Another of my weird questions - Counterbores and piloted D bits?

I came on the need to do another very odd operation. I have figured out a couple of ways to do this, but ... well, I view the people here as a collection of Obi-wan Kenobis. I need to do two counterbores axially on the same hole. The hole is 0.25", the smallest and deepest counterbore is 0.375" for 0.097 deep, and the larger, smaller counterbore is 0.670" and 0.05" deep. The odd double counterbore is clearance for an oddity on an existing shaft that runs in the 0.25" hole, and is really necessary, otherwise the hacksaw would already have spoken. My thinking was to face the part, drill the central 0.25" hole, then counterbore twice, once with 0.375" and once with 0.670"-ish. First approach: turn a drill rod with the right diameters including a 0.249" pilot/guide, then file (no mill!) flats and shape two flutes for a custom counterbore; then harden and temper. Second approach: turn a drill rod with the right diameters then grind across the first 0.5" or so of the bigger/counterbore diameters but not the 0.25" pilot to make what amounts to a piloted D-bit kind of thing. The advantage here is simpler making as my skills are limited. And for both of the above, I'm a novice at hardening and tempering. Third approach: take one old 3/8" drill bit and an 11/16" drill bit, grind the ends flat, and shape for use as counterbores. Counterbore the two bores, 11/16" first, 3/8" second, using the fixed setup with the 0.25" drill, then the two c/bores in the tailstock chuck. This ought to keep enough alignment for them. Any advice, oh Obi-wans?

The piloted D-bit approach should work just fine and would be easier to make. I believe I would try that approach, but you could make them both with .249" pilots, then do the larger counterbore first, then the smaller one after that. The only problem is that you might need to make a modified D bit where you only remove a quarter section of the diameter. This can be done with an end mill. Since the idea I am trying to get across may be a bit difficult to visualize, this drawing may help.

Unless the part is extremely large it sounds like a good job for a lathe. Even if it is odd shaped you may be able to hold it in a 4 jaw chuck. Drill your .250" hole and bore your shallow counterbores.

sent from my hand held hickymajig

melsdad said:

Unless the part is extremely large it sounds like a good job for a lathe. Even if it is odd shaped you may be able to hold it in a 4 jaw chuck. Drill your .250" hole and bore your shallow counterbores.

Click to expand...

I extracted out only the pertinent parts. It is on the lathe. The part is 1" round steel chucked into the spindle.
The process I came up with in the thinking experiment before throwing several attempts away like I normally do ... was:
1. set stock in chuck with length of part plus parting allowance plus extra so I don't run the cross slide and toll holder into the jaws protruding
2. Face the free end
3. Center drill the free end from the tail stock chuck
4. Drill to depth with 0.25" bit in tailstock chuck
5. Counterbore - either once with formed c-b tool, or once with 0.375" and once with 0.680", in some order
6. Once the smoke has cleared from the c-b debacle, turn the outside to form
7. Part off.

There's a whole lot of "change the tool in the tailstock chuck" in that process, and I was trying to see if I could cut it down, as well as doing some kind of real counterboring.

The outside profile prevents me from just drilling to 0.100" depth with an 11/16" normal bit and being done with it. There's not enough stock left for sturdiness after that and the outside diameter profiling.

I'm probably overthinking this, but my progression as a lathe operator is finally getting past simply making chips to thinking out how I'll do the whole part.

The drill to 0.25" diameter is only for 0.51" deep, so I might be able to dispense with the center drill step, I guess.

This whole process is kind of an exercise in doing things the way I've read they should be done, and adding skills: thinking out the steps, juggling steps to minimize set up and re-set up time, and picking existing or easily modified tools versus making custom tools. And I get useful stuff at the end if I do it right!

Oh. Material is 12L14 steel. I'm holding off on machining cubic boron nitride for now.

Oh[SUP]2[/SUP]. I see what you mean about lathe and boring - really just use a tool in the tool holder and bore. Doh! I get it. Lemme think about that. Off-hand, that becomes an exercise in grinding a tool bit that will cut the counter bores from the inside of the 0.25" hole, but that may not be so bad. Grinding one unique tool bit versus many changes and making other tools.

OK, now I'm pondering. Thank you.

Lathe is definitely the way to go. You can either grind a HSS tool with proper clearance to do the shallow counterbores, or take an end mill and put it on center with one of the flute end-edges horizontal, and plunge it. In fact, you could do both counterbores with the same end mill. I use them for boring all the time when I am in a hurry and the counterbore is shallow.

Tony Wells said:

Lathe is definitely the way to go. You can either grind a HSS tool with proper clearance to do the shallow counterbores, or take an end mill and put it on center with one of the flute end-edges horizontal, and plunge it. In fact, you could do both counterbores with the same end mill. I use them for boring all the time when I am in a hurry and the counterbore is shallow.

Click to expand...

How do you hold them? Are you putting these end mills in a boring bar holder?

The current paths I'm pondering are
(1) two different sized end mills held in the tailstock chuck; no problem plunging, as the center 0.25" diameter is already drilled

(2) a lathe tool (my lathe will hold 1/4" square ones) special ground to what amounts to a less-than-quarter-inch interior boring tool for about 0.15-0.2" on the end, mounted to the tool holder on the traverse and inserted into the 0.25" hole, and using that to enlarge the bore

For 1, I'd do the 0.375" bore first, then change to the bigger one. For 2, I'd bore the wider diameter first, then do the smaller one. Both based on smallest metal cuts on each cut.

I could not figure out how to hold end mills, either except in a boring bar holder.

Well, I don't know what sort of tool post you have, but I have an Aloris with some of the holders also incorporating a vee groove in the bottom of the tool slot. Works for boring bars or something round like an end mill. I only have actually bar holders for the larger sizes that exceed the 1" size. The 1" bars I have are milled flat to sit in a normal holder. 1 1/4" and up require a real bar holder.

If you have a turret you could grind a flat if the end mill did not have one, as long as the cutting edge ended up horizontal. Or, in a regular tool holder, if you push the end mill back all the way to the wall, the set screw will bear against it well enough to cut. I don't mean a 1/8" straight shank, but something like a 3/8 or 1/2" end mill.

One could put a collet chuck in the tailstock if they were lucky enough to own it.

Good Day

Bore it whilst you have it in the lathe ... I make little solid boring tools out of old high speed end mills ... after you have made the tool, make sure you have it set on centre height accurately ... I do it by setting it of a centre in the tail stock ... fine feeds and for the dia you are doing I would run at 1500 rpm in mild steel ... should take about 10 to 15 mins to do the job,
I can send you some photos of small boring tools I have made if you want them.
Have bored holes down to a bit under 2 mm.
Last week I made a tipped boring bar, I use old iscar parting tips, to do 8 off 8 mm dia holes throgh a harden die plate 50 thick ... drilled 6 mm dia holes with a masonary drills, have to be shapened and shaped properly, no good off the shelf sharpening, and then bored them to 8 mm dia, 3000 rpm .0015" feed ... got them within .002" and about a 001" taper then I lapped them to .0002" tolerance.

Batt