Homemade diff pin?
 

Which is better?

http://i420.photobucket.com/albums/p...a/100_1133.jpg

The shorter silver one(shorter because I cut it) says cobalt? Is that hardened steel coated in cobalt? The black one says hardened steel coated in black oxide. Which is what? and which is better? Thanx!

hardened steel just be sure to smooth all the edges

Cobalt is an alloying agent, not a coating. The black one should be a bit tougher, where the cobalt bit is harder, and probably more brittle.

They're both the same metal inside (HSS). Either one will work for your application. The different coatings (Oxide, Titanium, Cobalt etc.) provide different levels of friction reduction depending on the material you would be drilling.

Some great basic info here- http://www.wlfuller.com/html/steel_types.html

On cobalt: Cobalt Steel is very similar to High Speed Steel. Its identifier is M40CO or M42. Most drills made of Cobalt have a brownish gold tint and are marked with their identifier. Cobalt is a step up from HSS and offers better tool life than HSS. Since Cobalt is harder and therefore more brittle than HSS, Cobalt drills usually have a more rugged construction with less room for chips to escape in the flute area. Although they work great cutting materials like stainless steel and cast iron, they do not work well in wood or plastics because they do not clear chips well. In an application in which a good grade of HSS Drill cut 2000 holes before becoming dull, a Cobalt Drill might cut 2200 holes before dulling. Sharpening Cobalt Steel tools requires a grinding wheel made of stone or one that is Borazon plated.

Good info, but why then do my "Cobalt" bits have "HSS" stamped on the shank? Shouldn't they be stamped with the respective alloy?

Good question. Either you got ripped off or they are considering it a higher grade of HSS.

Be careful of co's using "Cobalt" in the name, in the same way they like to use "Titanium" for a stupid razor....

http://www.sears.com/shc/s/p_10153_1...?keyword=64086

They work well for me. I've only broken a few of the smaller ones. They seem to be fairly flexible although I don't really have anything to compare them to.

What's curious, there is no mention of cobalt on the packaging.... and split points are great if you don't wanna bother with a pilot hole. But they have a thinner flute by nature. I was going to mention, the easiest way to check relative hardness is with a file. A cobalt bit should file away very little, a regular black oxide HSS bit will be hard but a file will win. It's a bit of a misnomer right out the gates, because HSS tool steel is MUCH harder than "HSS" drill bits. Usually tool steel will contain cobalt in higher concentrations, a file will not touch it.

Yet another example of "standards" not really being standards...

Yes, but I should have mentioned that if the drill bits were made of true HSS they'd be unusable. There is a tradeoff made, lower hardness= increased ductility. If you've used a solid carbide drillbit you know how fragile they are. By compromising, you also lose some long wear characteristics. But think how annoying it would be to replace a shattered drill bit every time you had a snag. That's why the TiN coating caught on so rapidly, because the bits can have the flexibility of standard HSS combined with greater wear resistance.

I have used carbide bits... :grrrrrr:

I have a good one but it would just be too rude. Playboy, pencil d, etc

Wow great information here! Thanks guys, espcially redshift. Was about to install the cobalt one until I checked the thread! Great results! Runs like a charm for now! Going to the track in a few hours for the 5th time ever!

Fun huh? High speed and slow feed....

Glad it worked for ya Zack! And you got a little education whether you wanted it or not :lol: