Drilling Precision Holes

[mfarris]

I'm a complete newbie, so forgive me if this is a stupid question but.........How do you go about drilling holes to precision? In other words, if I have plans that call for holes that are .208, .157, .123, etc., and don't have drill bits for every size, how do you get the holes to the exact size? I realize you can bore larger holes, but holes that are really small would be difficult to bore wouldn't they?

 

[jtrout13]

First, In US units, there are 3 sets of drills:i
-Fractional (1/4, 1/8, etc) usually in a set from 1/16-1/2 in 1/64 increments
-Number drills in #60-#1 (bigger number = smaller drill)
-Letter drills A-Z

The letter and number series are the in-between sizes you mention. It depends on what you're doing with the holes. For tap drill sizes (pilot hole to tap a thread) you normally are going to need the correct size. If you're reaming, normally you can get away with a fractional set, just drill 1/64 undersize then run the reamer through the hole. For most other applications you can modify the plans to use a fractional size hole.

I have no affiliation with this company, but I buy a lot of tools from "wholesale tool company" www.wttool.com.

I recently bought a drill set from them which included all three drill series (a 150-some piece set) for around US$60. It is item #0192-9950. They're cheap import drills which I wouldn't use for production work, but they're just fine for home shop use, and are even TiN coated. This is a much better value than buying the sets separately, and honestly if you're not drilling hard alloy steels and production materials these drills are fine. A set like this guarantees you'll have all the drill bits that would ever be called out on a drawing, and will enable you to adjust your tap drill sizes (drill one drill size larger in tough-to-tap materials to prevent tap breakage.) I highly recommend one of these sets (but you don't need to spend the big bucks for a made in USA set, the import sets are just fine.)

 

[mfarris]

Thanks jtrout13....that's very helpful advice. HF carries a 115 piece cobalt set like the one you mentioned for $99. With a 20% coupon and purchasing in-store to avoid shipping, the cost of that one would probably be similar to the cost of the one you have. That's probably the route I will take. Again, thanks for the advice.

 

[rhitee93]

An option that has worked for me is to buy the cheaper drill set to get up and going. Then, if you feel it is necessary, just replace them with better quality ones as they get worn out or broken. The smaller drills are pretty inexpensive one at a time. Even the larger ones can be had for a few dollars.

 

[kvom]

Drill set selection aside, it seems odd to specify hole sizes that way. For a tapped hole, most plans would just specify the thread size and leave you to select the appropriate tap drill size. For example, a 10-32 thread might normally use a #21 drill (.159). However, for a harder material where the fit is not critical you could go up a size to #20 (.161) or #19 (.166) to make the tapping easier. Similarly, the plans can also just call out the thread size for the clearance holes allowing you to choose an appropriate diameter.

 

[jtrout13]

Drill set selection aside, it seems odd to specify hole sizes that way. For a tapped hole, most plans would just specify the thread size and leave you to select the appropriate tap drill size. For example, a 10-32 thread might normally use a #21 drill (.159). However, for a harder material where the fit is not critical you could go up a size to #20 (.161) or #19 (.166) to make the tapping easier. Similarly, the plans can also just call out the thread size for the clearance holes allowing you to choose an appropriate diameter.

That may have been true in the older days. But as engineers designing parts for outsourced production, we're taught to specify the tap drill size, but not as a decimal. We're taught to write a callout like "DRILL #29 THRU ALL, TAP UNC 8-32 THRU) so there aren't any lookup errors, and it makes it simple for outsourced production in Asian countries that aren't that familiar with US units and measurements. Also, the engineer will base the thread strength calculations on a prescribed thread minor diameter, which will dictate the tap drill size that must be used. Generally this will be the standard size, then the manufacturing dept. will complain about broken taps, then the engineer will re-check and see if the increase in tap drill size is permissible.

That's just my 2 cents from an industrial point of view. Doesn't really apply to home shop stuff. What most likely happened is the designer of the parts the OP is making designed them in SolidWorks or some similar industrial package, which by default always states the tap drill size in any hole callouts on the drawing.

 

[steamin]

The other thing to consider is that a drill does not make a round hole. You can drill under size and then use the size drill you wish for the final hole. That will give a truer hole than if you use the exact size you needed. If you need a "precision" hole then you have to use reamers. If the hole called out for is large enough then you can use a boring head in the milling machine for a "true" hole.

Another nickel and a half worth.
Larry

 

[ELM6061]

I'm a complete newbie, so forgive me if this is a stupid question but.........How do you go about drilling holes to precision? In other words, if I have plans that call for holes that are .208, .157, .123, etc., and don't have drill bits for every size, how do you get the holes to the exact size? I realize you can bore larger holes, but holes that are really small would be difficult to bore wouldn't they?

