Z-Axis slowly lowers at start of project

I bought the kit w/Rambo 5 driver. I built it and tested it without problems (square and 1mm = 1mm in any XYZ planes). First time using a CNC, let alone having one.

At first I had multiple small project that I completed without issue (small box, cut out letters, small etching).

Recently I have been trying to make a 2.5D sign made out of red oak for a house warming gift for a friend. Its a 25cm x 40 cm area with the entire area being milled to different depths. The tool path works but at the very start the Z-axis drops maybe a 1mm - 2mm over the course of 50cm (approx) while milling. As a result there are high points on the project. I have tried it on two different projects with three different tool paths and each time results in the same problem. At the end of the project the tool is suppose to return to its original position, but it is returning 1-2 mm lower.

DW660 spindle
3.125mm 2 - flute end mill bit
600 mm/min cutting feedrate

I am very new to CNC in general. I assume its one of two problems, either the plunge (which I’m using a 5º helical plunge) is too fast and the bit cant keep up and comes to a resting z-axis along the toolpath or something is wrong with the z-axis stepper. or I’m just entirely wrong all together!

The pictures are a little tough to see exactly but each pass of the tool is just a little bit lower.

Any help would be greatly appreciated!


What are your plunge and rapid rates? I suggest 3 and 5 mm/s to be safe. You can increase from there if all is well.

Plunge and ramp rates are at 5 mm/s, so i will drop them down to 3 mm/s. and try again this weekend

What do you mean by rapid rate?

Fifth picture down.