Tapping (full depth or reduced)

[Martin KyteParticipant @martinkyte99762]

Hi Chaps

Thought I would put this out there as something to muse on, distractions being more than welcome at this time.

Many people advocate reduced engagemant (oversize) holes for tapping, down to 65% in order to reduce breakages, which would seem logical as reduced cutting depth throws less torque onto the tap. In the last few+ years I have rarely/probably never broken a tap. Admittedly there were few threads as small as 10BA most beaing 6BA or larger and nothing in the 14BA range. I generally tap in the mill or using the pillar tool to ensure alignment but just now and again tap 'freehand'. I always use the sizes in the Zeus book which may be higher engagement than some would suggest.

OK so here is my question for discussion.

The most likely cause of a broken tap 'to me' would be a crooked start on the basis that good taps are perfectly strong enough to tap full engagement holes when aligned correctly. So when tapping freehand, does an oversize hole increase or decrease the possibility of a crooked start and thus a broken tap. Does reduced engagement only make sense for mechanically aligned tapping and does it actually make things worse if you are tapping freehand.?

Come on, I know you all have your own ideas so answers on a postcard as they say.

Best wishes to all, stay safe and well.

Martin

[Martin KyteParticipant @martinkyte99762]

Tapping (full depth or reduced)

[Oily RagParticipant @oilyrag]

Are you using Taper/ Intermediate / Plug taps? or do you 'short – cut' by just using Taper / Intermediate and then a Plug tap?

There are so many approaches to consider – material being tapped, type of tap (straight flute, spiral flute positive, spiral flute negative, let alone ground thread form or not, carbon steel, HSS, HSS coated,…….). As the barmaid said to the vicar "its also all about lubrication"…. Rocol RDT fluid? Ambersil Tuffcut paste? Russian Fat?

Every tapping job is different, and from my experience one of the most difficult has proven to be 1/4"BSW /UNC (never had the pleasure of doing anything less than a 5 BA) in 'sticky' steel. However, I agree that the tapping recommendations for the past erred on the side of caution and agree that a 65% thread engagement is generally sufficient and would also add that ensuring a 'straight' engagement is preferred above all else, for free hand tapping I use a 'bell' tapping guide, otherwise tapping from the pillar drill or mill as appropriate.

Now my question is what do you do about tapping an Acme thread?? Full engagement? 90%, 75% ???

[old martParticipant @oldmart]

Careful checking of the squareness as you start is important when freehand tapping. One thing you did not mention is using a taper tap to start off with, which helps a lot with self alignment. I would only drill oversize if the material was difficult. I had a 2.5mm thread to put in Densimet, which is a tungsten alloy, heavy like tungsten carbide, but not very hard. I wished I had a slightly bigger drill at the time as the new Europa tools green ring tap didn't feel like it was going to stay in one piece. After looking through the boxes of small metric taps that the museum has, I found one that finished the tapping easily. I have a feeling that I should have bought a tap designed for aluminium, rather than hard steel.

Tapping ACME depends a lot on the tap. I bought a 3/4 X 5 ACME tap which was a simple taper at least 5" long. When I tried it in a piece of aluminium, I gave up quickly as I couldn't turn it and feared breaking it if I did. Many ACME taps look like two extended taper taps in tandem to reduce the cutting forces.

Edited By old mart on 23/03/2020 19:15:20

[David George 1Participant @davidgeorge1]

Being a retired toolmaker who has seen the effect of insufficient thread depth failures. Like the side of a wax pattern die blowing out because apprentice drilled the holes larger to make tapping for helicoil inserts so the threads looked ok but the die press filled with a large amount of molten wax and took quite a lot of cleaning, it could have been more serious if the wax had leaked to operator. I know that all threads that i make are to drawing and will stand designed pressure. If you have a problem getting the tap square make guide block to tap through it only needs to have drilled hole square to the base through which you tap holding it down whilst it starts.

David

[Mick B1Participant @mickb1]

I don't think I'd want to start a tap freehand. If I can't tap immediately after drilling at the same setting I'd try to mount the workpiece on my drill press table and move it about till the reverse-chucked drill shank enters the hole to the bottom without hangup – then chuck the tap, bring down the quill and start it by hand, using the chuck key tommy bar in the holes in the chuck body if necessary. Once established, finish with a tap wrench. That *usually * works…

Edited By Mick B1 on 23/03/2020 19:23:57

[Martin ConnellyParticipant @martinconnelly55370]

I have seen M20 holes that were drilled 19mm before tapping. Considering these were for lifting eyes to be screwed into the supplier had to do remedial work by helicoiling at some cost to get it right then the part had to have concessions raised for the repair.

Martin C

[Martin KyteParticipant @martinkyte99762]

Just to remind you of the question.