For "precision" holes (and as you have not mentioned tapping) one should always use an undersized drill followed by a reamer or use a boring head/bar on a mill or lathe. Drills do not cut round "precision" holes.

Ed

 

[mfarris]

Does anyone have a recommendation on a good set of reamers? Are the adjustable reamers any good, or is it recommended to get a large set of reamers like the 115 piece drill set?

 

[robcas631]

I'm a complete newbie, so forgive me if this is a stupid question but.........How do you go about drilling holes to precision? In other words, if I have plans that call for holes that are .208, .157, .123, etc., and don't have drill bits for every size, how do you get the holes to the exact size? I realize you can bore larger holes, but holes that are really small would be difficult to bore wouldn't they?

Using a hole punch helps. However, it has to be done properly. If not your drill bit can "wander." Also, getting a good center drill set is a good idea. If you can't afford one used a countersink. Why? They do not flex and with practice it's not a bad option. When drilling do not oil because it promotes the drill to drift. I have read and perhaps senior members can establish this fact or not. A good set of reamers help. A lot of the decision is made on what you intend to do with the hole you are drilling? I too and relatively new and I am sure other members will have much to say.

 

[jtrout13]

Does anyone have a recommendation on a good set of reamers? Are the adjustable reamers any good, or is it recommended to get a large set of reamers like the 115 piece drill set?

For reamers, I'd start with a 7 piece set (usually 1/8 to 1/2 by 1/16's) and then get a 14 piece over/under set, which basically allows you to use the above 7 sizes in press and clearance fits. From the vendor I mentioned or HF, etc, you can get set up like this for $100 or so. I use the exact-size reamers for loctite fits, the undersize for press fits, and the oversize for clearance/running fits.

You don't need a large variety of reamers, as for HMEM type work, their main use is to create a hole for a shaft or piston to be pressed into or to run in, and these shafts will often be a stock size of drill rod, or if you engineer your own models, they will almost always be a multiple of 1/16" unless you just want to make things unduly complicated

 

[rkepler]

There are 2 critical things to understand about drills - they don't drill the size specified and they don't locate the hole well. As other have said to get a hole on size you need to ream or bore it, and to get a hole in the right location you need to spot drill first. You can kind of cheat on both - a screw machine drill with a split point will locate pretty well and hold size fairly well, both functions of it's short & stubby nature and the split point.

Fortunately most of the time a drill does what it's supposed to do - remove a lot a metal fast and give a hole within tolerances. You don't need +- .001" for a clearance hole, and most of the time +- .003 is OK for a tap hole (watch the little guys, though).

I made some jaws for a Verti-Vise - a series of holes located to about .0002" in 2 jaws to support jaw parallels between the jaws, each fitting 2 dowel pins into each jaw. With a DRO and spotting, drilling and reaming each hole it worked out - all the jaw parallels fit and were dead on (within .0001, anyway) across the mill table in the vise. I don't think it could be done any other way.

 

[kvom]

Does anyone have a recommendation on a good set of reamers? Are the adjustable reamers any good, or is it recommended to get a large set of reamers like the 115 piece drill set?

I got a complete 115 piece set of reamers when I bought my mill. However, I have only ever used the fractional ones, normally to fit pins (drill rod) or pistons.

I did acquire a set of fractional over-under reamers (i.e., over/under by .001) that are useful sliding-fit holes.

 

[mfarris]

As an example, I was just looking at the free oscillating steam engine plans from LMS, http://www.littlemachineshop.com/Products/Drawings/2593OscillatingEngineAssembly.pdf,

(i'm thinking this is going to be my first engine build) and on page 4, it calls for a hole in the piston rod sized at .142 or .144. It shows two sizes, so I assume this means .142-.144. Don't know how critical this hole is (as I'm a newbie), but it does seem like an odd size that you wouldn't be able to get with the 7-piece reamer set and the 14 piece over/under. Am I correct in thinking that this hole would not be critical?

 

[jtrout13]

You're right, those are the permissible limits of size. This is just a clearance hole for the 6-32 thread on the crank pin. You'd just run a #27 drill through there.


Or, if it were me, I'd do away with the thread on the crank pin, and just modify the design to use a pressed-in 1/8 dowel pin into the "crank wheel", then you could use an oversize 1/8 reamer on the piston hole.

 

[mfarris]

Yeah, I suspected that hole wasn't critical and a normal drill might be ok for that. Since it will be my first build, I'll likely stick with the plans though instead of making modifications. Thanks for all the advice though.

 

[rkepler]

You're right, those are the permissible limits of size. This is just a clearance hole for the 6-32 thread on the crank pin. You'd just run a #27 drill through there.

