Cutting height of our bits

I’m still a little puzzled by large bits being offered in multiple cutting heights.

Today I’m looking at purchasing an S=6mm, D=6mm, Z=1 bit to cut plastic. Amana offers these in 7 different cutting heights (from 8mm to 38mm).

For small diameters, minimising the cutting height reduces the risk of breakage and vibration. But with a relatively large bit, I don’t see that cutting height would matter that much so I buy the 38mm.

I assume that they offer the smaller heights for repetitive jobs. A professional shop, buying a lots of bits, may save a little with smaller cutting heights. On the contrary, as a hobbyist, I assume that the more versatile bit (highest cutting height) is cheaper since I only need to buy one.

What do our more experienced CNC operators think? Am I missing something?

Unfortunately, the longer the bit the more you are going to introduce a degree of deflection into your workpiece which in turn then translates into the spindle causing premature wear of bearings, collets etc.

I think most would recommend that you choose something with a length that is going to adequately cover a reasonable range of thicknesses (e.g. 1/2" 3/4", 1.0") that you would normally cut in your average day-to-day carvings.

In the industrial CNC world you generally try to have the absolute minimum of stickout for the reasons Dave shared and that deflection causes inaccuracies in your part. These deflection inaccuracies are in the .01mm range so that may not be a problem for woodwork, but the machine wear is definitely something to keep in mind.

Thanks but that’s part of what confuses me. If it has an impact on the length of the bit, I understand. But here I’m talking specifically about the cutting height.

In this example with the plastic bit, admittedly the 8mm cutting height is significantly smaller than the other bits. But the 6 others have pretty much the same length…

I would tend to go with the smallest shank length thereby reducing any deflection.

Stickout… a new concept for me!
I’m googling it and here’s what I found on CNC cookbook on my original question:

“I think maybe people think long flute lengths mean more versatility–you can use the cutter on more jobs. But truth is just the opposite. Deflection will limit what you can do with such cutters. Instead, get the shorter flute lengths and push them up into the tool holder as much as you can.”

The article goes into some length on the matter, interesting read.

And, indeed, it helps to keep in mind that accuracy with wood is not as critical as other material.

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Thanks, with the help of Nick, I’m beginning to the understand the importance of looking not just at the total length of the bit but also at the stickout.

So far, I thought it was best not to push the tool very far in the collet. It appears I was wrong!

I’m going to speculate a bit here and I’d love to hear other’s opinion.

Assumption: Minimizing stickout + maximizing collet engagement seems to be the goal for best tool rigidity and spindle longevity.

Speculation: The stubby collet on the Makita router (and clones) only allow for maximum engagement of ~0.5". Anything more than that is beyond the height of the collet. I hypothesize that maximum cutting tool rigidity is achieved as long as you have full collet engagement and anything extra is just bonus (theoretically unnecessary) tool length. Though no harm done in doing so.

If you have a spindle with an ER style collet your cutting tool engagement with the collet is double-ish that of the Makita and you’d benefit from a slightly longer bit to maximize that engagement.

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Having more cutting height can allow for deeper passes using the formula
shrink chipload by:

  • 20-25% - for depth = 2x tool diameter
  • 40-50% - for depth = 3x tool diameter
    The bits have more reach to accomplish these type cuts if you have a spindle that is capable of pushing them this deep.
    On the Amana bits there is a line that is on the bit that tells where in the collet it should stop. It is not all the way in.
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I believe the reason to have space between the bit and the bottom of the collet is so the bit takes the brunt of the vibrations from cutting. When it has solid contact with the bottom of the collet it transfers those vibrations to the bottom bearing. Which actually wears the tool out faster.
The router tool is designed this way because it was never intended to be a precision tool. Spindles that are designed for precision take those vibrations in consideration and have designed for them with more bearings and heavier parts.

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