View Full Version : StairStepped Z-moves

09-18-2005, 12:26 PM

I was cutting on a MDF project this morning, with a 1/4" DownCut Spiral, moved speed on the z-axis at 0.6 As you can see in the pictures, I have a serious problem with the z-axis movement. Each time that the z-axis would pick up and move to a new spot, it would lose distance. Why?


Can anyone give me some pointers on how to fix this problem?

09-18-2005, 12:27 PM

09-18-2005, 01:18 PM
Hi Jay!

I can tell you that I've observed the same on my Alpha tool... and I'm working on this.

First thing would be to check for basics; tool slipping, play in the Router/spindle bearing (Z axis), backlash/play in the Z drive (pinion, pinion to rack and such), right code in the file(s), hold down, "material stress", etc...

I have checked the accuray of my Z with a dial on a couple of tests and everything look fine (within 0.002" positioning)... I plan to run a long file to see if SB3 could be roundening numbers while reading the file... but I doubt it does... just got to check...

I've improved my hold down with the installation of the vaccum and I've make my Z-zero method more repeatable and I've noticed quite an improvement but I can still see some steps as you do on intricate pocketing (clearing around letters on signs) with more than one tool. What still confuses me is that I can still see (I'm talking here of about 0.005-0.010" steps that can be noticed by shadows from directionnal light) even with ONLY one tool!?!?!? I have been able to "eliminate" this with a 3D type of pocketing strategie only... both raster and offset from Insignia got one of my customer unhappy (me too as a perfectionnist)...

I am curious too to hear about other Botters about this "issue"...

Brady Watson
09-18-2005, 01:24 PM
2 things come to mind ~ 1) Your material is deflecting downward and springing back up as the downcut spiral pushes the material down. 2) Your Z-axis rack is worn. I made a routine to move the Z up to 7 and back down to 0 several times. While it was doing this, I noticed a few 'notches' where the tool was not moving smoothly and losses in Z-position. I had an extra rack and pinion here for the day that it would need replacing...well after installing it and really dialing in the Z, I can tell you that the Z now NEVER loses position or drops into my 3D files. I just machined 2 3D parts last night that took about 13hrs and they came out absolutely perfectly!


09-19-2005, 11:56 AM
Are you sure you're using a downcut? Those edges look pretty ragged, more like what you see with an upcut bit.

What happens if you run the file a second time? Does it clean things up?

Are your brushes arcing at all?

09-19-2005, 12:53 PM

I just ran a couple 3d files the other night and I didn't have this type of problem. Did you replace the whole z axis or just the rack and pinion part, new gear on the motor?


Why would you ask about the brushes arcing?

I am sure that I was using a downcut bit, but I will look at it again.

I did run the file a second time, it clean up part of the mess, but it was hard to tell if it was getting enough from the first one. I should have machined it a third time and adjusted my z-axis.

However today I am cutting four smaller ones. Using a 1/8" downcut bit. I am getting the same pattern. However I am noticing that its only doing this are the center part of the file, from top to bottom? I am cutting four of them right now, using the first 4'-0" of the table. I am cutting two on the upper 2'-0" and two from the lower 2'-0".

When these get done and I have them cut out, I will take a closer look at them, to see what may be causing it. I am sure that I will have another post here after lunch, asking more questions.

09-19-2005, 02:24 PM
How large is your project blank & how is it held down? Do the results get progressively worse? MDF will warp in the direction of the pocketing if not held down adequately.You are simply lessening the surface tension on the material which had developed during manufacture.Once cut, the pocketed areas of the material are also more prone to swelling in high humidity.

09-19-2005, 02:42 PM
I too don't have problem with 3D projects; only in pocketing or when I re-machine area at a later time within a file run.

I noticed this with MDF, Extira, HDU and insulation foams. Personnaly I'd say as Jeffrey; surface tension movements.

I wonder if this kind of file would simply need a complete re-machining (which can be long, so more costy for the customer and buisness time consuming for me) of the area?...

what making me unsure about this is that I usually rought out fast in HDU on thoses pockets and then run a finishing pass to remove the remaining 0.030-0.050"; which so far has not fix the "steps stair issue"...

my best fix as been to use a "3D" toolpath to make thoses pockets completly clean; this because (I believe) the tool won't come back to re-machine an area that's been already machined as most 2D strategies...

09-19-2005, 02:46 PM
Update... didit..didit..deet..


My first project was 4'x 4' the second ones were 2' x 2' They are help down on the perimeter by screws. An interesting point, the 4 x 4 I cut with a 1/4" bit and the second ones I cut with an 1/8" bit. They both exhibit the same problems in the same places. Looking at the large one, its got the same z-axis problems as the smaller ones. Each one of the small ones show in the exact same areas. On the last two of the smaller ones, I took the hold down bearings off completely, to see if they were causing problems, no change.


I ran the z-axis on a very length up and down program, moving from different locations and moving the z-axis at different speeds with no change. I plan on cutting into a 2 x 2 sheet, pocketing here, moving to another location and pocketing again and then moving back to the previous location and pocketing again at the same depth. I am thinking I will draw a bull's eye taking out every other ring.

Well, there is my update on my problem.


Brady Watson
09-19-2005, 04:44 PM
Make a little program that exercises the Z up and down the entire length of the rack. Look with a flashlight at the pinion making contact with the rack...listen for even the smallest of 'tick' sounds. The rack may be slipping in one or 2 areas like mine was. Is the rack lubed with grease? Is it worn on one side? Is it correctly aligned to the rack? Are there little steps on the pinion that are inconsistent with the rest of the gear? I replaced the rack (removed old from Z axis) and pinion...the results were night and day. Now it is running perfectly.

The other things that you want to check is that your Y rails are nice & clean...wipe them down with ISA and clean rollers of debris. Also observe that the X car is not deflecting under the weight of the router etc...Sounds obvious, but it's worth a look. Even though the material is being held down at the edge, you might be getting some springing of the material in areas where there are no screws (you said 4'...not 4" right?)

The illusive part of diagnosing the problem is that you may only have one or 2 bad spots in the rack that make the Z-drop in between teeth. This was the case on my tool. I would machine something 2" thick in 3D and never miss a beat...Got to do an area clear or 2D prof and the Z would drop about as much as your pics show. Turns out...it was only bad in a few spots. Keep in mind that the 20 tooth pinion on your Z is roughly 1" in diameter...this means that it will travel approximately 3.125" per rev...so if you do have a bad spot on the pinion or rack, you will only notice it in one or 2 spots on the entire Z travel.


09-19-2005, 09:10 PM
Jay, the reason I asked about brushes is that a couple of weeks ago our Z suddenly started changing depths toward the end of a run. The brushes looked to have a lot of life in them, but were arcing more than normal. After changing them out, things have been running fine.

09-20-2005, 12:12 PM
What is ISA?
We have had both polystyrene foam shavings, and mdf dust build up quite badly on the z rack. Brady's suggestion to look carefully at those areas is a good one.


Brady Watson
09-20-2005, 04:46 PM
ISA = Isopropyl Alcohol

...although I have used brake clean and denatured alky as well as a cleaning agent on the bot.