metric carriage bolt

**Don42** · May 16, 2020, 11:10 PM

Today I made a 4mm carriage bolt.

Four plastic discs are arranged to clamp two rods together at various angles on one of Milady's lamps. One of those plastic discs broke. No problem, I have some 1.5” dia black glass-filled Noryl rod from which I can make a new part. Noryl is an engineering plastic that machines beautifully.

The plastic discs on her lamp were assembled in a stack and secured with a M4 x 0.75 x 50mm metric carriage bolt. Yes, M4: about 0.157” dia. I planned to broach the square hole in the disc I’ll be making, so I made a broach and tested it on a scrap of delryn which is very similar to Noryl. It worked just fine. I stuck the carriage bolt in the broached hole. Purrrfeck! I thought I’d give it that last wee bit of nudge so I hit it smartly with a plastic hammer.

ZINGGG! That was the last I saw of the bolt. It instantly disappeared thru that portal to the 4th dimension that exists under my bench and behind the buffer. I uttered some words that only a drill sergeant could love, and then, after 10 knee-killing minutes on the floor with a flashlight, I resigned myself to making a replacement.

I turned some 3/16” mild steel rod to 4mm by doing it in short segments and taking very light cuts to minimize flexing and bending under the tool. I then used a die to cut 0.75mm pitch threads for half an inch or so. Then I turned a short stub 0.139” dia on the other end to fit in a hole in the head that I’d make separately.

I grabbed some 3/8” mild steel rod in a 5C collet spin index and milled a stub about 3/16” with a square cross section 0.157 (4mm) on a side, then took the rod to the lathe, drilled it #19 (0.140”) and parted it off with about 0.090” of 3/8” dia material remaining.

I then took that bit to the bench, fluxed the .140” peg on the rod with black flux, stuck the head onto the peg and silver-brazed them together with an oxy-acetylene jewelers’ torch. I used plenty of flux and didn’t overheat, so it cleaned up easily with hot running water. The molten silver brazing alloy wicked thru the whole length of the hole and peg, appearing at the other end so I know I got a sound joint. If the .1218 sq in joint area (0.140”dia x 0.277 length) can handle 50,000 lb/in^2 in shear (a conservative estimate per mfr specs) then that joint should have about 6000 lb of shear strength, which on a .140 dia peg would be torque of about 426 lbf-in or 35 lbf-in . The threads would strip long before the joint failed. I then chucked it in the lathe and shaped the head with a file, then gave the bolt a black oxide treatment.

metric carriage bolt-carriage-bolt-m4x0r25.jpg

I just realized that the required broach would be the same as the bolt, so I’ll just let the bolt be its own broach – and this time I’ll have a pair of vise grips on the bolt and use a hydraulic press instead of a hammer! I probably should put a GPS tracker on the vise grips, but nothing ever goes wrong when we plan for it to, right?

And now, the sequel to this saga: I just now looked for something in the second drawer of my tool cabinet – and there was the missing carriage bolt! I couldn’t find it on the floor because it never got there. Didn’t know bolts could laugh at us, did you? Me neither. Well, mine’s better, so screw you, Mr smartybolt!

Hm! The square part of my bolt is slightly larger than that of the original (by design), so now that I found the test blank I tried my bolt in it while observing safe bolt management practice. They say you can’t put a square peg in a round hole. You can with enough force! I didn’t even need hydraulics, just the Kurt vise on my mill.