I wasn't going to go out of my way to show what I've been working on but I've had a change of heart. I gave up full time building three and a half years ago when we had our first child. Since then, I've been a part time dad and part time builder. We're two difficult months into being parents of two children now and if there's anything I've learned in the past three years, it's that I need to make the best possible use of what little time I get when I get it and that means that I needed to figure out where I can best trim time from my builds.
Of the many areas I've analyzed, main tube mitering became one of my first targets. For the past six years, I've used an Anvil MTM which has been great. It's provides a super adaptable setup, fast loading/unloading and a pretty rigid platform from which to miter tubes. However, for my situation, I thought there were some things that could be improved. Until recently, I've had only a Wells Index Model 55 vertical mill. The model 55 doesn't have a ram so you can't move the head fore/aft which when coupled with a relatively small amount of Y axis travel, limits the usable setup space for a vertically mounted rotary table and the MTM. The offset (from the rotab ctr) of the MTM made it impossible for me to rotate it forward to cut the DT/ST interference miter without cutting through the fixed jaw. Even then, it was impossible to see the scribe line in most cases. Adding to the level of difficulty, the arm on the MTM isn't long enough to hold the tube dummy phase assembly when I attempt that miter on down tubes (meaning it was a trial to clamp the tube knowing it was phased correctly). IMO, this isn't a shortcoming of the MTM, it's just the situation with my mill.
Early on, I learned that I had to use delrin mandrels inside my titanium tubes to keep them from deforming to fit the square jaw of the MTM. After a few expensive mistakes in which the tube slipped in the jaws, I turned the mandrels and have been using them ever since.
I considered buying one of Jeff's fixtures but since I swage my tubing frequently, I would have had to use an adapter which didn't appeal to me much. It wasn't my intention to base my fixture off of Jeff's but I can't deny that I arrived there...by material constraints, minimal travel on both of my mills and the lack of CNC.
My goal was to make using this as easy as possible and some considerations were:
1. eliminate the need to scribe the tubing for cuts and make it so I didn't need a chart to calculate tube length changes due to miter angle (on the dummy end). This meant that the miter cutter needs to be on axis with the tube AND the dummy needs to pivot on the centerline of the intersecting tube.
My workspace is tiny and as much as I try to be neat, I'm not when I get into the thick of things. horizontal surfaces quickly become cluttered which meant that I had to make room on one of the two work surfaces I have before I could setup the station to scribe the tubing. I'm still going to attempt to keep my areas neater, but now it's not necessary to take the time just to scribe tubing.
2. eliminate the need for mandrels. Fairly easy to make round clamps for each size. Not as easy to make ones that work well on swaged tubes.
3. make measuring the length of the cut ludicrously accurate and based off of the c-c measurement. two digital caliper capture points were setup. There's a temporarily fixed point that locates on a dowel pin every 100mm from the rotab center and then another point on the carriage. If I want a 557.4mm tube length, the caliper capture part is attached at 500mm and the carriage is setup so the caliper reads 57.4mm.
I'm out of time. Here are some photos.
Phase keeper by VertigoCycles, on Flickr
Manual 4th axis by VertigoCycles, on Flickr
Nearly done with the new miter fixture by VertigoCycles, on Flickr
When I miss, it's a big miss. Rushed and misread the print. That lip is supposed to be .5" thick. by VertigoCycles, on Flickr
Untitled by VertigoCycles, on Flickr
One of the most important pieces to the new fixture by VertigoCycles, on Flickr
Slightly more successful by VertigoCycles, on Flickr
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