Using a taper tap, freehand. Does an oversize hole increase or decrease the chances of starting a crooked thread.?

regards Martin

[Anonymous]

The most common cause of broken taps (at least in small sizes) is accidental bending rather than too much torque. I normally drill for 65-70% engagement. A bit more for fine pitch threads and less for materials like stainless steel.

About half my tapping is by machine and half by hand. Machine tapping is done at low to high hundreds of rpm using professional tapping heads. For hand tapping I don't use tapping guides; a turn or two and look at the tap relative to the work in two perpendicular directions and tweak if needed. Probably a quarter of my hand tapping is with taper taps followed by plug if needed. But for most common thread sizes I use spiral flute taps for both machine and hand tapping. Haven't broken one yet hand tapping. Of course all the above is for small taps, say 1/2" or less. For larger taps it becomes more difficult to start by hand and keep alignment. In which case I use a centre in a drill chuck in the vertical mill to locate the end of the tap.

Andrew

[Anonymous]

Posted by Martin Kyte on 23/03/2020 20:14:29:

Just to remind you of the question.

Using a taper tap, freehand. Does an oversize hole increase or decrease the chances of starting a crooked thread.?

To answer the specific question, I doubt it makes any difference.

Andrew

[HOWARDTParticipant @howardt]

Increasing the size of hole makes no difference, you are placing a taper in a hole, if it was a tap ground with no lead threads them different matter. Reducing the diameter makes no difference until the tap won’t enter the hole.

i always deduct the pitch from the diameter for the nearest drill size, for metric threads which is all I use.

[Neil WyattModerator @neilwyatt]

In my workshop, the best cures for broken taps are (a) little offcuts drilled through and face square as guides and (b) using pin chucks instead of tap wrenches for anything under 8ba/2mm.

Neil

[Martin KyteParticipant @martinkyte99762]

Posted by HOWARDT on 23/03/2020 20:30:24:

Increasing the size of hole makes no difference, you are placing a taper in a hole,

You are indeed puting a taper into a hole. So for a smaller hole the biggest angle from straight in you would think would be limited to the taper angle (somewhere around 5 degrees). assuming that the tap starts to cut the first thread or so and pull the tap into the hole the action of the taper on the wall of the hole would be to straighten the tap.

With a larger hole it should be possible to introduce the tap into the hole at a greater angle with the possibility of the bottom of the taper digging in somewhat. One might think that this situation would demand more on the tap to straighten things out, particularly if backing off 1/4 turns are not strictly adhered to.

OK I admit I'm speculating here a little but I suspect there is an optimum hole size where the estabishment of the first threads is sufficient to preclude sucsessful realignment. After all drunken tapped holes can be made.

I'm sure we all know how to avoid misaligned taps but that isn't really the question. It's more, do smaller hole clearences more readily prevent drunken threads than larger holes.

regards Martin

This is purely a thought experiment for amusement, no one tries to start threads on the wonk.

Andrew is probably correct and it makes little practical difference but thinking about things  often brings some enlightenment about what is actually going on down the hole.

Edited By Martin Kyte on 23/03/2020 22:18:40

[Kiwi BlokeParticipant @kiwibloke62605]

Pushing a taper cock-eyed into a hole is different from just letting it fall in. In the first case, there's a sideways force trying to maintain the lack of alignment, so the end of the tap is subjected to a force it wasn't really designed for. A taper tap needs a push, to get thread engagement, and hence self-feeding, established. Larger holes allow more misalignment.

I hate starting taper taps by hand, unguided, especially littlies, because I find it almost impossible to feel when the tap is aligned properly. If the hole can be deep enough, one can counterbore at the thread's OD and start the thread with a 2nd tap. It will be pretty-well aligned. I find I rarely use taper taps anyway. One day, I'll make a set of short guides – just blocks or cylinders drilled the OD of the tap, to be hand-held on the work-piece surface.

If you really need to backwards-turn, try to resist until the thread-cutting is well established: very little swarf is generated in the first few turns. I have broken more taps when turning backwards than forwards. It's not surprising, if you think about what's happening. Spiral, or spiral-point taps are so much better.

Edited By Kiwi Bloke on 23/03/2020 22:44:49

[Howard LewisParticipant @howardlewis46836]

Wherever possible, I avoid tapping free hand.

AS already said, many tap breakages are not caused by torque, but by bending.

In the mill, I use spring loaded guides for the end of the Tap and the wrench, providing the torque by hand on a wrench sized to more or less match the Tap.

In the lathe, use a shop made sliding holder, using ER collets, i held in the Tailstock, For larger Taps, I get brave and use the "Jog" facility on the VFD. If any doubt, I either pull the chuck round by hand or use a Mandrel handle.

