Let's talk a bit about another tool very important to any woodworker be that person an amateur telescope maker or not: wood glue. A quality wood glue is essential for bonding parts where nails or screws are either not wanted or not really appropriate for the job. Fortunately there are several quality wood adhesives on the market today. I am mostly going to discuss those products that I have had personal experience with. They are all readily available at any hardware or big box home improvement store. One popular glue I have not used is “Gorilla Glue”. When it first came out I was interested but when I read some test reports I did not like the fact that when it cures it can swell out of the joint and is a bear to clean up.
First a short warning about common “white” glue. Do not use this for anything other than crafts like gluing popsicle sticks together. It will not hold up to heavier duty applications. All of the wood glues listed below can be cleaned up with water or scraped off the surface when dried.
For many years I only used TiteBond wood glue. It worked fine but it seemed to not be as strong as I would have liked. Then someone gave me a small bottle of Elmer's Woodworking Glue. Now this stuff was even stronger and I liked it better. I probably would have stayed with Elmer's but then TiteBond II came along. After reading some comparison tests I switched to this newer glue, Titebond II.
Titebond II Premium Wood Glue has a stronger shear resistance than regular Titebond. It has an “open” time of five minutes. This means the amount of time during assembly or clamping you can move or adjust the parts. After that it is very difficult to do anything other than to let the glue cure. Titebond II was the first one part glue to pass the ANSI type 2 water resistance test. However it is not to be used submerged or below the waterline. It is FDA approved for indirect food contact. A few years ago I took a 1/2-inch hardwood dowel and cut a 6-inch piece off with a table saw. I glued the pieces back together with Titebond II and let it cure for a couple of days. Then I took the dowel and whacked it on a thick board. After five or six slams the dowel broke...but not at the glue joint! This is one tough wood glue.
Titebond III Ultimate Wood Glue is even stronger when comparing shear strength. It has an open time of 10 minutes so it is a little more flexible when it comes to assembly time. It is also more water resistant but still should not be used submerged or below the water line. OK, I know we don't use our scopes submerged but let's say you are at a star party and left your wooden observing chair outside overnight and it rained. You would be glad you glued up your chair with a glue like Titebond III.
One more thing about these wood glues is that they have a shelf life of about two years. The containers are marked with a code that tells when they were filled but you may have to go to the internet to figure out how to read the code. When you buy glue write the date of purchase on a piece of masking tape and place it on the container. When yellow or white glue gets too old it will start to thicken and will eventually dry up. Keep these glues out of direct sunlight.
The other glue in the pic is Amazing GOOP. This stuff is actually pretty amazing. It looks at first glance like it is simply a silicone adhesive. It is more than that. While not strictly a wood glue it will securely glue wood, PVC, metal, glass, etc. It will glue just about anything to anything else. A few years ago I used GOOP to secure a secondary mirror to a shop made wood mirror holder. When I first used the scope it the image obviously had astigmatism. Immediately I suspected that GOOP was causing the problem. Checking the tube did not reveal any solvent that would allow safe removal of the secondary mirror. There was an 800 tech/assistance line so I called it. The guy confirmed my suspicions and said that when GOOP cured it “pulled” parts together even tighter. There was no solvent that would dissolve it but acetone would temporarily soften it. So, using acetone wicked behind the mirror and fishing line I slowly “sawed” through the joint. A quick clean up and I reattached the mirror with regular silicone adhesive and the astigmatism was gone. GOOP is one of my favorite adhesives and not just for wood.
One last note: a year or two ago there was a discussion on the Cloudy Nights ATM forum whether to use just good woodworking glue or glue combined with nails or screws to hold a dob base together. There was a spirited discussion but it ended when a well known builder of large dob style telescopes posted that once UPS crushed a package carrying a rocker box. The rocker was broken about everywhere EXCEPT along the glue joints!
About the Author:
Terry Alford has been an avid amateur astronomer since 1979. He is currently a member of two astro clubs: Bays Mountain Astronomy Club (founding member) and Bristol Astronomy Club. Since 2001 Terry has taught Astronomy Labs at East TN State University. His first ATM project was in 1979 and was an equatorial pipe mount for an 8-in reflector. His woodworking shop also turns out toys for grandkids.