# drill size vs reamer size



## joe d (Apr 6, 2008)

Hi

I need to end up with a nice reamed 3/16 hole through 1 11/16 of brass: would 11/64 be a reasonable sized drill to use prior to reaming? (I'm hoping yes, as I have a carbide drill bit in 11/64 that might "wander" less than an HSS of whatever size). All advice gratefully received!

Joe


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## rake60 (Apr 6, 2008)

Joe my answer is out of personal preference.

For a reamed hole in brass I drill 1/32" (.031) under before reaming.
For a 3/16" hole I would drill it 5/32"

Rick


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## joe d (Apr 6, 2008)

Thanks Rake

Would it make sense to drill it 9/64 (the only other size of carbide bit I've got) and then re-drill with 5/32 HSS, and then run the reamer through? My major concern is to try to keep the hole from wandering off all squint, and my HSS bits don't seem to be all that stiff.

Joe


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## rake60 (Apr 6, 2008)

Brass is fairly free cutting.
I have drilled the 1/16" angled holes in brass steam cylinders that didn't 
break through dead nuts on center of the air circuit below the steam
chest ports, but a 5/32 bit shouldn't wander.

Rick


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## BobWarfield (Apr 6, 2008)

Let me preface by saying I like Rick's way better, BUT, for those who want way too much information...

I am making the flywheel and con rod for the Team Build. Both involved some precise holes, so I'll need to do some reaming (heh, I like the sound of that!). Having the same question, I went looking around on the web and found this interesting page:

http://www.yankeereamer.com/Tech_Info/tech_info.html

They provide a method to decide what size bit, as well as what size reamer, depending on the material as well as the tolerance you want to hold.

As I say, I like Rick's method better because it's simpler, but this was an interesting page for me to look at. 

Best,

BW


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## joe d (Apr 6, 2008)

Rick: Thanks again, I'll go with that. 

Bob: Thanks for the link, I like Rick's simple answer too, but I do like to browse the complex ones... although it keeps me out of the shop too much sometimes!

Joe


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## BobWarfield (Apr 7, 2008)

Was re-reading "Machine Shop Trade Secrets" again last night. I hadn't looked at it in a year or two. Always some new thing to pick up, and I was suprised at how many of his tips I've managed to internalize versus the first time I read it and wondered, "Now why would he recommend that?" LOL

The rule of thumb he provides leaves very little material for the reamer. Based on final desired diameter:

< 1/8": Leave 0.001 - 0.002 for the reamer.

1/8 to 1/2": Leave 0.005" for the reamer.

For larger reamers, leave 0.005 to 0.010".

Run the reamer at 1/3 the speed recommended for the twist drill of the same size, feed 1/3 faster.

Cheers,

BW


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## rake60 (Apr 7, 2008)

Personally I have never drilled a 1/8" to 1/2" hole that didn't drill
over the nominal size.

Drilling a hole that is .005" under that nominal size would be very interesting
to see. It must be possible or it would not be printed as such.

Rick


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## BobWarfield (Apr 8, 2008)

I agree Rick. But maybe the guy knows how much over his bits go. I guess when I get ready to ream my Team Build parts I will take some stock of the same brass and run some experiments to see what kind of holes my twist drills are giving me before I try the reamer. I'm thinking I'll just try to get as close to the reamer as possible and still be under. I gotta believe it'll be more than 0.001-0.005" though.

Cheers,

BW


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## Bogstandard (Apr 8, 2008)

What people tend not to realise, reaming isn't the be all and end all.

When reaming in the lathe, if the tailstock is a finite amount out of true, you will end up with an oversized hole, and maybe a taper on it as well, just depends how big the reamer is and whether it can withstand the bending forces. This can be fixed in the most part by having a floating reamer fitting on your tailstock (I keep meaning to make one, but never got a round tuit)

If at all possible, it should be done on your miller. Drill hole, ream. Not drill all the holes in the components, and then remount to ream. A tiny amount out of true when remounting, and you can again end up with an oversized hole as the reamer tries to follow the previously drilled, now slightly out of wack hole. It won't be a large amount out, but it just might be enough to turn a nice sliding fit into a bit of a wobbler.

John


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