I Can't Drill Straight

[Ken I]

I've had this happen before - does anyone know the cause or how to counter it.

I'm making pistons for my elbow engine from cast iron, turned from 20mm barstock to finshed OD of 10.0, length 30.5mm with a drilled and reamed hole right through at 4.0mm diameter.

I made two pistons - centre drill, drill 3.0, drill 3.8, ream 4.0 - no problem.

Then I changed to my next piece of barstock (same grade / supplier bought 3 months apart) - first unit came out 0.3mm off centre at the ass end (0.66mm TIR) - useless.

Called myself a few names - checked my setup - 3mm drill a bit dull - silly boy - resharpened - outcome another bad one about 0.2mm offcentre.

I took extreme care with the next one - bad, repeated a fourth time - yet another bad one - Dammit that's four consecutive bad parts.

At this point I gave up and went to leaving the O.D. oversize with the intention to mount it on a mandrel to finish up concentric (which is probably what I should have done in the first place on a concentricity critical item like this).

I made 10 - all of which came out crooked to some extent or the other.

I tried centre drills, spotting drills, brand new drills, checked my tailstock centre ....... nothing worked.

Then I made two more from my remaining piece of the "old" barstock - and they came out spot on.

Coincidence or the material ? 4 out of 4 good ones from the old material, 14 out of 14 bad ones from the new material ?

I suspect (guess) there is a corkscrewing of the grain down the centre of the bar which makes straight drilling impossible.

Anyone out there know what the problem is - or how you can stop it ?

Heat treatment / normalising ??

Ken

 

[steamer]

Hi Ken,

Well annealing the material may help, but if I need a hole dead on for concentricity, I don't trust a drill. I would drill at least .4 (.03")undersize and then finish .2 (.010") under with a boring bar, or until it runs dead true and then finish with a reamer or D-bit. I've never had good luck drilling dead straight....always walks on me.....It's clear your a better driller than I!

Finishing the OD after the fact is another way to do it, but it still doesn't assure that you have a round, straight hole.
That sounds like a small hole, but I've bored holes down to 2mm ID on my 12" Logan, and if I can do it I KNOW you can... :bow:

Dave

 

[Ken I]

Dave,
Like you I don't trust drilled holes and go for boring etc. - that said "hope springs eternal in the breast of man" and I will often give it a try - in this case I was seriously led astray by the success on the first two.

Hubris leads to Nemesis.

The reamed part slides neatly on a 4.0 drill rod so I'm OK with that - longer might prove problematic.

I'm no cast iron expert and I was hoping someone might know what particular vargaries are plagueing me - just for future reference.

Ken

 

[steamer]

Hi Ken,
Depends on the piece of stock. CI is one of those materials that will lull you into a warm fuzzy place just before you find the spot that is harder than woodpecker lips!

My former employer years ago had a large faceplate casting on a VTL turning the OD and the old engineer came in and grabbed me, the "youngin" from my "board" and dragged me out to the floor to see it.

The operator had been blowing up carbide inserts like they were jellybeans!....finally figuired out why. There on the OD of this faceplate some 3-4 feet in diameter was the perfect outline of a railroad spike!.....and it had pulled in enough carbon to make it hard as glass.....the print said "CAST IRON" and that's exactly what we got!

He said to me "I wanted you to see this ....Always use Meehanite GC40 stress relieved to 220BHN +/-20%" If you use cast iron you get what ever the foundry shovels up off the floor!......"


Some 25 years later, those words are resonating......maybe there's a set of fiskar scissors in there or something? :big:

Dave

 

[greenie]

What Steamer said is true.
Customer came into the shop and asked where he could get some stuff cast, boss pointed him in the right direction, customer was a happy chappy.
Two weeks later he brings in this bit that had been cast at this foundry and I'm told to work on it.

Ok, everything goes well with the turning, now it's time to drill the guts before I hit it with the boring bar. Get the drill in about half way and all proceedings stop. Withdraw drill and have a look down the bowels, all looks OK, try again with a sharpened drill, instant blunt drill again. W-T-F is going on. Flip part and try again from other end, same deal at the halfway mark, sharpen drill again, blunt drill again real quick.
Take part out of lathe and shine torch down, turns out some moron had used swarf without checking it.
A diposable tip was embedded into the cast iron, had to use a punch to shatter it, before work could start again.

 

[Maryak]

I suspect (guess) there is a corkscrewing of the grain down the centre of the bar which makes straight drilling impossible.

Ken

Ken that seems the most likely option if it's spun cast, spin too slow, metal too cold. Maybe normalising would help straighten the grain.

Best Regards
Bob

 

[Ken I]

if it's spun cast, spin too slow, metal too cold.

Bob,
Thanks - spun cast - of course - didn't think of that - makes a lot of sense.

I'll try normalising a piece in a furnace and see if the problem goes away - simply for future reference.

Dave, Greenie - I'm well aware of the "junk" factor in castings - seen last year's carbide tips a few times.

Ken

 

[Swede]

That's wild... I've never heard of "junk" like used carbide inserts, or railroad spikes, ending up in CI. I guess it means that indeed, if you've got a complex project in CI to shell out the $$ and get the good stuff. It would be horrible to put hours and hours in only to find your material is polluted.