If you are willing to put a lot of work into it.
You attach a hinge to the work base. Attach a board of suitable length to the hinge. Attach a circular saw to the end of the board. ( clamp it or drill holes into the base and bolt it ) Now you have a saw on the end of an arm that goes up and down to whatever height you intended for the length of the board. I’d keep the vertical angle a bit low.
Mount a board vertical to the work surface that is at 90 degrees to the saw blade. Put a block at the end of the board so the length to saw blade is right. The board is the right height and position so the board with the saw attached rests on it but lets the blade extend downward enough to cut your tube surface.
Press the tube against the board, and the end stop. Lower the saw to cut. Rotate the tube.
I’d probably use my table saw. Use the fence for an end stop, clamp a piece of scrap wood behind the blade parallel to the fence, then just rotate the tube over the blade.
Thanks for the ideas everyone. A bandsaw sounds intriguing but it would have to have a fairly substantial sled to handle this large heavy piece (I said it was 5 feet long but it’s actually more like 6 feet, and 30 pounds).
I do have a table saw and I’ve been trying to think of a way to use it, but I don’t see how to accurately rotate the cylinder without the possibility of it slipping back and forth. Remember that the end is uneven and can’t ride against a fence.
The idea to use a jig triggered a realization. This tube is intended for a home built telescope. It will sit in a wooden cradle that rides on a movable mount. But I realized that the cradle itself, which I’ve built to hug the tube tightly, can be used as a cutting jig to guide a handsaw. At this point I’m thinking this is the best approach but I’m still considering other options.
A plumbing supply house might have a large diameter tube/pipe cutter they could use to cut your tube. In any case, how accurate do you really need the cut to be? I would think that the mirror mount would have sufficient adjustment range to account for a less than perfect tube.
I’m more concerned about getting a good reference for the secondary mount (spider) and the focuser, which have less adjustments than the primary mirror mount. However I agree it doesn’t have to be a micrometer scale accurate cut, I’m just trying to understand all my options so I can pick an approach that’s as accurate as possible without unreasonable effort.
As you’re thinking of options, try a few cuts with the different kinds of saws you have to get an idea of the quality of the cut. I have a feeling that table saw could leave a pretty rough edge because the material is so soft. I would guess there’d be lots of tearout. Just off the top of my head, I’m thinking you’d get the best cut with a fine-toothed hand saw.
One option would be to get two of those pipe clamp straps. They are those metal bands that have a screw you turn to adjust them. They’re typically in the plumbing section. Put them on the tube very close to each other so there’s only enough room for the saw blade. Don’t tighten them too much or else they may cause squeezing problems when the end is cut. They should naturally align perpendicular to the tube. Then you could use the hand saw just between the two straps and end up with a pretty clean and flat cut.
This might be a stupid suggestion, but throwing it in just in case: could this clamps approach also work using the type of knife that carpet layers use?
If I wanted a clean cut through 1/4" cardboard (especially not knowing how old the tube is) I would use a wrapped guide and cut with a utility knife. May take a few passes but it would be clean.
Not too hard to assemble a handsaw guide. Make a wooden box with no top or bottom. Run one side through the table saw completely. A full slot. Run the adjacent sides part way through. Now you have a slot for a handsaw to cut almost all the way through. Rotate the tube slightly to get the final cut through. 12 x 12 pieces should work fine. Though you could go narrower.
That’s actually easy, since it just takes a T-square.
ETA: And , yes, I know you were joking, but it’s an interesting problem. I have a number of very old machining texts, and they talk about things we usually don’t worry about - how to make a perfectly flat surface without perfect blocks, for example.
Agree, table saw. But (and maybe we are saying the same thing) I’d remove the fence. His tube is 5 feet long, and I doubt he has access to a table saw with that big a table. So I don’t think he’s going to get an end stop.
Clamp a long scrap of wood, perhaps a 2x4 on edge, just behind the blade but *perpendicular *to the blade. Use a square to get the alignment right. Lower the blade to perhaps 1 inch high (I’m just uncomfortable with big blades sticking up really high unless they’re needed.)
Clamp another scrap of wood, no particular size needed, to your miter gauge (to make it effectively ‘longer’).
Mark a spot on the ‘bad end’ of your tube, at which you want the cut to begin. Put the tube across the table, square it up to the miter gauge, and slide it up to the blade. BLADE IS NOT RUNNING! Align your mark with the blade so the cut will go in the right place. Now back the miter gauge / tube off so it’s just clear of the blade.
Turn on the blade, and while holding the tube firmly against the miter gauge, advance it through the blade until it stops against the 2x4 backstop. Now just gently rotate the tube between the backstop and the miter gauge until the cut is completely around the tube. The blade itself will function as an end stop, as long as you are careful in the rotation of the tube.
Turn off the blade and congratulate yourself.
I’ve cut 12 inch PVC pipe using this method and it works just fine.
I built a 10" Newtonian and it really doesn’t matter if the end of the tube is square as long as the three holes you will drill for the spider and the one for the focuser all line up properly. All you have to do is wrap it with a large sheet of paper and mark the holes at 120 degrees. You can then trim the tube within fairly loose tolerances. When you drill the holes on the other end to mount the main mirror, you can use the same technique. Leave plenty of tube behind the main mirror so that you can readjust it axially in case you fuck up, which is easy to do when dealing with optics.
This is pretty much what I was going to suggest. The only thing I’d add is that a cut-off disc (abrasive blade) will work better than an actual wood cutting blade in most cases. High-density cardboard doesn’t have a “grain” with wood fibers aligned in any particular direction. Even a fine finish wood blade may tear it. If you have a metal cutting blade with very fine teeth, that would also do the trick.
If you use a power saw to cut a cylinder I would not rotate the cyl around the blade. I would lock it down and change positions after each cut. Round objects can get out of control quick in a saw.
ZonexandScout, I suspect the composition of the cardboard (ratio of fiber to glue, specific glue used, packed density of the composite, etc.) will determine what blade will work best. Your suggestion of an abrasive blade does sound like a possibility great idea! But then again, it may burn as well as cut. A bit of finesse seems advantageous in the speed at which the tube is advanced into the blade. And a trial cut or two into the ragged ‘waste end’ using different blades will likely inform further attempts. If the OP tries it, I’m sure we’ll all appreciate a report.
