With the mouth cut, it’s time to start the metal work.
First step is to connect the two pieces of steel. Now, I don’t claim to be a machinist and there are likely better ways to do some of these steps. But I’ll post what worked best for me and you can change and adapt as your skills and available tools allow.
At the end of the piece of 1/8 steel, mark where you want to install the first machine screw. If you have layout fluid, that would be best. In place of that, I just marked over the area with a marker and scratched the lines through the black. Worked just fine.
I went about 3/4 in from the end and 3/16 up from the bottom in order to center the hole in the 3/8 base plate, scratched the lines with a marking gauge and knife and marked the point with an automatic center punch.
After that, both pieces were clamped to a squared up piece of scrap every which way I could and taken to the drill press with the appropriate sized drill bit (#25 for a 10-24 tap). If you haven’t drilled much steel, the best way I’ve found is to create a dimple using the drill bit dry, add a drop of light oil or cutting fluid to the dimple and drill some more.
Go ahead and drill through the 1/8 plate and dimple the 3/8. Using a squared up block clamped to the steel really helps stabilize it while you drill.
Then the 1/8 can be removed, a drop of oil added and drill down into the 3/8. Every 1/8-3/16, pull the drill bit up to clear chips and add another drop of oil. You’ll need to drill deeper than the length of the machine screw to allow enough room for the tap to work correctly and for chips to accumulate. While I’m sure there’s a standard formula for this, I just add 1/2 inch to the length of the machine screw and drill to that depth.
Note: When doing this, I decided that 3/4 machine screws were probably more than necessary and got some 1/2 inch ones instead. I’ve updated the first post of the series. You can certainly use the 3/4, it just means drilling and tapping more metal, along with the increased risk of breaking a tap.
So for my 1/2in machine screws, there is a 7/8 deep hole drilled into the 3/8 steel.
Now that hole can be tapped. As mentioned before, make sure to use good quality taps. Those you can buy at the big box or hardware store are fine for cleaning up existing threads or working with mild steel, but not the best choice for cutting new ones in tool steel. Take your time and go slow. Use plenty of cutting fluid, make sure to start the tap straight (make an alignment block if that will help) and reverse the tap frequently to break off the shavings. All tapping operations are time for patience—think tortoise over the hare. Impatience will probably result in a broken tap, followed by lots of cursing. (In case you haven’t figured out, breaking a tap sucks. I’ve done it three times and it’s a sinking feeling every time.)
I like to use a plug tap to start. It has a longer taper on the end that helps to get started straight. Every 1/8 to 1/4 turn, the tap gets reversed. When I feel it binding, it’s time to back the tap out and add some cutting fluid. Once the hole is well started, I switch to a bottoming tap. It allows cutting closer to the bottom of the hole, but with the sharper taper, is not the best choice for starting a hole.
Note: I had very good success this time switching back and forth between the plug tap and the bottoming tap. When the plug tap started to bind a bit, switching to the bottoming tap allowed it to cut the tapered threads to full depth without starting new ones. Once it started to get difficult, going back to the plug tap allowed it to just cut on the taper. This meant that both types were cutting less length at a time and therefore had less stress and were less likely to break.
Check frequently to see when you are at correct depth. Some people have feeler gauges for this, I just used one of the steel machine screws. When it goes all the way in, the hole is good.
Now the hole in the 1/8 plate can be enlarged at the drill press so that the machine screw passes through freely (for the 10-24, a 13/64 bit was the perfect size). Fasten the two pieces of steel together with the one machine screw and repeat the process at the other end. Mark, drill, tap, enlarge and fasten. Then the middle screws can be marked out . . . .
. . . . and drilled. With machine screws installed on both ends, you can drill through both pieces of steel at once. Here I have the steel clamped to two squared up pieces of scrap to help keep things lined up. The one in the back that cannot be seen is tall enough to support the side and base pieces.
Tapped and machine screws installed. Note that we’re still using the steel machine screws, as they will go in and out several more times, so you don’t want to mess up the softer brass ones. Save those for final assembly.
Again, go SLOW! I cannot emphasize this enough. While the earlier blog post about laying out for cuts took much longer to write than to actually do, this is the opposite. I’m sure someone with machine shop skills and tools could do this much faster and better, but this what I had to work with.
Now a very small countersink can be put on the holes-it doesn’t take much.
The best way to check is to use one of the brass screws and make sure that the bottom of the slot stays above the steel.
Once proper depth of cut is established, it can be set and transferred to the other holes. Reinstall the steel machine screws.
Next installment: Add some body to your work.
-- "Good judgement is the result of experience. A lot of experience is the result of poor judgement."