Modern taps.......

I picked up a job lot of very smashing looking taps a while back from my normal Sunday morning supplier.

Bear in mind I have been out of the engineering industry for many years now so anything new and strange looking is somewhat of a head scratcher for me.

The larger 15mm tap looks to me like a posh but all purpose coated HSS job. The smaller 8mm one however is weird as it has no flutes. I have pictured the tip separately which shows that the cross section is vaguely five sided.

What do I use one of these for then?

Rik

Edited By Rik Shaw on 28/09/2013 14:18:33

The lobed tap is likely a roll-forming tap, particularly common for use on thin material.

They work rather like ploughing a field (though not actually cutting) - tansferring material ploughed up from the furrow to form the crest of the adjacent thread.

Note that tapping drill size will not be the same as for a conventional cutting tap, but rather larger than that.

Edited By David Jupp on 28/09/2013 14:25:09

Roll taps are very useful on copper sheet material (which is very abrasive to cutting taps) and is good on any thinnish malleable metal as it compresses the thread theoretically making it stronger by deforming the grain structure rather than slicing it. They are considerably stronger too. Colin

Definitely a roll form tap. For use on ductile materials, like aluminium and stainless steel, less useful for things like cast iron. Roll form taps are often used for blind holes, as there is no swarf produced. It is important to countersink the hole a bit deeper than normal, as the first thread tends to get extruded upwards.

As David mentions a larger starting hole than normal is required. An accurately sized hole is also needed, as a small variation in size has a disproportionate effect on thread height. A tiny bit too small and the extruded material won't have anywhere to go; equals a broken tap. Some roll form taps have straight 'flutes' to allow penetration of lubricant.

For production you can buy machine screws with a similar lobal form; they form a thread as they are inserted, thus saving on the tapping operation.

Regards,

Andrew

I find the Thead- flow taps a delight to use. In small sizes they feel more solid, and give confidence, they go well in brass, and I'v used them to make some 10 and 7 BA nuts from steel. Ian S C

Prompted by the original post I searched the forum for "TiN coating" and was surprised to get no results. What do the knowledgeable say about this modern fashion?

Andrew Johnston: I have one of these thread-forming taps and read your comments with interest; does any one publish tables of recommended tapping sizes for these or do you just go on progressively reducing the hole size until a full form thread is achieved (and then stop!)?

Tapping size for roll forming metric taps is easy: Just take the average between nominal size and the usual tapping hole diameter. Say for M6: nominal 6 mm, tapping size 5 mm, for a roll tap use 5.5 mm.

Don't know how it works out for other thread forms, but the manufacturere should know!!!

Greetings, Hansrudolf

P.S. they work also very well in free cutting steel

Edited By Versaboss on 29/09/2013 13:04:08

Recommendations for tapping drill sizes will vary slightly from manufacturer to manfacturer, but here's a link to a calculator from Guhring:

**LINK**

Regards,

Andrew

I still can't transfer photos from my album, but I'v put a chart of recomended drill sizes for Sutton Threadflo taps, and recomended threading speeds, "you got to be kidding" 4BA 3500rpm for al cu soft brass, 2730 for die castings, and leaded steel, I did mine by hand. Ian S C

Many thanks to Versaboss, Andrew Johnston and Ian S C for their information.

PS

I realize I have committed the all-too-common sin of asking for information which I should perhaps have first looked up myself. The up side is that the many see potentially useful material rather than just the one.

I looked out my single 5/16" 24 UNF thread-forming tap and ran all three methods of determining the correct tap drill size with results varying between 7.3mm and 7.4mm. This might seem quite a large range but there is the variable of thread depth to consider; I used 75% for the Guhring calculator.

I also checked three other possible hardcopy sources:

Machinery's Handbook 1943 - apparently unknown then

Newnes Mechanical Engineer's Pocket Book 1997 - not mentioned

Tubal Cain's Handbook - not mentioned so far as I could see but he was very keen on the correct size for conventional taps

Harold Hall's Data Book (WPS 42) - he calls them fluteless and gives 7.3mm

Now off to the workshop to form a thread with the thought in mind that these taps presumably require a relatively large degree of axial pressure.

Try another computor, here is the tapping chart for Threadflo taps. Ian S C

Ian S C:

Sorry if I wasn't clear - I did find your table in your photo album. I see you have now been able to transfer the photo to this thread, something I have yet to attempt.

I did, however, produce a thread in softish aluminium without difficulty. As it happens, my Roltap brand tool turned out to be marked with a tap drill size - 7.4mm - and I actually used 7.35 which was my nearest drill and fell neatly between the high and low range figures. Under a lens the thread appeared to be not quite fully formed.

Harold Hall makes some interesting comments including that a good quality lubricant, not a cutting oil, must be used; my favourite Rocol RTD claims to be suitable for "forming operations"

Doing some M3 in Aluminium bronze last week. 100 pieces, blind hole and this stuff is like witches tits. First squeal you hear and it's a broken tap.

Changed over a few years ago to fluteless for this job, everyone says they are for ductile but nothing ductile about aluminium bronze but it works and works well.

I use one of those air tapping fixtures so they probably run at about 800 revs, Rock oil from ARC as a lubricant, good stuff this, slicker than owl snot, so far over a run of about 500 pieces no broken taps but I do replace then for every batch of 100.

Taps are funny things, they are consumables and wear but people seem to keep these for years and expect them to last. A drill won't last years without sharpening so why expect a tap to do the same ?

John S.

Ductility is the ability of a material to plastically deform before fracture. It's got nothing to do with how soft or tough a material is, or its machining properties. According to the Copper Development Organisation aluminium bronze is ductile, which is why form tapping works.

Regards,

Andrew