separate last pass

[Barry Anderson]

I have not used this option and would appreciate any advice. Is the allowance the distance for the last pass to make the final full depth pass? Do I need to reverse direction? I'm cutting letters out of 1/2" birch plywood. Would using the separate last pass help minimize sanding?

Thanks in advance.

Barry Anderson

[IslaWW]

Barry...
The last pass feature is most often used to eliminate witness or watermark lines for multiple passes. The initial profile passes are offset by the allowance amount, with the last pass cutting the full depth of the material, but only a few thousandths thick (the allowance setting).

Reversing direction on the last pass is usually used to allow much more aggressive feeds by cutting climb on the initial passes, then conventional for the last. This allows deflection to be in the waste and a good quality, albeit light cut for very good quality and accuracy. It allows accuracy and quality at a speed beyond what is the norm for machines that are not overly rigid

See page 203 in the Reference Manual

[Barry Anderson]

Thanks Gary. Not having a printed manual, I don't think of the help manual. Your reply was helpful and I found it under the help tab.
Barry

[martin54]

Just something else to be aware of when using the last pass feature Barry, because the machine cuts using an offset & the last pass will cut full depth the flute length needs to be long enough to cut the full thickness of the material, if your cutting 1/2" birch ply it's probably not an issue but worth remembering for thicker materials

[jeripens]

I know it's an old post, but I can't find this information anywhere. When IslaWW talked about watermark or witness lines, what is the cause of those? I'm cutting 2" thick HDU with a long bit and getting really bad lines on the sides. They are somewhat fixed by a last pass, but I'd like to know what the actual problem is. I can only think it's my "Z" out of square with table or the long bit is slightly imperfect and a bit wobbly on the very end.

[martin54]

What size is the bit you are using? If its a long bit then there is more chance of tool deflection especially with a small diameter bit. It may be that you are simply trying to cut to quickly or to deeply with the bit you are using or the bit is blunt. You could try slowing your feed rate or decreasing you deth of cut to see if it makes a difference. A larger diameter bit may help as well.

[tomgardiner]

While machine deflection might be present it should not be an issue with HDU. What is more likely is the bit tip being dull. The tips of the flutes will dull first. This leaves a notable line without the dimensional shift to the cut you would see with machine deflection.
Solution? Sharpen the bit or last pass.

[Tailmaker]

I think the waterlines, while somewhat dependent on bit type, is just a normal effect. The tip edges of a square end mill must cut not only in radial/tangential direction like the rest of the flute but also cut off some fibers in axial direction and I guess that may lead to more or different tear-out. Obviously, that can be made worse with a dull bit or when the bit flexes a lot.
The separate last pass is probably the only way to remove that