The other means, for smaller work, is a Tapping fixture, made from an old stand for an electric drill, and a cheap drilling Vice. The handle is a 6" length of 6 x 2mm injection pipe through the spindle, which carries a small Jacobs chuck to hold and drive the Tap.

The idea is that if anything jams, the Tap will slip in the holder, rather than break.

The neat Arc Euro version uses a handwheel to prevent excessive torque being applied.

Lubrication is either Bacon fact (Apologies to Vegetarians / Vegans&nbsp or Rocol RTD.

As long as the tap is backed out, to clear the swarf, breakages are minimal. The last was M10, because I didn't!

Afterwards, the Taps and Dies are cleaned with an old toothbrush, before being put back into their storage boxes.

HTH

Howard

[Martin KyteParticipant @martinkyte99762]

Well that was all a little inconclusive although there were certainly a few valuable tips as to how do get thing right instead of wrong. The biggest finding is probably that none of us really want to tap freehand especially in the finer threads which is probably not surprising.

To be fair I was really trying to get a curiosity discussion going based on my own musings.

Maybe others have ideas for a pithy conundrum or quirky question to ponder keep the interest up. (Perhaps not rear parting again ;O) Identifying wierd items works for me. Maybe Neil has some ideas.

Our Lab fully closed today except those doing virus connected work and essential services. I'm at home trying to protect my wife who is diabetic and has kidney desease, but we have a good support network so are managing so far.

Best wishes Martin

[Anonymous]

Posted by Martin Kyte on 24/03/2020 17:52:57:

Maybe others have ideas for a pithy conundrum or quirky question……

How about:

Why do model engineers keep saying they don't need to use industrial speeds and feeds but are quite happy taking lots of small cuts; and then get obsessed with quick change toolposts to save a few seconds?

Andrew

[Bill PhinnParticipant @billphinn90025]

Posted by Andrew Johnston on 24/03/2020 18:54:50:

 

Why do model engineers keep saying they don't need to use industrial speeds and feeds but are quite happy taking lots of small cuts; and then get obsessed with quick change toolposts to save a few seconds?

Andrew

Presumably because the fiddle of not having a qctp ranks higher for many people in the hierarchy of inconvenience than having to take lots of small cuts [as opposed to fewer big ones], and overcoming the first inconvenience by buying a qctp is more feasible for many people than overcoming the second, namely by buying and housing a machine that can cope with industrial feeds and speeds.

Edited By Bill Phinn on 24/03/2020 19:31:38

[Mick B1Participant @mickb1]

Posted by Andrew Johnston on 24/03/2020 18:54:50:

Posted by Martin Kyte on 24/03/2020 17:52:57:

Maybe others have ideas for a pithy conundrum or quirky question……

How about:

Why do model engineers keep saying they don't need to use industrial speeds and feeds but are quite happy taking lots of small cuts; and then get obsessed with quick change toolposts to save a few seconds?

Andrew

Nah, can't 'elp yer there – keep wondering sim'lar meself…

[old martParticipant @oldmart]

The only valid reason that was put in favour of QCTP for non professionals was because the hss tooling they used was always being resharpened and therefore the quick height adjustability was useful. I have accumulated at least 25 general turning tools and another 8 parting tools which have two things in common. They are all carbide insert type and they are all 17.53mm high. No messing with shims or the expense of all those QC toolholders. The only time hss goes on the lathe is in the form of twist drills.

[colin branniganParticipant @colinbrannigan54160]

I can't remember the last time I tapped a thread by hand, maybe finished tapping a blind hole but I always drill the holes in the mill and tap under power for M4 and above and below M4 spindle in neutral and turn chuck by hand, clean and square very important.

Never used QCTP even when working, standard four way tool post, one chamfer tool, one carbide turning tool and an Eclipse part off tool, the fourth position was free for specials ie threading, forming etc, etc

Now I have retired I run a Myford S7 with HSS tools, tangential for turning, chamfer, screw cutting forming and boring and a Chinese mill, both machines have DRO and the vice on the mill is my zero for X and Y.

Colin

[Martin KyteParticipant @martinkyte99762]

George Thomas seemed to posses several 4 way indexable tool turrets and he was a stickler for how long things took. Seemed to time everything. If you think about it swapping the tool post is a little like having a QC tool holder with 4 tools in it. I find the bug bear with QCTP's is getting everything set so the angle of the toolpost doesn't require resetting everytime I do a tool swap. If it's at the correct angle for the parting tool, all other tools must be correctly aligned so that they just swap in and out. It's only an issue with repeat work but it's taken me a while to get organised enough. They also are a little prone to picking up swarf under the hight restrictor on the clamping cam especially with brass so you swap to your parting tool and the tool high has all gone to cock. That and the mating faces have to be kept clean with a little brush. I'm not saying I would go back to a 4 way tool post but QC systems aren't a magic wand.

regards Martin

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