Drilling cast iron – where did I go wrong?

[Robin GrahamParticipant @robingraham42208]

Thanks for replies and apologies for delay in responding. Having a bad back I volunteered to be a subject for student training at an osteopathy school. I'm not sure if that was a good idea – it certainly took my mind away from workshop activities though*.

I think the mistake I made was changing too many things at the same time. I have little experience with CI, less with split-point drills and none at all with using pilots for a hole of this size. Normally I'd just spot or centre drill and go straight in with 10.5 mm, but I'd read that one should creep up to the final diameter and that split-point drills were the bee's knees. I thought it might be a good test ground as I didn't need to be very accurate.

All I want to do is make some matching holes For M10 screws:

 

They don't match even after drilling the clearance holes to 11mm. I really didn't think this would be a problem but I have to accept that I fouled up somewhere (can't blame the machine or tools). I'll think on it and make a new set of holes at 45 degrees to the first go in the hope that it'll look like it was meant to be like that…

Robin.

*Rib relocated – felt great at the time, agony afterwards.

 

 

 

 

 

Edited By Robin Graham on 18/05/2019 23:06:51

Edited By Robin Graham on 18/05/2019 23:18:57

[Graham RounceParticipant @grahamrounce14492]

Can you not clamp a piece of hardwood (20mm?) to the metal before starting the drilling?

Or instead of clamping, drill a couple of 4mm holes for bolts and nuts to keep it in place, if you don't mind an extra couple of holes?

[Robin GrahamParticipant @robingraham42208]

Thanks for your suggestion Graham. It was helpful in a way you probably didn't anticipate – I think the phrase 'if you don't mind an extra couple of holes' sparked a realisation that I'd gone about this project arse about face. For alignment it would have been easier to clamp or glue the pieces together and drill new holes through into the backplate. I have an irrational fear of modifying bought parts – in this case a D1-4 backplate- it creates a blind spot in my vision of how to make a thing. I need to get over that!.

Robin

[Robin GrahamParticipant @robingraham42208]

This is how it's ended up.

Following advice I tried drilling CI at the lowest speed (475 rpm) on my drill press without a pilot – just spotting with a 90 degree bit (wot I had) then going through with the 135 degree 10.5mm split point. No chatter and a neat hole. So learnt something there. Thanks.

I'd assumed that the misalignment was the result of my inexperience, but I wasn't too happy with working round the problem without understanding where I'd fouled up on my first attempt.

I put the pieces together again (they are prevented from moving against each other radially by a register), rotated them, and found that the holes actually line up well in one of the four possible orientations. Surely that can only happen if the holes in the bought backplate aren't at 90 degrees to each other. So maybe I didn't foul up, except by taking the accuracy of the commercial product as an article of faith.

Robin

 

Edited By Robin Graham on 21/05/2019 23:52:52

Edited By Robin Graham on 21/05/2019 23:57:25

[HopperParticipant @hopper]

Well done. You are learning the meaning of the old saying that a skilled tradesman is not one who never makes mistakes, but one who knows how to remedy/cover up his mistakes.

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