Need drill bit recommendation

[jfk92]

I have what seems to be a work-hardened bolt that snapped off in an aluminum swingarm on a dirt bike. I need a long bit - 8" or 12" with the chops to drill into and through this 5/16" bolt - I bought a harbor freight carbide drill bit (think twin tip even?) that seemed to be making some progress while other steel bits I had did nothing) - but ran out of length as it was a normal "standard" size length (barely cut in a dimple). Looking for drill bit material/coating etc recommendations from anyone that has had to drill through some very hard (or hardened) steel. Thx

 

[cvairwerks]

Try Brown Tool for 12" cobalt bits. Just be aware that they require slower rpms and greater pressure to cut. Spin them too fast and you dull them instantly.

 

[gizardlizard]

Are you using cutting fluid? Highly recommended especially for hard to drill materials

 

[Rusted Nut]

Any high speed steel bit should work, as long as others have said, you go slow (like as slow as the drill will turn) and use cutting oil and keep drill bit tip wet.

 

[ThePostman]

You want a cobalt bit, I've drilled numerous grade 8 bolts out of vehicles. Get a good center punch and use it if you can find a way to get it in there. I use Norseman cobalt 135 degree split points. In your position I would look to see if you have a Fastenal in your location, if you have one, they will have the best stuff, and you will probably pay cheaper than online. And they have tons of long drill bits. I paid 150 for the jobber set that is 200+ online from them, without even asking.
That said. Drilling with these, sometimes slow and low is best, sometimes leaning in high speed is. I have a mason jar with gL5 that I dip the bit in. Seems to work cleaning metal off and keeping things lubed. But always have extra bits, 27/64 to tap m12x1.25 for example, when you're chewing through 3/4" of a 20+ year Toyota truck brembo brake caliper screw.

 

[neophyte]

Try Brown Tool for 12" cobalt bits. Just be aware that they require slower rpms and greater pressure to cut. Spin them too fast and you dull them instantly.

I’ve seen conflicting recommendations as far as proper speed for cobalt drill bits.
Some suggest higher speed drills, since the cobalt handles high heat fine, but tends to be more brittle.
The same would go for carbide tipped drill bits.

The main issue with the high speed might be work hardening in the material being drilled.

 

[RoninB4]

I prefer cobalt drills over plain HSS when the material makes drilling tough. Quite important to ensure the drill isn't dull. I know this sounds rather basic but if there's even a slight dulling of the cutting edge it makes a difference. If you've work hardened the material then even cobalt may not be good enough. Work hardening is a result of excessive "rubbing" (usually from a dulled cutting edge" and heat generated. The advice to slow the RPM and use a good cutting fluid is sound advice. Check the cutting edges often, resharpen as often as needed. I've resharpened a drill several times to get through a job.

The length of the drill suggests you're down in a hole. If so make sure to frequently blow out the chips so you're not re-cutting the chips. If the HSS/cobalt drill isn't doing the job (short pecking cycle with heavy pressure at lower RPM) then you've fully work hardened the surface and carbide will be required until you get past the hardened area. Be wary of using masonry drills with brazed in carbide tips. If enough heat is generated, prolonged drilling cycle time, the heat may soften the braze enough to let go of the carbide in what you're drilling. Problem just got bigger.

Let us know how it went, good luck.

 

[cvairwerks]

Last resort is find a machine shop with a tap burner and have them pop a hole into the both that you can follow drill.

 

[MBfreak]

+5 on cvairwerks statement


Last resort is find a machine shop with a tap burner and have them pop a hole into the both that you can follow drill.

Ola

 

[cannuck]

IF you are going to drill, what has been inferred but not said is you need to have a low enough speed and high enough feed along with adequate coolant to CONTINUALLY make chips/curls. If the bit is not cutting (that takes the heat away from the cut in the chips) it is sliding where the heat from friction will soften and dull the cutting edge immediately, thus making the smoother work hardened indent where you need a hole. What kills 99% of bits and holes is too much speed and no where near enough feed pressure or rate.

 

[darkzero]

Being in a deep hole, steel bolt, & in aluminum, I wonder if Alum will work?

I've never tried it myself yet but apparently machinists back in the day used it to get broken taps out. Won't work if it's in steel or if the bolt is plated. Mix Alum with hot water, it's supposed to react with the steel & start bubbling, the steel slowly dissolves/breaks down, then you can chip it out. It's a slow process so you need patience.

You can find Alum at the grocery store in the spices section. Again I've never done it myself so no idea how well it works if at all. But if you do try it please let us know what happens!