Boring a slotted shaft

You won't be able to get this bore accurate unless you set it up in a lathe. Best way is to chuck up the part and simply go at it with a boring tool. It's going to be an interrupted cut so go easy until you are at the bottom of the slot, at which point you can then drill to open up the hole to the desired depth, and continue on after that with a boring bar.

Jim

======================== This is one of the very few cases where you can successfully use an end mill in a jacobs style chuck. I suggest you get a 4 flute [two flute end mill or drill will tend to grab in the slot] 1/4 inch end mill, and slowly and carefully feed the endmill in the required depth.

GmcD

Just remember doing it his way you WILL get a hole larger than the endmill so be prepared to custom make the pin you press in.

If the hole has to be *exactly* on size (or a touch under for a press fit) I'd under drill it with a smaller end mill and then go at it with a boring bar.

--.- Dave

I would respectfully suggest this is a 'bad idea.'

If he is really interested in a bore in this shaft that is concentric, and to size, and end mill in a jacobs chuck is very nearly the worst way to do this.

Even glossing over the interrupted nature of the cut which will tend to pull the jacobs chuck off its taper, if it is Morse taper mounted.

Jim

Thanks, but let me make sure we're on "similar pages"...If there was no slot, I would have used a #3 center drill followed by a letter size "C" drill then finally a .2495 chucking reamer. With the slot, I'm thinking of starting with a "D" drill (just slightly larger than the shaft of the center drill) to a depth just short of the flat then continuing with center drill on the flat, then "C" drill then chucking reamer. Is this similar to what you had in mind? Already have the parts mentioned (I do a lot of 1/4" boring) and yes, I'm using a lathe.

This approach does not give good concentricity. Unless you bore the part before reaming, you stand a sizeable chance of getting a hole that is several thou off center.

Jim

I'd plug the slot with a bit of steel and soft solder it in place, then proceed as for a non slotted bar. When finished, soften the solder and remove the two halves of the plug. If you don't like the idea of soft solder, use Loctite.

Cannot recall getting more than .002 runout on the pressed shaft (done without the slot that is). My headstock and tailstock were aligned especially for drilling with a lathe like this. Also, I use stub drill bits instead of jobbers. Probably shouldn't have used the word boring since a boring bar isn't being used. At first I had my doubts too and would press a 5/16" shaft then turn it down to 1/4" but the difference in runout wasn't worth the effort. Other than possible headstock/tailstock misalignment, what else do you see wrong with drilling/reaming with a lathe?

... any machine tool...

Simple,

2) Reamers follow the existing hole.

I like the use of a two-flute end mill..... but certainly not to size. That's guaranteed to fail!

Personally, I'd plunge to the bottom of the slot with a 7/32 2-flute endmill, centerdrill, drill to 7/32, bore to clean up, leaving about .010, and then ream to size using a floating holder.....

(It wouldn't be a *bad* idea to forget the reamer and bore to size!)

Long story short....Not an issue provided the center drill has an unobstructed path to the flat. My only concern was how best to present the flat to the center drill. Don't see any reason for using an end mill to clear a path if a normal drill will do, and so far no one has suggested that it won't.

Just face off a spot with a regular HSS tool bit for your center drill.

That's it in a nutshell. If he doesn't care that the hole is truly concentric, a reamer will make a hole, someplace in the shaft, when it follows a drilled hole. Kinda hard to tell where.

Jim

Actually no.

Doesn't matter even if it *does* have a faced surface to start the C-drill on. The twist drill that picks up that centerdrill will make some kind of hole that cannot be guaranteed to be better than a few thou within center.

If it's off by five thou, your reamed hole will be off by five thou.

Have you considered the tolerance you need for the application?

Jim

Will do. Thanks.

Actually, I'm more concerned with the runout of the pressed shaft. The hole doesn't have to be concentric provided an oversized shaft is used then turned down to the desired diameter **and** the difference in diameters is much greater than the eccentricity of the hole. In fact that procedure should provide the most accurate shaft since it caters to a less than precise shaft as well. But I digress since that isn't even necessary in this case.

Where's Harold when you need him.

You are correct that your plan will work. But it's just as easy, in fact, *easier* to do it the right way, which is to form the bore for the add-in shaft with the correct geometery to start with. As a long-time bodger myself, I recognize that when projects start to include features that 'can be corrected later in another stage' that's a sign that maybe things need re-thinking.

Jim

I guess I'm missing something in the terminology.

I don't understand what an "unobstructed path to the flat" is, since my mental picture requires that both the centerdrill, the drill, and the reamer make an interrupted cut during the first .063 - .188 inch.

If this is the case, I guarantee unacceptable run-out in the finished hole.

Likewise, as another poster posited, I can't visualize the act of "Just face[ing] off a spot with a regular HSS tool bit for your center drill" unless, of course, you intend to face off the entire slot... That is what I was proposing with the 2-flute endmill. It could be chucked into a jacobs chuck and plunged to the appropriate depth.... all of 20 seconds...

What am I missing?

Easier.....Right......All relatives as far as I'm concerned. You seem to have a major misconception when it comes to precision drilling on a lathe. Just to prove a point I chucked a 1/2" diameter 3" long aluminum shaft, checked the runout then proceeded to drill/ream a 1/4" diameter hole 1/2" deep in it. I then pressed a 3" piece of 1/4" precision steel shaft in the hole and checked runout.

It took exactly 10 minutes between the starting of the center drill into the aluminum shaft and completion with the pressing of the steel shaft. The runout of the pressed shaft was less than .001 up to 1.25" from the point of entry. It was .0015 at the end of the pressed shaft at 2.5" from the point of entry.

You're blowing hot air buddy. Here's a shot of the final setup;