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Snapping taps

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MW21/03/2016 13:32:57
2051 forum posts
51 photos

I know that this problem is as old as the hills but i seem to have developed an expertise for breaking HSS taps on a regular basis, i'm talking around the M4, M3 variety that i've gone through about 6 or 7 of. I can only use wrenches as i don't have a pillar tool and i'm not keen on machine tapping into blind holes.

I seem to find that while HSS may be very good at cutting harder materials like stainless, i'm finding myself wanting taps in carbon or silver steel now just because theyve got a bit more give. Do you reckon this would be better for smaller sizes when youre not cutting anything harder than mild steel?

Michael W

John Stevenson21/03/2016 13:41:47
5068 forum posts
3 photos

In the smaller sizes, try roll form taps or flute-less as they are sometimes known but you have to select the correct drill for them.

With no flutes they are a lot stronger.

AlanW21/03/2016 13:58:55
83 forum posts
10 photos


Forgive me if I'm about to advise what you already know.

Steps to avoid breaking taps when using 'free hand':

a. Ensure that the tapping hole size is big enough for the intended thread:

b. Ensure that the tap starts square, either by sight against a visual (square) reference but preferably make a proper guide;

c. Always use a 'first' tap on tougher materials;

d. Use appropriate lubricant, ie proper tapping compound or neat cutting oil for steel, paraffin or similar for aluminium, dry for cast iron;

c. Back off a 1/4 turn for every forward turn;

d. For blind holes, remove the tap completely occasionally and, if possible, knock the debris from the hole and clean & relube the tap (I keep old tooth brushes for cleaning taps).

If a tap twists much in use it is clearly making hard work of the job and likely to break, so check (a) and (b).

Personally, I never pay the mark-up for HSS, on the basis that carbon taps are more than adequate for the amount of use they are likely to get in MY workshop.

Even following the above advise, you will still break the occasional tap; I think most of us do, even engineers that have been using them for years, which serves to highlight the use of carbon steel taps.

I hope that helps, if only to let you know that you are not alone.


Edited By AlanW on 21/03/2016 14:01:17

Neil Wyatt21/03/2016 14:06:39
18776 forum posts
733 photos
80 articles

I can help on this topic, I have real expertise at snapping taps.

For M4 and M3 and below the t-handle type of holder are much better.

For really small taps below 8BA/M2 either a tiny t-wrench or just a pin chuck are good.

It's not just limiting how much force you can apply, smaller wrenches are lighter and have less leverage if you occidentally apply bending forces.


Martin Kyte21/03/2016 14:08:40
2558 forum posts
45 photos

A few suggestions. You may already be doing some/all of these.

1. Use a suitable tapping fluid for steel and parafin/wd40 for aluminium.

2. Don't use overly large wrenches.

3. Turn 1/2 then back off 1/4and repeat.

4. Always start with a taper tap.

5. Ensure correct size hole.

6. Counter bore to clearance size for the first mm or so.

7. If you have drilled in the mill use a centre on the back of the tap/tap wrench to get a straight start when everything is still aligned.

8. Buy good quality taps.

9. Hold the wrench with your fingers not your fist.

10. With blind holes remember the swarf gets down the hole so clear it out before you reach the end.

Can't think of much more.

regards Martin

Muzzer21/03/2016 14:11:24
2904 forum posts
448 photos

To understand what you are doing currently - what tapping drill sizes are you using for M3 and M4?

JA21/03/2016 15:37:50
1222 forum posts
73 photos

In addition to the above if I think the taper tap is becoming tight I remove it, clean out the hole the best I can and then go down with the second or bottom tap until it starts becoming tight. Then remove tap, clean out the hole and start again with the taper.

I made a nice little tapping wrench which I always use for 8BA and smaller threads. It is simply a suitable brass rod with a hole drilled in one end and then gently hammered square over the drive end of an old tap. The other end is silver soldered into a 1/2" diameter knurled cylinder. The wrench is used between thumb and forefinger. I have used it to tap 12BA open holes in mild steel with success.



Edited By JA on 21/03/2016 15:51:13

Mick Henshall21/03/2016 15:48:13
557 forum posts
34 photos

As far as lubrication goes I have always used tallow, nice clean well defined threads every time and never snapped a tap


MW21/03/2016 15:50:28
2051 forum posts
51 photos

I know the "regular drill" for tapping, turn backs, keep straight, lots of oil but its just the last little bit of blind holes and before you know it, it just goes, and it always snaps so close to the hole that you cant extract the miscreant out. Your sole option remaining, is to drill with a carbide drill and re tap/threadsert. I dont have those so i just have to accept the inclusion until i do get a thread repair kit.

I was just drilling 8 blind holes M4 X 0.7MM, 6.35mm deep, 25.4mm eqi-spaced, center drilled all of them, drilled to depth + point angle with 3.2mm, tapped 6 successfully with molislip thick metalworking compound and the last two just the slightest twist and it was gone. I was only using a little BA tap wrench, the ones you can tighten up with a metal pin into the small Vee groove.

Now, the shank of the tool is actually smaller than the cutting thread itself, presumably this is run-off for clearance purposes into very deep holes, however, it isnt all that strong and quite brittle when its half way down a hole. This wouldn't pose much of a problem if the shank was thicker, i don't know about you but i find the bigger taps tend to last alot longer because of this. What you need is a shank nearly as large as the tapping thread to provide that strength. I am going to experiment by making silver steel taps to see if using a softer material than HSS provides less stress on the shaft and allows it to "bend" slightly to take up the give rather than break, which i think is preferable because even if the tap is bent slightly you can always take it out and chuck it away rather than have a stuck tap that you cant get rid off without bringing out the big guns.

