Cut length of bits questions

[skintigh]

I'm a little confused on a basic concept: the cutting length of bits. Say I have a Onsrud 57-260 with 3/16" diameter and 3/4" cutting length. I believe I'm only supposed to cut plywood around 3/16" deep per pass.

So what is the rest of the cutting length for? Is it re-cutting/cleaning the cut from the previous pass? Is it just removing material and not cutting (maybe that's more applicable to an upcut bit)? Is it there just in case you want to cut something soft at 3/4" per pass, but otherwise it's doing nothing at all with plywood?

I think the shaft on that bit is as wide as the flutes are, so theoretically I could cut much deeper than the cut length, but I assume that would be bad for the bit and/or burning the makerspace down. Is that correct?

Thanks!

[Chuck Keysor]

Hello Bob..... You are referring to a general rule of thumb, which is used reduce the chances of snapping off a bit. If you are cutting Styrofoam, you could of course take much deeper passes. If you are cutting brass, well, you could only take cuts of maybe .01" per pass. But for cutting run of the mill wood, plywood, plastic, well, not having the pass depth not exceed the diameter of the bit is a reasonable starting point. And, I am no expert. Many people here are experts, and this topic has come up many times before.

I order to search this form to look for the information you want do a google search for: bit cutting depth site:talkshopbot.com
Well, the general format to do a google search on this forum is: SUBJECT XYZ site:talkshopbot.com
You can put quotation marks around important phrases, or use * as a wild card.

Anyway, even what I just told you here, is what I have learned here on the forum. And, I have a notebook for Shopbot, where I write down important things I see here on the forum.

Good luck, Chuck
PS: I didn't know your name, and I have never met a Bob I didn't like, so I called you that...……….

[steve_g]

“Bob”

Some of us cut very thick material… 3” solid pecan is not uncommon for me. You’d like your cutting length to be as deep or deeper than your material so that you can do an “onion skin” cut. When stepping down 1-bit diameter at a time through thick material, you’re left with a cut edge that has “water marks” or horizontal lines reflecting each step down. You can eliminate this undesirable look by making your last pass very shallow and a few thousandths inside (or outside depending on the situation) the original path.

If you were cutting 2” material with a bit with 1” CL, if doing an onion skin cut, you’d be rubbing on the shank and either break the bit or get a tapered cut.

A bit properly sized to your material will last longer… you don’t buy all 3”CL bits in case you might some day need the length. A very long bit cutting ½” material is likely deflecting.

A 1”CL bit is less costly than a 3”CL one…

These are three quick reasons for using properly sized bits… I’m sure there are more!

SG

[skintigh]

Thank you both!

Steve, if I'm not doing an onion skin cut, is there a danger from cutting deeper than your length? Most of what I'm will be sealed, primed and painted anyway. I searched online and half said that cutting deeper than the cut length was fine and half said chips would get caught in the slot and rub against the shaft.

I ask because I need to cut a lot of thick exterior corbels, some 3ish some 5 inches thick. I could cut them out of 4x6s, or glue up douglas fir 2x12s and cut them, or cut 2x12s then glue them. I'm leaning towards the latter, but either way I need to buy a new bit, and I don't have a jointer so I'm worried about the 2x12s warping.

Thanks again and I have a name now,
Seth

[steve_g]

Seth…

What I do, and what works for me isn’t necessarily gospel!

I don’t go deeper than CL when using a down cut bit. There are enough members here that have experienced fires when using down cut bits, especially when chips are trapped, that it would seem unwise to do so. Trapped chips are a certainty when drilling…

I do cut deeper than CL (when cut diameter is equal to or greater than shank diameter) But carefully watch for good chip evacuation. I’ll use an air hose to assist them if I’m not happy with how they are ejecting.

It sounds like it wouldn’t matter much if you got a little bit of chatter from chips getting caught between the shank and the material.

Those are nice corbels you show in your other thread!

SG

[coryatjohn]

If you're not doing production work, you can certainly go deeper than CL. As SG mentioned, just keep an eye on the cut and have an air hose standing by if the chips start to clog up the kerf. Forget downcut bits for this too, unless you like the smell of wood smoke.