[ghardy]

Hi All,

I'm a newbie and recently started machining a few projects in wood. I started off machining small stl models and now I've tried to do something a bit bigger. It was a a celtic skull approx 250mm by 200mm by 14mm in American Cherry. I did the roughing cut no problem and then started the finishing cut but because the finishing cut was taking awhile I stopped it half way through. The next day I came to resume the the finishing cut and checked that the x,y and z were as before. When I started machining again there was a slight but noticeable lip/edge between the cuts over the two days.

The finishing cut was using a 3mm ball nose.Now I guess there are various reasons why this could be happening and I just wanted some advice and ways in which I can avoid this sort of thing happening. I have had a bit of a trawl through the forum and I'm sure people have had this before but I couldn't find anything.

As the lip was consistent around the project from where the finishing cut resumed I'm more inclined to think that it was due to the bit or parts of the machine expanding due to the heat.

Should I be creating vectors of smaller areas and creating toolpaths based on those? Then depending on time I machine the toolpaths that I can until the whole project is complete.

Any advice would be appreciated.

[ger21]

Two main issues.
1) Do you have accurate home switches, and did you re-home the next day to get the tool in the same place? If not, then most likely the tool was in a slightly different position.

2) Wood moves when you machine it, especially a 3D carving. A big enough piece can expand or contract a LOT overnight.

[ghardy]

Thanks for that Gerry.

I'm fairly certain the home switches are accurate. I will double check though.

On the wood side. Can the wood expand or contract uniformly? Reason being because lip was the small all around the project as the toolpath was cutting in a circular path.

If I had been cutting for several hours is it possible the heat generated played a part? I've got the machine inside an enclosure to keep the noise down.

[ger21]

Wood moves when it's moisture content changes. When you start removing the surface, the interior of the board will usually either gain or lose moisture, causing it to shrink or expand.

Wood movement is mainly across the grain. Length doesn't really change at all.
I'm not sure how it could be a consistent offset all around the perimeter, unless perhaps the Z zero is not the same.

Trying to do a project in small sections over several days is always going to be problematic I think, unless your machine is very repeatable, and the wood your cutting is very stable.

[Tom-AMS]

spindle operation temps can cause small "deflection" of cut path, depending on the design / quality / cooling mechanism ( if any) of the spindle.

[ghardy]

Thanks all for the advice and guidance.

In the last few days I wanted to mount a depth control on my z axis and I noticed the z axis mount was not sitting perpendicular. Don't know if there was some slight shift in it at some stage but I've since straightened it and tightened it. Hopefully this was the cause... will give it a test when I've finished my v-carving projects.