Tag: CNC

After taking the incandescent lamp socket off its base, I drilled the tapped (yeah, in plastic) 6-32 holes out to a firm press fit for the knurled 6-32 inserts, buttered the inserts with epoxy, and pressed them firmly in place:

Fast forward a day and they’re stuck in there like they were glued. You can see a bit of the epoxy around the right rim of the insert; I wiped a bit more off around the other one.

Putting The Right Amount of epoxy on the insert requires dialing back my “The bigger the blob, the better the job” enthusiasm, but wasn’t all that difficult. It’s certainly more tedious than just ramming the inserts into a printed hole and might actually produce better retention. I doubt that will make the least difference for (almost) anything I build.

On the whole, they look good…

Tagging along behind the metric inserts, a sack of knurled brass inch-size screw inserts arrived:

The nice stainless steel screws on the right range from 4-40 to 10-32, which suffice for nearly everything I build around here.

Unlike the splined metric inserts on the left, these inserts have actual knurls and ridges that should hold them firmly in place. The specs give hard-inch dimensions, of course, that (seem to) correspond to the root diameter of the knurls. You can find nice engineering drawings of precise tapered holes (by drilling down into the Heat-Set Inserts for Plastics item on that page), but a few metric measurements of the actual parts on hand should suffice for my simple needs.

Thread: overall length x small rim OD x (knurl length x larger knurl OD)

• 4-40: 5.8 x 3.9 x (4.0 x 4.6)
• 6-32: 7.1 x 4.7 x (4.6 x 5.5)
• 8-32: 8.1 x 5.5 x (5.9 x 6.3)
• 10-32: 9.5 x 6.3 x (7.0 x 7.1)

Rather than fussing with a tapered hole, just punch a cylinder with the small rim OD (to clear the screw) through the part and put a cylinder with the knurl OD x length at the surface.

Using cylinders without diameter correction will make them slightly undersized for heat bonding. The usual 3D printing tolerances don’t justify anything fussier than that.

Using PolyCyl diameter correction will make the holes nearly spot on for epoxy bonding: butter ’em up, ram ’em in, pause for curing, done.

That’s the plan, anyhow…

These seem like they ought to come in handy for fastening things to 3D printed objects:

The assorted screws come from the Small Can o’ Small Screwlike Things, all harvested from various dead bits of consumer electronics:

These would benefit from a heated staking tool that slides them into the hole parallel to the axis and flush with the surface. Such things are commercially available, of course, but for my simple needs something involving a cartridge heater, a wall wart, and a drill press may suffice.

It would be better if the inserts had actual knurls, rather than splines. So it goes.

For the record (thread x length x Knurl OD x Body OD):

• M2 x 4 x 3.5 x 2.8
• M2 x 6 x 3.5 x 2.7
• M3 x 4 x 4.5 x 3.8
• M3 x 8 x 5.0 x 3.9
• M5 x 10 x 7.5 x 6.9

The actual measurements seem to vary within ±0.02 of nominal and I doubt the manufacturing consistency justifies any assumption tighter than ±0.1 mm.

The M3 inserts really do have two different ODs.

The M5 insert was listed as “7 mm OD” and measures 7.5 mm, which suggests a typo in the description.

The polygonal hole adjustment I use produces dead-on diameters for small vertical holes:

HoleWindage = 0.2;

module PolyCyl(Dia,Height,ForceSides=0) {	// based on nophead's polyholes
  Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);
  FixDia = Dia / cos(180/Sides);
  cylinder(d=(FixDia + HoleWindage),h=Height,$fn=Sides);
}

So an ordinary cylinder() with the nominal knurl OD or a PolyCyl() with the nominal body OD should suffice. Horizontal holes can probably use a plain old cylinder() with the nominal body OD, because they need reaming anyway.

Perhaps a dab of epoxy would bond better with the plastic around a nominal-size hole than forcing the insert into an undersized hole or heat-bonding the insert. Some experimentation is in order.

Ten bucks for the entire collection (five bags of 50 inserts each = 250 little brass doodads = 4¢ each), shipped free halfway around the planet, seemed reasonable, given that inch size knurled brass inserts run anywhere from 50¢ to upwards of $2 a pop and a Genuine Helicoil 4-40 insert sets you back just shy of a buck.

An Amazon vendor offers 4-40 inserts for $0.24 each in single quantities, but with $9.25 shipping. [le sigh]

Inch-size inserts with knurled rings intended for ultrasonic bonding seem to be 5¢ to 15¢ on eBay. I think the straight-side versions will work better than the tapered ones for heat or epoxy bonding.

It knurls my knuckles that we here in the US haven’t gone solidly metric. Yes, I have a goodly assortment of metric hardware in addition to the harvested fasteners shown above, but it definitely wasn’t cheap & readily available.