feature of the binding post and tightening the nut down.

So just a round hole is not really an option for that reason. Thought about a nibbler, but the spacing is tighter than my smallest metal nibbler. The only other method is drill .250" holes, then hand file them out. I want four sets of binding posts and hand filing 8 holes them seems like a lot of work and error prone.

I haven't found an anchor washer for this kind of hole. Sometimes you can find an anchor washer with a singe-flat D-hole. The other option might be drilling the .328 holes in the aluminum panel and adding some metal strips to create the flats. A little aluminum stock and epoxy would do the trick.

Or layout a simple PC board with the appropriate double-D holes, have it fabricated and mount it behind the aluminum panel.

I could just go buy four of the double sets which the pair mount in round holes and hold themselves in place.

I think the tool would be a lot more than a $100. Would have to be a press-punch and die, and that gets expensive. Not to mention the press.

Was hoping to brainstorm up with a cheap/free method, since I have a old unused LMB enclosure and a bag full of these red and black binding posts hanging around the house.

Looks like either the PC board idea or just buying four sets of double terminals is the way to go.

[xtian]

No, the terminals need the flats to keep the post from turning when using the "binding" feature of the binding post and tightening the nut down.

The nut will keep the post securely fastened to the chassis even without the D shaped holes. Use toothy lock washers for even better grip. The nut just has to be tightened with more torque than the end user will apply to the binding post with their fingers.

[solderhead]

If there's a transformer on the chassis then magnetostriction can make any fastener come loose. These doulbe-d fittings were designed with this in mind.

I'm with nuke on this -- essentially, this all boils down to professionalism vs. hackery. I know that's kind of harsh, but the reality is that there's a proper professional chassis builder's way to do what he is trying to do and there are amateurish DIY kludge methods. I understand completely why he would want a professional chassis mount and not have to resort to a hack.

The double-d system was designed to provide a fail-safe professional system that would not fail under the worst possible circumstances and allow a binding post to rotate and compromise a wire. If you're dealing with binding posts for high voltage connections, ie: kilovolts (which is what some banana-jack type fittings were originally designed for), and you use anything less than a double-d fitting, then you're literally betting your life that your hack method won't fail by coming loose and kill you when your chassis and it's ground system go hot. OTOH if you're just using binding posts to make a low voltage speaker connection, the risk is a lot lower -- you may kill a transformer when the connection fails, but at least it won't kill you.

For most of the building that we're likely to do, kilovolts aren't a consideration so it's common for people to think up alternative solutions without contemplating extreme risk scenarios.

From a practical standpoint it seems that nuke's options are:
a) drill and file
b) drill then rivet rigid flats along the path of the holes to create the double-d sidewalls
c) have a PCB laser-cut
d) buy new double connectors instead of using what he already has
e) buy an expensive punch (if he can find it)

I don't like spending money to buy alternatives to devices that I already have on-hand, so I'm not likely to buy double binding posts if I've got a bag of single bindings posts in-stock. Spending money on tools that I really need doesn't bother me at all -- I'm the kind of guy who would spend what it costs to buy the punch. I know that once I've got it, I'd take advantage of having it, and having it would likely effect all of my future projects. For me it would never be a single-use item.

Nuke, is there the possibility that a local tool and die company might be able to help you? Sometimes custom work is worth it.

[nuke]

Yeah, the problem with the tighten-and-pray method is that the metal nut is threaded onto the plastic insulator with the double-d shape, meaning, it is missing threads on the flat sides, leaving less than half of the already weak plastic to hold the nut.

Being a 5-way post, in addition to banana jacks, one would naturally want to tighten wires and lugs on the post using the big insulated knob.

There are panel fabricators here in silicon valley, but you can't even walk in the door without dropping a $100 bill just to say hello. A punch press with the right punch and die would bang these out all day long in seconds. But I ain't got one of them. Found a die set in the shape in an industrial catalog, but let's just say, the price was also very industrial.

I'll draft a circuit board and see what it would cost from PCB Way or some such. It would do the trick, just layer it behind a panel with the .328 holes drilled in it.

Or maybe Send-Cut-Send might be worth a look.

Or just order four of the double binding post sets. Just the cheap skate in me wants to use up the stuff in my spares box, since I already have the resistors, posts and enclosure gathering dust.

[Phil_S]

IMO, you're overthinking this.

Suggestion #1. Drill the holes. Then fix a strip of sheet metal on the inside (where it won't be seen) or the outside to create the D shape. Toothed washers and thread locker. I think that will do it. Not sure of the best way to fix the sheet metal. I'm thinking 5 minute epoxy, however, epoxy may soften with heat.

Suggestion #2. Drill smaller hole. Find a half-round file of (nearly) the correct size. You may need a smaller file. Work the hole until it's right. Will this work?
https://www.mcmaster.com/4221A11/
Small round chainsaw file is another possibility. Make a rectangle hole with a flat file. Expand the round parts.

