Drill size of 6.5mm from overseas manufactures (LEGO)

I’m working on a new set of PCBs that use Lego studs to mount the boards. During my prototype runs I found that using non-plated holes at a size of 6.5mm worked best and gave me the most consistent fit. Plated holes were too inconsistent. My prototypes were done though OSHpark and they support drill sizes up to 265 mils (~6.731mm) for plated holes. I’m assuming they must have drill sizes up to that size and used them for the boards.

A couple days ago I got a whole load of boards back from a manufacture in China. The non-plated holes are too small, they measured at about 6.3mm. It turns out the maximum drill size they offer is 6.35mm. Because this was a prototype service run, I’m guessing they just moved down to the drill size they had available. Which I understand and don’t blame them for.

I’ve now checked with several other manufactures in China and it looks like the 6.35mm maximum size is very common. A couple have told me that they can route out a 6.5mm hole, but then the tolerance goes up and I’m not sure that will work for these. Or I’m worried it will work the first time OK then not the next.

Any one have ideas? Or used a manufacture that has drill sizes larger than 6.35mm?

  • Eric

Pretty much this. And yes, been there & done that :wink:

Only idea I’ve got - get a tapered reamer that ends in 6.5 mm or that includes the final diameter you want but can start with holes that come from China and set up a drill press (depth limited) to do this for tests/prototypes.
Dunno about series though, but I guess you’ll be able to find some service who can do the same.

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This looks like they have a chuck size limitation

Is that really the “Lego” plastic building blocks sold as kids’ toys, or is “Lego” a slang term for a mounting scheme with a similar appearance but a different formal name?

I don’t have direct, personal, experience with your question on hole sizes and tolerances. A few incarnations ago the place I worked had a board on which they imposed some tighter-than-normal dimensional tolerances. It was sometimes mentioned as an example of something that design engineers should NOT do to ensure company profitability, employee bonuses, etc. I got the impression that pretty much any shape, with any tolerance, was possible . . . for a price.

I think that’s a valid concern. Where I’m working now, we see tool wear problems on carbide router bits and we’re cutting HDPE polyethylene. I imagine that a drill bit used on FR-4 fiberglass will make measurably different hole sizes across its service lifetime. You may end up calling out a specific hole size, and tolerance, on your drawing or purchase order and paying a surcharge for the vendor to comply with the requirement.

Or, like @Joan_Sparky suggested, the best approach in the long run may be to call out an undersized hole and finish it to size locally. This might be done by your own company, or jobbed out to a machine shop. And, it may be better to call out a significantly undersized hole - say, 5.5mm or 6mm - so the final drill or reamer can get a good “bite” in the material rather than simply deforming its way through the pilot hole.

What are the actual requirements of the mated holes and studs - an easy assembly fit, a close (no play) fit, or snug (locked in place) joint? Does the hole diameter have to be sufficiently oversize to absorb locational tolerances? Do you have a good idea of what the usable hole dimensions are? I’d guess that Lego blocks go together so well at least partly due to the draft (taper) forced onto the mating studs and sockets by the casting molds. Getting a snug fit on tapered surfaces is MUCH easier than machining a line-on-line fit with straight surfaces. The tapered reamer idea may be a good choice!


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I’ve seen some new things online… I suspect that yes, the PCB is intended to be mounted with Lego blocks.

Of course, I may be wrong.

First thanks to everyone for you replies!

I have some bits on order, I’m hoping I can make a few different sizes to see what tolerance the parts can tolerate.

I had not thought about the chuck size, that makes sense.

Yes I should have made that more clear, I’m using “Lego” plastic building blocks. Photo here:

I will have photos of the PCBs in many colors soon.
The actual requirement would be to something that is easy to assembly because the customer will be doing the assembly. Thanks for the suggestion on having someone else do the finishing I’ll look into that.

You guessed it!

Just a follow up to what I’ve found so far and what I plan to test.

Fixing the boards with a smaller hole
At my local hardware store I found a t-handled reamer in the plumbing section. This works great and I’m able to make the holes work OK and to test out different sized holes.
Looks like this one:

Production holes
Of the well known PCB manufactures in China I’ve not found any that will drill a hole larger than 6.3mm. So that leaves me with having the holes routed. Many of manufactures have a tolerance of +/-0.2mm on routing. From the testing I did I don’t think this will work. One manufacture PCBcart.com, who I have used in the past, says they can hold a tolerance of 0.1mm. For holes sizes I found the following:
6.45mm = little bit tight but works OK
6.55mm = fits well
6.65mm = little bit big but works OK

I’m planning to run some protos with a hole size of 6.55mm. PCBcart tells me I don’t have to specify anything in particular but I’m planning to add a note that these holes are routed within a tolerance of +/-0.1mm. Just to make sure it’s clear.

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Lego needs pretty tight tolerances.
What about drilling smaller holes (about 3mm) which are a bit bigger than the notches on top of the lego bricks and then putting 4 of the round studs on a big Lego base board and glueing the 4 studs to your board?

For my own experiments I’ve been thinking to glue a breadboard on a piece of wood and glueing a piece of velcro nect to the breadboard.
Idea is that you can put different boards on the velcro next to the breadboard depending on the project you’re working on but it would be a bit wobbly.
Lego base board and some Lego bricks glued to the custom pcb’s would be an improvement on that.