Tapping drills for cast iron

Does the the size of the tapping drill need to be altered because the thread will be in cast iron? I have tried finding the answer by searching but with no luck.

The reason for asking is I have drilled two blind holes 5.5mm (5/8" deep) to tap 1/4" BSF in a small block of cast iron. Before I actually tapped I happened to offer up the 1/4" threaded rod and found it very neatly entered the hole without the thread being cut.

In this case I can solve the problem with Loctite but for the future should I use a smaller drill for cast iron than that shown in the tables?

Thanks

Martyn

I have tapped CI many times with holes from 2BA, 6mm, 8mm, 1/4 bsf 5/16 bsf and unc and 3/8 unc, plus 3/8 bsf helicoil. I have not noticed any particular looseness with any of them. I do use top quality drills, not hand sharpened, as there is a tendency to drill oversize unless you are very good indeed grinding them. I do use a taper tap if possible followed by a plug tap if the hole is blind.  If possible, do some tests with very slightly undersize drills, to find out how close you come to breaking the tap. Tap CI dry, no lubricant and a drill guide to hold the tap square at first does help.

Edited By old mart on 18/11/2020 20:50:06

First thing I would look at is the 5.5 drill. Is it ground unequally making it cut oversize ? Try another hole using a 5.10 or 5.20 drill as these are nearer to size required.

In a word - maybe. I've tapped dozens of 1/4" BSF holes in cast iron on my traction engines with no problems. I'd use 5.4mm rather than 5.5mm as a tapping drill. When hand tapping the thread form is slightly extruded as well as cut. This tends not to happen with cast iron, so I normally drill a few thou smaller for the tapping hole. I use quality (Dormer) drills that drill within a thou or two of nominal. I use a spiral flute tap as I can't be ****ed to use a taper tap and then have to repeat with a bottom tap.

Andrew

The important point is the size of the hole. I suggest you check the actual diameter of the holes you think are 5.5mm. Clearly you need to select a drill which will cut a 5.5mm hole. Yours could be cutting as much as 6mm, perhaps more? That could be due to the drill bit, the machine or even the operator.

Using a threaded rod, as a measuring device, may not be regarded as a good method.🙂

Edited By not done it yet on 19/11/2020 07:47:44

Particularly as commercial threaded rod is often undersize or not well formed having flat crests meaning even less thread engagement if your tapping size hole is already a bit large

Many thanks for all your helpful replies. I take the points about the drill size, 5.5 may have been a trifle too large for cast iron and I certainly should have been much more careful when actually drilling to ensure I got the size of hole I wanted.

I will concentrate on what I am doing when I next tap cast iron. NDIY was more than half right when he mentioned "operator error".

Martyn

Hi Martyn, personally, I would have used 5.3 as stated in the Zeus data chart.

Regards Nick.

Might I suggest the following:

First, look at the two cutting lips on your selected tapping drill. Use a magnifying glass if necessary, or even attempt to measure them. Recently I drilled a 4.3mm hole which turned out to be about 4.7mm diiameter and on inspection, the cutting lips were easily seen to be noticeably unequal.

Second, drill a hole in a piece of scrap material using whichever tapping drill you wish to use.

Third, measure the diameter of the hole so produced.

Now, measuring the hole is an almost impossible task so what I would suggest is that you use the shanks of a set of metric drills rising by 0.1mm until you find one which does not enter the hole. With luck, this will be 0.1mm above your chosen tapping drill, and hopefully the 5.6mm shank. You can always then measure the diameter of the shank to determine the exact size of the shank. This isn't an exact method, but given the difficulty of determining small diameters will get you somewhere in the correct region.

In the instance I briefly described above, the lips were obviously uneven, so no real need to measure, but I did and discovered how far out the drill was, so it wasn't surprising that my (5mm in my case) tapped holes were almost non-existant. Careful resharpening of the 4.3mm drill produced equal lips, a hole which would not allow a 4.4mm drill to enter, and a correctly tapped hole.

Good luck,

Peter G. Shaw

Couple of problems working together maybe. In order of increasing likeliness:

• Drilling hard with a blunt drill can open the flutes up, causing an oversized hole.
• Cast iron can be soft, coarse grained and prone to crumble. Drilling and tapping can open soft materials up.
• 5.3mm is recommended for ¼" BSF in my book, so I would probably use 5.4mm to reduce strain on the tap whilst accepting a slight reduction in strength. Whilst 5.5mm isn't insanely loose, it's a step further than I'd go.
• If a drill is blunt, or unevenly sharpened, it will drill oversized. Also if it hits a hard spot, or goes in at an angle and is straightened during drilling by the operator.

On a bad day more than one problem could bite. Say the drill is a little on the big side for the tap, and cuts big because it's unequally sharp, and the cast-iron frays a bit, and then the operator has to straighten the hole slightly...

Dave

I've had better results by drilling slightly undersize first (e.g. 5mm in this case), followed by a check to see how it turned out and then a light cut with the size I actually want for the tap. This with cheap drills and an ancient drill press.

You have all been very helpful. In fact my drill was a brand new split point one so it was 'operator error' that caused the problem, though maybe the 5.5mm size did not help.

I certainly have noted all the points made and will treat cast iron with more respect next time.

Many thanks again,

Martyn