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.
Bottom side of drill fixture showing clamping screws.
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
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.
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.
I'm anxious now to get started on the job and see how everything works! Material will be another week.
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.
Bottom side of drill fixture showing clamping screws.
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
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.
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.
I'm anxious now to get started on the job and see how everything works! Material will be another week.
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