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Drilling cast iron - where did I go wrong?

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Robin Graham15/05/2019 23:11:32
727 forum posts
178 photos

I have almost no previous experience of working with CI, so this is very much a beginner's question.

I want to make some counterbored M10 clearance holes in a slab of CI to take cap head screws. I bought a counterbore tool and a 10.5mm 135 degree split point drill to suit the pilot. Holes were marked out and centre punched as usual, then started with a BS3 centre drill and opened up to 7mm with a standard 118 degree drill. No problems so far, everything spot on. But when I went in with the 10.5 drill the work chattered horribly and I ended up with a hole with a ragged entrance and centred over 10 thou from where it should have been.

I've done the same sort of thing in mild steel many times (albeit with standard 118 degree drills) and ended up with clean holes within 2 or 4 thou (depending on the weather) of where they're meant to be.

I was going at around 1100 rpm for all the ops.

Any thoughts?


Buffer15/05/2019 23:21:06
160 forum posts
46 photos


I haven't had this problem in cast iron and I have made some bigger and smaller holes than that.

1100rpm sounds fast to me but i'm no expert. Have you tried going slower? I would personally go quite a lot slower when I do it.



Emgee15/05/2019 23:22:50
1541 forum posts
219 photos

Hi Robin

Try reducing the spindle speed considerably and feed rate accordingly.
Perhaps there is a hard skin on the cast plate that would upset things a bit.


Hopper16/05/2019 00:19:15
4650 forum posts
101 photos

Try drilling using the same procedure in a piece of scrap mild steel and see if the 10.5 drill still plays up. These days you can't be sure a newly purchased drill bit is actually ground correctly. There is a possibility it is unevenly ground. If not, try slower spindle speed as suggested. Otherwise, I'd get a standard 118-degree drill bit. They work just fine in cast iron. I've never used anything else.

Also, bolting the vice down firmly to the drill press table might help.

Pete.16/05/2019 00:31:57
222 forum posts
36 photos

Try a smaller pilot, 7mm is quite big for a 10.5mm drill, You've only got just over 1.5mm of the drill actually being used at the edge, so the taper of the drill doesn't seat as nicely in the pilot hole, I'd try with a 4 or 5mm pilot.

Mark Rand16/05/2019 00:57:05
899 forum posts
5 photos

Why were you using a pilot drill for a 10.5mm split point drill? A spotting drill is ok (use a spotting drill rather than a centre/slocombe drill), but there is no need or value in a pilot drill for that size.


With such a large pilot hole, there is little to restrain the drill from grabbing ant it will chatter unless used in a heavy drill press or milling machine.

Edited By Mark Rand on 16/05/2019 00:59:55

John Reese16/05/2019 01:39:53
842 forum posts

I agree with Mark. There is no need to pilot drill doe a 10.5mm drill unless your equipment is too light.

not done it yet16/05/2019 06:53:33
4739 forum posts
16 photos

If I had chatter with a drill, I would revert to an end mill. Far better dimensional control.

HOWARDT16/05/2019 06:54:07
568 forum posts
15 photos

As Mark said, drill from solid or a spotted hole. Using too large a pre drill will cause chatter as the drill or bends as it rotates grabbing at various points during rotation. The alternative is to use a core drill in a predrilled hole. Core drills are three or four flute drills which are meant to be used for opening holes out, they have no point only chamfer on the leading edge so stay centred on a pre drilled hole. Also they are run slower at greater feed.

JasonB16/05/2019 06:58:01
18310 forum posts
2024 photos
1 articles

At 1100rpm the 10.5 will jump about as it had no real guide, drop to about 300rpm

Simon Williams 316/05/2019 09:00:19
512 forum posts
80 photos

Pilot hole is always a good idea, needs to be 1/3 to 1/2 of next drill size, not more. Give the next drill something to do or it will grab.

Drilling down a pilot hole in cast iron always needs the drill rake modified to be nearly neutral, a standard drill grind will always grab and chatter or rip the corners off the drill. Same goes for green brass. Standard 118 deg point jobber drill perfectly OK for CI if the rake is backed off. Negative rake not necessary, neutral or about +5 deg is OK.

Speed - 1000 revs is too fast as others have said. Drilling cast iron keep the speed low and the drill cutting. Chips should come out as discrete pieces - say half the size of a match head - not dust. Slowness not critical; to success (that's in the grind of the drill) but suggest anywhere below 500 rpm for 10 mm hole is right ball park. Slower the better, keep the feed going.

If drill wanders and chatters on entering the pilot hole (leaving a nasty serrated edge as it throws the work piece around) fold up a small scrap of rag (old jeans ideal) and wedge it in the pilot so the advancing drill touches it first. This is sacrificial, but will stop the chatter and grab as the next stage drill edge meets the face of the work.

HTH Simon

Paul Lousick16/05/2019 09:35:14
1451 forum posts
555 photos


You said that you wanted to counterbored M10 clearance holes in a slab of CI but no specific details. Often castings have a hard outer layer which is difficult to machine and you have to get under this to do any good machining. Some CI is a dream to machine, others are a real B##*&%%$!!!!.


John Haine16/05/2019 09:56:10
3174 forum posts
171 photos

Why would they make a 10.5 mm split-point drill if it was expected to be used with a pilot hole for which the split point is useless? Actually for this sort of job I use "pilot point" drills, that have an end a bit like a slot drill with a conical split-point projecting out from the centre. I start by using a pilot point drill the right size for the counterbore, it generates a flat-bottomed hole except with a conical dip in its centre, which I drill to the required depth. Then use that to locate the point of the clearance hole drill and drill right through. Using the biggest drill you can is a good idea, the drill gets disproportionately more rigid as it gets bigger and is less likely to wander. 1100 rpm also too high for CI.

Ian S C16/05/2019 13:10:07
7468 forum posts
230 photos

110 would be better than 1100, if you are not sure what speed, go low speed to start with. Well that's my way!

Ian S C

Dennis Pataki16/05/2019 16:04:18
8 forum posts

Split point drills work best when guided with a hardened drill guide bushing, the type used in fixtures for production drilling work. They are also useful after the drill has gone through the spotting the BS3 or similar center drill produced.

The difference in angles between the 135* split point drill and the 60* center drill can cause some wandering as others suggest.

Cast iron can be delightful or terrible to machine. Sometimes chills occur in the casting if it was not allowed to slowly cool in the mold, or if questionable quality material was used. These chilled hard spots can cause all kinds of machining problems.

Alistair Robertson 116/05/2019 18:16:03
92 forum posts
6 photos

When drilling decent cast iron then any pilot hole should be no bigger than the chisel point of the drill.

When I was an apprentice our foreman would not allow a pilot hole for any hole that was smaller than 1 inch! "What do you think they put that sharp bit on the drill for? Cutting thin air" he would say!

I remember him drilling 2 1/4" holes in 4 inch thick steel plates with no pilot hole on a horizontal driller. There were 100's of holes to be drilled and he said if he had drilled a pilot hole the 2 1/4" drill would have chipped and probably smashed on the broken bits. He had the ability to sharpen the chisel point to his own special shape and 100 holes without having to re-sharpen the drill was the norm. The steel spiral swarf had to be seen to be believed!

Pete Rimmer16/05/2019 18:18:15
734 forum posts
50 photos

Pilot holes in anything - 1/4 to 1/3 the size of the next drill. For cast iron use slow speed and plenty of feed. If you're manually drilling on a drill press Keep up a decent pressure even if you're pecking, never let the drill turn without feeding.

John Haine16/05/2019 21:51:08
3174 forum posts
171 photos

I repeat - why do they make larger drills split point if they expect you to drill a pilot hole? I think there is model engineering dogma here.

Mark Rand17/05/2019 00:01:34
899 forum posts
5 photos

Wot he said ^ smiley

Hopper17/05/2019 02:07:14
4650 forum posts
101 photos

I generally don't use a pilot drill if the hole size is half inch or under. Over that, it just makes life easier, ie less effort on the drill press handle or lathe tailstock wheel, to use a pilot hole.

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