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Reamer Sizing - Am I missing something??

C

ccater1

Aluminum
Joined
Mar 12, 2017
Need to drill and ream for interference fit for some 3/8" dowels. Digging into the Machinery's Handbook I've found a recommended max ream size of .375" to a min of .3745" for a 3/8" standard, hardened, ground dowel pin.

Now looking at the dowel pins on McMaster, they are sized nominally at .375" with a diameter tolerance of +.0001" to .0003".

Looking at a 3/8" reamer on McMaster and it has a cutting diameter of .3750" with a cutting diameter tolerance from 0" to .0002".

If my math is correct, this means I could end up with a reamed hole diameter of .3752" and a dowel pin with a diameter of .3751", which equals a .0001" oversized hole to me.

Looking at undersized reamers and the next closest size that I can find at Mcmaster is .3740", which is .0005" under the min recommended size in the Handbook. Browsing on MSC's website, and I can't find anything in between, and they don't even spec the cutting diameter tolerance.

What am I missing? I'm missing something.

Thanks!

Casey
 
Welcome to reamer hell! Reamers are fuzzy math at best. For interference fits, I start with a 0.0005" under target, use fast feed rate, and don't let the reamer dwell too long in a couple of test pieces first. Also coolant coolant. Good luck.

Best Regards,
Bob
 
For most materials, you would be fine reaming to .374/.3745. Hell, even to .3748".

Get yourself the .374" reamer and have at it. Drill to around .365, a little more or less is fine.

As far as the "cutting diameter tolerance" goes. That's more determined by the amount of material left for reaming imo.
 
ccater1 said:
Need to drill and ream for interference fit for some 3/8" dowels. Digging into the Machinery's Handbook I've found a recommended max ream size of .375" to a min of .3745" for a 3/8" standard, hardened, ground dowel pin.

Now looking at the dowel pins on McMaster, they are sized nominally at .375" with a diameter tolerance of +.0001" to .0003".

Looking at a 3/8" reamer on McMaster and it has a cutting diameter of .3750" with a cutting diameter tolerance from 0" to .0002".

If my math is correct, this means I could end up with a reamed hole diameter of .3752" and a dowel pin with a diameter of .3751", which equals a .0001" oversized hole to me.

Looking at undersized reamers and the next closest size that I can find at Mcmaster is .3740", which is .0005" under the min recommended size in the Handbook. Browsing on MSC's website, and I can't find anything in between, and they don't even spec the cutting diameter tolerance.

What am I missing? I'm missing something.

Thanks!

Casey
Click to expand...

Fine tuning a reamer for size is a dying art. Everyone wants to pick something from an on-line catalog, letting someone else carry the knowedge.

It much like never using your brass reamers on steal.
Or using a reamer that has finished a few hole before it is used on "the good work".

A reamer can be "pushed" to cut a little large or smaller.
Much on the order of a chefs steel, or putting a burr on a cabinet scraper.

IT's not hard, nor risky. But it helps to have a good HARD piece of steel, and a strong hand glass.
 
As mentioned, the number etched on the reamer does not guarantee you will get that size hole. One way to cope is to have dowel pins that are over size, on size and under size. These are stock items, sold by McMaster and others.

McMaster-Carr

They make reamer sets in standard fractional sizes minus .0005" and plus .0005" and even make sets of nominal fractional sizes. Coupled with the three classes of dowel pins, you have a wide choice of press to slip fits easily obtainable. You can also find reamers sized in .001" increments in about any size possible.

Larry
 
L Vanice said:
As mentioned, the number etched on the reamer does not guarantee you will get that size hole. One way to cope is to have dowel pins that are over size, on size and under size. These are stock items, sold by McMaster and others.

McMaster-Carr

They make reamer sets in standard fractional sizes minus .0005" and plus .0005" and even make sets of nominal fractional sizes. Coupled with the three classes of dowel pins, you have a wide choice of press to slip fits easily obtainable. You can also find reamers sized in .001" increments in about any size possible.

Larry
Click to expand...

Excellent explanation! Seems like it would be wise to purchase all three sizes of reamers and dowels for each fractional size each time I'm aiming for a given size. Would be more expensive, but at some point you would have everything you need when working with common sizes.
 
I would avoid non-standard dowel pins if at all possible.
You would just be setting a trap for the next guy.
Been there, done that.
Non-standard dowel pins are last resort.

Get a pair of over/under reamers, and another pair of dowel pin reamers.
Shouldn't be hard to find.
Those four reamers will handle almost all your requirements.

There is some good advice posted here on technique.
Pay attention.
 
