I decided to use stainless steel for the tailgate on my scope. Although 6061 Aluminum is an excellent choice for the bars and triangles in the mirror cell (a much better option than steel despite Kriege’s advice on the topic), aluminum isn’t a good choice for the tailgate. It won’t hold threads worth a darn and it is difficult to weld.
The Dobsonian Telescope recommends using “heavy-walled” square stainless steel tubing for the tailgate, with quarter-inch barstock for the side rails. That’s what I decided to use. Of course, the description “heavy-walled” doesn’t refer to any particular size of tubing — a fact you’re bound to discover if you ask for it at your local metal supplier. A veteran of commercial lumber yards, where aged and sarcastic yard men have perfected the technique of rolling their eyes in amazement at naive questions, I didn’t make this mistake. After considering the square tube options available at Metals By the Foot (where the sales people are both friendly and helpful), I opted for 1-inch square stainless tubing with an 0.12 wall thickness. It’s just under 1/8 inch thick. It’s important to get thicker tubing if you want to thread the tubing for collimation bolts. Some people simply weld nuts onto the tubing frame after drilling it out. If you go this route, you could probably use thinner tubing and save some weight.
This image shows the tailgate components drilled out and mounted in the jig ready for welding. A LOT of work was required to get to this point…
David Kriege offers excellent advice about making the tailgate in his book. First, it is a good idea to have the metal supplier cut the stainless steel parts to size. They have equipment designed to cut all sorts of metal and will be able to cut the parts more accurately than you will unless you have a metalworking shop. You’ll probably need to round the part size to 1/8th inch measurements, though. Give them some measurement in 1/32nds, 1/64ths, or 1/100ths and you’re likely to get one of those “We’ve got a newbie here grins.” When I realized I had the side rails cut a little too long, I actually took the stock back to the supplier and asked them to recut it. I was buying more material (threaded rod, if memory serves), and they didn’t hesitate to make the cuts. They were dead on, too.
Kriege’s second recommendation is to drill the parts out before welding them. Pretty much an essential from my point of view. What is not discussed in Kriege’s book, however, is the difficulty of drilling stainless steel. Nightmarish is not too strong a word!
It’s not that stainless steel is harder than regular steel. The problem is that it hardens dramatically if you overheat it in the drilling operation. This is called work-hardening. It’s very easy to do, and it’s guaranteed to destroy drill bits and drive the inexperienced metalworker to the brink of insanity. At least, that was my experience. Here are some tips I picked up on my way to getting the tailgate parts successfully drilled out.
Tips for Drilling Stainless Steel
- If you don’t own a drill press, hire the work out to a machine shop. It will be the best money you ever spent.
- Use cobalt drill bits. These are designed for stainless steel and last much longer than other bits.
- High precision drill bits do not have flats ground into the shanks. They are designed for machines that have holding collets which are much better than the chucks found on the Delta, Jet, Rigid, etc. drill presses owned by most woodworkers. If you own one of these, which I do, use drill bits with flats ground into the shanks. Otherwise, the bits will slip, the work piece will overheat, and the bit will die a sudden and painful death.
- Clamp the work piece rigidly in place. If you have rubber protector feet on your clamps, remove them. You don’t have to worry about denting stainless steel, trust me. Clamping is critical because any sort of vibration reduces cutting efficiency and increases the heating effect. Do not attempt to hold the piece without clamping — you’ll lose control of it and get hurt.
- Use a good quality cutting lubricant designed for stainless steel. This is critical to success. It’s required both for lubricity to keep bits of steel from clogging the cutting edges of the drill and to dissipate heat. The latter function is especially important. You can find it at hardware stores, but metal suppliers stock better brands.
- Set your drill press for the slowest speed possible. Formulas are available for calculating the optimum bit-speed based on drill-size, material being drilled, etc. Ignore these. They are for production machine shops that need to produce work as quickly as possible. The faster the drill speed, the more heat generated, and the more problems you will experience.
- As soon as the bit touches the work piece, you must exert enough downward pressure to cut metal. Do not under any circumstances permit the bit to rub on the work piece without cutting. Doing so will cause overheating, the stainless steel will harden and become unbelievably difficult to cut, and the result will be a trip to the hardware store for a new bit. As soon as you stop cutting metal, raise the bit away from the work piece.