Michael W

John Stevenson21/03/2016 15:56:41
5068 forum posts
3 photos

3.3 or even 3.4for M4

Martin Kyte21/03/2016 15:58:31
2558 forum posts
45 photos

Well for a start you should be drilling 3.3mm not 3.2 and you could use 3.4 unless you really need full engagement.

It may be advisable to use a new drill too as you are having problems just to ensure a correct sized hole.


Martin Kyte21/03/2016 15:59:02
2558 forum posts
45 photos


Michael Gilligan21/03/2016 16:07:45
18932 forum posts
943 photos
Posted by Michael Walters on 21/03/2016 15:50:28:

Now, the shank of the tool is actually smaller than the cutting thread itself, presumably this is run-off for clearance purposes into very deep holes, however, it isnt all that strong and quite brittle when its half way down a hole.


I have always thought that style of tap was for making nuts ... very deep holes are generally best 'designed-out'.


JasonB21/03/2016 16:10:14
21436 forum posts
2448 photos
1 articles

As the others say simple rule of thumb is dia less pitch so on a M4x0.7 you get 4mm - 0.7mm equals 3.3mm tapping size.

Andrew Johnston21/03/2016 16:13:11
6266 forum posts
677 photos

If you're regularly breaking HSS taps changing to carbon taps isn't going to help. It must be the technique that is wrong.

To start with drilling 3.2mm for M4 is too small. For M4 I normally use 3.5mm, may be 3.6mm in stainless steel. If the hole is only 1/4" deep then the standard set of three hand taps aren't really suitable. The lead on the taper tap is too long to get any sort of thread formed before moving to the 2nd and bottom tap. Generally I use spiral flute taps for most work, although they are designed for blind holes. You only need one tap, they thread to with a couple of pitches of the bottom of the hole and you don't need to faff around backing off the tap every fraction of the turn.

I've recently hand tapped 36 off 9mm deep blind M3 holes in heatsinks, which are not the nicest grade of aluminium to machine. They were tapped dry using a small, ~3", tap wrench. Each hole was tapped in one go, no need to keep backing off the tap.


I.M. OUTAHERE21/03/2016 16:16:54
1468 forum posts
3 photos

By tap wrench do you mean you are using the conventional type or one of those tee handle types ?

I will only use the tee handle type in desperation as thier extra length makes it easier to transmit the turning force off centre causing the tap to flex , this makes breaking a small tap easier .

3mm x0.5 pitch = 2.5 mm drill

4mmx 0.7 pitch = 3.3 mm drill

Depending on what you are using the thread for you could go up 0.1 mm in drill size but if you stick to the thread diameter minus the pitch you should not be having a problem .

Also measure the drills you are using you wouldn't be the first guy to cop an undersized drill or one with worn flutes if in doubt peg it in the bin and get a new one .

You can make a simple tapping block out of scrap by drilling a hole the same size as the thread in a small block of aluminium (use a drill press ,mill or lathe for ths so the hole is at 90 deg to one surface ) this will help get the tap in the hole nice and square .

If you are tapping a hole and you feel the tap starting to bind back it right out of the hole and blow the hole out with compressed air re - apply your cutting fluid if your using it and continue , i have found that chip build up can cause the tap to bind especially with cast iron , aluminium and copper .

Buy only good quality taps the extra expense is worth it - trust me i have broken enough taps and had to scrap a part more times than i care to think about to ever scimp on quality ever again !

Harold Hall 121/03/2016 16:54:52
418 forum posts
4 photos

I agree with JS 3.3mm or 3.4, even 3.4 gives 70% thread which is plenty for all but the most arduous situation. I also agree with his suggestion for using roll thread taps. For metric threads I hardly ever use anything else. No swarf left in the tapped hole and the screws just float in. You do of course need to use larger drills, even here I tend to go up one size in some cases.

Since I took note of Tubal Cain's (not the Youtube variety) suggestions on drill sizes for tapping holes I have never broken a tap.


Neil Wyatt21/03/2016 18:55:43
18776 forum posts
733 photos
80 articles
Posted by Harold Hall 1 on 21/03/2016 16:54:52:

Since I took note of Tubal Cain's (not the Youtube variety) suggestions on drill sizes for tapping holes I have never broken a tap.

Sore point!

The number drill he gives for 8BA in the handbook is a size too small!


Chris Evans 621/03/2016 19:31:23
1960 forum posts

When I was in the die making game we did lots of small 16 impression tools. Each piece of H13 or in some cases 420 stainless had 4 M4 holes 15mm deep. That is 64 tapped holes in the die side and another 64 in the punch side.

Lessons learnt, use a 0.1 oversize drill/use tapping past not liquid/ throw the tap away when it starts to bind when you wind it out. I did these by the hundred and rarely broke a tap. Have a look for one of the eclipse flat bar style tap wrenches they give a nicer feel. With the advent of CNC mills we moved on to machine tapping on all sizes down to M4, none of us brave enough to go any smaller !

Muzzer21/03/2016 19:58:28
2904 forum posts
448 photos

One benefit of using a slightly oversize tapping drill is the reduced torque for tapping. And the challenge with hand tapping with a tap wrench is keeping the tap straight.

I like to use a cordless drill driver (with adjustable torque setting) for smallish (M3-M8) taps. You can increase the torque setting until it's just capable of tapping the whole. With an oversize hole, there will be more margin between the torque you need to drive the tap and what will break the thing. It's also easier to line the tap up with the hole.

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