Reaming 13mm Holes..what Size Drills?

sorry, my mistake, I was looking at 33/64". Yes, 31/64" is .4844 12.3 mm
The 13 mm is not that critical. I just want a good 13mm reamed hole for the rods
that I'm using and the size of the brass square. I may powder coat the through rods
so that will add about .006". I have a nice Guhring 1/2", 6 flute endmill that should work well for boring a clean pilot hole.

Thanks for all the great replies, ideas and technical info.

I will of course do some tests first. :)
 
Last edited:
mikey said:
Odd ... 31/64" = 12.3mm, not 13.1mm. This drill should give you 0.003" to ream and while that is probably on the minimal side it should be enough to guide reamer adequately.
Click to expand...
31/64" drill (.4844) will leave .003 material left for a 13mm (.5118) reamer..?
.5118 - .4844= 0.0274
 
31/64" drill (.4844) will leave .003 material left for a 13mm (.5118) reamer..?

equals .0274, I suspect .030 was intended.
 
T Bredehoft said:
31/64" drill (.4844) will leave .003 material left for a 13mm (.5118) reamer..?

equals .0274, I suspect .030 was intended.
Click to expand...

I think so too, Tom, and I suspect that most of us wouldn't attempt to remove that much material with a reamer :) It was a decimal point error, I guess - in practice, we'd notice that the pilot diameter was way too small and correct the problem.
 
randyc said:
I think so too, Tom, and I suspect that most of us wouldn't attempt to remove that much material with a reamer :) It was a decimal point error, I guess - in practice, we'd notice that the pilot diameter was way too small and correct the problem.
Click to expand...
Yes, .030 is way too much even for brass. A 1/2" drill will leave .030. I'm going to test the 12.9mm drill Guhring sent for me to test.
 
if you are ever in the same situation again and dont want to buy an expensive special size drill:
take the next smaller one you do have and grind it assymetrically one lip longer than the other
you will find it will cut oversize
trial and error will bring you up to the size you need
 
Thank you. Guhring was nice enough to send the 12.9mm drill free of charge.
 
janvanruth said:
if you are ever in the same situation again and dont want to buy an expensive special size drill:
take the next smaller one you do have and grind it assymetrically one lip longer than the other
you will find it will cut oversize
trial and error will bring you up to the size you need
Click to expand...

That is an excellent tip and I can easily see how it would be effective for a new hole but wouldn't the drill tend to self-center if there is an existing pilot hole ?
 
I don't know. I've never done that. I think the holes would come out elliptical and not round.
My tolerance is +0.020 - 0.000, so I think the 12.9 mm drill will work. Thank you all.
 
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