Drilling a flat

[[JP]]

Hi all,
sorry for the lack of technical words but being a foreigner, albeit being here for 16 years, some words still fail me, therefore the ask for help here.

I have a large pillar drill, a Record one, drill anything without a problem. it takes MT2, but most of my drill bits are MT1, which I then use an adaptor.

Anyway, my question here...
I have a thick metal plate, say about 8mm.
On this plate I want to drill a hole for a 1/4 bolt to go through
So far easy enough.
Now.. I would like for the bolt head to be recessed in the plate
For this I would need to cut a recess with a say 3/8 flat drill bit deep enough for the bolt head not to be sticking out.

Now.. I have no clue on the words to use to search for this type of drill bit.
All I can find is bits for a milling machine... which takes me to a second question, can I somehow adapt one of those bits to my pillar drill?

note that I don't want to do any cross side cutting, just a simple flat.

thanks for the help.
JP

 

[MattH]

You want a counterbore.

http://www.ebay.co.uk/itm/HSS-End-M...hash=item3d26f655ac:m:mYy2eitIavAhpN0ahm70GOw

Drill the through hole first and then use the counterbore to cut the recess. The counterbore runs in the through hole.

 

[MattH]

P.s. make sure the workpiece is firmly clamped down otherwise it will be jumping around all over the place.

 

[[JP]]

thank you very much MattH!
I do have all the clamping apparatus on the drill bed, so making sure things don't fly away is not an issue.

Counterbore! that was the word I didn't know.. thanks again.

 

[[JP]]

Just another thing, looking at that link and others on ebay, using that type then I'm restricted to what through hole I have?

what I mean is, say i want a 8mm through hole and then a 15mm flat. the bit on the end of the 15mm counterbore drill won't go through the 8mm hole..
does that make sense?

is there other way?

 

[colnerov]

Hi, To avoid confusion do you want a counterbore or countersink?

Colin

 

[MattH]

Just another thing, looking at that link and others on ebay, using that type then I'm restricted to what through hole I have?

what I mean is, say i want a 8mm through hole and then a 15mm flat. the bit on the end of the 15mm counterbore drill won't go through the 8mm hole..
does that make sense?

is there other way?

Yes. They will have to match up. If you want a bigger through hole then it's no problem as you can open it up after cutting the counterbore but if you want smaller then you can't.
In that instance you'd need to use an endmill in a milling machine to create the flat.

 

[[JP]]

@colnerov - definitely the one on the right, the counterbore as it's to hide an allen cap bolt

@MattH - Ok, got it. I'll what's available, was more of a question really, as I wouldnt need the bore to be much bigger than the through hole, as usually you dont get an 8mm bolt with a 15mm head on it lol

thanks

 

[8ob]

Worth looking at the step drills as well, there may be one that will do the job. There are companies that will custom grind one the size you want.

http://www.sharptool.net/a12.htm

 

[Wendelspanswick]

If you have a decent bench grinder it's possible to grind a normal drill into a counterbore, it's easier to do on big diameter drills.

 

[Maxakarudy]

Couple of Q's
Does the bolt need to come out?, why a hexagonal head and not a allen head cap bolt?
If I wanted a recessed head I would mainly just countersink one

 

[outofthefire]

Couple of Q's
Does the bolt need to come out?, why a hexagonal head and not a allen head cap bolt?
If I wanted a recessed head I would mainly just countersink one

The OP said it was for a allen not hex head, and countersinking would be wrong because it is a bodge, a flat base on a bolt head should have a flat surface to bear down on, a countersunk head needs a countersunk hole.

 

[Pete.]

Counter-boring a 1/4" socket cap flush into 8mm plate doesn't leave much under the bolt to take the load, you might consider countersinking instead.

 

[[JP]]

the 8mm plate was just as an example, as I didnt think was relevant the accurate measurements as my question was more focused on how to do it and what its called.

but, it's in fact 5mm plate, where a 1/2" bolt will be driven through. this bolt can be allen cap or hex, not important, its just to hold the plate in place.
the plate will be small. I'd like to recess as much of the bolt as possible as then it will take another plate over it, couple of holes around with grub screws (basically to hide the bolt)

 

[Maxakarudy]

So if you drilll the hole, could you just weld some thread rod in.
BTW @outofthefire I never implied I'd counterbore a hole & put a countersink in it

 

[[JP]]

weld thread rod in the hole? dont get it...?
but no, dont think that is possible with what im doing

what I'm doing is, imagine it's like caps on the top of a motorbike suspension leg.
this leg is threaded 1/2-20.
make a plate with the recess, bolt this plate to the fork leg, then put another plate on top, with grub screws, to hide the bolt.