Snapping taps

"I have read that only three or so thread pitches are actually taking load, as the screw stretches slightly as the load increases. So deep threads do not have a greater carrying capacity than shallower ones. "

That is true Andrew which is why counter boring to just over 1 thread pitch gives you a stronger fixing by burying the threaded portion. I know its not possible on thinner sections but I always do it when I can. It looks tidier as you can put a tiny chamfer on the edge of the hole and the surface stays dead flat when you tap. If you look really closely at a new tapped hole straight into a block the surface deforms upwards slightly around the tapped hole which a quick touch with a file or abrasive will reveal especially in soft materials. For heatsink applications this can compromise the flatness contact surface although I tend to just do it generally because it looks nice.

regards Martin

Sounds like a good reason to use studs, they could have been screwed into the softer aluminium to the full depth available then the stronger nuts would have coped better with the short engagement to start with.

J

As Martin has said there is no need to tap to deeply. There is a lot of information out there on calculated strength of threads. You only have to look at the thickness of a standard nut. The nut thickness takes into account a chamfer each end of the hole and a given number of full threads. I was always told that in steel you have the depth of full thread equal to the diameter and one and a half times for aluminium. Always chamfer the hole to allow for the radius under the screw head and to remove the burr raised during tapping. The blind hole depth depends on the type of tap used, if you are hand tapping with a bottom tap you can obviously get closer to the bottom of the hole than machine tapping. Have a look at Emuge tap catalogue there is lots of information which may help to analyse what your particular taps are capable of.

Howard.

Simple design rule:

1. Nut, washer and bolt - good and easy
2. Stud - OK
3. Screw into tapped hole - full of problems (but looks nice).

JA

Not really! First there is no room on module moulding for nuts, or for the means of tightening them. As it was we required special low profile head SHCS. Even if we did manage to use nuts the studs would have stuck out from the module once tightened down which would have compromised creepage and clearance, and may well have shorted a busbar to the chassis. Since the inverter output was 600V DC that could have been spectacular.

Andrew

British Standard BS3643 part 1has a table for metric thread engagement. The table has short, normal and long columns against rows which have a range of diameters and pitches. For an M2 x 0.4 thread short engagement is up to and including 1mm long, that is 2.5 pitches. M3 x 0.5 short engagement is up to and including 1.5mm, that is 3 pitches. This, presumably acceptable, short engagement matches Andrew's post regarding only 3 pitches taking the load. Long thread engagements for these two threads are over 3mm and over 4.5mm respectively. Anything between the long and short values is classed as normal engagement. I think the load and materials involved would determine which engagement would be specified if you were doing a drawing for someone else to follow.

The usual guide of one diameter of thread engagement does not always fall into the normal range in this table, for fine pitches one diameter can be classed as long engagement but it is never a short engagement even for coarse pitches.

Martin

Most of the 1 to 1 1/2 diameters thread engagement rule is down to pitch errors and that is for high precision screws and bolts and tapped holes too.

What happens in practice as the engagement gets longer is the the pitch errors accumulate so some threads can finish up taking very high loads and a lot of distortion as a result. They may even strip in the extreme. This sort of corrects the problem after a fashion but isn't a good idea really. What it all means in practice is that maximum holding strength will be achieved in the recommended range - the materials don't really matter when viewed this way.

I bought some high precision high tensile socket cap heads not all that long ago for holding things down on the mill and in the lathe at times - they cost rather a lot more than the usual run of mill stuff and seem to be hard to find. I asked for the equivalent of old unbreako's in a screw nut and bolt shop.

People talk about finer details and don't measure the tools they are using even to make the hole and add the thread.

John

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In industry theoretical calculations are based on using the standards for the fixings, i.e. unless the fixings are to the standards then the calculations do not apply. Also any additions specified such as lubrication. Where strength is critical then the fixing standards have to be specified for purchase and may include testing requirements. All this is a little OTT for the model engineer. We have to accept what we get, and in most cases it doesn't matter that the fixings are from some unknown source. As John said trying to get fixings to specified standards is not easy as the market is flooded with cheap imports with no traceability to source. I will stick to 1 to 1 1/2.

Howard

As the thread (pun intended) has drifted off into thread length engagement etc it is useful to look at the different thread types used in different materials.

For example, automotive applications where a cylinder head bolt or stud is inserted into an alloy casting. Metric threads are not really suitable where high tensile loads are required, but UNC and Whitworth are superior. I once had a discussion with one of our young mechanical engineering guys who insisted his VW car had metric threads throughout. I told him he was wrong, there are plenty of non-metric fasteners used on those models. I told him to pull out a cylinder head stud and measure it. Sure enough it was UNC but the top bit where the nut screwed on was metric fine.

Another screw type on cars which is not metric is self tapping screws, as far as I am aware there are no metric versions of these.

I don't think there is all that much theory in this area. What standards have done is to establish fits that in some cases in my view are worse than they need be. The fact that things generally produce holes larger than their diameter seems to have gone - pure "shaft" based tolerancing. So an M6 tap for instance is highly unlikely to be exactly 6mm dia it will be over and a matching die will be under. Standards tend to do this sort of thing - or parts manufacturers tend to make use of any tolerance that is quoted and want and get limits when the standards are established which will always make them looser than they think they need be to ease manufacturing problems.

