Reaming 13mm Holes..what Size Drills?

[EmilioG]

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 ?

 

[mikey]

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/

 

[EmilioG]

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

 

[Doubleeboy]

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

 

[EmilioG]

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

 

[mikey]

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?

 

[stupoty]

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

 

[EmilioG]

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.

 

[randyc]

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

 

[mikey]

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.