Another broken tap thread

[chris stephensParticipant @chrisstephens63393]

If you find yourself breaking taps on a regular basis, and you have drilled the holes the right size, could it be that you are not tapping straight but on the p*ss. Quite often , if you are on the piss, you can go in quite a way then things tighten up on you., then OOPS.

When tapping holes I almost always use either a drill stand to keep the tap straight or I use a block with a squarely drilled hole, that the tap just fits, to do the same job. I can only recall breaking two taps in the last ten years or so, one 10BA the other M2.5 both were hand held! On the 10BA, I blame the weight of the tap holder. the other was pure clumsiness on my part.

chriStephens

[Hugh GilhespieParticipant @hughgilhespie56163]

Sid,

 

For ISO metric threads the formula to calculate the percentage engagement is:

 

percent engaged = 92.38 * (major diameter – pilot diameter) / pitch

 

So, for M6 x 1 using a 5 mm pilot drill gives 92.38 % engaged.

 

Most people recomment around 70% so, using a 5.2 mm pilot would give

0.8 * 92.38 = 73.9%.

 

The formula is from Machinerys Handbook by the way.

 

Regards, Hugh

[Gone AwayParticipant @goneaway]

Thanks, Hugh but engagement vs tap drill dia wasn’t the issue in my question (see top of page 2). It was engagement vs part dia for dies.

 

And yes I have Machinery’s Handbook (probably the 5th or 6th copy I’ve had over the years – the latest being the desk version of the 28th edition which has large enough print I can still read it) and I realise it contains the data for most cases. I just didn’t want to have to dig it out and calculate it for everything I use (mostly unc/unf, metric, some BSW/BSF).

 

The point was that there are tables available from at least two sources for engagement relating to taps. Why not similar tables for dies?

 

Actually, I did make an error in my reply to David Littlewood which might have confused you”

 

When I said ” For larger tap sizes ….. ” I should have said “For larger thread sizes ….” which makes more sense in the context. Too late to edit it now.

 

 

Edited By Sid Herbage on 26/01/2012 22:53:39

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

I once came across a professional welder that was a wizard at removing broken taps . All he used was a cutting rod in an ordinary arc welder .

 

When the hole with the broken tap in was intended to be redrilled anyway he just burnt the tap and threads out together . His party piece however was to burn a hole down the centre of the tap , hammer in a piece of Allen key and then just screw out the now very loose remains of the tap leaving hole undamaged .

 

He had a very steady hand and could do this type of work at least down to 6MM taps .

 

The cutting rod was just an ordinary off the shelf one except that it was kept short and reduced in diameter as needed .

 

The only problem I saw was that particularly deep holes didn’t discharge the burn residue very well and it was nescessary to have an occassional clean out . There didn’t seem to be much problem with the heat damaging the rest of the workpiece

 

I thought when seeing a demonstration that it would be possible to put together a contraption , based on an old drilling machine , that could do this type of thing both more easily (ie without great skill) and work down to smaller sizes .

 

We’ve talked about spark erosion before and how slow it can be but what I’ve described above works in a few seconds .

 

I’ve seen taps removed with oxyacetylene before now but never as cleanly as that welder did it .

Edited By MICHAEL WILLIAMS on 26/01/2012 23:23:52

[Speedy Builder5Participant @speedybuilder5]

Get hold of December / Jan issue of Engineering in Minature, and make a spark eroder for about 15 quid. Give it to the club when you have finished with it. It really is a simple job to make it and only has about 6 electric / electronic bits.

[Michael HornerParticipant @michaelhorner54327]

John There many ways of digging out broken taps. But before I begin are your holes ‘through’ or ‘blind holes’?
Method 1 The chemical method. This depends on the fact that hardened steels are more readily attacked by chemicals than mild steel. I generally use this method for taps smaller than 3.2 mm. Get hold of a solution of Ferric Chloride (the stuff used to etch Printed circuit boards). Clean out the cutting oil with isopropanol then drip in the Ferric Chloride. If it is a open ended hole stop up the bottom with something. This method is slow.

Richard.

How slow is slow? I have a broken 2mm tap I wouldn’t mind removing. It’s in a blind hole but it full length about 8mm.

 

Cheers Michael

[Gordon WParticipant @gordonw]

Special welding rods are available for removing broken studs ,and taps, I’ve used them and they work. The coating is a special, and protects the inside of the hole, just poke the rod down the hole and form an extension on the stud. These are available in UK in small quantaties, not cheap tho’, sorry but can’t find the address now.

[Douglas JohnstonParticipant @douglasjohnston98463]

Robert,

I went out this morning to look for an issue of Engineering in Miniature but the February issue was the only one on the stand. Is the spark eroder article in the December and January issues since I will need to get a back issue/issues.

 

Doug.

