Central hole in round stock

[Andy BrocklehurstParticipant @andybrocklehurst85292]

Probably a daft question but I’m still learning lathe techniques.
Im turning down some bronze that needs a perfectly concentric hole in the centre. To achieve this should I turn the bronze down to my finished OD then drill and ream my centre hole or drill and ream the hole first while the stocks thicker and more ridged and the turn my stock down to my required OD or doesn’t it make any difference and I’m over thinking things?

Thanks

[Howard LewisParticipant @howardlewis46836]

If the central and external machining are done without disturbing the workpiece, the two should be concentric.

[JasonBModerator @jasonb]

Although it will be concentric at one chuck setting there is a slight chance of teh OD increasing if the wall thickness is thin due to the bronze being soft and a less than sharp reamer could make the OD swell.

Rough out the OD leaving 0.2mm on diameter, drill & ream then finish the OD with a sharp tool.

[Charles LamontParticipant @charleslamont71117]

What Jason says, but for really accurate concentricity you should drill, then bore undersize (0.2mm to 0.1mm on diameter, but other people will have different ideas), then finish by reaming. A drilled hole can wander off centre, and if it is off, the reamer will follow the hole. Boring will bring the drilled hole true.

To make something with a long hole that needs to be concentric, a spindle, for example, then the bore is best finished first, and then the job mounted between centres on a true-running mandrel for finishing the outside diameter.

[Charles Lamont]

On Charles Lamont Said:

What Jason says, but for really accurate concentricity you should drill, then bore undersize (0.2mm to 0.1mm on diameter, but other people will have different ideas), then finish by reaming. A drilled hole can wander off centre, and if it is off, the reamer will follow the hole. Boring will bring the drilled hole true.

To make something with a long hole that needs to be concentric, a spindle, for example, then the bore is best finished first, and then the job mounted between centres on a true-running mandrel for finishing the outside diameter.

My finished ID is 2.37mm (depending on the pin stock im using I use various sized reamers around that size) can I bore holes that small? Finished OD 4.75mm. I’m making bushings for folding knives.

[Charles LamontParticipant @charleslamont71117]

Ah, probably not.

[Peter Cook 6Participant @petercook6]

You can bore that small, but given what you are making – I presume less than 10-15mm long – I suspect any discrepancy is likely to be fairly small. If you spot the centre before drilling, and use as short a drill as possible that should eliminate any initial wander.

If the length of the bush is less than 10mm, I would consider using the small carbide drills sold for PCB drilling. They are stiffer than steel drills and less likely to wander – but they do snap easily if abused.

[bernard towersParticipant @bernardtowers37738]

Make a 2.37mm d bit

[Andy BrocklehurstParticipant @andybrocklehurst85292]

Thanks everyone, some great advice.

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