Tapping a Thread

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alec

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I need to tap two threads which I have never done before.

One is 4mm in bronze and the other is 8mm in about 5mm thickness stainless.

I have ordered my cheapo set from ebay but googling suggests weird drill bit sizes before tapping. Do I have to stick rigidly to these tables ?

Any help or tips would be very welcome.

Thanks in advance.
 
Cheapo sets worry me. If the tap breaks off in the hole, not easy to sort. Esp in the stainless, except at 5mm you should be all right. Tapping bronze is a doddle but be carefull to apply the twist without sideways lean, 4mm is pretty small, so be carefull and back it out every few turns. Depending on the loads, it will be worth sticking to the recommended drill size.
In stainless it need the best quality taps, cheap ones just wont do it.
A
 
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IOne is 4mm in bronze and the other is 8mm in about 5mm thickness stainless.

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Yes, the tap drill sizes are very important. Too small and the tap will struggle to remove the material, and probably break. Too big and you'll get a partly formed thread which may well fail under loading. So you need to get it right, even if that means a trip to an engineers merchants for the right size drill. Whilst you're there get a decent quality tap for doing your stainless.
Just a thought - 8mm thread has pitch of 1.25mm. In your 5mm stainless that's only going to give 4 engaged threads which doesn't seem like much to me. I hope it's not going to be a highly loaded fixing.
 
Thanks for the responses. Clearly there is more to this than I thought.

Bodfish !

Thanks for the response !

Yes. The stainless is one of those infernal Lewmar genoa cars which is threaded and you screw down to change positions. The threads are more or less gone now and I thought of re-doing them in a larger size. I am too skint to buy new ones.
 
As others have said, drill size is important and so is making sure that you put a good clean hole in. It is much easier with a pillar drill, or be very careful. Taps are usually tapered -- you have to run them through the work until the taper is well out of the other side otherwise the bolt will bind. Always use oil, and back off the tap as you go; don't try to go right through with one go. It's a bit of a knack and I strongly recommend you to do a few gash threads before you tackle the work.
 
Question about the bronze.
Is the item to be screwed into your newly tapped hole an existing fitting of some sort? Many bronze threads may look like a metric one but are infact a UNC thread. Follow the advice above, drill and tap a bit of gash material (mild steel will do) and try your threaded screw item in that first just to be sure.
 
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Question about the bronze.
Is the item to be screwed into your newly tapped hole an existing fitting of some sort? Many bronze threads may look like a metric one but are infact a UNC thread. Follow the advice above, drill and tap a bit of gash material (mild steel will do) and try your threaded screw item in that first just to be sure.

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Hi Les !

The bronze thread is a new one. It's going through a bronze spigot to stop the cutless spinning.
 
One trick I use is to put the tap into the chuck of a drill press (pillar drill) The tap and chuck are turned by hand or use a spanner on the exposed square of the tap.
The drill press ensures that the tap is always vertical and pressure can be exerted via the feed handle.
If you have doubts about the drill size then try a smaller size but stop if the tap gets too tight. Typically for the 4mm in bronze turn the tap with a 6inch crescent(adjustable) any bigger spanner can lead to snapping the tap. For the 8mm I reckon you should turn it with no more than a 10 inch crescent (adjustable) spanner.
Once it is cutting do no more than half a turn before backing off to clear the thread before advancing again.
good luck olewill
 
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Taps are usually tapered

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Decent quality taps should be supplied as a set of three - a taper tap, a second tap and a plug tap - and are used in that order. The plug tap has no taper, apart from a small lead on the first thread. On thin or very free-cutting material you can miss out the second tap.
Use decent quality taps - eBay cheapies will break - buy some from someone like Axminster Power Tools instead. While you're at it buy some thread cutting compound as well, something like Trefolex, to help give the thread a decent finish.
When you do the tapping, you need to turn the tap 2 half-turns clockwise, then back it off 1 half-turn to break off the swarf, then repeat. If you just carry on turning clockwise all the time the tap will break, even a good quality one.
It sounds involved if you've never done it before though in practice it's a piece of cake, but I agree with the advice above to have a practice on some scrap first to get the feel of it.
 
You need to use the correct tapping drill diameter for the thread you intend to make. The hole must be true and clean.
Taps come in sets of three usually, one starting tap, which has a relatively long tapered section, an intermediate, which has a shorter tapered section and a plug tap which has no tapered section. Start with the starting tap and ensure that the tap follows the hole witout wobble. Use even pressure and turn the tap slowly. Back the tap off every couple of turns - you'll feel some resistance when you do this but continue until you feel the swarf break away. Once through with the starting tap follow up with the intermediate and then the plug tap. It often helps to use a cutting fluid to lubricate the tap, particularly on hard materials like stainless steel. If drilling stainless use a slow speed and plenty of pressure to avoid blunting the drill with the associated heat build up which will work harden the steel and make it impossible to drill or tap. I would strongly advise against using cheap taps in stainless.

