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Old 31-12-2015, 09:46   #1
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trouble drilling deeper dimples into prop shaft

I'd like to drill the prop shaft dimples deeper, but I don't seem to be making much progress. I'm drilling at 1/2 speed with cobalt bits, dipping them in water to keep them cool, but even with a 1/8" bit I'm not getting much metal shavings coming off. Maybe my bits are burnt out? Any ideas?
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Old 31-12-2015, 09:52   #2
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Re: trouble drilling deeper dimples into prop shaft

Cutting oil.
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Old 31-12-2015, 10:32   #3
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Re: trouble drilling deeper dimples into prop shaft

Sounds like you have work hardened the SS surface (assume your shaft is stainless?) you're trying to drill.

I have had better luck drilling stainless using good quality high speed steel bits that alloy or coated. Mainesail highly recommends the Viking or Norseman bits (same product under different names) for drilling SS. Here's a comment from their website. And yes, using cutting oil can help.



High speed steel (HSS) is a form of tool steel; high speed steel bits are hard, and much more resistant to heat than high carbon steel. They can be used to drill metal, hardwood, and most other materials at greater cutting speeds than carbon steel bits, and have largely replaced carbon steels. A Ultra Bor Super Premium drill with its thin web will penetrate the work hardening stainless fast enough to continually cut beneath the chip which is hardening from deformation. This means the drill is cutting softer steel. The cobalt drill, with its thick web, cannot be fed at a fast enough rate to cut beneath the area which is hardening. As a result the cobalt drill is continually drilling into hardened steel
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Old 31-12-2015, 10:48   #4
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Re: trouble drilling deeper dimples into prop shaft

Give the hole a whack with a center punch to break through a work hardened spot. For that matter, use a center punch often. It makes drilling faster sometimes.
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Old 31-12-2015, 12:59   #5
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Re: trouble drilling deeper dimples into prop shaft

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Cutting oil.

+1


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Old 31-12-2015, 13:16   #6
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Re: trouble drilling deeper dimples into prop shaft

You could also try a left handed bit to break the work hardening.
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Old 31-12-2015, 14:22   #7
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Re: trouble drilling deeper dimples into prop shaft

Stainless steels work harden very rapidly. That means that when you try to drill them, if you allow the drill to rub on the steel without digging in and cutting a good thick chip the rubbing will very rapdidly ( a couple seconds is all it takes) harden the steel to the point where the drill cannot cut it and the hardened steel instead takes the edge off the drill.

Of course you should use oil (any oil will do) but oil will do nothing for you once you have work hardened the surface.

Stainless is cut with very low speed (ie RPM on your drill motor) and heavy feed. 1/2 speed on a typical drill motor is WAY too fast. Success here requires fully sharp drills, very low RPM, and high force on the drill to force it to cut a good, thick chip IMMEDIATELY.

Cobalt drills will cut materials that will eat up high speed steel. That is what they are made for.

You have thoroughly work hardened your hole bottoms. You probably will not be successful with a hand drill now. Drilling new holes in new locations is a possible fix if you can do this with your hand drill. A drill press/milling machine and good drills will probably get thru the work hardening OK.
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Old 31-12-2015, 14:58   #8
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Re: trouble drilling deeper dimples into prop shaft

I've had luck using a tungsten carbide grinder bit to break a work hardened surface in SS. You can also get a diamond bit for a Dremel to do the same thing. Then start over.

Yes, very slow speed and high pressure is the way. Don't have a rpm counter on my drill but looks like 50-100 rpm max when I hand drill holes in SS.
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Old 31-12-2015, 15:23   #9
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Re: trouble drilling deeper dimples into prop shaft

A tungsten drill bit designed for metal will drill into austenitic stainless like a metaphorical hot knife through butter if sharp and done correctly. Half speed sounds to fast. Slow is the go. Another vote for proper cutting oil being a must here, too.

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Old 01-01-2016, 06:02   #10
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Re: trouble drilling deeper dimples into prop shaft

Thank you for all the responses.

Yes, the shaft is SS. I see now that I must have work hardened the spot and likely dulled my cobalt bits in the process.

I think I'll get some new bits and try in a new area.
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Old 01-01-2016, 10:21   #11
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Re: trouble drilling deeper dimples into prop shaft

The old salts used lard as cutting/cooling agent. Works marvelous.
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Old 01-01-2016, 11:11   #12
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Re: trouble drilling deeper dimples into prop shaft

Quote:
Originally Posted by Guy View Post
Give the hole a whack with a center punch to break through a work hardened spot. For that matter, use a center punch often. It makes drilling faster sometimes.
I had not heard that before but it makes good sense along with the right bit, slow speed and plenty of cutting oil. Thanks for the tip.
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Old 01-01-2016, 11:29   #13
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Re: trouble drilling deeper dimples into prop shaft

A portable drill press is a great aid to providing some force & control to a portable drill.
I have an old Sears Craftsman that are still available on ebay but the base is large (8" x 8" ? ) for tight spots.

Here is a link to a Cadillac version Strong Arm 5 Portable Drill Press | Versatile portable drill press, leveraging device!

Cheers/ Len
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Old 01-01-2016, 14:35   #14
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Re: trouble drilling deeper dimples into prop shaft

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Originally Posted by asahi View Post
Thank you for all the responses.

Yes, the shaft is SS. I see now that I must have work hardened the spot and likely dulled my cobalt bits in the process.

I think I'll get some new bits and try in a new area.
Do yourself a favour and get a carbide bit drill if you can. Will work much better than a cobalt drill for s/s.

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Old 01-01-2016, 15:08   #15
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Re: trouble drilling deeper dimples into prop shaft

What alloy of stainless is the shaft?
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