Getting ready to drill a big hole in a bigger piece of steel.

KeithB

Resident Half Fast Machinist
Got a job coming up that requires us to start with a 2" diameter 8620 hot rolled steel blank 8" long. We need to drill a 1-9/32" ( 1.281") hole halfway in from either end. The last time we did this job we drilled the holes on our CNC lathe. Had to use four different drills to get it to size, the lathe has plenty of horspower but that axis has a limited thrust capability thus the step drilling. Since then we have bought a large radial arm drill with lots of power AND thrust, it's designed for serious drilling. (Rated for a 2" hole in solid mild steel!)

Bought the right size spade drill holder and several drill bits off eBay. Also bought a 1.281" drill bushing. Spade drills can make very accurate holes in one step if they are guided properly. So when this job came up again I had all the components to make a drill fixture. Basically it is just a cap that will fit over one end of the upright part. Two screws will clamp it in place. The hardened drill bushing will guide the spade drill. The drill bushing is held into the fixture by a cap screw that bears on a semicircular notch on the rim. There are special screws made for this but a standard 1/2-13 socket head cap screw works just fine. Just for reference. the basic part is 3.5" diameter and 2.5" tall, with a 1.980" hole bored 1.5" deep in one end.

Drill fixture and V notch vise jaw with 6" caliper for comparison.
drill_fixture01.jpg

Bottom side of drill fixture showing clamping screws.
drill_fixture03.jpg

A spade drill bit has notches in the cutting edge to break up the chips, note there are four on one side and three on the other. It also has a point profile that comes to a 0" diameter sharp point which reduces the thrust load.

1-9/32" spade drill bit
drill_fixture05.jpg

After I got the drill fixture done I decided to make a special V notch jaw for our Kurt/compatible 6" machine vises. Our vises do not have extended mounting flanges to clamp them down, they are flat on all sides and can be clamped down on their base, either side, or end. I decided to use the Chinese vise we normally keep on our manual mill to hold the parts for drilling. It will be mounted on its side and clamped down flat to the top of the knee table on our drill. The V notch will hold the part upright. By the way, the movable jaw is not as wide as the base so there is no binding when its mounted on its side.

Machine vise that can be mounted on its base, sides, or end.
drill_fixture02.jpg

I do not trust holding round parts subject to serious torsional loads with just two point contact. Three point contact is the way to go. With two points of contact being the V notch jaw and one point being a standard flat jaw I feel petty sure that will be plenty of clamping force.

The jaw was made from a 2" x 2" piece of mild steel shop scrap. I milled it down it 1.6" thick and squared up one end with an extra long mill, the turned it around and squared up the opposite end to make the jaw 5-3/4" long. (I don't want it dragging on the table top.) Drilled and counterbored the two holes for the clamp screws, in this case a 17/32" hole with a .780" counterbore. Used a 1/2" four flute carbide hogging mill to cut out the slot to depth and width -.005". Then the sides of the V were cut by lowering the table .050" and moving it sideways .050", then cut full length, rinse and repeat 12 times, then do the other side... Lots of little sharp stairsteps!

I then switched to a 1/2" 90* V-point mill, and cut the sides again, this time with a stepover of .1". The surface is a lot better and even though it looks rough in the picture it is really pretty smooth and flat. I'm going to work it over with some Scotchbrite pads on my little Milwaukee grinder. It is certainly good enough to clamp a piece of round stock.

Finished V notch jaw prior to deburring and smoothing.
drill_fixture04.jpg

I'm anxious now to get started on the job and see how everything works! Material will be another week.
 
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glassparman

"OK, OK, I'm going as fast as I don't want to go!"
Wish I had all that setup! I have an early Craftsman (Atlas) 24" bed lathe that needs the jaws trued up on the three jaw chuck.

In the mean time, I struggle getting the four jaw just perfect.
 

KeithB

Resident Half Fast Machinist
Showoff!

Yeah, would have been the perfect tool for the job wouldn't it? Well, if you don't have the right tool, you use what you got.

If I need to recut my V block to make the surfaces flatter now that it is to final shape I'll just set up my sine plate to 45* and flycut both surfaces. But I'm not going to overthink this whole thing, it's just a drilled hole that leaves enough stock for the lathe to finish out. The holes we drilled on the lathe weren't all that straight once we got to the center of the piece, I honestly believe that this will give a straighter and more concentric hole.

Now that I've got a large drilling machine I have been getting those thoughts again of making a cannon...
 
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Bret4207

At the casting bench in the sky. RIP Bret.
Wish I had all that setup! I have an early Craftsman (Atlas) 24" bed lathe that needs the jaws trued up on the three jaw chuck.

In the mean time, I struggle getting the four jaw just perfect.
You need an internal grinder, or to farm it out to someone who has one. Not a big job. People have done it with a Dremel in a tool post holder.
 

glassparman

"OK, OK, I'm going as fast as I don't want to go!"
Keep practicing with the 4 jaw. Taking advantage of its accuracy potential takes less time than changing chucks.
Yeah, I'm no machinist but I'm learning how to use that dial indicator and rotating 180 each time or a little less to get it in there. Just a tedious process for the unskilled.

I have a primer pocket cutting tool and I'm taking Surplus Czech 7.62x45 small primer Berdan brass and making them large rifle primer pockets. I just can't get that chuck dead perfect for a tight primer pocket. Even half a thou ruins it.

Mike
 

Bret4207

At the casting bench in the sky. RIP Bret.
Keep practicing with the 4 jaw. Taking advantage of its accuracy potential takes less time than changing chucks.
As long as the 4 jaw doesn't have one jaw with a hump in it. Been there, done that, got the tee shirt. More frustrating than the scope mounting jig that had a chip of metal between the base and one jig support. Took me a while and 2 rifles with misaligned holes to figure that one out!
 

KeithB

Resident Half Fast Machinist
Thought I'd update this post with some photos of the fixturing and tooling in use. Scott got the blanks sawed off and turned down. I told him how I envisioned setting things up and running the parts and he did a very thorough job of getting things set up. He bolted the V-block jaw in the vise and then clamped the vise down on it's side on the drill press knee table. He clamped a blank in the vise and dropped the drill fixture on the end of the blank. A couple socket head screws hold the fixture in place.

The drill was run at 156 RPM and .009"/rev feedrate.

Vise on knee table with spade drill in spindle
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Closer shot of drill fixture on end of blank
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Before and after
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The misalignment in the middle is very minor, less than .015", which is well within our requirements for the next step. The holes are nicely centered on the ends.
drill_fixture09.jpg