How to mount a change wheel to work on it's bore size?

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Eugene:
This again is some pre-planning, this time for making up a complete set of change wheels.

I have five cast iron change wheels that are the correct tooth form for the existing half set on my Myford M type, but the bore sizes are a gnats too small, and vary a bit. In addition the locating pins and matching holes are also not right. I'm trying to think of a way of mounting the wheels in the lathe so that I can bore them to correct size and drill new locating holes.

I can see one long winded way; turn a close fitting mandrel to each wheel between centres, then put an angle plate on the cross slide, slip the mandrel through a convenient hole and clamp up to that using spacers. De-mount the mandrel and we're good to go. The centre of the wheel bore will be on centre height, and if the cross slide mic is set to zero the holes can be drilled dead opposite each other at the correct distance apart.
I suppose if I started with the largest mandrel size first, it could be turned down a bit to suit the next required 'ole, avoiding the need to turn up five items.

Is that the best I can do, or is there some cunning plan? See below the red wheel that's one of those requiring some mods, and the dirty green original.

Eug

John Rudd:
Clamp to a faceplate and centre up with a dti on the existing bored hole

Eugene:
Thanks John,

I don't have a lever type dti, just the plunger version, but assuming I did and went your route, I can see the boring out would be OK, but how would I be able to drill the holes the correct distance apart? I don't see where a datum would come from.

You can tell I'm a bit green on this stuff! :scratch:

Eug

awemawson:
Once bored to size, slide the victim gear and one with correct holes onto a snug fitting spindle, and spot the holes through with a spotting punch

Lew_Merrick_PE:
Eug,

One method would be to turn a (I assume Morse Taper) shank to fit your spindle with the other end turned to be a close (i.e. +.0010/+.0005 inch or +0.02/+0.003 mm) slip-fit to your existing bore.

Another approach would be to make your own indicator. This would be a pair of (say) 1:10 levers that pivot on precision pins and bores in the case. The first lever would have a hammerhead shape that will "follow" your part. The second lever, driven off of the 10X tail of the first lever, becomes the indicator needle for your display. I believe that one of the people from the old Home Shop Machinist magazine (Rudy K..., if I remember correctly) wrote an article with a "how to" on this subject.

It has been nearly 50 years, but this was one of the tools I made as an apprentice machinist. I also made a nice maple box for it, but it was one of the tools that did not survive a (1974) house fire. At 1:10 to 1;10 lever ratios, it would resolve things into the .00025 inch range (though, I admit, my Last Word indicator supplanted it in very short order as my lives in my tool chest instrument). Now, mind you. the one I made pivoted on jewel bearings (one of the points of the exercise) and the one I remember running across a decade or so ago in Home Shop Machinist did not go to that level of detail, but the article brought back warm fuzzy's of my hours spent finishing things off in the useful pursuit of skills & knowledge...

And, finally, there is always shelling out the lucre for an off-the-shelf indicator. .0005 inch resolution inexpensive ones run in the $20 range in my neck of the universe. Good and accurate ones still are fairly spendy (though, having had my set of indicators for more than four decades, I admit to being unaware of their current pricetag).

In terms of finding your datum for your pin & hole, my first question is, How will you make a tool follow your center and not the existing one? Otherwise, it is a case of finding a center and exploiting it. One method would use an indicator. Another would involve making a pair of concentric disks and using them to locate the centers of your new locations. There are others, but they are all variations on a theme from there.

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