Jim's PVC Focuser Adventure

For the time being, this is an ongoing project page. It shouldn't take me too long because the project looks pretty easy (why else would I be doing it?). But check back from time to time just in case it takes longer and really funny stuff happens as I work.

You can find links to a lot of other great projects at Build Diary, a great webpage run by a very good friend of mine. Dave's one of the best and he married well, too!

Wednesday, March 2, 2005
This whole thing started when I decided to duplicate a project for a telescope focuser made of short lengths of PVC tubing and a few odds and ends. You can find the original page for this project by clicking here.

But there are some things that this page doesn't tell you.

Thursday, March 3, 2005
I started the real work on the focuser today. The documentation part of this project might be a little overkill but I'd rather have more information that I don't need than be missing something later that I can't go back to retrieve.

I started this project with all ten fingers. Let's see how many are left uninjured when this is finished.
Here's the first section of PVC just cut off the stock pipe. I wrapped it with a piece of paper cut to 2 1/8" so I'd be sure to cut a straight line. I figured, if I was cutting paper, I was drifting.
The metal rod you see there will be cut to length to make the pinion.
Cut to length. I used a file and sandpaper to get a nice, level surface on the cut end.
By the way, if you use a file on PVC, have a nice, wire brush handy to get all the gunk out of the file when you're done.
I marked the 1/2" and 1 3/8" lines on a piece of paper and re-wrapped the piece. I also marked it a 90° intervals. Using the awl on my Swiss army knife (there is a use for that thing), I marked some pilot holes for drilling later.
Cut and drilled. This is now ready for shaping. I'll probably save that for tomorrow because I want to make a special jig to guarantee the shape won't be distorted when I heat the PVC.
I decided to try to shape an interior ring just to see what kind of time and temperature I'd need to make this stuff workable. It is definitely a learning process.
It only needed about 30 seconds before it got pliable. And you don't have that long to work before it cools down again.

Don't you just love that avocado stove top? That's what happens when you rent.

Here's the ring when I inserted it the second time. Even now, it's not quite matched up and it's a bit warped because there is nothing in the center to press outward to make it conform to the shape of the outer tube.
Before my third attempt, I rushed out to the porch and cut off a short section of the drawtube (I was going to need it sooner or later) and used it as a form to help the ring keep its shape as it cooled.
This time, it worked. I'll also use a section of the drawtube stock to help hold the rings in place when it comes time to glue it all up.

All in all, a pretty good start for the few hours between getting home from work and going to bed. That includes the time to write this section of the diary.

Friday, March 4, 2005
I'm working a split shift today so I'm taking the hours between coming home and heading back to do a little work on the project.

FIRST BLOOD! The saw skipped out of the groove and slid across my left thumb just a bit. Not bad as far as my project injuries go but it's a good demonstration of why I don't own a lot of power tools.
Back at the stove again. Notice that I have the drawtube at the ready this time.

Although the PVC becomes pliable within about 30 seconds, I actually let it sit in the water for two minutes. I figured this might help with its heat retention.

By the way, the water only has to be at a low boil.

I picked up the ring with the tongs and dropped it into my gloved hand. I picked the focuser body with the other hand (now gloved) and slid the ring in. Then I inserted the draw tube.

And it worked on the first try this time.

Here's all the tools I thought I'd need for the next step. I've got a bar clamp to hold the arms of the focuser body to the little block of wood to make sure I'd have nice, parallel sides.

You'll also notice the drawtube/ring assembly. Turns out that when I tried it the first time, it warped the body out of round. This took care of that but there were other problems.

All clamped up and ready to cool, you'll notice the wood along the sides of the focuser body. This was to help spread out the load of the clamp so the arms would be nice and straight after it cooled. In the next picture, we'll see if this worked.

It was a handful to hold all these pieces in place while tightening the clamp.

What an ordeal! This thing wanted to bend and flex in all three dimensions. After using a series of clamps, shims and a special jig I made, I think I finally managed to get the arms parallel without deforming the tube. We'll see when I get home.

While the water was heating for my last attempt, I used a rasp to clean up the ends of the arms.

Saturday, March 5, 2005
It looks like this project might finally be coming together. Until I get a full day off (which won't be before Monday), I can only dedicate a little time to it each day.
It didn't turn out as well as I'd hoped but it will work. It's also given me ideas for improving the design which will come at a later date. As you can see, I've drilled the holes for the pinion.

