Problem with setting Z

[Call-Tech]

I have had my shopbot for over a year and seem to always have a problem with accuracy on setting my z axis. Most of the time I am cutting sheets and just cut an extra .050 into my spoil board, which I hate doing. But every now and then I will resurface my spoil board and try to NOT cut it all up as it reduces vacumn. When I first got my shopbot I made a Z-plate. It measures exactly .170. I altered the z program to subtract the .170 and zero my z to table top. But I never seemed to cut all the way through material. I even altered the program to subtract a little extra, but still always had problems cutting through and just stopped using it. Now I just manually set z and undercut most of the time to make sure I go all the way through.

Today I needed to cut some 1/8 thick acrylic and I needed to cut some dado grooves 1/2 way though it, and then cut out the pieces. I manually set my z to top of acrylic using a sheet of paper. (my default method) I bring z down in .005 increments until I pinch paper and zero z at that point. The 1/8 acrylic actually measured .105 and the protection paper on it was .004 for each side totaling .113. I use artcam for my toolpaths and z to top of piece. dado grooves were cut to .056 and profiles were cut to .116.

Everything should be great, not! dado cuts are shallow and profile did not cut all the way though. I broke the piece in two and checked remaining thickness with micrometer and it still has .017 left. So I bring up keyboard control and move x axis slightly off table and move z to -.018 and zero z. Grab new piece of acrylic and Run file again and it still does not go through. I run z to -.005 zero z again. Repeat file. Still no joy. Subtract yet another .007 from z and zero again. Repeat file. again profile cuts fail to go though. So I have lowered z by a total of .030 when only .018 had been remaining.

I would assume that I screwed up if I have not been through this many times. and measured everything many times.

Does anyone have any ideas on what I could be doing wrong? I have been working around this problem for a year and would really like to fix it or at least understand it.

Sincerely, Fred
Call-Tech Signs

[knight_toolworks]

I get this sometimes too. so I use the nudge feature to get the depth right. while cutting a file check the depth then use the nudge to get it perfect.

[bill.young]

Hey Fred,

The support guys could probably help you figure it out better, but here are some ideas.

1) Is your Z axis pinion gear good and tight? If you try to lift up on the Z while the motors are on, can you lift the Z at all or feel any slop in it?

2) What kind of bit are you using? When I'm cutting something depth-critical like scarfs that have a series of steps that are .02" thick, I alway use a straight bit instead of an upspiral or downspiral....an upspiral always seems to cut a little too deep and the downspiral a little too shallow. I don't know for sure but I assume the upspiral lifts the material a little and the downspiral is either pushing down on the material or lifting the armature a little, because of endplay in the router shaft


Bill

[butch]

Make certain the bit isn't slipping. I had the same problem and started marking the bit with black marker then checking after the cut. Found that the bit was slipping and was being pushed up into the collet. Changed collets and tighten a fit more, and rarely get that problem any longer.
Butch

[Call-Tech]

Bill, I will try checking for slop in my z axis.

Butch, I have had problems with small bits slipping when I am using a collet adapter and 1/8 diameter bits. but in this case I was using 1/4" shank with 1/8 end mill cutter and I have never had problem with those slipping. But I will give your suggestion of marking bit a try to make sure.

Has anyone seen a 1/8" collet for the porter cable?

I have done some really fine work like engraving on glass and mirror. But I have done those things by using scrap product and constantly moving down until sample is cutting correct and then replace substrate with final piece. I have cut 8 point outlined fonts on glass that rivaled any engraver. I have been very surprised at the accuracy of my bot at times at least as far as the x and y axis goes. It just seems I have always had a ghost in the z axis.

Steve, You know, I am probably at fault here somehow or another as I seem to always learn by hard knocks. I am not sure I know what the nudge feature is. How does that work? I would probably know if I had spent more time reading manual and less time trying playing with machine. I do use a feature in the keyboard control that I think is nudging. I have it set to .oo5, I use it when setting my z. I run z down close. then turn on the .005 feature and move it down till I pinch paper. Sometimes I call that good and sometimes I will manually move it up and down till paper slides and then move in .001 till paper does not slide.... It just doesn't matter how accurate I try to be. Z is never right.

Most of my work is simple flat stock. what little I have done in 3D has been done without bit changes so .050 has never been a problem.

I left out the part above on working on the 1/8 acrylic that in the end I dropped z another .035 in irritation and then proceeded to dig in too deep on first cut and promptly decided to work on something else for the day.

Thanks for everyones help,
Sincerely, Fred

[harryball]

Check your router bearings too. I had a problem with a Porter Cable on my router station, and it was a new router. I would set the depth of cut then when I cut it would be too deep (i.e. like I set the bit too high) I finally figured out when you power on the router the bit would raise up ever so slightly. Turned out to be slop in the bearings, sent it to PC under warranty and it's all fixed now.

Robert

[knight_toolworks]

the nudge feature is handy when you hit enter when a file is running the second to last box is nudge, you use it to change x,z or z. it will change it till you rezero. I use it when I need to dial in onionskin

[Call-Tech]

Steve, when I hit spacebar or enter while running file I get a dialog box and the second to last box is insert command. I use this often to change jog speed. But I do not know how to insert command that alters the zero of x,y or z. How do you do that?

I did a few tests today to see if I could see any problems with slop in z axis. With bot powered up, I tried to lift up on both bit and whole router. I thought I could see a little movement so I attached a dial indicator to z assembly. I could move whole router about .003-.005 with a little bit of pressure. If I really pushed hard I could make it move about .020 but I could feel it trying to return and it did return to zero when I released pressure on it. I then attached the dial indicator to my table and placed it touching bottom of collet to see if I could measure any movement in the routers bearings. I believe that I could measure about .002 play. Now I'm not sure, but I believe these measurements are fairly tight and should not be affecting my work.

I then tried to zero my router to surface of my acrylic with router running. I did this by using keyboard control and moving router down very slowly a couple of thousands at a time until I saw the protective paper tear away. I thought this might tell if the router was lifting bit when spinning. I then ran my file again and still did not go through acrylic.

I then placed dial indicator again against the top of the z assembly and using the keyboard controls. I moved the router up and down in .005 increments and measure if dial indicator agreed with keyboard command. It was not perfect, but close. I saw movements of .004 to .006. I had set dial indicator to 0 and moved z down total of 8 increments of .005 for what should be a total of .040. I did this up and down about 10 cycles and after the first cycle, the dial indicator showed .001 at top of every cycle and .038 at the bottom of every cycle.

Again, I believe that this is within the specs for shopbot and not sufficient to cause my problems.

My next test is going to be to take a thicker piece of acrylic and cut stair steps in it and see if the increments I set in artcam are equal to what I am getting on bot.

I welcome any feedback on this process and let me know if anyone sees any flaws in my logic.

Thanks again for the help.
Sincerely, Fred