Merry Christmas everyone! couple of questions when anyone gets a chance to answer or help in anyway, that’s be great! I’ve measured out the actual distance moved with the program available on the open builds control that is offered to get the most accuracy possible... but I recently cut out a simple box, both dimensions and cutting speeds were the exact same for the top and bottom... when the box was cut out on my mini mill, the top wouldn’t fit into the bottom half.. any ideas on how to dial it in anymore? I’m still fairly new to this, but I am already wanting to upgrade to a Bigger machine.. I just want to make sure I know what I am doing before I drop the money. thanks everyone, have a great Christmas!
Did you measure the exact diameter of your endmill? Very few are actually the diameter they are listed at. If the diameter is smaller and this figure is not used in your CAM software, outer cuts will be a tiny bit larger, and inner cuts will be a little bit smaller. If this is the case, parts with tight tolerances may not fit together properly. Merry Christmas to you as well.
also, if you cut a 2" wide hole and a piece of wood 2" long, the piece will not fit in the hole (with a hammer yes, but not just 'fit in'). it also matters which way you cut. if you cut around the outside clockwise this is called a 'climb' cut and will tend to leave parts large. if you cut around the outside anticlockwise this is called a 'conventional cut' and will tend to leave parts small. these size variations are because of tool forces pushing the tool away or toward the desired cut line. every machine flexes, even multimilliondollar machines, so this is nothing unusual (needs to be fixed if a circle is visible oval though ) The usual way of counter acting it is to leave a little material during the rough cut and then remove it with a finishing cut. Also avoid slotting as this is the worst cut for tool deflection. Another factor is chip removal. Re-cutting chips is to be avoided at all times, it creates heat, bluntens the tool and reduces surface finish. A blunt tool deflects more.
Assuming the machine is cutting precisely and you have the precise diameter of the bit, then what David said is quite important. I have spent way too much time fussing about "allowances". I've been typically oversizing my holes by .2 mm to get a snug, non-forced fit in wood. Plastic can be a bit tighter. Yet, there are cases whe you have wood that doesn't mill very cleanly and you need even greater allowance. Also, wood will change size with humidity and temperature so you need to take that into account. For example if you have a 4 inch crossgrain piece of wood that fits in a hole, it may expand as much as 0.04" depending on the species. That could translate into a stuck part or even split wood.
