Relative axis movement

[ohnocottrill]

I have a shark pro plus and have cut around 40 signs. On two separate occasions, the Y-axis has moved incorrectly during a g-code and ruined the stock.

On the first occasion, I was using a 90* bit to carve the name and boarder. The machine cut the "Pe" in Penny then moved to the year. During that move' the Y-axis "slipped" and moved up about 1/4". The year and "nny" of penny were then cut incorrectly. It then proceeded to cut the boarded, which was done correctly. The results are as followed:

I scraped the board and started over. This was the final product:

This cut worked perfect EXCEPT at the end of the profile cut, the machine returned to about he (0,1.5,0.25) position and said (0,0,0.25).

I then cut a few more plaques with no problem, then today I was cutting a plaque out of oak (which is identical to a plaque that i had previously cut on the machine with no problems) and I had the same problem. I was using a 3/8" straight end mill to clear out a large area. It cut the first 1/8" depth with no problem then during the 2nd 1/8" pass it moved the Y-axis again on me. The result was a total loss of my stock.

My question is, why is it doing this? It is not near it's limits of motion and I never run it greater than 75% speed. I saw a video on YouTube that talked about a set screw on the X-axis being loose, could that be my problem? Please help, I can't afford to keep ruining boards.

[ohiococonut]

You may have answered your own question. It wouldn't hurt to check all of the set screws on the drive motors.

[4DThinker]

Threaded rod or the motor shaft slipping in the coupling would be my guess.

[baby15]

Threaded rod or the motor shaft slipping in the coupling would be my guess.

yep that happen to me. tonydude fixed it

[ohnocottrill]

Are there any good retrofits to help out? Seems to me the motor drive should have been made square to prevention it from spinning. The threaded rod looks to small to drill and put a cotter pin through.

[rungemach]

You can replace the set screws in your couplers with cap screws and get a lot more tightening force, The allen wrench size is much larger for a cap screw and you do not end up bending the wrench or stripping it when you try and get a good torque on the screw.

The motor shaft should have a flat on it , as well as the lead screw. to prevent rotation.

This thread may be helpful.
http://www.cncsharktalk.com/viewtopic.php?f=9&t=1422

set screw and cap screw wrench size comparison

[Eagle55]

ohnocotrill,

Keep in mind we are all guessing here based on what we have to go on mixed with our own experience. As you have heard and read the set screws can be a problem and from everything I have detected the replacement with cap screws solves that problem. Personally I haven't had that problem since the first week that I had my Shark and tightened the screws, although I have the cap screws on hand and ready as soon as I either have spare time and get motivated OR have another occurrence and do it out of desperation. I wouldn't tell you not to replace them because you should. And don't do an "Eagle55" thing and ove rthink the problem, the cap screw solve the problem without any need to drill, dimple or anything else. However, I'm not sold on the idea that this is your problem. Again, guessing a little here but I am going to suspect traveling too fast for the amount of cut that you are taking. All things being relative, you say that you never go above 75%, but you have to consider that it is 75% of what?? 75% of 30ipm it a lot less than 75% of 100ipm. And 75% (of whatever) might be fine for pine, but marginal in a harder wood like oak. I sense that the stepping motors are being given the step command and not able to overcome the force holding it back (ie hard wood, knot or just a tough spot). Lubrication or I should say a lack of, could be causing resistance in the leadscrew/t-nut, which has been seen before, so rule that out by making sure you have it lubricated, which the y axis can be tricky to get to. Past that I would post the feed rate of the tool that you have designed that toolpath with and the depth of cut per pass etc and lets see if it is within reason. My guess is that you are just getting marginal and maybe tough spots in the wood are pushing it past the point of causing the motors to slip. Example, If you are using the default of 100ipm feed rate for a 1/4 end mill and doing 75% of that, then you are cutting a approx, 75ipm. Making 1/4" pass is double the energy that taking an 1/8" pass. I would probably recommend that you take smaller pass depth (depending on what you are doing now) and slower feed rates (probably the easiest to do) and see if that cuts out the occurrence of the problem. If everything has been good and these 2 occurrences have been quite recent then I would look a little closer at the lubrication issue. And that's my guess of the day

Roger