Repairing a hole

I'm not a beginner, but this seems to me to be a topic that beginners should know as much about as possible - as soon as possible. Clearly, I didn't...

The complicating issue in this case is that the hole is in hardened steel (a moving vice jaw). The hole diameter is perhaps 2.5mm (haven't got the thing here to measure), is about 10mm deep, and is blind. Apparently, it got chewed up by attempts to remove a roll pin. The first few mm are certainly no longer cylindrical, perhaps .5mm oversize, but it's probably as manufactured, deeper down.

I have access to a not-particularly rigid mill (Emco FB2). I have been asked to 'make the hole round again', so a removable dowel can replace the roll pin. Its diameter doesn't matter too much - it could be 1mm oversize, but the centre position should be retained as well as can be managed. I'm thinking of using a drill bush to locate the hole and constrain the cutter, as it will be cutting off-centre, at least to begin with.

The first question is: what cutter, drill, burr, whatever, would be best? I have broken a few carbide twist drills, in the past, so am nervous about brittle, small-diameter cutters, particularly when cutting 'on the corners'.

...and the second question, the answer to which, had it been known earlier, would have saved all the trouble: how the hell are you supposed to get (small) roll pins out of blind holes?

Spark erosion or second choice carbide slot drill, but 2.5mm is pretty fragile.

Emgee

+1 for spark erosion, don't know much about it but I've had much the same problem - albeit a bigger diameter - solved by someone who had the kit and knew how to use it.

Drilling is asking for trouble, and offers every chance of making the problem worse. Next paragraph potentially should be a thesis on how to remove a broken carbide drill from a blind hole. (Spark erosion!)

Can you soften/anneal the workpiece. If you coat it in soap and then wrap the item in iron wire most of the oxide will be kept off it. Then you might (perhaps, with a following wind and a whole bag of luck) stand a chance with a drill bush and a reamer. Perhaps. Now re-harden. Oh yeah!

Make another jaw from scratch. Spark erosion is suddenly looking economic.

As for removing a roll pin, rumour has it if you can stuff plasticine or putty down the hole in the middle, then insert a wire piston (tiny twist drill?) and give it a whack. The hydraulic forces will push on the bottom of the pin and (maybe) expel it. Perhaps. Seems like a bit of a fairy tale, but it works with bigger things like bearing outers.

Simon - not a fairy tale, the hydraulic method definitely works if the dowel is hollow. I've also drilled and angled hole down to where I thought the end of the pin was, and done the hydraulic trick, for solid dowel pins fitted blind. (they never should be, but some machine designers insist on doing so.)

Heavy grease is often used for removing bushings and bearing races this way in car and truck parts.

Heating the surrounding metal fast with an A-O torch, then using vise-grip / mole grip pliers to twist and pull them out, can sometimes free small blind pins.

Depending on how hard the jaw is... I bought a set of "cobalt HSS" drills and they are great for this sort of job - went straight into my Dickson toolholder block to make a blind dowel hole. Your steel may be even harder though...

Locate the work with a ground steel rod the original diameter into the undamaged lower end of the hole. Drill a bit oversize and replace the dowel with a roll pin.

Hi I have done something similar to make a bush by loading it on a face plate to hold the part in the correct place then I used a small 1.5 mm diamond rotary cutter in the dremmel attached to the toolpost. Then as the lathe rotates slowly feed in the diamond wheel cutter to enlarge the hole and make it parallel slowly.

David

Premium brand solid carbide centre cutting end-mill ought to be up to the job.

Used one to remove 3 broken off 2.5 mm taps in cast iron with a centre cutting carbide end mill. Machine was a Bridgeport and hole was clean enough to complete tapping with no apparent issues. Maybe a bit slacker than I'd have liked but did the job just fine.

Don't recall if it was 3 flute or 5 for that job as I got both. Not cheap but worth it to rescue about 2 days worth of work. Unfortunately both eventually died on other rescue missions, mostly due to me getting a bit over-confident in their capabilities, so I can't go and look up the brand.

Best guess is Nachi via E-Bay.

Clive.

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Forgive me if I am being dim, but ... My understanding of the opening post is that the pin is already out, and Kiwi Bloke just wants to make the butchered hole round again.

MichaelG.

Kiwi Bloke 1, I had to open out an 11.2 mm hole to 12 mm on a Dickson type toolpost, I used cobalt drills and carbide drills on my Chester V20 mill, they were wrecked. I think it all depends how hard the hardened steel is, mine was bloody hard and the only tool that would work was a 12 mm diamond core drill. I think you will need diamond to make any headway, sorry to be the bearer of bad news.

Dave W

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Since the pin is out, there's no need for a slot/centre cutting endmill. I'd go for a 3 or 4mm three or four flute carbide endmill, after having centred the hole. I would cut from the 'good' side, so that when the end mill got to the damaged end there was enough support to discourage the flutes from cutting the sides of the hole. You could also use a carbide drill from that side, but might want to control the feed to lessen the load on the 'weaker' drill.

Thanks folks for all the suggestions. They've freed up the gummed-up neurones. I'd looked at the job and decided that it would be difficult - probably because of the small diameter - and then seemed to have just developed tunnel vision, seeing the only possibility as some sort of guided re-drilling procedure. I'd never considered what might be called a generation method (internal cylindrical grinding, boring, etc.). Now the neurones are free again, all sorts of alternatives present themselves.

Sorry for the unclear, rambling initial post. I tend to post after I should have gone to bed... The roll pin is out (not sure how it was done). It was in a blind hole, and punched below the surface, so there was nothing to grab. Why do designers do things like this? I wouldn't have thought hydraulic methods would work with roll pins, because of the large leakage path, but I've removed bushes hydraulically - very satisfying! Perhaps Plasticene and taking it by surprise would have worked. Must try to remember that idea for the next time...