Bi-Metal vs Safety Blade

[BruceH]

We just picked up the Donek D2 drag knife and I'm eager to get going with it. I've read through the documentation, and watched a couple of the Donek videos, but I have yet to see a clear example of bi-metal vs safety blades. It's clear in the documentation what some of the structural differences are, but it doesn't go into much detail on telling the difference or when to use which. I've done a web search on it, which has only lead to more confusion. Shaving razors, box cutters and utility knives flood the results. I've also see on Irwin's own site, an item listed as a "Bi-metal Safety Blade." !?! So...

What is the difference (how does one recognize one from the other if they're not labeled)?

What kinds of projects benefits from bi-metal vs safety?

Thanks!

[Brady Watson]

BiMetal blades have less of a tendency to snap off like a regular blade. By combining metals, both hardness (so it stays sharp) and flexibility (so it doesn't snap) can be enhanced over that of a regular blade.

Test out both kinds of blades, then report your results to the group so others can learn.

-B

[BruceH]

Thanks. That much is stated in the documentation, and it makes perfect sense, but I still have no idea how to visually tell the difference or why I would ever opt to use a safety blade over a bi-metal one. Do they support different feed rates? Is one more agile? Is one more expensive? Why does Irwin sell "Bi-metal safety blades" if they are ostensibly (according to the documentation) mutually exclusive?

[Brady Watson]

I don't know the answer to all of your questions. However, the bimetal ones are more expensive, as are the induction hardened regular blades (DeWalt I think) - they last longer - and stay sharper longer.

As with anything else on this rock, you get what you pay for. The best value would be the one that performs best for the longest amount of time. The bimetal blades may be a little thicker, which may not perform as 'tightly' with some designs, when compared to the regular issue blades. For the most part, I believe that the blades are nearly if not identical in shape. So, aside from kerf/thickness, there shouldn't be much consideration for this.

Rather than get caught up in analysis paralysis, I would try these things out for myself - doing your own research - with your specific material and designs. As with most things CNC related, help is pretty much limited to getting you into the proverbial ballpark. There are too many variables to get super specific - so testing on your part is in order. "I hear and I forget. I see and I remember. I do and I understand." -Confucius

You might want to send Sean Martin an email or PM and ask for his advice. He is the one that made your drag knife & he knows best.

-B

[BruceH]

I sent Donek an email request for clarification. Rest assured, I'm not letting this get in the way of testing things out. I'm just still in the RTFM stage at this point, and said manual is murky on the subject. In fact, they don't even list what blades come with the D2! They're not labeled either way. There are significantly different offsets associated with each blade type, so I'm guessing there is a visual difference. Maybe rounded or flattened tips, or maybe a steeper trapezoidal slope, or maybe a shallow rake. We'll see what Donek says...

[Brady Watson]

Went out to the shop and opened a pack of Irwin's & compared to the DeWalt & Stanley ones. They are identical in thickness, shape with a length deviation of about .03-.04 between the Irwin & others (Irwin is shorter).

The difference in corner radii from what I recall is the difference in the D1 and D2 holders. Unless you put a blade in there that has a radically different shape, I don't think that you are going to see much difference from one to the other.

If you think about it, the blades can't vary that much from one manufacturer to another because they have to be able to fit in a variety of knives and holders.

-B

[BruceH]

Thanks for taking the time to check into that. Curious. According to the Donek Cutting Radius and Depth chart that came with our drag knife, the D2 & D4 blade offset with a "stock" blade is .140", and with a safety blade is .090". (the D1 & D3 are N/A for offset) So, why such a difference if the blades' profiles are the same? Still no word from Donek.

[Brady Watson]

Hey Bruce,
Check out page 44 and note the 'round tip' utility blades. They elude to them being 'safety blades' and just a quick look shows how that would affect the corner radii, compared to your garden variety blade. My guess is that it is a lot stronger with less tendency to snap off on smaller radii, but I am no expert on drag knives.

-B

[BruceH]

Good stuff. Here's what I was missing. I finally found it, and it confirms your suspicions from above. (8:51 in, if you care to see it) So "bi-metal" and "safety" are not mutually exclusive. Thanks for helping me work through it.

[Brady Watson]

Cool - thanks for the link that makes sense.

BTW - Welcome to the SBF.

-B