Hole size for reamer

[Davewild]

Hi guys

Could you please help me out, I have just bought myself a set of reamers, and the question is how much smaller than the reamer do I drill the hole? some of you may remember I work offshore and have put together a portable workshop (Sherline )this is the second trip I will be taking the sherline and all the other tooling etc offshore, I will post some picture off the set up once its all together, still waiting for some tooling to arrive in the post, it has become much heavier than I expected, the reason for the above question is that I want to make sure I take the correct drill bits with me, I have purchased all the metal stock I require for my next project, will post some pics very soon.

Regards

Dave

 

[cobweb81]

Dave: I typically use a drill 1/64 smaller than the reamer size. So if you have a 115 pc drill set that should not be a problem, just use the next smaller fractional drill size below nominal.

Adam

 

[kvom]

I do 1/64 for holes up to 1/4, and 1/32 for larger holes.

 

[Brian Rupnow]

I generally go 1/32" under finish size, sometimes even more. I bought one of those 100 piece drill sets and about half of the drills aren't particularly straight. As a consequence of being not straight, about half the drills drill oversize. Trouble with drilling in a lathe is that the drill is stationary in the tailstock chuck, so you can't visibly see that its bent, as it doesn't rotate. I hate that sick feeling you get when you drill 1/64 under finish size and then when you ream the hole you immediately see that the reamer isn't cutting anything.---Brian

 

[jack620]

I hate that sick feeling you get when you drill 1/64 under finish size and then when you ream the hole you immediately see that the reamer isn't cutting anything.---Brian

Yep, that's what happened when I reamed the wrist pin holes in my ML Midge pistons. Instead of using 2.5mm drill rod for the pins I had to make 2.58mm rods and then make a 2.58mm D-bit reamer to enlarge the hole in the top end of the conrods. :wall:

 

[aonemarine]

I hate that sick feeling you get when you drill 1/64 under finish size and then when you ream the hole you immediately see that the reamer isn't cutting anything.---Brian

Yep been there done that more times than I care to mention. Ive been going 1/32 under, then ream 1/64 under, then ream the last 64th. I have found that I get a much more accurate hole and location by doing it this way. Maybe one day Ill by some shorty drill bits and see how much they help...

 

[Davewild]

Thanks everyone, as I'm using metric I think I will go 1mm under and then ream out the last 1mm in two stages 1/2 mm per reamer, or do you think this is too much, 1/32 is just under 0.8 mm, aonemarine what are shorty drill? Again thanks everyone.

Dave

 

[Swifty]

Dave, I think that 1mm under is way too much for reaming, I only ever allow 0.2 /0.3 for any reamer. I drill undersized first them use the reaming size drill, I have a set of reaming size drills that I have rounded the corners off slightly with a diamond lap, this helps them to cut on size.

Paul.

 

[gus]

Dave, I think that 1mm under is way too much for reaming, I only ever allow 0.2 /0.3 for any reamer. I drill undersized first them use the reaming size drill, I have a set of reaming size drills that I have rounded the corners off slightly with a diamond lap, this helps them to cut on size.

Paul.

Hi Paul,

I think this is reason why my reamed holes are always slightly bigger as I use 1mm under. Coming from a veteran Tool & Die Meister,I valued your expert advice.
My next project,the Rupnow engine will be better made than the Webster.
Must drop to visit Paul,talk engines and soak up his expertise. And Brian too.

 

[jwcnc1911]

I guess it all depends on what drill you have on hand and how they run (over/under size).

Having said that, every tooling rep who comes in the plant uses 2 to 3% as a starting point. When I'm finding a drill for a reamer in my home shop I find 2% and 3% and then find one in my box that is in between.

Say, for an 18mm drill:

Low side: 18*0.98=17.64
high side: 18*0.97=17.46

or for a 1/4in:
0.25*0.98=0.245
0.25*0.97=.2425

I how ever prefer to err in the way of MMC.

 

[Swifty]

Hi Gus, would love to catch up with you next time you visit family in Melbourne, I'm not far away.

Paul.