Tapping Cast Iron

Firstly, my apologies if this repeats what I wrote in my Test bars post, but that was dull and rambling so hasn't see much attention!

I want to make a tailstock adjuster for my lathe (like this one). To do that I need to tap the adjusting bolt into the tailstock. My single foray into tapping cast iron (in the headstock with an M6) left me nervous as the threads stripped easily.

I have a rubbish tap and die set so expect to get a proper tap for this task.

The internet claims you should tap it dry. Or with oil, or grease or something.

the internet claims that the fine threads were invented for cast iron, but my observation is that it chips and breaks easily so fine sounds like a bad idea.

I was planning to try and use an M5 fine thread so one turn gives .5mm movement, but concerned it may not work.

I gather that you can make a drilled block to ensure the tap is vertical which sounds like a good idea.

Can anyone offer advice - wet, dry, fine coarse, and perhaps anything I should look for in a tap for cast iron.

IN anticipation of the usual flurry of helpful responses....

Iain

You should use a coarse thread in cast iron,and tap it dry (there is graphite in the cast iron)

.

Iain,

Don't trust "the internet" ... Your own observation is better !!

Depending on the quality of the iron, 1mm pitch [or coarser] is likely to be safe.

If you want a finer thread, make an insert.

MichaelG.

.

P.S. ... Alternatively, [if I understand the design correctly], it looks like you could use Loctite [601 or 638] Retainer to secure the screw into a plain hole in the casting. 

Edited By Michael Gilligan on 25/01/2016 18:48:11

As the central stud does not actually rotate why not thread one end of a 6mm rod M6x1.0 and screw that into the casting and then thread the rest of it with your M6 x 0.5 fine adjusting pitch.

I nearly always tap it dry from 10BA upto 1/2" BSP, not had a problem with stripped threads.

Edited By JasonB on 25/01/2016 18:40:16

.

10BA

You must have access to some remarkably good Cast Iron, Jason

... I bet they don't make lathe tailstocks out of that !!

MichaelG.

Only a TE cylinder casting, nothing special and they were only 6 of the 63 tapped holes in that part and on a curved surface just to make it interesting. They are the cleading retaining screws on the two flanges.

I've not threaded the lathe but have put a few M3 and M5 threads into the mill and it was quite nice stuff, obviously the chinese melt down quality scrap metal

Hi, Jason.

I'd not heard of 'fine adjusting pitch' - M6 fine is 0.75mm (which would have been my fall back).

Apparently I can buy a die for £4.25 from china (link). Or what looks like a decent one from RS for £7.50 .

Now to spent 15 quid on some tools I will probably only use once! Ah the joy of model engineering.... (Argh!!! the tap to go with the RS one is 15 QUID on it's own!)

MIchael - I did think about gluing it in, but it feels like I'm letting the side down...

Iain

Tracy tools ones should be reasonable quality and their carbon steel dies are cheaper, you will need a tap for the adjusting nut. Or you could screwcut the male thread.

The M6 x 0.5 would be termed Extra fine, I just said fine adjusting pitch as its the fine pitch you will be using to adjust things

Edited By JasonB on 25/01/2016 19:36:14

When I was an instrument maker,the threads in iron castings ranged from 1/4 to 3/8 whit,and 2 to 10 BA when I first started holes were tapped dry,later on tallow was occasionally used as a lubricant, most threads were tapped on a drilling machine,with tapping attachment, the 10 BA threads were hand tapped,about 50 holes could be done dry then the tap needed re sharpening any more and it would break, we were taught when say threading a small batch of microscope castings by hand to tap the smallest holes first,and work up in size, if you work down you put a heavier effort into pulling on the tap wrench,and tend to loose "feel" when you get to say 10 BA a tap is easily broken as you pull too hard on the wrench and cannot easily feel when the tap is about to stick or break. Tapping cast iron with an M5 tap should not be a problem,of course a good quality tap should be used, it may be expensive but why risk spoiling the thread or breaking a tap it can cost more than the price of a tap to rectify the spoilt work. I do a lot of restoration work and keep my good expensive taps for cutting new threads and use old taps for cleaning out holes in old vintage parts.

No-one told me not to use 10BA in cast iron! There are 10 BA studs all over Norden, including two holding the valve rod to the CI eccentric.

M6 coarse is 1mm pitch, you should have no trouble adjusting to 0.001" / 0.025 mm with M6, 1/40 of a turn is more than you would expect.

Neil

It is not really about the size of thread but the load you intend to apply which dictates what thread diameter and pitch you use.

Iain,

To make a drilled block to ensure the tap is vertical just drill a hole through a round or square block, make sure the hole provides a good sliding fit over the tap and the block/rod is faced square with the hole. Thickness will need to be enough to reach at least beyond part of the parallel portion of the tap.

To use pass the tap through the block and locate the point into the hole to tap, lower the block to the surface and hold steady whilst tapping, once partly tapped you can release the block as the tap will be at 90 degrees to the surface.

For the cast iron thread I would use M6x1 with M6x1 threaded length of rod for adjustment.

Emgee

For any tapping but particularly in iron or aluminum I would recommend good quality spiral point taps (aka gun taps). Osborn blue wizard spiral point taps are the best I have found. FEW from Switzerland (I think) are also excellent. No connection t these firms except as a satisfied customer. Good luck. JD

.

Iain,

Remember that 'Engineering' has the same root as 'Ingenuity'.

MichaelG.

Hi everyone

Seems to me the original thread started with the understandable nervousness of tapping small threads into cast iron.