Don’t overthink this, get the .3745” or even the .3740” reamer. Do the holes, push the pins in. Do you have a decent sized press to work with? You only really need to worry if these pins are going into something hardened.
 
Leg17 said:
I would avoid non-standard dowel pins if at all possible.
You would just be setting a trap for the next guy.
Been there, done that.
Non-standard dowel pins are last resort.

Get a pair of over/under reamers, and another pair of dowel pin reamers.
Shouldn't be hard to find.
Those four reamers will handle almost all your requirements.

There is some good advice posted here on technique.
Pay attention.
Click to expand...

Much appreciated, thanks!


newtonsapple said:
Don’t overthink this, get the .3745” or even the .3740” reamer. Do the holes, push the pins in. Do you have a decent sized press to work with? You only really need to worry if these pins are going into something hardened.
Click to expand...

Gotcha. Consider the .3740" reamer in the cart. Yes I have access to a large press, and I'm pressing into .5" aluminum. Any chance I could press this .5 ID x .75 OD drill bushing into a (.5" thick aluminum) hole reamed with a .750" reamer or do I need to get the .001" oversized reamer? The OD of the busing is oversized to .7515" to .7518": Link to bushing They are quite proud of .75" reamers lol.. Would rather get the standard size if it will work. If not, I'll just have to buy what's needed.

Thanks
 
ccater1 said:
Much appreciated, thanks!




Gotcha. Consider the .3740" reamer in the cart. Yes I have access to a large press, and I'm pressing into .5" aluminum. Any chance I could press this .5 ID x .75 OD drill bushing into a (.5" thick aluminum) hole reamed with a .750" reamer or do I need to get the .001" oversized reamer? The OD of the busing is oversized to .7515" to .7518": Link to bushing They are quite proud of .75" reamers lol.. Would rather get the standard size if it will work. If not, I'll just have to buy what's needed.

Thanks
Click to expand...

Its aluminum, you could press that bushing in if you only drilled the hole to 11/16ths. It wouldn't be pretty, but you
could do it. Its ALUMINUM.


And just for fun. When I'm making fixtures and I need a press and a slip. Say 1/4". I'll just put a .2505" reamer in, and
use it for the press and slip.. Then take a 1/4" form tap and run it into the "press" holes with a cordless drill... works great shrinks it up just enough so that the dowel bites good.
 
Do you think that reaming .0015" to .0018" undersize for the drill bushings will affect the operation of the jig in any way? I would think it wouldn't make a difference, but if I knew, I wouldn't be asking lol.. Here's a rendering of the jig. It's 1/2" aluminum plate with drill bushings and dowels as locating pins.


 
ccater1 said:
Do you think that reaming .0015" to .0018" undersize for the drill bushings will affect the operation of the jig in any way? I would think it wouldn't make a difference, but if I knew, I wouldn't be asking lol.. Here's a rendering of the jig. It's 1/2" aluminum plate with drill bushings and dowels as locating pins.


Click to expand...

Hmm A drill jig..

If the "reamed holes" come out over size, there is always loc-tite.
 
CalG said:
Hmm A drill jig..

If the "reamed holes" come out over size, there is always loc-tite.
Click to expand...

Well my dilemma is that I don't currently posses a 3/4" reamer of any size, so I have to buy one. The cost for a 3/4" reamer from McMaster is roughly $80. So I'd prefer to buy a nominal size reamer that I'm more likely to use in the future than to purchase the .7510" reamer that may not see much use after this project. If I can get acceptable results using a .750" reamer for a press fit bushing with an OD of .7515" to .7518" then that's what I'd prefer to do. If not, I'll just fork out the dough for something that will probably only be used once.
 
don't worry

the 3/4 reamer will be fine with 3/4 drill bushings.


You are WAAYYY over thinking this thing.

It's a freak'n drill jig! Do you think a guided drill will hit on the money within a tenth? And how to you propose to get those jig pins into anything. Butted up to edges I hope.
 
A vertical mill with a boring head can produce precise holes in a very wide range of diameters. You don't have to have a reamer for every size hole you want to make.

It is also possible to drill a hole with a 47/64 bit and follow it with a 3/4 bit to produce a hole close enough to .751 to hold your bushing. And Loctite is your ace in reserve (or even your ace in the hole, I suppose).

Larry
 
Just another thought...couldn't you just circular interpolate the holes sizes with an endmill? Personally, I'd make the press fit no more than .001. IMHO, any more than that, and the bushing could lose accuracy in final placement due to material displacement, (especially if the top edge isn't countersunk a bit)
 
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