Problem printing circles

So when you print a 20mm cube and measure it, it is 20mm? What are your steps per mm on your X and Y axis? It is long belts that cause backlash issues by the way not short ones. So all your long belts need to be the same tightness. Able to be be pressed or pulled the same distance from a reference point. Also have you confirmed your frame is truly square along the X and Y planes?

Rods that can rotate are fine. In fact in most printer designs the rods are not fixed to allow rotation. They are allowed to rotate as this prevents wear from occurring from the bearings running in the exact same spot over and over. I have never seen a printer where the rods where not allowed to rotate. The reason is that in most cases the balls in the bearings are harder than the rod and they will score the rod if they are allowed to run on the same surface.

The main thing to assure is that rods are straight, parallel, and not allowed to move on their opposite axis. For example you do not want a Y rod to be able to move at either end in the X axis. Like wise with the X versus Y. Rods that rotate are the easiest to find if they are bent or not. Just place the head over the bed, rotate the rod by hand and note if you see the head move any. If you do then that rod is bent. That is also another reason rods are allowed to turn.

The shop I used to work in even had rods that would turn on their large CNC machines. By binding the rod where it can't turn you are doing more damage than good.

We are trying to remove issues here, not create more. When you have an issue like the OPS, you have to focus on the mechanics and physics that can cause the issue. A rod that can rotate as long as it is straight will not and cannot contribute to the OPs issue.

While you think this may have been an issue for you in the past it was probably something else you did while messing with the rod that fixed your issue.

Carl, Can you explain how it would be possible for the x carriage rods to be able to rotate but not to slip in the x dimension when using the Lawsy carriage design? I think he even built in a hole for a lock screw to hold them in place. This also prevents rotation of the rods when used.

I see this thread is about a different printer design so maybe this is not applicable here.

Rods that slip along their axis is fine and prevents no issue as long as the slippage is only along their axis and so much that they can be pulled from their mounts. The only time I can see a rod slipping in its own axis being an issue is if the home or max end stop was mounted directly to the rod. Then that slippage would allow for the end stop positions to change and this would cause dimensional issues.

My Ct'c dual head has rods that move along their axis by about 1mm total. They do not have any play in any other direction though and they do spin. I can print perfect circles and boxes with it at all times. I know this machine is reliable and trust it to all my jobs.

So again rotation and small play along it's axis should not cause issue with a rod and a print. Again the exception is if the endstops are hard mounted to the rod and not the rod mounts.

In the case of the Lawsy carriage design, that slippage would affect your belt tension in X if the rods are not locked. I get how it would be a non-issue with a different design.

I do agree about the wear. My Y rods are fixed in all dimensions and the bearings are wearing traces in them. I'm actually shopping for replacements now and manually rotated them as a temp fix.

You're right, the fact that I have been using and building 3d printers since 2008 when they first became home items and only a hobby means nothing. Your outstanding experience is far superior to mine.

All of your machines seem to be low tier sub $500 machines and that would explain the use of rods that are too small for their length and the need to bind them. The binding also prevents them from flexing as easy under their length. My Ct'c not to be confused by the umbrella brand of CTC is an all metal $900 unit that has 10mm rods through out and sealed bearings. It is a very well made but very easy to repair and find parts for machine. I don't know why they stopped making it and went for the cheaper wood frames. It is one of the best machines I have owned next to the D7. It is so quite I have to check to make sure it is operating every now and then.

Just realize your fix is not the fix for every machine and while it may work to have a rod bound on one machine it may not for others. If the rods are allowing flex due to diameter versus length then that is a poor design and the fix would be to increase the rod diameter.

Not a fix for some of the less technical but still would be the proper fix. Another fix would be to use case hardened rods and not just drill stock. Case hardened rods which is what should be used are much more ridged per given length than drill stock which is the most common used rod in cheaper printers.

Mine are case hardened on my CT'c which is why I guess there is no flex in them on the long axis of 300mm You can test them by removing one and using a dremel with a cutoff wheel on the very end you should have a very hard time cutting them if they are case hardened. Again though not something the less technical might want to try.

Then perhaps you should sit back and be quiet for a change - and maybe learn something from those that DO have the experience.

Just sayin...