Counterboring to Precise Depth

[SteveHGraham]

I'm still working on an aluminum motor mount for my CNC lathe. I am amazed how long it takes to make such a simple part. It's astounding that people have ever been able to afford anything machined by hand.

Anyway, I drilled and reamed a 1/2" hole down the center of this thing, and the plans call for a 0.625" counterbore at one end, to a depth of 0.188". This doesn't need to be precise at all, but I would like to do it right anyway. What's the best way to do it? My mill has an automatic stop on the down feed, but I don't know how trustworthy they are. On the lathe, they aren't precise.

RE measuring, is this where people tell me I should have gotten depth micrometers by now?

I suppose I could do it with a 4-jaw on the lathe, using thin cellophane to find the surface of the part. But the shape of the part would make chucking it a little hairy.

Reaming was pretty interesting. I got lots of chatter for the first 3/16" or so. Is that because long reamers don't like hanging out of a drill chuck? I believe the chattery part will mostly disappear when I bore it.

[ctwo]

I would bore it on the mill. You have a z-axis DRO? Maybe reference the top surface as you said and bore to depth on the DRO? I imagine the auto stop to be fairly repeatable, but it might be hard setting the precise stop, but I have not used one in so long (and that was just because I don't have one)...

[SteveHGraham]

The big problem is getting a flat shoulder or whatever it's called. I have to move the boring bar out in jumps, which will leave grooves or humps. I have a 5/8" end mill, but it's about 6" long (long story), so the chatter is off the charts.

I guess I could make something out of 1/2" tool steel.

[SteveHGraham]

I think I'll pop the chuck off the rotary table and clamp this thing to it. Then I can use an end mill while turning the table. I can set the depth using cellophane and raise the part using the knee.

[ctwo]

Soon, I hope to get used to having a knee...

[John Evans]

I think I'll pop the chuck off the rotary table and clamp this thing to it. Then I can use an end mill while turning the table. I can set the depth using cellophane and raise the part using the knee.

Steve: Keep going and a machinist will be made from a attorney! Figuring the best way to hold and tool needed for the job from what is available seperates the sheep from the goats.

[arthur.marks]

RE measuring, is this where people tell me I should have gotten depth micrometers by now?

Naw. Use the depthing rod on a good caliper. Alternately, mount a rod type dial indicator over the workpiece. Take a measurement on the top surface and one at the bottom of the counterbore. Subtract the figures for a precise depth reading.

[SteveHGraham]

Here is what I did. Hmmm...the board says the "attachment quota" has been reached, so I guess I'll just have to tell you. I stuck the part on the rotary table with no chuck, and I cut it with a 1/4" end mill.

Indicating the part over the bore in the table was a nightmare. I think shooting for under 3 tenths runout was a little obsessive. I can't really measure the counterbore's diameter with a gauge, because it's too shallow, but the calipers say I'm around 0.630", so I'm happy.

The depth is about 0.191", if subtracting one micrometer reading from another is correct.

I learned something important: don't blow into a bore to knock chips out.

You amateurs may not be aware that aluminum foil is the proper tool to block chips from entering a rotary table bore. It's too bad you can't see the photo so you can learn the proper way.

I'm working on it, John.

[rudd]

Take a 5/8" drill bit. Start the C bore with the drill enough to get the point fully in and a little more, enough so that taper of point is about where you want the flat. Take drill bit out of machine, grind point off *flat*. Reinstall drill bit, make flat at bottom of hole. Go re-sharpen drill bit, or keep it flat and get another with a point.
I'm getting a good little collection of ground off drills for flat bottom holes.

[Harold_V]

Hmmm...the board says the "attachment quota" has been reached,

Thanks for the notice. Administration has been advised accordingly.

Harold

[Harold_V]

Naw. Use the depthing rod on a good caliper.

That's certainly not a great way to measure a depth, although it likely is better than using a ruler. I'd not trust a reading if it had to be precise, as it's all too easy to add error by tilting the caliper slightly. They're just not a great instrument for measuring, not by any of their features.

In my estimation, a caliper is to a micrometer what a ruler is to a tape measure. Better suited for measurements, and fraught with error.

Harold

[SteveHGraham]

Here's the photo the system regurgitated.