We've all bought one of those universal kits of every tap size and matching dies for £12.50, only to discover they're brittle as hell, and Murphy's Law applies (the more expensive the part, the more difficult it will be to extract the broken tap...) I have a theory that some of these sets haven't got any relief, so they just bind up, but when you're starting out it isn't clear that these sets aren't HSS and if you're new to the game what are you spending all that extra money on anyway.

Those of us who persevered past this first disappointment spent a fortune on HSS, blue wizard or some such, and anyway breaking a few taps taught us quite a lot on how to feel when it's all about to go horribly wrong. Once you've thrown those cheap and brittle carbon steel taps in the scrap - along with the work piece you just ruined - and bought some decent HSS taps you find there's nothing to it really, just a bit of practice.

In fairness to Tracy Tools, the grade of CS they seem to use has always delivered the goods for me, But I would be very nervous of tackling an M5 thread in CI if all I had to hand was a CS tap. Depends on the grade of CI, and how granular it is. Bigger threads, or M5 and smaller in (most grades of) brass with a CS tap by all means.

Cast Iron isn't difficult to tap, but it's got to be done with a sharp tap. Lubricant is irrelevant if the tap is sharp and cuts freely - hence my comment about relief on the cutting edges. And lubricant won't make a dull tap cut, but it might help you rescue the tap out of the hole just as is begins to bind up. A sharp good quality HSS tap won't break in CI unless you force it off axis or collide with the bottom of the hole (under power - don't ask, took me three weeks to dissolve the tap remnant out of a block of aluminium I'd invested a week's work in with battery acid, and tapping 4 x M5 holes was the last operation) And any metric (coarse) thread in CI is doing exactly what the designers invented metric coarse to do.

And spiral point taps are a dream come true, particularly in the sizes below about 6 mm.

So - to sum up - bite the bullet and buy at least one HSS good quality tap (is one allowed to say EUROPEAN?) , preferably spiral point but it doesn't have to be. Doesn't matter if it's a shiny polished one or a coated one for what we're looking at here, but if does need to be new (= sharp). Make an alignment block to ensure the tap starts true. Make sure you drill the tapping size hole the right size (4.20 mm for standard M5 x 0.8 pitch coarse) and that the drill you use actually cuts that size. There's a whole different thread about playing with the size of the tapping drill in other materials but that's irrelevant here. My recommendation is not to use lubricant, but if you feel you should use something thin but oily. Spit works well, as does butter.

I don't fancy a very fine thread in cast iron - if it's subject to movement it's going to strip sooner or later, and if it's not then a coarse thread is designed to give the optimum holding capacity (whatever than means) for the given size of thread. If you really want to put an adjustable thread into cast iron and make it as fine as 0.5 pitch then you're looking at an insert of some kind (do helicoils exist in this size?).

I used to work with a bloke who'd creep up behind you as you were just into threading a hole. He'd just lean over your shoulder and mutter "Give us a ping". Worked every time.

Best of luck, let us know how it goes!

Simon

Interestingly, Whitworth is still a commonly used threadform for threads in cast iron, because of its relatively coarse pitch compared to its diameter; and because of its nicely rounded root and crest profile which means there are no sharp corners to act as stress risers, and no sharp vee points to crumble. Whitworth is still used in the metric zone for that specific purpose. Compared to, say, M6 x 1, 1/4 Whitworth is 20TPI and has a pitch of 1.27mm, so it is significantly more coarse than the nearest readily available ISO metric thread. 5/16 Whitworth has a pitch of 1.41mm which is greater than an M8 x 1.25 thread. If you prefer to stay with metric, on the basis that the taps will be more useful for other jobs, I would be inclined to choose M8 x 1.25 instead of M6 x 1 in cast iron. It probably depends on the amount of metal left around the threaded hole, because too little leaves the uncomfortable prospect of cracks in the main casting; so M8 might be too large for your tailstock.

Marcus

If you're only going to buy one tap then spiral flute is more useful than spiral point. While spiral flute taps are designed for blind holes they work just as well for through holes and also work perfectly well when tapping by hand rather than under power.

On my traction engines I have tapped lots of holes M4, 2BA, M6 and 1/4" BSF and have never stripped a thread or worried about over-tightening the screw. And I don't bother drilling for 100% thread engagement, for cast iron I am for around 65%.

Andrew

i would agree with the need for a coarse pitch for cast iron, whether finely cast, or not so good.

i wont repeat my bad experiences of tapping cast iron 10BA!

loctite can solve all manner of problems, especially in dodgy miniature loco cylinder castings even with coarse whitworth threads.

cheers,

julian

Hi Iain

For screw that is going to get a lot of use Cast iron is not the ideal material, it wears away quickly. particularly with a fine thread. I would be inclined to take a different approach and make an insert.

Starting with an M10 or M12 machine screw (or Whitworth thread, if that is all you have to hand) Just axially drill and tap the machine screw for your M6 fine thread, then drill and tap the cast iron to match your chosen bolt.

A high tensile steel bolt would be even better as long as you have a good quality tap to make the M6 fine thread.

The insert can be epoxied or Loctited in place.

You have the option of leaving the rather large hex head or cutting it off leaving a sleeve like the one below, This one has screw slots, although they are not necessary, just use a bolt and nut set the sleeve in place.

The image below was found at a site in China, however making one is trivial and much more satisfying. I would use steel in preference to brass. It will last for a long time.

Regards
John

**LINK**

The screw shown in Ian's link won't have to move once it's in the CI. Lets face it most of the imported lathes that just have a rubber grub screw either side of the tailstock to adjust the position use M5 or M6 Metric coarse.

Sounds like I must give up using those 1/2 & 3/8 x 40 tpi threads in cast iron not to mention all those spark plug threads