Indicating Rough Cast Iron

[SteveHGraham]

So what size should I make the bore for a 4.125"-diameter object? I was thinking maybe 0.030" over, to make sure I don't get any bite marks.

[spro]

I just don't get this. The periphery isn't even close to being round or grip able. If you can't come in between those spokes by lets say blocks of aluminum or something and expand the chuck to grip... All you need is enough shimming to locate and turn the boss accurately. Once that is done everything else falls into place. You will be able to grip it for all the other operations.
I don't see carving out new soft jaws for an irregular object.

[SteveHGraham]

The soft jaws worked fine, although I had a weird problem calculating the diameter of the gripping area. The wheel was 4.1" wide, and the object I used to space the jaws out was 3.25" wide. I took 0.85" off the insides of the jaws, and the opening was too small. I managed to measure using the wheel as a gauge, and I found I needed to go to 1.05". No idea why.

Anyway, now I'm in the wheel-tapping business. Will post a photo. Because the soft jaws make this job so easy, I may get another wheel and tap it. The work I did previously was okay, but now that I have a proper tap, I can do better.

[GlennW]

No idea why.

Because it was Imperial dimensions.

[SteveHGraham]

Got a photo.

[John Evans]

The old dog learned a new trick Huh??

[SteveHGraham]

I couldn't fight it any more!

[spro]

Me too. Don't trust the first pic.

[Harold_V]

So what size should I make the bore for a 4.125"-diameter object? I was thinking maybe 0.030" over, to make sure I don't get any bite marks.

Like I said, check my comments in The Resource Library, in regards to soft jaws, and how they're used. You'll gain some very useful information.

Mean time, you DON'T make any allowances for size. You bore the jaws so the part(s) to be held are a snug slip fit. That way they conform to the part, and leave no marks. Holding size is important, as the scroll in almost all chucks isn't true--so if you tighten past the point (or before the point) where the jaws are machined, you can expect to lose concentricity. How much depends on the amount of clearance the scroll has on its spigot, as well as how much error was created when the scroll was ground to fit.

If the wheel isn't machined, it's important that you align the jaws in the same position when you remove and replace the part. That way concentricity and perpendicularity will be maintained, assuming the part is parallel. If it isn't, you may have issues with that feature. If that's the case, take a very light skim cut on each side, so you have a parallel surface to use as a reference.

H

[Harold_V]

The wheel was 4.1" wide,

You've been watching too much Stargazer. The wheel is 4.1" in DIAMETER. Wide would relate to the thickness of the wheel.

and the object I used to space the jaws out was 3.25" wide.

It, too, is diameter, not width.

I took 0.85" off the insides of the jaws, and the opening was too small. I managed to measure using the wheel as a gauge, and I found I needed to go to 1.05". No idea why.

Because that's not how you accomplish the task. Unless you have the capability to measure three points of the diameter, you can use the difference as a guideline, but stopping short, to see how the part fits. In the end, you sneak up on the size, and try for a snug fit. It's a good idea to take a small amount out, but only to a shallow depth, then test the fit. If it's loose, but it was too tight before, back off and take the cut to full depth. Shoot for a snug fit, not too tight, not too loose. Won't matter much for this project, but when concentricity is tight, it can spell the difference between success and failure.

Anyway, now I'm in the wheel-tapping business. Will post a photo. Because the soft jaws make this job so easy, I may get another wheel and tap it. The work I did previously was okay, but now that I have a proper tap, I can do better.

Interesting. Is that an admission that soft jaws are actually a good idea? Sounds to me like that's exactly what you mean, and I have to say, you've been damned hard to convince.

H

[SteveHGraham]

No, soft jaws are still awful. Worse than imperial measurements.

[SteveHGraham]

Sorry about the improper use of the word "wide." I was just trying to use the same nomenclature I use when measuring my bench grinder stones.