Ream and Drill Size

seal killer · Post by **seal killer** » Thu Aug 06, 2009 10:21 am

All--

I've reamed holes in aluminum before with mixed results. They were mixed due to my total inexperience and the fact that I used a less than stable setup: a ChiCom drill press.

Now, I need to ream small holes in brass and bronze and stainless steel and drill a couple in stainless steel. I don't know what size of drill bit to use for each different diameter hole. Nor, do I know if the workpiece material makes a difference. Therefore, I have some questions. As you will quickly see, most of the questions are the same, differing only in materials and diameters.

By the way, the book from which I adopted the plans merely called out the materials as "brass", "bronze" and centerless ground stainless steel for the piston. (I don't even know what centerless ground stainless steel might be, so I hope to use a stainless steel dowell pin for the piston.) The links below are to the workpiece materials I have already bought from McMaster-Carr.

Bronze 932

I need to ream a 3/32" diameter hole. Should I use a 5/64" drill bit and then use a 3/32" reamer to take the remainder?

I need to ream a .250" diameter hole. Should I use a 7/32" drill bit and then use a .250" reamer to take the remainder?

Brass 360

I need to ream a 3/32" diameter hole. Should I use a 5/64" drill bit and then use a 3/32" reamer to take the remainder?

I need to ream a 5/32" diameter hole. Should I use a 9/64" drill bit and then use a 5/32" reamer to take the remainder?

Stainless Steel Dowell Pin (I've got a big set of them from McMaster-Carr.) This will be the piston. It needs to be hollow and have a crankpin hole drilled in it.

I need to drill a 3/16" blind hole in the stainless steel dowell pin. This will make the piston hollow. Should I just use a 3/16" TiN-coated drill bit?

I need to ream a 1/16" hole in the stainless steel piston (dowell pin) for the crankpin. Shoud I use a 3/64" drill bit and then a 1/16" reamer to take the remainder?

Stainless Steel (I need to buy a length long enough to fit in my H/V saw vise and cut off a 3/16" long x 1/2" diameter piece. Looks like I will have a "little" left over!") This little piece of stainless steel will be a disk 1/2" in diameter and 3/16" in length. It will be the crank of the crank shaft.

I need to perform a press fit using a 1/16" diameter length of stainless steel (or a 1/16" drill bit). How large should the hold be in the stainless steel workpiece to accept this 1/16" press fit?

I need to perform a press fit using a .093" diameter length of stainless steel (wonder if a #42 drill bit [.0935"] would work?). How large should the hole be in the stainless steel workpiece to accept this .093" press fit?

What drill bit and reamer materials would you use to perform these operations? Stick with HSS? Something different depending on the material?

Finally, I will have plenty of material with which to experiment, so I can screw up multiple times without loss. However, I hate to break bits and reamers. If I can only break a few on this project I would be happy!

Thus, these questions. (I understand that I may break more than a few working brass, bronze, and ss.)

Thanks (a lot)!

--Bill

Willy B · Post by **Willy B** » Thu Aug 06, 2009 10:45 am

Bill, can't answer your specific questions. In general, I try to drill within a 1/64 of the final size, regardless of material. Never have reamed stuff as small are you will, I'd try to get within a few thou on the small stuff.

As to the press fit hole holes, machine reamers are available in "over and under" configuration, usually one thou. Very handy things. Ream a hole "under", drive the pin, ream the mating part "over", perfect sliding fit on the stock pin (or drill rod). I'd just go with whatever is available.

Bill

abarton · Post by **abarton** » Thu Aug 06, 2009 11:17 am

Seal Killer,

I just bought some 1/8" dowel pins from McMaster-Carr. McMaster described them as being just slightly "over". Like .0001 to .0003 IIRC. So I reamed a hole to .125 and pressed the pin in and it stayed put. (I was exploring new skills that I might use on an upcoming project.)

But, my point is, you may not need a just-under reamer for your pins.

seal killer · Post by **seal killer** » Thu Aug 06, 2009 12:58 pm

Alexander--

Thanks. I had forgotten that they might be over. I've had them a while. I'll be sure to mike what I intend to use and THEN order the tools, if I don't already have them. (I won't have those little reamers!)

--Bill

seal killer · Post by **seal killer** » Thu Aug 06, 2009 1:05 pm

Alexander--

0.0002" over!

So, what is a good press fit for a 0.2502" pin?

--Bill

kvom · Post by **kvom** » Thu Aug 06, 2009 1:23 pm

Bill,

I suggest you mosey over to one of the forums dedicated to model engine building. I hang out on both http://madmodder.net and http://www.homemodelenginemachinist.com/. Youy will find lots of people willing to advise on engine material and construction.

