|Two threaded cores screwed in to each pipe end|
Although the cores were a fairly tight fit, I wanted to be sure they wouldn't move, so I thoroughly cleaned the inside of the tube ends, then added some strong metal adhesive as I screwed them in. Let the adhesive cure completely before attempting to assemble the frame, even if it takes a couple of days, but do screw the bolts in occasionally to ensure the thread doesn't get gummed up. If you can't find cores that fit exactly, you could try crimping the tube very slightly to stop the cores from migrating out the end of the tube during assembly.
|Please read this! Why use bolts in this manner? The tension in the bolt compresses the joint. It's this clamping action that provides most of the strength in the joint. Don't be tempted to cut a thread in the vertical tube, or to just glue it all together, since you'll actually be reducing the joint strength, and it will be difficult to correct any flexing. The key is to ensure the threaded inserts can't move (screw them in very tight and be sure to let the glue set completely). This way, you can even tighten the bolts later should you need to - and of course, you can take it all apart too, for storage or travel.|
Use a rasp or curved file to shape the ends of the tube to fit the vertical tube nicely, so the join is flush.
|Hex bolt and washer test fitted to the threaded cores|
It's important to get the location of the join exactly right, unless you want to make the frame fully adjustable by drilling a sequence of holes. If your dog is over a certain weight, I wouldn't recommend that, because too many holes will weaken the tube at a point where it might be vulnerable to flexing. Get someone to help your dog to stand naturally, or as close as possible given his/her likely back-end trouble, and hold the frame so you can judge the right point for the join. Most important is that the horizontal frame be slightly higher that your dogs belly, so that resting his/her belly in the sling will result in your dog's back being naturally level. See the 'belly sling' section for important information about its role and design.
With the correct points marked at the exact same distance from the ground, clamp the frame in a vice and use a punch (or a large nail) and a hammer to notch the tube. This will help the drill bit start without wandering all over the curved tube surface. Use a small bit first for a pilot hole, then a drill bit that is exactly the right size for the bolts you're using. You don't want the holes to be larger - it'll introduce unwanted movement in the joint. Clean up the holes and assemble your frame... then do a little dance!
Update: I revisited Ace Hardware to track down some info on the parts I used. I have no affiliation with Ace, I'm just supplying this info as a courtesy.
Metal thread inserts (8/16" male, 1/4" female)
#329-4 (found in one of the red-marked trays)