Screw Threads

[oafpatroll]

Close to 2mm is uncomfortable small to do anything with. If you do a test first on a 3mm or so screw, try super glue on a brazing or welding rod close to screw diameter as a test to see how easily the super glue breaks off, if you are satisfied then you may be able to get that screw in and secured. That entire assembly is tiny enough, if I remember correctly the first he made were welded on, how I don't know.

Anyway the above has helped me a few times if you really need to try.

Thanks, that sounds worth trying. The grub screw is supposed to be permanently locked in place once you 'tune' it to the safety lever shaft. If I file off the boogered allen key head and am able to bond it to something I should be able to get it flush with the sear body. Is Loctite 270 the stuff to use for something like this that isn't intended to come apart? Anything better?

[Sean KZN]

270 is as good as any for that application. Doesn't set too fast and sets pretty tight when cured.
The Cajun trick is a slick operation, much better than filing as would normally be done.

I've seen the filing go wrong, it's on my Canik, so it can fire with the safety on, but only once as there is some internal interference that will cause a cluster on round 2, like something broken.

[Sean KZN]

270 is as good as any for that application. Doesn't set too fast and sets pretty tight when cured.
The Cajun trick is a slick operation, much better than filing as would normally be done.

I've seen the filing go wrong, it's on my Canik, so it can fire with the safety on, but only once as there is some internal interference that will cause a cluster on round 2, like something broken.

Before that I had fitted a CZ adjustable trigger, too much locktite although it tested ok before I put it to bed. Next morning was a Tyro shoot, trigger had some locktite leaked onto the pin and else where, loaded and pointed, squeezed trigger, fork, solid as a rock, so I was tjop of the day.

[oafpatroll]

270 is as good as any for that application. Doesn't set too fast and sets pretty tight when cured.
The Cajun trick is a slick operation, much better than filing as would normally be done.

I've seen the filing go wrong, it's on my Canik, so it can fire with the safety on, but only once as there is some internal interference that will cause a cluster on round 2, like something broken.

Ja, it really is a well executed design. When set properly and locked the safety is properly positive. Despite my fat fingers I was able to achieve a quality of trigger pull with drop in parts and a bit of polishing that really surprised me.

[Brian40]

This popped up in my feed.. I found it interesting..

[Eben Wiid]

All in accordance with my experience. I found that taps from 4mm up don't break too easily but from 3.5 down the break very easily. Cause of breakage is almost always too much torque needed because of blunt tap, no lubricant or hole too small. Let me repeat, in respect of hole diameter, recommended drill size whether on the tap box or from a chart, is invariably too small, usually 100% engagement which increases torque dramatically higher than partial engagement. There's a reason why commercially tapped holes are partial engagement. There's a very useful chart in the real Tubal Cain's book "Drills taps & dies" but I don't know where you'd find such a chart elsewhere. Any amount of partial engagement can of course be calculated from a thread diagram.

Material properties must be taken into account and if you increase your pilot bore to bigger than it is supposed to be, the easier your tap goes in, the easier the thread will strip out because you decrease surface area which increases pressure with the same force acting on your bolt. One other contributer is thread depth

If you drill a 5.5mm hole if you want to tap M6 1 series alluminium with a tensile strength of 100MPa, the thread will shear out like butter with very little force. If you drill the same 5.5mm hole in a piece of EN24 with a tensile strength of between 850 and 1000MPa, your partial thread will be much stronger if tapped to the same depth. For load bearing threads I would never increase my nominal drill size.
I have an excel thread calculator that I put together for when I cut my threads on the lathe, I will be happy to share it with anyone that may be interested. General rule of thumb for tapped hole nominal drill size is to just subtract pitch form diameter. An M6 x 1 mm tap needs a 5mm hole, M8 x 1.25 needs a 6.7mm hole, M10 x 1.5 needs an 8.5 mm hole, M12 x 1.75 neeeds a 10.2 mm hole and so on and so on. These sizes are the drill sizes listed on your tapping charts. For non standard threads I use the calculator that takes the thread angle into account with the flats on the hills and valleys which is different for shafts and nuts.

[Dick]

My foray into commercially tapping a lot of 3mm holes ended recently because of my business partner not playing an honest game. I had been doing at least 60% of the work from day one, but it came to a head when he did 60 and I did 250 of a batch of 310, but he divided the payment 50/50. I terminated the arrangement because it was clear that that's what could be expected in the future. It would have suited me to continue alone as the earnings were a very useful supplent to my meagre pension. But, as he was the primary contractor (it was his client) he not I ended up with the work. At that point we had drilled and tapped about 1050 pieces = 10 000 holes. The tapping head was ten times faster than hand tapping (I timed both) and the first tap was still going strong with no sign of wearing out. It was of course in aluminium and a tap won't tap as many holes in steel, but it was still good performance. The tapping head is quite forgiving - I found that perfect alignment with the holes wasn't necessary - as long as it was reasonably close there was enough sideways tolerance built into the tapping head, and 10 000 holes without a tap breaking proves it. Those tapping heads are almost magical. This one was a Vertex made in Taiwan and I'd buy another if I needed one without hesitation. Cost R7000 and paid for itself in a few days. Just as a final note, we used a 2.60mm drill. 2.50 would have needed too much torque and if I'd been tapping steel I probably would have drilled 2.70.