If your engine is designed to run on air, then you have lots of choices for materials, and tolerances are less critical than for IC and steam. Usually it's easier to bore or ream the cylinder and then turn the piston to fit, rather than vice-versa. For a brass/bronze cylinder an aluminum piston will work fine and be easier to machine than SS.

LivingLegend · Post by **LivingLegend** » Thu Aug 06, 2009 1:48 pm

Here's the deal....

To ream a hole for a dowel pin, you use an undersize reamer. These reamers are commonly refered to as "Dowel Pin" reamers and are usually .0005 or .001 under the nominal dowel diameter.... In other words, a reamer for a .250 dowel will be .2495 in size. There are also oversize dowel pin reamers which allow for clearance on the pin. The oversize dowel reamers are usually .001 oversize.

Reaming a hole for a dowel pin using an on size reamer.... example: a .125 reamer for a .125 dowel or a .1875 reamer for a .1875 dowel.... will result in what will usually turn out to little more than a tight push fit at best.

LL

Post by **Harold_V** » Thu Aug 06, 2009 1:57 pm

Here's a few tips that can save you a lot of grief.

If you want holes to come out on size, don't use the finish diameter to establish the hole. Those of us that have worked in industry, where hole size is often tightly controlled, use drills that are slightly undersized to drill the hole, then open the hole with the desired drill. That guarantees the hole won't be oversized, and often yields a better surface. Great care must be exercised in brass or bronze when taking the hole to size, for it hogs easily. Most other materials will double drill without issues.

On small items, anything more than a couple tenths press fit is likely to lead to problems you can't overcome. If you provide a thou press for a 1/16" pin, it will crush before it will fit as hoped. Half a thou press fit for a ¼" pin should be looked upon as a maximum press, especially if pressing in steel.

When reaming, as the hole gets smaller, less material should be provided for the reamer. 1/64" for a 3/32" reamer is too much material, and likely to bind the reamer in the hole if deep holes are reamed without clearing chips. The less material you can leave and still get a cleaned up hole, the better. Use number drills, not fractional drills, when working within the range you speak of.

Drills almost never provide a hole that is acceptable for a press fit. They rarely cut straight or round, and rarely cut a size that is acceptable when looking for proper fits. Even double drilled holes tend to be disappointing.

Centerless ground stock isn't generally all that good. Such material, unless purchased as being precision sized, isn't ground in a centerless grinder, but in a centerless belt sander. It tends to be less than round and straight. Your idea of using a dowel pin for stock is actually a very good choice, assuming you can buy the grade of stainless that is desirable. You will enjoy greatest success if the material is 303 or 416.

Dowel pins are sized in a centerless after they have been machined to length. A centerless grinder is far different from a centerless sander, so the pins will be round, with a superior finish, and will be held on size, unlike bar stock.

Harold

Willy B · Post by **Willy B** » Thu Aug 06, 2009 2:03 pm

Bill, here is an "Over and Under" reamer set. (second set on the page)
http://www.shars.com/product_categories ... mers__Sets

I currently have a couple of undersized reamers and like them so much I decided to go whole hog. Ordered a set Sunday, arrived yesterday but have not had time to even open the box!

Bill

seal killer · Post by **seal killer** » Thu Aug 06, 2009 2:19 pm

Bill and Harold and LL and kvom and Alexander--

I really appreciate all the help. Now, I have to digest it to figure out how to procede on this project.

Thanks!

--Bill

BMyers · Post by **BMyers** » Thu Aug 06, 2009 2:53 pm

All you need to know about sizing a hole to ream
http://www.yankeereamer.com/Tech_Info/tech_info.html

seal killer · Post by **seal killer** » Thu Aug 06, 2009 2:58 pm

Harold--

If I am correctly interpreting your advice, on all the small holes on which I need to press fit, what I should actually do is drill undersize by a "certain amount" and then finish drill at the correct diameter.

What would a "certain amount" undersize be for a 0.0625" (1/16") diameter hole, 0.0938" (3/32"), and a 0.15625" (5/32") diameter hole?

Would the "certain amount" undersize drill bit simply be the next higher numbered number drill bit?

In other words, like this . . .

Use a #53 to open the 1/16" diameter hole.
Use a #42 to open the 3/32" diameter hole.
Use a #23 to open the 5/32" diameter hole.

Or would you move up a couple more numbers on each?

Thanks.

BMyers, I received your link while writing this. I will go study it while awaiting a reply. Thanks!

--Bill
ps By the way, if I am understanding your advice correctly, it should be a LOT cheaper, too!

[edit: "drop down" to "move up"]