Indexing options

Hello all,
I have a very simple problem that probably calls for a simple solution. That said, I am open to "doing it right" if that is an option with
what I have available.
The problem? Given two rings, OD=2.468", two different IDs, and both 0.440" thick (all pretty much within a thou or two), I need to drill and tap for radial 8-32 set screws. The plan is to have holes at 120 degree intervals.
Setting aside my pride and desire to learn new techniques, all I really need is to get the holes to be radial; I can do that with a vise and a well-placed stop. It would be nice if they were reasonably accurately placed around the circumference.
The closest thing I have to a dividing head is my H/V RT. I have a tailstock for it, but I doubt that will be of any help because the "tubes" are so short. A lathe chuck comes to mind, but I do not have one for it, and am not in a hurry to spend money on one given how close I am to buying a lathe. My understanding is that the chucks are different (plain back vs. backed for the spindle), so I will probably end up with both, just not all at the $ame time.
Given the circumstances, how would you locate the holes?
Bill
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On Tue, 20 Nov 2007 16:11:54 -0500, Bill Schwab

For reasonably accurate with not much to work with, I'd scribe a concentric circle on the face of the parts with dividers. Without changing the dividers, step around the circle and lay out six evenly spaced points. Clamp the part in the vise with two of the points that define a diameter flush with the top of the jaws. Center and drill, unclamp, and use the next appropriate pair of points to reclamp, repeat.
A more accurate technique is to use a hex collet block with an arbor turned to hold the rings. If you have 5C collets, the collet block sets are a great bang for the buck - I probably use them 20 times for every time I use a rotary table or indexer.
You could also scab together an arbor to attach the rings to a large hex nut and clamp the nut in the mill vise.
--
Ned Simmons

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Good suggestions.
This device is my favorite for milling or drilling something made on the lathe:
http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO06547&PMT4NO3228729
It's much more rigid than a spin index as long as the collet closing ring and the spindle clamp are snugged up *tight*.
You can get 3 and 4-jaw chucks on a 5C shank that let you move larger work like these rings between the lathe and the mill. The repeat accuracy is pretty much the play between the collet slot and the locating pin.
Center it, or the collet block, under the spindle by putting a milling center finder in a collet and another one in the chuck and aligning the points. You could also use edge finders. Slide the collet one in far enough to grab the sliding portion. Once you've aligned it you can drill a small hole in the top to make it easier next time.
A simple free way, if done carefully, is to draw on paper three radial lines at 120 degrees and a circle the OD of the part, align the part with the circle, and use a small square block as a guide to scribe the lines onto the ring. I've done similar layouts on a freshly plotted CAD drawing that were accurate to better than 0.010". That's how I lay out the mounting holes for a power supply or amplifier when transfer punches won't pass through.
Jim Wilkins
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Jim,

My wallet was cringing, but that's cheap. Definitely on the to-buy list.

Interesting. Of course, that is starting pile up some metal. However, you have just added on to my bag of tricks, **if** I am smart enough to remember it ;)
Enco has an $80 4-jaw lathe chuck that I will probably end up getting for my RT. I would have to make a backing plate for it, but should be easy enough (now I've done it<g>).

Got it - I think.

That is a great idea. I use the 1:1 printed drawing trick fairly often. Recently, I ended up cutting a drawing as I worked down into a stepped part, leaving the part that told me where to cut next. More accurately, it told me which dial revolution was correct - a poor man's DRO.
Thanks!
Bill
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Bill, Here's another way.
Put a peice or two of mdf on the rotary table, screw it down, and place face up on the mill. Face off the surface, to provide a flat and true surface. Machine a round pocket so your rings will index on the inside of the pocket. the pocket is about .005 less depth than the thickness of your ring. Machine outside the register pocket, so your drill will not be starting on an angle. Mill a 1/2" hole in the exact center of the mdf. Stand the rotary up, put a 1/2" dowel pin in the center hole and dial in center, and the surface, remembering to allow for whatever crush was allowed. Place the ring in the register, and clamp thru the 1/2" hole. Drill radial holes, and tap.
Another way, I'll assume you are using a lathe to make these, and a three jaw chuck: After machining the OD. Put a 1/8 spotting or center drill in a 1/8" pencil die grinder, mounted in your boring bar holder. Using a dial indicator, locate one jaw, the indicator travel must allow jaws to pass, so you can get to the next 120 deg position. Drill using low pressure to keep from burning the drill. You will still need to finish the hole at the drill press, but that angle is easy to fake in. . DJ
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Don't get too excited. The $45 price is for a blank indexing ring. The actual indexer that is shown in the photo goes for $260.
Doug White
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MSC had a good image. The less expensive import version I have is sometimes on sale for around $50-$70. Unfortunately they don't have a unique common name like "spin index" and are harder to search for.
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I'd do it one of two ways. 1. face clamp it to the RT, zero it in, move off the R of your BC, drill, tap, move 120 deg on the RT and rinse & repeat. 2. Clamp it in a set of V-blocks in a vice (supported), center up, then trig out the 3 bolt hole centers.
--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
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Anthony,