[Speedy Builder5Participant @speedybuilder5]

Doug Johnston – pleas check your personal messages for update on broken tap spark eroder.

[John CoatesParticipant @johncoates48577]

Update

 

Using a punch and a BFH didn’t work! Guess the thread depth of 0.5mm the tap has cut means it is in there pretty fast

 

Anybody got this EIM article on a spark eroder for £15 ?

 

John

[Douglas JohnstonParticipant @douglasjohnston98463]

For spark eroder plans look at this http://www.stockportsme.co.uk/Membersprojects/Spark_eroder.html

 

Doug

[Keith Wardill 1Participant @keithwardill1]

I guess you’ve all seen this. Lots of other interesting stuff as well (no connection )

http://modelengineeringwebsite.com/EDM_1.html

[Harold Hall 1Participant @haroldhall1]

Sid

I must apologise for having been slow in answering your question regarding possible lists on my website for the effect of producing external threads being made with reduced diameters, in short the answer is no. Its an interesting thought though as I find when producing threads on the lathe beyond M6 my three jaw is unable to withstand the torque required.

I suspect many readers of this thread have noticed a contradiction between my figures for percentage thread depth in relation to drill size compared to those as calculated using the Machinerys Handbook formula as mentioned by Hugh. Using a 5.2mm Machinerys Handbook results in a quoted thread depth of 73.9% whilst I state only 65%. The reason for this is as follows and I can only leave it with the viewer to chose which method suits the home workshop situation.

The reason lies in the quite different thread form of both Metric and Unified threads compared to earlier standards, BA, Whitworth, etc. I produced my DOS based spread sheets to calculate all the values about 20 years ago when the MEW data book was first appearing in the magazine. with this being so long ago my memory of the method was very hazy to say the least and have had to go back to view the formula that I wrote into the sheets, hence my delay in replying.

Incidentally, for anyone interested in using DOS, I have on my Vista PC a Virtual Windows 98 PC which has the facility of exiting to DOS, still back to the thread problem.

Considering first the external thread and the root of this. This conforms to the norm having sloping sides with a radius at its base. However, the external thread that mates with this has flats on its inner diameter so that there is a gap between the two. This being just like tapping any other thread form with a larger drill than the thread core diameter.

For my calculation therefore I have used the core diameter of the external thread and Machinery Handbook the larger internal diameter as produced by the flats on the tips of the internal thread. I have calculated this to prove that this method is what they are using.

As it is the norm to produce internal threads using oversize drills in the past and thread depths have been based on the core diameter of the external thread it seems illogical to use the internal diameter of the internal thread when considering tapping drill sizes.

Incidentally, using my method, Metric and Unified threads start with a thread depth of 88.2%

Whilst it has no bearing on tapping drill sizes, Metric and Unified threads also have a small flat on the outer tips of the external thread and a radius on the mating internal diameter. Because of this you will find that the outer diameter of a tap is larger than the thread’s quoted size. Typically, an M6 tap will have a diameter in the region of 6.1mm (amount quoted from memory).

Even though the standard quotes flats, the tap and die manufactures are permitted to add a radius to ease manufacture though these must stay within the quoted dimension and not be on the top of the flat.

I intend to stay with my method of defining thread depth as I consider it more logical, also noting that Tubal Cain appears to have taken this approach.

Harold

[Gone AwayParticipant @goneaway]

Posted by Harold Hall 1 on 29/01/2012 22:40:04:

Sid

I must apologise for having been slow in answering your question regarding possible lists on my website for the effect of producing external threads being made with reduced diameters, in short the answer is no. Its an interesting thought though as I find when producing threads on the lathe beyond M6 my three jaw is unable to withstand the torque required.

……..

Incidentally, for anyone interested in using DOS, I have on my Vista PC a Virtual Windows 98 PC which has the facility of exiting to DOS

 

No apology necessary Harold we all have lives

 

I too have trouble with the chuck not being up to the torque (not to mention my so-called muscles) hence my interest. TC’s book is really curious in that it goes into taps and tapping in exquisite detail while covering external threading with dies rather cursorily.

 

If anyone is interested in the virtual PC software (which allows you to install virtually any windows OS – or MSDOS directly I believe – not just windows 98) it’s a free download from Microsoft. “Microsoft Virtual PC 2007” will get you a version that runs under XP and Vista. “Windows Virtual PC” will get you a version that runs under Windows-7

[John CoatesParticipant @johncoates48577]

Sorry to resurrect this but the acid hasn't worked (been at it three days) so I'm going down the cobalt drill route (set ordered) and need to know what speed to run the drill at. The set I've bought goes up to 6.5mm from 1.5mm in 0.5mm increments so which should I use (original hole was 5mm for the 6mm tap)?

Cheers folks!

John

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