small bit chiploads

[angelosart]

Couldn't find chiploads for 1/8" & 1/16" on the SB chipload calc?

[dana_swift]

The diameter of the bit is not part of the calculcation (up to a point). The size of the chip should be the same, however, the other limiting factor is the strength of the bit.

As the bit gets smaller, the depth of cut becomes critical because the bit flexes as it is cutting, and thin bits build up heat and fail quickly. To solve that, make more passes. Shallow cuts minimize forces on the bit. With thin bits I use the radius of the bit as a good starting point for the pass depth. Even less if bit is thinner than 1/16".

If the bit is under 1/8" I also back off the chip load to minimize the force on the bit. The chip size should be much smaller than the chip channel in the bit. The bit will shave off a chip the length of half the circumference of the bit on each turn. That much material must fit in the gulley with room to move around, then the chips will clear if its an upspiral bit.

A straight or downspiral bit needs more room for the chip in the gulley than an upspiral bit.

Summary:
Cut shallow to minimize bit forces if the bit is thin. (New toolpath needed to change this value)

Lower/raise feed rate until the chips clear the gulley freely. (You can find this value while the machine is running with shift > and shift < )

Hope that helps-

D

[knight_toolworks]

it also depends on the material you are cutting.
I cut 1/8" bb plywood in a single pass with a 1/16" downcut bit at 1ips. sometimes I can cut for 14 hours on a single bit a few times only 4 hours or so. same file same material.

[angelosart]

Thanks for feedback guys. The forum and the people on it are very helpful for people like me who are new and have 5,000 simple questions. :eek:

[dana_swift]

That was a most excellent question! Everybody has to figure it out, but how many bits should it cost?

Too many times I have put in a very small bit 1/32 or so to have it cut about three inches before breaking. Its always my mistake for being too greedy in my cuts. Or switching materials and not bothering to set up a new definition in Aspire for the new material. Cutting walnut with the same settings I used for cedar. Breaks small bits every time.

The ability to adjust feed rate while the machine is running is not something most beginners realize the shopbot can do.

So chipload calculations are bit size independent, up to the point where the chip fills about half the gulley.

Pass depth is dependent on horsepower requirements for the current material and bit flex/strength. The same size bit extending a long ways below the collet is more flexible than the same bit pushed deep into the collet. Pass depth has to get shallower to lighten the load on the bit.

As I said eventually this all makes sense, and you figure out a good starting point for your machine and materials. You will learn what your machine sounds like when it is biting off as much as it can handle (or too much). Then its fairly easy to adjust toolpaths for repeat production with more appropriate cuts. Start light when in doubt. Also examine the chips! Take a caliper to them.. are they close in size to the chip load computation? Are they shavings or dust?

Glad to have helped!

D

[bill.young]

There's a bunch of good info on cutting with small bits on the Precise Bits website

www.precisebits.com