# Reaming 13mm Holes..what Size Drills?



## EmilioG (Apr 9, 2015)

I need to ream some through holes in brass square 3/4", what size drill bits can I use
before reaming? Could I use 1/2" drill size then ream 13mm or is that too much to ream?
Is 12.5mm drill size better?  Is there a drill/reamer chart ?


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## mikey (Apr 9, 2015)

I like the Yankee Reamer method to gauge drill size because it takes the machinability of the material into consideration. You can go here to view the method and to download a pdf guide:

http://yankeereamers.com/tech-info/


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## EmilioG (Apr 9, 2015)

For a 13mm reamed hole, the closest drill I could find is 12.5mm.


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## Doubleeboy (Apr 10, 2015)

What ever drill is about .003 to .007" smaller is what I would use.  I like to have enough metal to remove that I get a good bite, but not so much as to strain the reamer.   You could also bore the hole and then ream.

michael


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## EmilioG (Apr 10, 2015)

I found a Guhring 12.9mm drill bit on Msc but it costs $49!


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## mikey (Apr 10, 2015)

My concern would not be so much about the pre-reamed hole size, but the finished hole size. If you need 13mm and you use a 13mm reamer you may come in oversize on the hole. If that happens then you need to buy an undersized reamer or resort to boring the hole. What are your tolerances?

If the hole size is not that critical then I would probably use a 31/64" drill; this will leave you just under 0.003" to ream out and it might be satisfactory. Maybe try it in some scrap?


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## stupoty (Apr 10, 2015)

As mikey says do some test holes and check the sizes.  I had left my drill index at work the other day and pre drilled with a 9mm drill As it was the closest I had to hand, then reamed It to 0.390 which is about 30-40 though reamed, it was just a 10mm thick piece of mild steel and it went through like butter.

When ever I try and get too close to final size with drills the hole dosn't come out cleanly.  I probably need better drill bits 

I don't use brass very often though and I think I have only reamed brass once, I dont remember it being traumatic though.

Good luck,

Stuart


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## EmilioG (Apr 10, 2015)

mikey said:


> My concern would not be so much about the pre-reamed hole size, but the finished hole size. If you need 13mm and you use a 13mm reamer you may come in oversize on the hole. If that happens then you need to buy an undersized reamer or resort to boring the hole. What are your tolerances?
> 
> If the hole size is not that critical then I would probably use a 31/64" drill; this will leave you just under 0.003" to ream out and it might be satisfactory. Maybe try it in some scrap?



Unfortunately, a 31/64" drill is 13.10 mm, larger than the finished hole size.  I'll need a 12.9 mm drill bit.
or 12.7 mm.  They're expensive.


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## randyc (Apr 10, 2015)

Like Stuart, I rarely ream brass but the last time that I did, I seem to recall that the reamer cut just a teeny bit small-ish.  I like Michael's suggestion of boring the pilot if it isn't too deep, this will insure a straight reamed hole.

My personal preference for reaming is sort of complex but as a HSM, I have plenty of time:


Drill a pilot hole that is about 1/2 the diameter of the reamer.
Slowly plunge an end mill (held in the tailstock drill chuck) through the pilot hole.  End mill size should be approximately the next fractional size up from the pilot drill.  If the end mill flutes are not long enough for the pilot hole, just plunge it as deep as possible and call it good.  (The idea is to produce a straight starting bore for the final pilot drill.)

Drill the final pilot hole, generally .002-.003 under the reamer size.  If no appropriate drill is available and boring is not an option, perhaps make a "D" drill.  You'll have it next time you need the reamer.

Ream at slow spindle RPM, plenty of cutting oil, pushing the tailstock by hand along the ways to feed the reamer.
The end mill acts like a boring bar and "straightens" out the first pilot drill.  The last pilot drill will tend to follow the hole bored by the end mill rather than wandering.  This will give you a better starting hole for the reamer.

Reaming can be a real PITA and it's possible that holes can end up larger or smaller than the nominal reamer size.  The type of cutting oil can influence the finish diameter as can the work material and the pilot hole.  As several other members have suggested, no matter how you decide to do this and what size pilot drill you use, it's a good idea to run a practice part first


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## mikey (Apr 10, 2015)

EmilioG said:


> Unfortunately, a 31/64" drill is 13.10 mm, larger than the finished hole size.  I'll need a 12.9 mm drill bit.
> or 12.7 mm.  They're expensive.



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.


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## EmilioG (Apr 10, 2015)

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.


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## EmilioG (May 2, 2015)

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.


31/64" drill (.4844) will leave .003 material left for a 13mm (.5118) reamer..?
.5118 - .4844= 0.0274


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## T Bredehoft (May 2, 2015)

_31/64" drill (.4844) will leave .003 material left for a 13mm (.5118) reamer..?_

 equals .0274, I suspect .030 was intended.


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## randyc (May 2, 2015)

T Bredehoft said:


> _31/64" drill (.4844) will leave .003 material left for a 13mm (.5118) reamer..?_
> 
> equals .0274, I suspect .030 was intended.



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.


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## EmilioG (May 3, 2015)

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.


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.


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## janvanruth (May 3, 2015)

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


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## EmilioG (May 3, 2015)

Thank you.  Guhring was nice enough to send the 12.9mm drill free of charge.


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## randyc (May 3, 2015)

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



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 ?


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## EmilioG (May 3, 2015)

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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## TOOLMASTER (May 3, 2015)

i use 1/2" ..works fine


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## EmilioG (May 4, 2015)

For a 13 mm reamer, that's a lot of material to remove, .030.  That would put a lot of stress on a reamer
and shorten it's life I would think.  I wrote to Yankee Reamer and I'll let you know what they write.
For now, I have a 12.9 mm drill bit that I'm going to test first. Thanks.


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## w9jbc (May 4, 2015)

typically 0.015 is what was considered reaming tolerance on the automatics


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## EmilioG (May 4, 2015)

Yankee reamer wrote back and suggested using a 12.5 mm drill.  He wrote that most drills will
leave an oversize hole, or use a 31/64" drill followed by an undersize 1/2" reamer then finish with the 13mm reamer.
The 12.9 mm drill leaft the 13 mm hole oversize, 13.02 mm, which is fine for my project.  It's a loose slip fit manifold type part.


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## EmilioG (May 6, 2015)

Here are my results.  I used a 31/64" stub length drill bit to open the initial starter hole.
Then  I used a 1/2" AP undersize reamer and finished with an AP 13mm reamer.  I got a nice smooth hole
.006 over.   The 12.9 mm drill was giving me .020 over.  In the future I will use a 12.5 mm drill instead of the
1/2" undersize reamer.


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