Spyderco Forums

I've always liked the Atwood PerfBaby tool, pictured below. I'm wondering if a "PerfBaby" Mule with a pair of translucent printed scales would look neat, or no scales at all for ultralight backpacking use. I think that drilling holes in hardened steel is relatively straightforward with carbide bits, a drill press, and solid clamping, but I have not actually tried it yet. If I get three sizes of carbide bits -- 1/8, 3/16 and 1/4 -- I could make a design as sketched below.

I found a feed/speed chart for carbide tipped tools and it reads 100-150 RPM with coolant for "high strength steels" and 65-105 RPM for "high temp stainless steel alloys". The only thing that changes for diameter of the bit/hole is the feed rate, but since I'll be doing feed rate by hand, I can't control it precisely. I go with a firm downward pressure somewhere between "bit squeal" and "bit break."

Those of you who have used carbide bits to drill into hardened steel...any hints, tricks, or advice before I blunder into this project? I might try it on a straight spine mule.

I think you need to see what the hardness is for your mule. Tungsten carbide may not work. If so, diamond baby.

Oh dear. I think SPY27 is run around 63-64, no? Is that too high for tungsten carbide?

Looking on McMaster Carr, I see diamond bits for masonry and glass/ceramic. The latter are hollow and $130 each and require piped-in coolant using a $250 adapter. That's a bit rich for me.

Depends on the carbide bits, some advertise it can drill up to 55, 60, 65, ..., Hrc.

I dont know how "realistic" these claims are. I know however that it is better to go for the highest quality directly. Even if it is to use them once, you can always resell afterwards.

(Spoken with the wisdom of the idiot)

Bolster wrote: ↑

Thu Feb 23, 2023 12:34 pm

I found a feed/speed chart for carbide tipped tools and it reads 100-150 RPM with coolant for "high strength steels" and 65-105 RPM for "high temp stainless steel alloys". The only thing that changes for diameter of the bit/hole is the feed rate, but since I'll be doing feed rate by hand, I can't control it precisely. I go with a firm downward pressure somewhere between "bit squeal" and "bit break."

Those of you who have used carbide bits to drill into hardened steel...any hints, tricks, or advice before I blunder into this project? I might try it on a straight spine mule.

I recommend using solid carbide bits to drill holes in your Mule. They worked on my Rex76 Mule so you should be fine. There are a couple of things you’ll want to do just to make sure there aren’t problems.

First thing is lubricant. Oil or wax should work, lately I have started using CRL tube wax which I like because you can load up the bit and it will melt down as you build heat. This makes it so you don’t have to stop to add more oil.

Second, I get the best results by pecking with the feed pressure rather than slow and steady. It just build less heat and the carbide seems to cut nicely this way.

Third, for solid carbide bits you need a drill press. They are too brittle to drill by hand and will snap if bent.

Fourth, I would recommend backing your Mule with a piece of steel as a spoil board. This way as it cuts through the blade steel it is backed by a hard surface. The one issue I had with my Rex76 was that I had it clamped to wood on the first hole and when it went through the bit chipped and it cracked the knife. Thankfully it was a spot that was being cut away, but I haven’t had this same problem since I started backing it with steel.

Fun side note, the carbide bit that chipped will still drill through hardened steel. If you look at my folding Mule thread you will see where the Rex76 cracked.

Best of luck!

I just wanted to slightly amend my previous comment, but decided it would be better to not just edit it.

Regarding using a drill press, in my opinion that is the bare minimum with solid carbide bits. A mill would be way better, but I figure if you have a mill you’re probably ahead of me experience wise.

This leads to my second edit which is in regards to pecking with the bit vs. slow and steady pressure. I’m pretty sure the reason this works best for me is that the slowest speed on my drill press is way faster than the ideal cutting speed. Basically this is a bit of a workaround for using a drill press instead of a mill, which could deliver way lower RPMs. Likewise, I have found that the cutting wax works better for me with the tools I have on hand.

Again, best of luck! Please let me know if you’d like me to clarify anything for you.

