While making some fidget spinners (http://www.sawmillcreek.org/showthre...idget-Spinners) I wanted a good way to securely hold a flat disk with a 22mm hole. At first I made a wooden jam chuck but I decided to modify some steel Nova pin jaws to hold the disks. The pin jaws are about 25mm I think, too big for the 22mm hole. If I turned them down to 22mm I could also machine a square shoulder on the jaws so I could register the blank precisely when turning the back face.
Although I have metal machining equipment, I decided to try machining it on the wood lathe using standard woodturning tools. The biggest reason to turn it on the wood lathe is that is a simple way to ensure the final jaws are perfectly concentric to the wood lathe since the chuck is already threaded for the spindle!
I have turned a bunch of much softer aluminum and brass but never tried steel. I'd heard it was possible so why not?
I was a little concerned since one good way to destroy a sharp edge on a tool is to accidentally run it into the chuck jaws! Putting the tool to the steel on purpose concerned me a little until I considered this: most of the tools we use on the metal lathe are made of HSS and are ground very much like scrapers - they call the grind angle the relief. There is both end relief and side relief: http://www.steves-workshop.co.uk/tip...olgrinding.htm In fact, the hand-held metalworking tool called a graver looks more or less like a scraper to me.
I already had a 1/2" Thompson scraper ground with both end and side relief for a special purpose, so I just used it on the steel.
Cutting the steel was slow but it worked fine. Here is a picture:
I sharpened tool on both the end and side, honed, then pushed the tool straight into the steel at the center line. I cut a square shoulder to help register the blank. It did leave very thin needles instead for shavings so a word to the wise - don't touch!
Guess what, turning steel dulled the tool quicker than wood. Duh.I made several trips to the grinder!
This picture shows a Cocobolo blank held for turning:
I did a several of things to make these jaws better for the very specific job of turning these little spinners. First, I made the distance from the end to the shoulder exactly the thickness of one of the 608 bearings commonly used in the little spinners. This way I can turn the hub to the thickness needed without ever having to measure.
Second, I made the jaw diameter 21mm for clearance but compensated to make a perfect fit at 22mm. We know the best holding power of jaws whether with a recess or a tenon is when the diameter matches exactly. If the diameter is too small, each jaw is only holding by the rounded center portion. If the diameter is too large the jaws will only hold by the "corners". To make them expandable to a perfect 22mm fit I first turned them to 21mm for clearance. Then I opened the jaws and tightened them down on some shim stock and shaved a tiny bit more to take off the "humps".
Lastly, I don't intend to take these jaws off the chuck. I like to have a chuck for each the jaws I use the most and I have several with the pin jaws. I have never used one in the expansion mode, just in the compression mode to hold small things. Modifying this set for a special expansion use doesn't affect their compression use at all.
Now when I open them on a 22mm hole they are snug and secure even with very little tightening force. This is important on the fidget spinners if wood is removed close to the center. A tiny bit too much expansion force and the wood will crack. I learned the breaking force the hard way!
Back to turning steel: I did try a few other tools. One was a 3/8" bowl gouge, another was a skew with a 45-deg total sharpening angle. While both of these (and others) work very well with aluminum and brass, they don't work so well on steel! The blunter edge of a the scraper was perfect.
JKJ
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Originally Posted by Marvin Hasenak
