Hi. I just finished up 3 attempts at a small project. It involves a pocketing operation followed by an outer contour of said pocket that is 0.25" offset. Picture a small odd shaped bowl with a 0.25" thick wall.
Anyway, on all of my attempts I seem to be losing steps left or right. The odd thing to me is that it’ll do 2 or 3 passes of the pocket at the first few DOCs but then on the 3rd or 4th pass, it shifts left or right. As you can imagine, when I got to cut the outer contour, my 0.25" wall gets cut into because of this shifting. (in my last attempt I stopped the operation before cutting the outer contour)
I’ve attached the file I used on my last attempt as well as a photo of thee workpiece. I haven’t had this issue before and recently cut out similar shapes just fine. Any ideas what could be going on?
I would agree with this. The DOC in your file probably works alright with the 1/4" bit, but I think it’s definitely too much for the 1/8" bit - especially at a feedrate that’s nearly as fast as for the 1/4" bit & cutting a slot.
Slot cutting is not advisable for such a small diameter bit & going nearly 1" deep.
1/4" bits are much more capable of slotting, but you still have to be careful that you are not pushing it too hard.
If you really want to make the 1/8" bit work, you will probably need to create an outline shape that is a little wider than 1/8"(like 0.15") which can then be Contoured or Pocketed out. That little bit of extra space prevents the bit from rubbing on both sides of the slot as it goes deeper & allows chips to move up & out. And I would reduce the DOC to 0.100" to start & feedrate to 40-50ipm.
Thank you for the reply and that info. I’ve run both of these bits at these depths and speeds without issue in the past. Of note, the attempt in the top of the picture shows only the 1/4" bit’s attempt at cutting; I stopped before switching in the 1/8" bit. In the bottom of the photo, the slotting with the 1/8" bit stayed on the mark; it was the 1/4" bit that lost steps cutting only about .155" deep at a time which doesn’t seem so crazy of a cut.
Anyway, I have to agree that must be the reason. I updated Carbide Create but also switched to cherry (vs. the harder white oak) and it worked fine. I suppose I’ve been used to pushing these bits through MDF. Need to temper expectations with white oak.
I did a seach for the Janka Wood Hardness scale. White oak is 1350 and cherry is 950. So 1350-950=400 So 1350/950=1.5. That makes white oak 1.5 times harder than cherry. That is a considerable difference. White oak is a very dense hardwood. I have cut a lot of cherry and it is a very machinable material. Oak in general is a much harder material to cut. It tends to be stringy and very hard. Red Oak is 1290 on the Janka scale. So it is close to white oak but the white oak is still very hard. There is always a +/- to each individual tree.
They used white oak to build sailing ships. White oak is rot resistant and the straws that make up the oak is a different structure than red oak. Red oak will suck up water through the straws of the fiber (tree rings) and will also rot more readily than white oak.
The hardest hardwood on the Janka Scale is African Blackwood at 3670 and the softest is Basswood at 410 to give some prospective on hardness scale.
See this website about Janka Hardness scale of different woods.
