Softjaw question

[EOsteam]

Ok, I'm sure this is going to be a stupid question but I'm drawing a blank, so here goes. A job requiring softjaws is about to commence. I have softjaws and a spider but how does one measure the diameter of the softjaw cut. All of my micrometers measure at 180 degrees whether they be calipers or micrometers. How does one measure the diameter an inside circle at 120 degrees from each other.

I'm sure this is self evident to all except me.

Thanks,
HJ

[Harold_V]

You don't, not unless you have Brown & Sharpe bore micrometers, and precious few have them.
Not a problem, however.
What I do is judge the diameter by placing a part next to the bore. Hook one jaw, then see how much overlap there is. Translate that to thousandths, and take a cut, although less than the amount you have determined. You don't want the part to fit, not yet. Bore a few thou deep at that setting, then try the part. If you're close ( say less than .015"), take the bore to depth. If not, take a few thou more and repeat. It's desirable to have just a few thou to remove for a final pass, so the finish is quite good.

When you get very near size, take a thou, and check. If the part doesn't fit, take another thou and check again. What you're looking for is the part to start in all three jaws, but not easily. When you achieve that condition, take the pass to depth. Check the part once again. If it is an easy slip fit, the jaws may or may not run true, depending on the chuck and how the scroll relates to the jaws. The ideal condition is a snug slip fit---which supports the part by full contact of the jaws, and grips without doing any scarring.

By using the idea of taking a shallow cut with each change, if you go oversized, all is not lost. Simply back off a few thou. The very shallow oversized portion won't be troublesome unless you have very thin parts.

This sounds like a slow process, but it's not all that slow, and the small amount of time it requires should reward you with jaws that run true. You should expect no more than a half thou runout if you've done your work well

Edit:
There are times when the jaws in use are wide (pie jaws), at which time you can still measure them. That's one reason to make your soft jaws as wide as you can.

H

[John Hasler]

Ok, I'm sure this is going to be a stupid question but I'm drawing a blank, so here goes. A job requiring softjaws is about to commence. I have softjaws and a spider but how does one measure the diameter of the softjaw cut. All of my micrometers measure at 180 degrees whether they be calipers or micrometers. How does one measure the diameter an inside circle at 120 degrees from each other.

I'm sure this is self evident to all except me.

Thanks,
HJ

Measure points 120 degrees apart and use trigonometry.

Some HASS videos:

https://www.youtube.com/watch?v=dRMm9P9XdbY

https://www.youtube.com/watch?v=-AyMQNoaBjc

Nothing about measuring for obvious reasons, but some good points such as that you should machine a relief at the base of the cut, a CNC machinist using the cigarette paper method, and an interesting use of engineer's blue.

[GlennW]

I'm a bit OCD sometimes when using thin wall stock, so I bore a piece of scrap, measure the bore, and then use the dial or DRO to go from there to the size I want the jaws bored to.

[Magicniner]

I use Diatest bore gauges, I was very lucky and picked up a range of very good second hand sets from a closing down sale

[Harold_V]

How do the Diatest bore gauges get around the fact that the typical soft jaw setup present jaws @ 120° intervals, thus not always offering a direct reading? Do they not require 180° opposed surfaces in order for proper measurements to be achieved?

H

[SteveM]

There are bore gauges that have three measuring points at 120 degrees apart, like this one:



Because of the design, you can't just use one and get extension rods, so you have to buy a bunch to cover the range you need.

If you make your jaws wide enough, you can find points 180 degrees apart (one side of one jaw, opposite side of the next one).

I just use the slip fit method.

Steve

[BadDog]

How do the Diatest bore gauges get around the fact that the typical soft jaw setup present jaws @ 120° intervals, thus not always offering a direct reading? Do they not require 180° opposed surfaces in order for proper measurements to be achieved?

I was also lucky enough to find 2 sets (ranges) of these cheap as well. They are very nice and handy for bore measurement, but they do require 2 points of contact at 180*. As nice and handy as they are, I generally just use t-gages. I think they are really more of an inspection tool.

[Harold_V]

There are bore gauges that have three measuring points at 120 degrees apart, like this one:

They are quite similar to the B&S types, which I mentioned in my first response.

H

[EOsteam]

SUCCESS!! Thank you to everyone but especially Harold. I used Harold's instructions and everything turned out just perfect. I am now a big believer in softjaws. Since I have no machining experience or anywhere locally to gain experience, this board has become my apprenticeship. It's awesome to have great mentors to keep me on track when I hit a wall.

Harper

[Harold_V]

Good news! Congratulations on your success, and thanks for your kind comments.

H

[SteveM]

Harold made me a convert years ago.

Soft jaws are so accurate and flexible, you should have at least one chuck for them.

Look for a chuck with replaceable top jaws that are missing, and beat the guy down on price because new hard jaw tops cost a bloody fortune, then go to US Shop Tools and get some sets of soft jaws for cheap.

Steve