By the way, no build diary page is truly complete without a toy robot.

Here is the pinion with one of the wheels. We have a hobby store that's about 150 yards from here as the crow flies so I stopped by there for these little wooden wheels after we finished supper.

They came in a small poly bag that contained four of them. Usually they're a buck but there was a sale so I got them all for 50¢.

It's actually starting to look like a telescope focuser now. However, at the moment, it doesn't work. I'll need to put in the bearing (the strip of milk carton plastic) and do a bit of trimming on the inside front of the tube. Either that, or I'll have to get thicker tubing to attach as the contact points on the pinion.

But the drawtube moves pretty well inside the body tube. The action is nice and smooth. I figure it can only get better with the bearing.

Wednesday, March 9, 2005
No pictures today but I managed to get the focuser working. The slide action works really smoothly. More smoothly, in fact, than the brass rack and pinion focuser I'm using now. I had to use a round file to work the inside of the tube to allow for the milk jug plastic bearing and now it's working just as I wanted.
Thursday, March 10, 2005
After much, much searching, I managed to find some plastic to use as a base. It's a form of expanded vinyl called Cintra and I love the stuff. It can be cut with the same kinds of tools used for woodworking and it can be heated and formed.
Because I didn't want to try to drag this fairly large scope into the kitchen, I left the heated Cintra immersed in the hot water and brought the whole lot into my workspace. This way, there was little heat lost between taking the plastic out of the water and forming it on the tube.

I slipped the heated plastic under some bungee cords I'd wrapped around the scope. The three lengths of cord will hold the plastic in place and tightly to the tube until it cools enough to hold its shape.

Incidentally, that's my high-tech, high-priced Goodwill Store plastic bag scope cover you see at the top.

I'll pull the bungee cords off tomorrow. I hope to have first light by Saturday night.
Sunday, March 13, 2005
Ever get fed up with the way a project was going and decide to take a different approach? That's what's happened today.

After trying to get everything to bend just the right way, etc., I realized that unless the person who built the original PVC focuser (see the link above) has access to tools that I don't, I simply can't keep the optical path nice and straight by doing any kind of bending on the main body of the tube.

Once again, Cintra comes to the rescue. I've decided to make the focuser's arms from one piece and attach them to the body tube. Unfortunately, this means chucking everything I've done up to now but I figure that was just part of the learing curve.

Here's the jig I built for shaping the arms. It's just some 1" scrap pine nailed to a back board. There's a small test piece of Cintra so I can make sure my idea is sound.
Here's the new arm piece in the jig. I've had to add another tool to the bending process. The hammer is there to gently coax the piece to match the curvature of the jig.

I put the body tube on top of the jig to see if the curvature of the arms matches the tube. So far, it's looking pretty good.

I'll take it out of the jig after it's had a few hours to cool.

And it really, really works! The curvature matches just as it should and the arms are nice and parallel.

As soon as I get this project tested and finished, I'll put together a list of parts and tools with a measured drawing for download.

I've drilled some holes on both sides for the section that needs to be removed. If anyone knows a better means of cutting PVC pipe lengthways, please let me know.

The up arrow helps me keep track of which end is which. It's pencil so I can just erase it when I'm ready to finish up.

Cut and ready to attach the arms. Almost, anyway. Naturally, I'm going to sand some of the rougher edges.
I think that's about enough for a day. Considering I drafted the idea this morning, built the jig with nothing but a saw, pocket knife and a file and I've gotten this far in a day, I think that's pretty good.

More to come... I'm just getting started.
Here's a partial list of the tools I've used on both versions of the project:
  • Miter box
  • Carpenter's square
  • Steel ruler
  • Fine tooth hacksaw
  • Dovetail saw (works well with the miter box)
  • Visegrips (for handling hot PVC/Cintra)
  • Hammer (for times when you need a little finesse)
  • Pot of boiling water (if you've come this far and don't know what that's for, you need to work on your reading comprehension skills)
  • Gardening gloves
  • Eye protection
  • Dust mask
  • Swiss army knife

Click to drop me a note.

All images, text and JavaScript in this document
©2005, James C. Craig.
All rights reserved.