[Dmattic]

Hi Guys,

This is my first post and would like to know if anyone can assist with my issue regarding pocket cuts.

I've got a retro fitted BF20 Clone CNC (like a Grizzly G0704) and for some reason my x-axis shifts slightly every time it plunges which gives me a stepping effect on right hand walls. It only occurs with pocket cuts. Pocket cuts that are 3mm deep will shift about 0.5mm to the left. I'm doing climbs cuts but the stepping effect will swop to the left side walls if I do conventional cuts (I don't do conventional cuts but wanted to see what happens).

This is not the best photo but you can see in the top right radius that it has steps which smooths out as it goes to the left.


I've checked the following:
- Gibs are good
- Leadscrew backlash is 0.12mm (x-axis) which I think is acceptable for a retro fit.
- Spindle head is perpendicular with the bed.
- Pulley set screws are tight.
- Belts are tight.
- Belts aren't rubbing against anything.
- Machine vice has no play.

I did add backlash compensation in the Centroid Acorn Setup Wizard which does solve the problem but only for a few cuts and then it gives me the stepping effect again. I did notice ,while measuring the backlash, that the backlash value will be inconsistent by 0.01mm-0.02mm. What I mean is that the backlash value will be slightly different after I do a cut so it's impossible for me to enter a spot on value. The y-axis does the same but I don't get the stepping effect in this direction.

Has anyone had this problem before?

Machine Specs:
Retro Fitted CNC BF20 Clone
- Centroid Acorn Controller
- Wantai DQ542MA Driver (Nema 23)
- Wantai DQ860MA Driver (Nema 34)
- Nema 23 X and Y Axis
- Nema 34 Z Axis
- GT2 Belts and Pulleys connected to lead screws and stepper motors.

[steelcogs]

I'd put my money on it being backlash, especially if it's inconsistent. I typically see that backlash comp shouldn't be used for that much backlash but I may be wrong. If it's inconsistent than I'd think it'd be even worse. Your best bet would be to use ballscrews instead of leadscrews. I have ballscrews and get a pretty consistent less than 0.04mm without comp. I also have my motors directly mounted to the ballscrews. Not sure if that would change much for you but it's very possible. IMO the less you have between the motor and the screw, the better. Less room for error. The Y axis has the same amount of backlash and is also inconsistent?

[Dmattic]

Thanks for the response.

Yes the Y-axis has similar backlash and is also inconsistent but I don't get the stepping effect. I only get the stepping in the X-axis even when backlash comp is turned off in the X and Y axis. I did read an interesting post on the shapeoko forums where they had the same issue which was caused by a faulty 300 watt spindle. He replaced it with a 600watt router and the problem was solved. I don't think anything is wrong with my spindle and I obviously can't just replace it but maybe I get this error because I'm not ramping. I'm plunging straight into the material (only 0.2mm plunges). I don't use ramping because I'm not plunging very deep and it adds to the machining time but I'm going to give it a try.

I agree that ballscrews with directly mounted steppers is the way to go because you always want to reduce backlash mechanically as much as possible but I can't afford it at the moment. I actually want to try and use IGUS anti backlash nuts that fits onto my lead scews (https://www.igus.co.za/product/?artn...AB-2835-TR16X2). It's a cheaper option but I'll still have belts on my axis and the these nuts will cause more friction despite being made from a low friction plastic.

[dazp1976]

I'd say backlash or head drop when leading out.
May be worth trying a Z move to raise the head about 1mm before leading out to see what happens.

I'm building mine atm and my new ballscrew is crap. Has a fair backlash and the head does drop a little so I'm looking at using some gas pistons to keep it stable until I can afford a decent screw/nut.

[steelcogs]

Weird the Y doesn't do it, maybe because it's over a shorter distance? The person who had a problem with the spindle probably had an EMI issue that was messing with their steppers. That's happened to me before. If you're using the stock spindle/motor that's likely not an issue since (I don't think) any of these mills come with a VFD which is usually what causes EMI.

[he1957]

If the backlash is similar in X and Y, try reversing the orientation (transpose X and Y) of the part to see if the "step" pattern moves to the other axis. This will prove if it's a mechanical issue (which sounds like it is).

[LongRat]

If the backlash is similar in X and Y, try reversing the orientation (transpose X and Y) of the part to see if the "step" pattern moves to the other axis. This will prove if it's a mechanical issue (which sounds like it is).

This is a good suggestion. Also, look at the head on the machine. Just grab the spindle nose and try to rock it in X, and then Y. If there is any free play in the head mounting, gibs, or (less likely given your details of the issue) the spindle bearings then it could cause this.

[Dmattic]

Thanks for all the input.

@steelcogs: I'm using the stock spindle so it's probably not an EMI problem but that makes sense why it worked for the other guy.

@dazp1976: I don't think it's a head drop issue because my depth of cuts are actually spot on.

@he1957: I actually have done this and the stepping is always in the X-axis. It's only with pocket cuts so if I cut a pocket and finish the the hole with a contour/profile cut then it would cut on the last tool path of the pocket cut. In other words the contour cut is offset from the zero position that was allocated before starting the job. When I first did the conversion I got the stepping issue on the x-axis and it disappeared with backlash comp. Then it came back and I noticed that the pulley set screws with gone and there was a lot of play in the axis. It was actually making a knocking sound. I then locktite new set pulleys, adjusted the leadscrew nut backlash with the two screws, ensured that the lead screw nut was secure on the carriage, checked the gibs and readjusted the backlash comp. The problem was gone and I cut one part which came out perfect. So mechanical adjustments did solve the problem but the steps returned on the second part which is where I am now.

@LongRat: I've followed your instructions but the head/spindle is solid. There is no wobble or play.

I tried ramping and I think it improved it a little bit but the problem is still there. I did notice a grinding sound coming from the right-hand side bearing on the x-axis (where the lead screw slides into and where my pulley is mounted). I think something might be wrong with the bearing because the other side is silent and the same for the Y-axis (It's silent). So I'm going to inspect this more.

[LongRat]

Sounds like you have narrowed down the source of the problem, X axis bearings/mount or coupler. One change I can wholeheartedly recommend would be using clamp-type couplers rather than the set screw type. Way, way more reliable.

[Dmattic]

I seemed to have solved the problem and it's weirdly not mechanically related in this case.

I tried cutting a test model that I 3D modelled in fusion 360 and it came out perfect. I normally 3D model my parts in Solidworks and import it into Fusion 360 to setup the CAM. So I realized that the test model was obviously centered (the origin was in the center of the part). I never moved the imported part to the origin and once I did that the stepping effect was gone. Hopefully it stays like this now.

Regarding the bearing: I read on another forum that the preload on the X-axis leadscrew is adjusted on the left nut on the X-axis. So I loosened it slightly (increased my backlash by 0.02mm) and the right-hand size bearing is now silent. The consistency of my backlash on the X-axis is now also better.

Thanks for all the advice.