[solderhead]

Yeah, the problem with the tighten-and-pray method is that the metal nut is threaded onto the plastic insulator with the double-d shape, meaning, it is missing threads on the flat sides, leaving less than half of the already weak plastic to hold the nut.

Part of the problem with tighten-and-pray is that the threads that you're tightening are designed to do nothing other than to prevent the connector from lifting out of it's hole -- they are not there to prevent anything other than the vertical degree of freedom. They are certainly not designed to prevent rotation.

If you try to use the threads to prevent rotation, then you're over-tightening the plastic threads in an effort to make them prevent an unrelated degree of freedom that they were never designed to prevent. The plastic threads just aren't up for that. IF you try to force them to do something they're not designed to do, they will fail.

Part of the problem is that things like binding posts and single-D power cords are remnants of past designs. That makes it hard to find punches for them, and when you can find them they are expensive/uncommon parts. This is one of the reasons that I don't fab my own chassis to use a single-D power cord grommet. First, I prefer a detachable cord; second if I have to buy an expensive punch I'd rather buy expensive current technology (IEC punch) than expensive obsolete technology (single-D power cord punch).

One thing that you might consider is finding a local tool and die shop that can cut the tool steel for you, and then have the cutting head welded onto an arbor press. That's the cheapest solution I can imagine that would allow you to punch your own sheet metal ad libitum. When I built my turret-staking press I welded the Keystone staking tools onto an arbor press from Harbor Freight Tools.

PS - "your hundred dollars just to say hello" rule of thumb is spot-on for this sort of thing.

[nuke]

IMO, you're overthinking this.

Suggestion #1. Drill the holes. Then fix a strip of sheet metal on the inside (where it won't be seen) or the outside to create the D shape. Toothed washers and thread locker. I think that will do it. Not sure of the best way to fix the sheet metal. I'm thinking 5 minute epoxy, however, epoxy may soften with heat.

Suggestion #2. Drill smaller hole. Find a half-round file of (nearly) the correct size. You may need a smaller file. Work the hole until it's right. Will this work?
https://www.mcmaster.com/4221A11/
Small round chainsaw file is another possibility. Make a rectangle hole with a flat file. Expand the round parts.

Suggestion #1 I was already thinking as a way to do it. Simple enough, and I'd just epoxy the filler strips to the back of the panel and space it all out so I only need three strips, and only one needs to be of a particular width.

Filing I'd probably do for one hole, or maybe a pair. But 8 holes is a lot of filing and I'd almost certainly mess a few up and get the spacing off.

I think I'll fire up KiCad, see if I can find a "footprint" for these, or just draw a foot print with the shaped hole. Set down 4-pairs of them correctly spaced on a board. Just stick it behind the panel and let the nuts hold it in. Heck, I can make second pcb as a drilling template. Get a quote for it anyway.

[nuke]

Part of the problem with tighten-and-pray is that the threads that you're tightening are designed to do nothing other than to prevent the connector from lifting out of it's hole -- they are not there to prevent anything other than the vertical degree of freedom. They are certainly not designed to prevent rotation.

If you try to use the threads to prevent rotation, then you're over-tightening the plastic threads in an effort to make them prevent an unrelated degree of freedom that they were never designed to prevent. The plastic threads just aren't up for that. IF you try to force them to do something they're not designed to do, they will fail.

Yup, exactly. The flats keep them from turning, and they'll turn since that's how one locks a wire or lug down. The threads are just strong enough to hold them in while pulling out a banana plug. It isn't possible to tighten these down to the point where the normal use of the post will not work loose.

May be a relic from the past, but binding posts are still convenient for a lot of bench top tasks.

PS - "your hundred dollars just to say hello" rule of thumb is spot-on for this sort of thing.

Yeah, been there done that. It is a lot more fun spending someone else's money though.

[Stevem]

Hit a hardware store and pick up a couple of 5/32" drill bits and a small rat tail file for metal.

When you lay out the holes space them so to they butt up to each other .

You will end up smaller then the needed .328" x .250" size you need so you can file right up to the size you need

If you have. Dermal tool and a small stone that will make short work of getting your needed D shape.

I quick note on drilling anything but wood.

The center punch you make must be the full width of the nose of the drill bits your using .
A bit cuts with its outside edges so if the punch does not allow the edeges to touch the work surface the drill takes forever.

If you can't make a wide and deep enough punch make then start off drilling with a much smaller bit .

[jabguit]

$100 or so for special punches that you'd use once?
mount them in a round hole and call it a day...


cheers,

No, the terminals need the flats to keep the post from turning when using the "binding" feature of the binding post and tightening the nut down.

that's the job of the lock washer. you don't NEED a round hole, but if you Must have it what I would do is make a template to mark out the necessary hole shape and drill 1/4" then extend the ends with a Dremel and needle files. a whole lot less $$ than the custom tool...


cheers,