Really appreciate the advice Casper. I will obtain some solid carbides and avoid the carbide tipped ones. I have both stinky sulfurous cutting oil and some new-fangled wax type cutting lubricants, check. I will peck, check -- that was something my ol' machinist teacher taught, also, so you're in good company there. I am most definitely NOT ahead of you experience wise but I do have a smallish (400 lb) mill, and mill table clamps, that should keep the setup rigid, check. Backing with steel -- would not have occurred to me, many thanks for this hint.

I use carbide bits to mill hardened knife steel with no problem, but I try to avoid having to drill holes in it. After burning up too many $$ in bits I've decided I don't want to do that anymore. Milling a notch or jimping is easy enough by getting your feed and speed right and just taking small bites with each pass. Making a new hole in hardened steel is much harder and the larger the diameter the harder it gets. My best suggestion is use lots of lube, don't ride the drill, take small bites and let off the pressure frequently to help control heat... be patient.

Bolster wrote: ↑

Thu Feb 23, 2023 12:34 pm

I found a feed/speed chart for carbide tipped tools and it reads 100-150 RPM with coolant for "high strength steels" and 65-105 RPM for "high temp stainless steel alloys". The only thing that changes for diameter of the bit/hole is the feed rate

Hmmm... you might want to rethink this. Optimum cutting speed is measured in surface feet per minute. For drilling operations, figure out the circumference of your tool in inches, convert to feet, and then figure out how fast it has to spin to meet your target SFM.

There's an optimum feed rate, but you needn't get too worked up about that. Essentially, you want enough pressure on your tool so it cuts continuously and doesn't build up too much heat, but not so fast that you chip the tool.

You mentioned high sulfur cutting oil. That's what I would use. Apply liberally. Keep your work drenched. If you let the work get hot, you risk work hardening it, and you'll be unhappy.

Some chilling replies. Now contemplating whether a PerfBaby mule is worth the risk.

Bolster wrote: ↑

Fri Feb 24, 2023 2:03 pm

Some chilling replies. Now contemplating whether a PerfBaby mule is worth the risk.

Risk? Nah, you got this.

You have all the knowledge, so there's a good chance of success. If you end up breaking all your tooling, at least you got to learn something and have some fun. If the handle of your blade gets buggered, just put on scales as you normally would.

P.S.
I just looked up the PerfBaby and PryBaby. I'm torn. Pocket prybars are cool, but I don't really want to carry more junk in my pockets. And I don't want to spend a ton of cash on one. Maybe I need to see if I can find a chunk of stainless in the garage and go carve one out. Might be fun...

RustyIron wrote: ↑

Fri Feb 24, 2023 2:25 pm

P.S.
I just looked up the PerfBaby and PryBaby. I'm torn. Pocket prybars are cool, but I don't really want to carry more junk in my pockets. And I don't want to spend a ton of cash on one. Maybe I need to see if I can find a chunk of stainless in the garage and go carve one out. Might be fun...

Thanks for the advice!

Now here's some for you: Avoid Atwood tools. They're addictive, expensive, and other than the titanium PryBaby on my keychain, they're drawer candy--too small to be actual useful tools IMO. The one good thing I can say about them is that they go up in value/price over time, and there seems to be steady demand.

Atwood was, I think (?) the inventor of the category, but legions of imitators now exist, at much reduced prices.

If you make a pocket prybar, please post it somewhere I'll see it! Would be interesting. If you re-handle a mule using a hidden tang design, maybe you could make your prybar out of leftover mule handle.

I don’t know if you are trying to retain the heat treat but a plasma CNC machine will cut circles out of hardened steel like it’s butter. My brother has one for on-site fabricating forklift parts and it is amazing. I don’t expect most folk would have one just kicking about in their shed, but I know of several machine shops near me that will do ad hoc work during their lunch breaks for beer money. You may be able to find someone locally who could help you out, although I completely get that this would likely defy the objective of doing it yourself for fun.

Just thinking out loud.

Have a great day,
Al

sorry Bolster, I misread your post - I thought you were planning on turning a mule into a pry bar. In my defence I’m still recovering from illness so my brain is fried. A plasma cutter would make a right old hash of a knife’s heat treat.