# Hole size for reaming?



## rodw (Feb 6, 2013)

Hi guys, I am fitting a BXA toolpost to my new lathe and tomorrow's job is to drill and ream a 12mm hole about 40mm deep in the centre of the 16mm bolt I bought today. I've already drilled an 8.5mm hole right through the piece (as an 10 thread is required as well) and have a set of unused chucking reamers to do the job with.

I just wanted to check the hole size to use. From what I have read, it seems that an 11.5mm hole is the right size to go for. That is 0.5mm or 0.002" undersize. 

It just sounds too easy to use a standard sized drill. Should I be using a fractional drill? I do have a set of imperial Letter and Number drills that I can use for fractional sizes if required.

Anyway, I just wanted to check this before diving in and breaking something.


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## Woodster (Feb 6, 2013)

.5 mm is 20 thou undersize. That may be a bit much. 11.8 mm would leave you 8 thou.


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## Tin Falcon (Feb 6, 2013)

IIRC rule of thumb use one drill bit size smaller ie 1/64 or 0.015 undersized run reamer at 1/2 speed or drill for machine reamers a few thousands for hand reamers. 
http://www.mwdropbox.com/tutorials/ARMY-TC-9-524/ch4.pdfpage 4-24 covers reamer use.

Tin


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## Brian Rupnow (Feb 6, 2013)

Be aware though--Unless you have really good top quality drills, the slightest bend AT ALL in a drill 1/64" undersize will drill an oversized hole and you won't have any material left to ream. I have learned thru hard experience that it is far safer to go 1/32" under finished hole size. This doesn't seem to impair the function of the reamer nor the size of the finished hole in any way, and it keeps from scrapping parts where the size of the reamed hole is critical.


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## MuellerNick (Feb 6, 2013)

Assuming you are reaming steel (do you?) up to 850 N/mm^2, undersize should be 0.2 &#8230; 0.3 mm.


Nick


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## kvom (Feb 6, 2013)

I was taught to go 1/64 undersize up to 1/4" hole, and 1/32 for larger.


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## gus (Feb 6, 2013)

Tin Falcon said:


> IIRC rule of thumb use one drill bit size smaller ie 1/64 or 0.015 undersized run reamer at 1/2 speed or drill for machine reamers a few thousands for hand reamers.
> http://www.mwdropbox.com/tutorials/ARMY-TC-9-524/ch4.pdfpage 4-24 covers reamer use.
> 
> Tin



Thanks.Tin .D/loaded into my Machinshop folder for reading. Covers topics I missed out in Trade School. Like part on drill grinding.My reaming practice is hit & miss. OK So its 1/64". 

Come tomorrow will windowshop at TokyuHands,Osaka,Japan.The Japanese have very good reading material on metal machining.Too bad my Japanese not the best.Only good for reading Train Station Names.


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## rodw (Feb 6, 2013)

Thanks everybody, awesome information provided while I was sleeping on the other side of the world!

I can see I need to get a set of metric fractional drills. I will go with 29/64 which is 0.019" under, 15/32 is only.004" under. When I consult my drilling chart, I can see the best fit would be 11.8mm which I don't have and I don't know where to grab one locally.


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## rodw (Feb 7, 2013)

This worked a treat, so thanks everybody again. Full details here!
http://www.homemodelenginemachinist.com/f28/yet-another-qctp-thread-bxa-al320g-20078/#post209450


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