- When you see smoke (the lubricant burning off), raise the bit instantly and apply more cutting lubricant. Do not keep drilling! This was probably the single most important thing I learned while drilling stainless steel. I cut each hole in a series of short pulses, adding cutting fluid after each. Drilling through the 1/4 inch bar stock required 4-5 repetitions of this. Once I learned to drill in this manner, the work went smoothly and quickly with no additional drill bit casualties.
I used 3/4-inch MDF (Medium Density Fiberboard) to build my tailgate jig. This turned out to be an easy job — a welcome relief coming on the heels of my adventures with drilling stainless steel. I cut a back panel large enough for 2&1/2-inch edge boards around the tailgate components. I clamped two of them to the panel, making certain they were at a precise 90 degree angle. When I was satisfied with their accuracy, I drilled and screwed them in place.
From the full scale drawing I made of the mirror cell, I determined the precise spacing between the rails in my tailgate — 7.97 inches — and cut an MDF plank to this width on my table saw. Then I cut two the plank into two spacers. I have a quality Delta cabinet-style saw. It weighs a ton and makes precise cuts. A saw like this is a real asset for a large dob project.
To build the jig I put a rail in position against one edge board, and laid a rung against the other. Then I placed the first rail spacer, added the next rung, added the second rail spacer, and then the final rung. I laid the second rail and pressed the whole assembly into position with the last two edge boards, clamping them first to check for squareness and then screwing them in place. An effective way to ensure squareness is to measure two diagonals. If they are not the same length, things aren’t square.
If I were making it again, I would use shorter rung spacers to allow more area on the inside edge of the rails for tack-welding (which is done while the components are in the jig). My jig worked, but it would have been easier to weld if I had left more room. I don’t have any experience with welding, so I approached a friend at work about helping me with the job. Paul is an absolute wizard when it comes to working on automobiles (and just about anything else he wants to do). He bought a house last year in Peculiar, Missouri “where the odds are with you” as a local billboard proudly announces. One of his primary criteria was that the house have a large, column-free Mortenson building for his shop (shown in the following image).
Here is the tailgate after it has been tack-welded together and removed from the jig. The purpose of the jig is to hold the tailgate pieces square until the can be fixed in place. The assembly has to be removed from the jig to complete welding. One thing we noticed when we laid the jig on this steel work table is that the jig flexed slightly because the table wasn’t completely flat. The result was that the edge rails rocked up and down ever so slightly. To fix the problem, we inserted a piece of 1-inch steel barstock under one side of the jig to raise it off the table surface. Our quick solution worked perfectly.
Paul used a MIG welder for my tailgate — one of a number of welders he owns. The name “MIG” comes from Metal Inert Gas. The welder uses a coil of wire fed through a tube which also discharges gas (I think Paul said argon) during welding. The gas reduces oxidation around the weld, making a cleaner and more precise joint. Paul is shown here with the business end of the welder getting ready to finish the inside of one rung joint. The next picture shows the welder in operation. An extremely dark shield is required to protect your eyes during welding. The welding light is blindingly intense. Even when using the shield, long periods of welding can result in a sunburn effect which causes your eyes to ache. Of course, the shield also protects your eyes from metal thrown off during welding. My grandfather, who worked at the Ford plant in Claycomo, Missouri, was an “uphill welder.” He could puddle melted welding material and move it upwards — a challenging skill to master. One time, however, he lifted his shield too soon and was hit in the eye by a piece of cobalt thrown off a joint he was welding. He lost his eye.
I used a flash for the welding picture above, which reduces the apparent brightness of the welding operation. Here is a picture taken without flash that gives a better idea of how the work actually looked.
The welded joints have to be cleaned up with a grinder to make them smooth and remove small blobs of metal thrown off during welding. Using the grinder without eye protection isn’t the recommended procedure. When I made a comment to that effect, Paul grinned and asked if I was going OSHA on him. I wasn’t really worried about it — Paul knows what he’s doing and I was too busy trying to get a good shot showing metal sparks flying all over the place. This picture is a pretty good one.
Here’s Paul with the finished tailgate.
And here is Paul’s “Green Machine,” which he is converting into a hybrid of the Urban Assault Vehicle from Stripes and the Death Car from Animal House. At least, that’s my theory…
