Can I get away with knurling over rust?

[shild]

Things are not going too good for me right now! I did the stick method with the 2 knobs I needed on that stick. The knurl's came out very satisfactory, but then I had to put it back in the 4-jaw holding it by the knurl. So I wrapped several layers of masking tape around the knurl to protect it from the jaws. While trying to do finishing cuts the piece moved in the 4-jaw! Now I'm going to have to indicate off the knurl!

[shild]

And yes I am using a live center to hold it but I can't take all the jaws out of the 4-jaw and reverse them so I can compress the part up to something hard in the chuck with the live center. It just slides in the tape when I crank the tailstock wheel hard. Maybe the 3-jaw will work?

[Harold_V]

The discussion over the use of a four jaw can get highly controversial. For sure, it allows for gripping most anything, but it isn't necessarily the best choice, especially if you must handle more than one part, or if you need help in registration. For example, holding lengths, where you may benefit by using a rigid stop.

That said, yeah, a three jaw would be far better in this case. However, only if you have the ability to run soft jaws. Here's why.

Soft jaws offer the ability to grip a part without doing damage. Even with steel jaws, if the jaws are machined precisely (a snug slip fit for the part), it grips over the entire surface, so the part is held without damage to the knurl. With steel jaws, you do risk blunting the very tips of the knurls, however, assuming you don't truncate your knurls. If the jaws are machined with a cavity instead of a through bore, you gain a permanent stop that allows for establishing lengths effortlessly.

I just went through this exercise when I built a pair of stainless knurled knobs (two days ago). I have a mix of steel or aluminum jaws, but for this application I chose the aluminum jaws, as I didn't want to risk flattening the very tips of the knurl. I like my knurls sharp, not truncated. By using aluminum, the sharp knurl indents the aluminum ever so slightly, thus assuring a very positive grip. I could install and remove each knob quickly, each time being assured of no damage, and having the knob run dead true, in spite of gripping over the knurl. The material was round and was gripped so it ran true when I applied the knurls, also by the stick method (a knob on each end of the short piece of material). Once knurled, and the ends chamfered, the material was parted in half, yielding both knobs. The threaded end was then machined, holding the knob by the knurl.

Don't rely on masking tape to protect a surface to be gripped. That rarely works unless the required action doesn't create much pressure. If you hope to grip a surface without damage, use metallic shims, and make them thick enough to absorb the pressure without transmitting the jaw pattern to the part. In days gone by, one would use 1/8" thick copper, but cost has made that rather prohibitive. You can achieve the same end by using aluminum, and it doesn't work harden the way copper does. Better yet, aluminum soft jaws, machining a cavity instead of through boring the jaws. The cavity yields a register that guarantees the part is gripped dead perpendicular, and you can install and remove a part quickly.

Use aluminum soft jaws. They work.

H

[shild]

Alright, guess I'm going to spend some time at the bandsaw cutting some aluminum to wrap around my jaws then. Thanks Harold!

[Greg_Lewis]

A trip to the hardware store for a short piece of copper pipe will yield enough material for some jaw wraps.

[shild]

A trip to the hardware store for a short piece of copper pipe will yield enough material for some jaw wraps.

Already have a 5/8" diameter piece. Will that do? I'm thinking do a single straight cut lengthwise and unroll it to a sheet.

[Greg_Lewis]

A trip to the hardware store for a short piece of copper pipe will yield enough material for some jaw wraps.

Already have a 5/8" diameter piece. Will that do? I'm thinking do a single straight cut lengthwise and unroll it to a sheet.

That's what I've done, more than once. Be sure to anneal.

[John Hasler]

For the four-jaw I cut off four bits of aluminum extruded angle and put one under each jaw.

[pete]

I've seen lot's of Youtube videos that show machining heavily rusted steel. Seems ok since the finished part will no longer have rust on it right? Only until I learned rust is a close relitive to carbide and is much harder than the steel it came from. I never turn or mill rusted steel never mind chew up good expensive knurls with it. The rust is always removed to get the steel as clean as I can get it with a wire wheel first. Even then I'll usually cover the ways with a shop rag at least until the first pass is made. The dust and rust particles WILL get in where you don't want it and mix with the way lube. Once the soft carriage ways or table way surfaces get hard material embeded in them you have exactly the same as a lap. That hard material will even wear hardened way surfaces if you've got them. And you'll never get it all out. Regrinding or scraping is the only way to fully remove it. Rust might even be worse than the hard outer surface on cast iron castings that everyone knows to protect the machine tool from.