On thread pitches BSW was aimed at ductile materials - wrought iron mainly. Cast iron too for different reasons. BSF was added later for steels. UN is pretty similar but forgets the difficulties of having sharp corners on cutting tools. Threads are often rolled these days as well.

When I mentioned metric fine to an instructor he more or less exploded and said even metric coarse is too fine really but what do you expect from a bunch of watchmakers, they are not engineers. Bit biased really but I understand they did make use of imperial threads in the past. For some reason even the extended range doesn't offer coarser pitches but some table may do some where or the other. He was probably concerned about mild steel.

I built a kit car once John. It called for lots of self tappers. I used trilobal, wonderful things but do need a min thickness of metal to hold really tight. Metric would be good for that.

John

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A classic example involved the Brussels Parliament deeming that only metric threads would be legal tender. This caused a huge argument and much taking of sides. The NATO standards for all radio frequency connectors all use imperial threads. When NATO sat down and worked out how much it would cost to replace every rf connector and mating connector it ran into billions of $. The fact that no equivalent metric thread connectors existed and would have to be designed and certified apparently was unknown to the Brussels politicians! Even the Russian equivalent rf connectors are direct copies of the NATO pattern and was due to the fact that during WW2 a lot of American and British radio and radar equipment was given to the Russians. As they became the accepted pattern throughout the world there was no sane reason to change.

Aircraft bolts etc are usually imperial for the simple reason they are stronger.

The very first metric fastener system used in Britain was the BA, introduced in 1884, which is a true metric system, but due to the average mechanical engineer not being au fait with the metric system they were specified in imperial dimensions. The largest BA thread is the 0BA being 6mm in diameter and a thread pitch of 1mm, the same as a metric coarse M6 today. The difference was the metric thread angle is 60-degrees whereas the BA is 47 1/2-degrees. BA threads were based on a Swiss pattern adopted earlier by instrument makers where small diameter fasteners were needed.

I bought a Taiwanese mill/drill machine a while back, about 25 years actually. It has metric dials and it is claimed to be a metric machine. Well, it isn't metric at all.

Every single screw is imperial including the lead screws are 10tpi. One rev of the handle is supposed to give 2.5mm movement. It gives 2.54mm ie 0.1 inch. Thinking something was odd I measured the lead screw pitch and sure enough it is exactly 10tpi.

The table locking screws are UNC and the others are Whitworth.  Actually a good choice as metric threads in cast iron isn't a good idea as the pitch and thread depth is not suitable for these applications.

Edited By John Fielding on 25/03/2016 08:07:43

I see lots here about how to use taps but as far as I can see nothing about removing taps that are broken in a job. Years ago in M.E. there was a mention in a letter that one should try ALUM. I was presented with a job with a broken 4BA HSS tap & submerged the offending part in a saturated solution of ALUM- bit of heat now & then & bingo- the tap more or less crumbled to a black dust after about 3 days. Give it time & a bit of heat- it works. The job was in mild steel & had no more than a grey finish on it. I was told by a chemist who used the same pub that there was likely something like a nickel-iron cell formed- if memory serves!

If anyone wants to guarantee not to break a tap, this is the way forward.

Some years ago I had a job where I needed to tap 1100 m3 holes through 15mm steel bushes with an ID of 6mm. since I was very tight for time and figured I was going to break taps I ordered 5 sets of Lyndon M3 taps.

The holes were drilled 2.5mm, I used a battery drill and did the whole lot with 1 tap.

Regards.

David.

That sounds very interesting, John, i'd like to know where you could buy that if you don't mind me asking?

I've tapped a few holes since writing this and i'm pretty sure it was because the starter drill i was using was 0.1mm too small, doesnt sound much but makes a bit of a difference.

Michael W

Here You Go, alternatively try your local Indian/Asian supermarket for Fatakdi powder.

Thanks Peak, i presume by a solution he means just mixed with water? I find it perplexing how it just disintegrated the HSS after 3 days of being submerged in this powder and then heating it til it crumbles. Amazing.

Michael W

The Alum, or it may be found as Aluminium Sulfate is used in a saturated solution, and works best warm. aluminium Sulfate may be found in (usually) 1kg bags in the garden shop. the stuff you get in the Super Market is more refined, and dearer, and the Chemist will have even more refined and sterilized medical grade.

John Duffus 1, yes I think the process is electrolytic, it works fastest in brass, and I have found it a bit slower in aluminium. Wouldn't it be nice if there was a similar method of getting broken taps out of ferrous metals.

Ian S C

It also is sold in the canning supplies of the local grocers. I tried it once and it would not dissolve the tap I had after keeping it warm (or hotter) on a wood stove for a week. I had heard it would not work on some taps and I guess I had one of them.

Micheal W,

The action is electrolytic, two dissimilar metals forming a cell. If you cannot help with added heat, it still works but a lot slower. Removed a broken tap from a copper boiler took days at ambient temperature and covered it with cling film to reduce the water evaporation, black powder removed from time to time with a "pick" and finally blown out with an air line.