[oafpatroll]

That must have burned! Sadly shaky people walk amongst us and they aren't always easy to spot until they vibrate a bit under pressure or lust for money.

I wouldn't have believed it possible for a 3mm tap to last like that. The few times I've needed to tap stuff that small in the last decade I've bought a spare every time. That head really must be something.

[Dick]

Material properties must be taken into account and if you increase your pilot bore to bigger than it is supposed to be, the easier your tap goes in, the easier the thread will strip out because you decrease surface area which increases pressure with the same force acting on your bolt. One other contributer is thread depth

If you drill a 5.5mm hole if you want to tap M6 1 series alluminium with a tensile strength of 100MPa, the thread will shear out like butter with very little force. If you drill the same 5.5mm hole in a piece of EN24 with a tensile strength of between 850 and 1000MPa, your partial thread will be much stronger if tapped to the same depth. For load bearing threads I would never increase my nominal drill size.
I have an excel thread calculator that I put together for when I cut my threads on the lathe, I will be happy to share it with anyone that may be interested. General rule of thumb for tapped hole nominal drill size is to just subtract pitch form diameter. An M6 x 1 mm tap needs a 5mm hole, M8 x 1.25 needs a 6.7mm hole, M10 x 1.5 needs an 8.5 mm hole, M12 x 1.75 neeeds a 10.2 mm hole and so on and so on. These sizes are the drill sizes listed on your tapping charts. For non standard threads I use the calculator that takes the thread angle into account with the flats on the hills and valleys which is different for shafts and nuts.

Guys, this is completely at odds with Tubal Cain's advice. I don't care who taps 100% thread engagement but 75% engagement is universal in industry because torque is 80% less = less tap breakage, less tap wear and faster production with zero loss of strength or utility of the tapped hole. Industrial tapping is of course done with machines and taps are kept very sharp, but 65% is as good for hobbyists and professionals like gunsmiths with 40% less torque.

Eben Wiid's recommended drill diameters are 90-100% engagement which is lousy advice. Drill diameters that will give 65% engagement for the same tap sizes with only one eighth of the torque and less tap wear with no loss of pull out strength are 5.3, 7.1, 9.0 and 10.80.

As for strength, the thread is stronger than the bolt ie the bolt will shear before the thread strips and nearly all bolt failures are bolt shear not stripped threads. And that's true irrespective of thread engagement.

I think I'll stick with Tubal Cain whom I consider the last word on this topic. Those who need more convincing should google his credentials. Better yet, buy his book - nothing I can say will convince you better than that.

[Dick]

That must have burned! Sadly shaky people walk amongst us and they aren't always easy to spot until they vibrate a bit under pressure or lust for money.

I wouldn't have believed it possible for a 3mm tap to last like that. The few times I've needed to tap stuff that small in the last decade I've bought a spare every time. That head really must be something.

Yeah, but I didn't lose money, just got paid less than I earned. It was worth it for the experience - who knows when knowing how to use tapping heads might be useful again.

The tapping head is one of the nicest pieces of kit that I've used. It worked flawlessly and was very forgiving which is what saved us from tap breakage. If you think about it, its not practical to work a tapping head with jigs and drill guides - too complicated and too slow. How, for example, would you rig up a jig to drill 100 holes in a custom clamping plate as I saw in a video? It's speed can only be enjoyed on the basis of positioning the hole under the tap visually and thus less than perfectly. The tap is secured against slipping up down or sideways by two small grub screws engaging two opposite flats on the square wrench flats at the top of the tap. Below that the body of the tap is gripped in a rubber collet which allows it some sideways movement. The obvious question would be "if the tap moves sideways in the collet it won't be perfectly vertical and thus risk breaking in the hole." I can't answer that - I can only say is that worked with amazing ease, no hitches or glitches. I tapped in batches of ten pieces, ten holes per piece, time 30 minutes. We used Tapmatic fluid by dropping a drop into each hole. If the volume of work had increased substantially I would have looked into some sort of semi auto fluid feed to speed things up a bit. Tapmatic is way too expensive to use as flood coolant but feeding manually we tapped 750 pieces with the first can. I can't say whether the Tapmatic extended tap life but logic suggests that it must have. The other item that avoids tap breakage is a torque clutch. It is set for the size of the tap, more torque resistance for bigger taps and presumably different settings for different material. First tap a few holes with low setting so that the clutch lets the tap stick in the hole, then tighten it just past the point at which the tap works properly, at which setting it will let the tap stop turning if it detects too much resistance. Its a lovely piece of kit.