Looks to me that the crankpin is expected to be 9/64 with a 6-32 on one end, and 9/64 to be a nice fit in the hole. So that hole isn't one that you'd want to simply poke through with a #27 - with a .144 nominal I'd bet that would leave a hole .146-.148. IOW: the .141-.142 should be a 9/64 reamed hole. You might get away with a 9/64 drilled if it's spotted OK and the drill is new and maybe split point (I'd try it with a screw machine drill and see).

Or, if it were me, I'd do away with the thread on the crank pin, and just modify the design to use a pressed-in 1/8 dowel pin into the "crank wheel", then you could use an oversize 1/8 reamer on the piston hole.

A decent solution and likely to work. It would require a 1/8" under reamer and maybe an over.

 

[mfarris]

In thinking about it, since the plans show two sizes for the hole, .142 and .144, that would seem to indicate that the size for that hole is critical, since they give you a size range. Most holes only show one size. Is that an accurate assumption?

This is my very first build, so I'm sure you experienced builders have built models much more complicated with many unusual sizes for holes. What do you guys do? Do you have a reamer set that matches the 115 piece drill set? Adjustable reamers? Do you ream the hole as close as you can get it with the size reamers you have, and then make sure you match whatever fits in the hole the same size? I still can't decide if the 7 piece reamer with the 14 piece over/under is the way I need to go.

 

[jtrout13]

First, 0.002" is a fairly large tolerance on a hole of that size. Honestly if you drill it it should be fine. Since the 0.142 OD that will be on the crank pin is not a standard ground stock size, you'll be turning that OD down on lathe. I'd suggest drilling the hole in the piston however you see fit. Then, leave an extra thou or two on the crank pin OD and use some files/sandpaper while it's spinning in the lathe, and keep testing until you get a good running fit in the piston hole.

Usually reamers are expensive, so you work to the sizes you have. If you wanted to retain the threaded stud/locknut configuration that the plan uses for the crank pin, you could: Take some 3/16 drill rod for the crank pin, then turn down the threaded part to the correct length and diameter, thread up to the shoulder. Then, the threaded part would be the right diameter, and also the part that actually needs a good fit is a standard size stock, so you could just go through the piston with a 3/16 reamer.

I'll also note that the tolerance on this hole is larger than usual for a reason: If the hole through your piston is slightly crooked (easy to do for beginners that didn't spot drill the hole properly), or maybe your piston bore isn't perfectly aligned, or maybe the pivot hole isn't perfect, etc, the crank pin - piston interface is the place where the engine will bind. Having a few extra thou of tolerance makes it easier for a beginner to get a running engine even if they made some mistakes along the way (didn't have their vise aligned right, etc). At high speeds this can make the engine a bit more noisy, but shouldn't be a problem.

So as a recap you can just drill it and make the pin to fit, or you can re-engineer the design a bit to use standard reamer sizes. Either one will work just fine.

 

[rkepler]

In thinking about it, since the plans show two sizes for the hole, .142 and .144, that would seem to indicate that the size for that hole is critical, since they give you a size range. Most holes only show one size. Is that an accurate assumption?

Probably. The (more) usual practice is to indicate the error bar as in " .143 +- .001 " meaning ".143 give or take .001" or in this case " .142 +.002 " meaning "at least .142 and no more than .144". On a hole taking something you give the minimum and on the something you give the maximum with a minus as in " .141 - .002 "

This is my very first build, so I'm sure you experienced builders have built models much more complicated with many unusual sizes for holes. What do you guys do? Do you have a reamer set that matches the 115 piece drill set? Adjustable reamers? Do you ream the hole as close as you can get it with the size reamers you have, and then make sure you match whatever fits in the hole the same size? I still can't decide if the 7 piece reamer with the 14 piece over/under is the way I need to go.

If I were in your situation I'd buy just the reamers that I needed, unless you found a heck of a deal. It'll slow you on projects but over time you'll look for the right reamer for a new project and find that you have it already. If you want a start buy a used set in an index on eBay, something like 1/8-1/2 by 16ths or 32nds and an over/under index that you fill over time. 98% of your "imperial" size reamer work would be covered by those cases being only half full.

As for drills I do suggest getting a 115 drill set but try and buy a decent brand of known quality. As the drills dull and break replace then by buying some quantity of good drills. You can never have enough #43 drills around. After a while you'll need some specialist drills - carbide, airplane, etc. and you can get those in singles or start filling indexes again.

And on "adjustable" reamers - there are 2 types, one with a screw in the front to increase the diameter of the flutes, those are meant to be reground after change. The other type have looseish blades that are on a taper. I've never had much luck with those but do know some who have reported successful use.

As for matching parts, it's something you figure out over time. You ;look at how and why they mate and make the parts to fit that requirement. If you're making parts blink you really need the dimensional tolerance and direction and use that to determine what method to use. If I see a bolt clearance hole specified to .001 I know the designer either is an idiot or forgot to change the defaults, it's different on a pin that's sliding in a moving hole.