Bugger beaten to it /forums/images/graemlins/ooo.gif
 
Cutting fluid? Tallow is what you want - the animal sacrifice involved propitiates the gods of mechanical things and ensures straight threads and unbroken taps.

When you've finished, Loctite "Studlock" is good if you don't want to undo, "Nutlock" if you do.
 
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Yes. The stainless is one of those infernal Lewmar genoa cars which is threaded and you screw down to change positions. The threads are more or less gone now and I thought of re-doing them in a larger size. I am too skint to buy new ones.

[/ QUOTE ] Thread can be repaired to the same size using a helicoil or similar thread repair kit. Always better to repair to the same thread if you can unless there are absolutely no interchangeability or functionality issues. A thread repair is done by drilling out to a larger size and tapping with the special size tap and screwing a stainless "spring" into the thread. You must use the correct kit, drill and tap but the repair is as good or better than new. ebay is a good source but make sure you know what the thread size is, not always as easy as you might think.
 
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When you do the tapping, you need to turn the tap 2 half-turns clockwise, then back it off 1 half-turn to break off the swarf, then repeat.

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That reminds me of a company I worked at many years ago, I was putting a thread on some stud and the production manager walked past and decided that he would show me the 'correct' way of doing it, he proudly stated (in front of a bunch of engineers): "you must always do half a turn forwards followed by half a turn back........" as far as I am aware he is still trying to get the same thread finished to this day /forums/images/graemlins/grin.gif
 
M4 tapping drill is 3,3mm and M8 is 6,8mm. As previously stated, these sizes are critical. Good quality drills and taps will be requires as cheap ones tend to have the same properties as chocolate! You really should use a tap wrench to give you even pressure while turning. A spanner tends to skew the tap ruin the threads. The last thing you want is to get half way and the tap break or seize up. Cutting compuond is also required or you will get a ragged thread as the tap binds. Sounds complicated but once you get into it its a doddle! Just remember one turn in half turn out.
 
Re tapping size of drill. For critical engineering purposes, yes, the drill size is very important.

For ordinary use metric threads, simply deduct the pitch from the diameter and you get a 'common' drill size that gives you at least 80% of the theoretical thread; more than accurate enough for ordinary applications.

In general, if the screw will be in tension, three complete turns of thread are sufficient. This is because the cross section of material will be greater than the cross-sectional area of the core. In other words, no matter how many extra full turns you put in, the screw will fail at the core, not through the stripping of the threads. This assumes that the screw and the threads are of the same material. When threading material that is softer than the screw, increase the number of full turns accordingly.

Always buy HSS taps from a good source. You can tell the better types by examining the first tap. The crest of the threads will show as a flat for practically the whole length; this ensures that the tap will not cut to full depth. Cheaper taps will just have a long taper, with the last third showing a full thread.

For stainless, get the best that you can afford. Personally, I always use cobalt taps for this material. Not cheap, but well worth it if you are using them regularly.

Whatever you do, NEVER leave the tap in the material halfway through tapping. If you do this in stainless, chances are that the tap will bind in the material and WILL break when you try to remove it.

On the bright side, there's nothing to it once you are used to the proper way.
 
Tracy tools in Dartmouth will sell you the HSS taps you need, no need to buy a whole set. Also advise you on drills!
When tapping, I don't stick religiously to 1 turn in half turn out, more a matter of when you feel it start to tighten, back off until you feel the swarf break off. I have had good results using aerosol chain grease as a tapping compound.
 
quality taps and quality drill that are the correct size are a must. the size is worked out by the pitch of the thread being taken off the nominal size. ie M8 x 1.25 mm requires a 6.8mm drill. i would suggest buying dormer taps and drills. they all have a habit of breaking but cheap ones are worse. if you do break a tap you can sometimes get mole grips on the stub or you can sometimes use a center punch to break the tap up further and push it through the hole. worst case you have to have the hole plasma cut out and the hole welded up. this is then even harder to drill and tap because the work piece has actually been hardened by the welding process.
 
as an afterthought: when you buy your drill bits and taps if you buy from an engineer's supplies also buy a "Zeus book" just beforehand because it gives ALL the drill sizes for clearance and tapping holes both imperial and metric. I wouldn't be without mine and it is only a cheap pocket booklet.

It also has loads of trigonometric formulae plus it lists most, threads, pitches etc. Very worthwhile.
 
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