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Open Segment Indexing Fixture

Brent Dalrymple

PRO Member
I had a request to post a picture of my open segment indexing fixture. The first version is shown in P1000955. The white, 12" indexing graph is glued to a 1/8" acrylic disk, and is changeable using two screws with threaded inserts in the 1/2" MDF mounting plate. The graph was made using Title: Free Online Graph Paper / Polar, and printed at Kinkos using one of their wide format printers. Printing rather than enlarging guarantees thin lines, which helps accuracy. I made 6 different graphs for different numbers of segments, using both sides of the acrylic plates. I found that getting the MDF mounting plate exactly centered on the turntable was difficult and this one was off about 1/32", so I added a second smaller plate, shown in P1000959, that is fixed to the larger one with bolts and threaded inserts. The holes in the smaller plate are 5/8" so it is adjustable and can be exactly centered. The upright is also adjustable and exactly perpendicular in both X and Y. I designed this jig after studying those used by Tom Lohman and Dennis Edwards. Mine requires no metal machining since I used a standard 1"-8 tpi lathe adaptor with a 3/4"-15 tpi internal thread (from Grizzly), which is secured to the 1/2" MDF mounting plate.


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Mine is already built. I had started with a small Driskell style fixture with index wheels about 8” diameter. It seems OK up to about 24 segments but over that I don’t feel the spacing is accurate enough (index plate needs to be larger I think).

My new fixture is similar to the fixture Tom Lohman, with my own twist on things.
I used linear rails and bearing for the vertical, and a computer operated stepper motor instead of using an index plate.
Did a drawing of it in Sketch-up
Index Fixture-3 (1).png

Working on my first bowl with it, 60 segment 35 rows 2040 pieces

IMG_20170313_195915358_HDR.jpg IMG_20170313_195929260.jpg

Few things to work out yet, but we are operational
Not happy with my first row, the ID is way to big. Did not pay enough attention to the first couple of row when I was designing it in Segment Pro. Ended up moving the last dot in the profile designer portion of the program and re-did the first row. Much happier with the results.

IMG_20170315_134619097.jpg IMG_20170315_134645096.jpg
Bob - nice jig! I did something similar, but on my lathe. I added a stepper motor that turns a gear on my headstock. I originally used it for routing decorations, but I've also used it with open segment bowls. I also added a laser engraver that lets me laser around a snare drum shell. I have given thought to making a flat version for open segments, since that would be more enjoyable than on the lathe. I used an arduino to control my CNC breakout board and I wrote an interface using chilipeppr.com. I'm curious what you are using to control your stepper?

Here's a test video I did of the laser engraver:
Neat - the funny thing is that I actually have all the gear you used laying around - a few spare rods, bearings and mounts. The only thing I am missing is a threaded spindle - what did you use? I do have a threaded morse taper. I've thought about adapting that with a 3d printed gear, but it might be a challenge to get it accurate,
Also - if you wouldn't mind sharing a screenshot of your interface, that would be awesome. Here is a link to mine. It is pretty lame so far, but it works. If you go to it, you'll see some errors since you don't have a device connected. Title: ChiliPeppr - Hardware Fiddle

If you would rather not share, that's no problem!
Bob - that looks awesome! I like how you did the phase function and mimicked a real lathe index plate. I have one last question and then I promise I'll stop bugging you: How do you handle zeroing out your faceplate. The only idea I have so far is drilling a small hole in my wooden face plate, so that I can stick a small nail in it. Then make a holder for a micro switch so that when the nail is installed, it will hit the limit switch. That way I can get the faceplate back to zero if I shut the computer off. Thanks again for your responses!
Justin, don't worry about asking questioning that's what the forum is all about.

I tried this first
I had a hacksaw cut into the side of the faceplate that a piece would pivot down into and a wood block that everything would push up against. It was OK but not positive enough.

This is what I am using now:
IMG_20170406_080154206.jpg IMG_20170406_080204762.jpg IMG_20170406_080426776.jpg

Basically what you were saying about the nail only using a bronze bushing and a 1/4" pin
Neat - the funny thing is that I actually have all the gear you used laying around - a few spare rods, bearings and mounts. The only thing I am missing is a threaded spindle - what did you use? I do have a threaded morse taper. I've thought about adapting that with a 3d printed gear, but it might be a challenge to get it accurate,

The main shaft was a 3/4" - 10 UNC bolt which actually is under size a little and matches pretty close to 19 mm. Then I used a 3/4" to 1"- 8 adapter from Oneway to get to the faceplate.

You might want to look at my posts here:
Title: Open Segment fixture (Manual)

They may answer some of your questions
Great info. Good idea on the 3/4" - that should be a lot easier to find than a 1 1/4" x 8. I think I'll give my idea a shot using a momentary switch to mechanically zero my setup on the lathe. If I click to home the stepper with the rod in place, then it will rotate back to the rod. I'll have to see how repeatable it is. I like the idea of the bronze bushing. I think I have some copper tube and rods. I think that would be more accurate than a nail. I'll check out the other post you mention!
Hey Bob, I was able to bang together a prototype using a rocker arm limit switch to determine home. Here's a video:

It would be a lot easier to implement in a flat jig such as yours. When you click home, the stepper rotates until it detects the switch and then jogs back and forth and sets the zero. Theoretically at least, I can take the face plate off to work on another project and then put it back on. No matter where the faceplate ends up on the spindle when I tighten it down I should be able to get a repeatable zero location.