Straightening a wayward hole

I have been machining some main bearing caps today from a casting.

I centre drilled then put a 2mm pilot hole through each cap which will be used to spot through to the crankcase and then opened out to 4mm for the mounting bolts.

Now I have discovered the holes [ 10 of them ] have all run out of true , I suspect I used a badly ground drill bit.

I need to straighten these hole as I open them out to 4mm and was thinking of using a BS1 Centre drill as this is quite stiff and only 1/8 diameter.

What do you think

you will most likely snap the end of the centre drill off in the job. Use an end mill instead and take it very slowly as it will easily deflect.

Should I put a M2 mill through or open it up with an M3 .

Use 3mm as that will be stiffer and longer.

2mm is a bit of a slender pilot hole and if there were ant hard spots or voids in teh castings then that will throw the drill off.

Edited By JasonB on 27/02/2019 18:20:54

Hello David,

I suggest that the likelihood of success of BS1 centre-drill to 're-drill' your wayward holes depends on how much out of true your existing 2mm holes are. The point diameter on a BS1 centre-drill is 1.2mm, provided that point doesn't touch the walls of your already drilled 'wandering holes' at any point when drilling through then with the work firmly held and the centre-drill securely oriented and progressed into the job with great delicacy and patience your scheme will work. So, the success of your proposed centre-drill method depends on the waywardness of the existing holes. You do need to be confident that the point will not 'snag' the hole-wall at any point as should the point of the centre-drill touch the wall of the already drilled hole at any point as it progresses then it is likely going to fail.

If you prefer a less risky method then I would suggest a suitably sized slot-drill (likely 3mm?) to re-orient your wandering holes. Once again rigidity, delicacy and patience will be the watch-words.

Best of luck(!) & regards,

Phil

Edited By Weary on 27/02/2019 18:30:09

+1 for the slot drill. Use high revs too to keep the tooth load light.

I'd go straight for a 4mm endmill. Use a 3 flute centre cutting carbide one,. Carbide is about 3 times stiffer than HSS.

Andrew

Andrew has it right. Definitely go for the largest diameter, carbide preferable too. Probably does not need to be centre cutting because your exiting hole has removed the bulk of the metal.

IanP

Thanks, I will have a go tomorrow with an End mill .

I have not used castings before and did not realise how hard it can be to

drill through even being aluminium.

Are these the caps for your Gypsy?

They should not have been a problem to drill through which may point to a problem with Your drill bit or methods. Is it a quality name brand bit or unknown, new or has it been sharpened by hand. How are you drilling - just with them floating about on a drill press and using the dimples to locate the bit or something better? Are you getting aluminium stuck to the tip of the drill?

Yes it is the Gypsy , the drill bit was from Screwfix [ Erbauer ] and was new.

I am using my Chester Champion mill to do the drilling , I centre drilled first then followed

with the 2mm drill bit.

I have been getting aluminium sticking to the drill so withdrawing it to clean regularly.

You can not see it on the picture but this is how I discovered the holes tapered inwards.

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Erbauer Drills are really only good for a bit of DIY.

Get something better, Dormer or Guhring with split points, if you want slightly cheaper the ones from ARC are not too bad and use a small amount of paraffin or WD40 dabbed on with a brush to stop the metal sticking.

As Barry says spot drills are a lot better, keep the Ctr drill for when you actually need the 60deg cone to take a ctr.

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Thanks everyone, put an End mill through each casting this morning and it actually

cut through really easy . All holes are running parallel now.

I think I will be investing in some quality drill bits now.

If you are using a drill as small as 2mm, they are not very stiff, so too much end load will cause the drill to bend and wander. With 20/20 hindsight, maybe 3mm taken slowly might have been better, as a pilot.

The main thing is that you have now got things back as they should be.

Howard

I think the lesson that I must take from this is remember 'when it matters like this drill one hole then check it has gone straight