That sounds like what I would do for holes parallel to the axis??? These are radial. Sorry if I'm missing something - thanks either way.
Bill
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No, I missed it Bill. I thought the holes were with the axis. If you have a lathe available, turn out a stepped mandrel that you can face clamp to the RT that gets the part far enough away from the RT faceplate that you can get the mill quill close enough to the part to do the holes. To clamp the ring to the mandrel, I'd split the mandrel 4x-6x depth of the part you want to machine with a saw and put in a couple of set screws on one side of the split, pushing against the other. Center up mandrel on RT, slide ring on mandrel, tighten set screws, and have a go at it.
If you don't have a lathe....
Pair of V blocks to hold the part, angle plate (or parallel) narrow enough to fit between the v blocks to square the part up to. You would have to trust your layout skills to mark the 120 angles.
Also, just as a note, when you are trying to find the center of a round part on the mill with the part horizontal (looking for the highest point), just running an indicator across the top will not find it accurately. There is a significant distance across the diameter (depending on diameter), where you will see no perceptable movement of the indicator as you travel. For really accurate pickup, you want to move the indicator to some distance away from center where you are actually getting indicator movement as you move the mill axis. Then set the indicator to 0, take note of your axis position, then move to the other side of the round, and move til the indicator reads 0 agian. Take the total distance moved of the mill axis and divide by 2 to get the true center of the round part from your present position.
--
Anthony

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Good point. I generally use an edge finder and then move the radius plus 0.100 but runout or out-of-roundness will throw that off. Lacking a DRO I can't locate both sides and average the readings.
For usually adequate pickup, balance a 6" aluminum scale across the round piece, like a seesaw, and carefully lower the center drill onto the center line. Move the table until the seesaw is level. You could stack a few pieces of wood etc on the table for a reference to sight on.
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Anthony,

No problem.

Good - I wasn't crazy :) I did not come up with quite that solution, but getting the part parallel to the RT in its vertical position did occur to me.

I am working on it, but have not taken the plunge yet.

Are you thinking of using the v-blocks to hold it like a long cylinder? The parts are probably too short for that (0.440"), or would it still work? Perhaps that is why you stressed something narrow? My guess is that I am envisioning something different than you are describing.

Understood.
> For really accurate pickup, you want to

Interesting.
My thought was to measure the OD, and arrange a stop to make contact at the 9 O'clock position. However, the flaw might be that there is no way to be certain about the contact between the stop and the curved surface. I would find the edge using my super-nice Indian made v-block against the stop. That makes sense for a squared work piece, but probably not for a round one.
After that digression, I suspect you are going to recommend the indicator approach. Sounds fine to me.
Thanks,
Bill
[*] It's so flat and square I keep looking over my shoulder for NIST agents :)
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Yup, like a cylinder laid down in the vice. And that is the reason for specifying the word narrow. I don't recall exactly how big of a diameter you said, but the purpose of the parallel end or angle plate is to get the part square and vertical to the vice. You could actually machine notches for the V's out of a piece of square stock slightly smaller than the clamping diameter of your part, this way you catch most of the face of the part you want to machine, plus you could clamp it in place on the vice and use it for a location stop along the x axis.
This way you can be assured of the part being square to the vice.
Yes, I advocate the indicator approach. I have to use this method often when setting up gauges to measure round parts. I have a +/- 5 micron tolerance on the parts, so centering has to be _exact_.
--
Anthony

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Anthony,

Given that they are more rings than tubes, it strikes me as easier to let the vise grab the flat surfaces, and use the indicator to locate the center along the x-axis. I sometimes use squares to get parts perpendicular to the table, but just don't trust it (or myself??) as much as a firm grip with a vise and/or snug parallels.

I think I follow. But again, if considered to be tubes, the parts are so "short" that it makes sense to grab them on the ends using a vise. They will have little choice but to be square, and an indicator will let me find the center along the x-axis.
In terms of machining, the remaining step is to drill radial holes. Not (yet?) having a spin indexer, I will probably resort to the CAD drawing idea.

=:0
Thanks!
Bill
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