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inconsistent cuts when turning CB 1220 XL

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korecoa

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Hey guys, picked up my first ever 3-in-1 smithy It's the CB 1220 XL. I paid 500 bucks for the unit, the bench and all the tooling. So i couldn't say no. I went through the machine and everything seems tight. I adjusted the gibs and modified the cross slide drive screw. I added two springs to the drive nut adjuster mechanism to help keep tension on the drive screw. This is helping a little but am finding it loosens after a bit of use. so i get play in my cross slide. I did add the AXO quick change tool holder, I made a custom extended M10x1.5 bolt to mount it. It's identical to the origional mounting bolt but longer.

Anyways, the main issue i'm having is when I go to take a 10 thousands cut, it seems to be pretty close. I measure the cut depth with calipers and it seems good. When i go to take 15+ thousands it becomes wildly inaccurate. for example, a 15 thousands cut will be 28 thousands when i measure the cut depth. Is there anything I can do about this, or is this just the nature of the beast with these machines? I was thinking my best bet may be to add a DRO, or Maybe zero my cut depth every single cut instead of continuing the turn as i go.
 

markba633csi

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Do you think it could be a poorly made feed screw or something going on with the feed nut? Sounds like you need to revisit the drive nut adjuster mechanism perhaps? I'm not familiar with what that looks like but it sounds suspiciously like the source of the problem
Mark
ps if either the screw or the nut is moving around then that would cause the problem- you shouldn't need to go to a DRO if the mechanism is secure
 

Bob Korves

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It sounds like things still may be loose enough to move under a higher loading. The cross slide needs to be solid under load, as does everything else. Springs may hold it under light loads, but not under heavier loads, which is what you are describing. You also installed a new tool post. It needs to be mounted rigidly. The t-nut pulls upwards when tightened. When the "T" portion of the nut pulls up when tightening it down, does it clamp to the compound rest, or does it clamp to the t-nut? To be properly attached, you should have a gap between the t-nut and the bottom of the QCTP when tightened down. 3 in 1 machines are less rigid than ordinary lathes of the same size, but can turn out good work if you keep the cuts light and everything else as stiff as possible. Also, you need sharp tools, properly presented to the work at center height. Minimize stick out and leverage on everything possible.
 

korecoa

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I was thinking about that, and you may be right. I think i'm going to take it apart again, I will take some photos go give you guys a better idea of what i'm dealing with. New to all of this and although i'd consider myself mechanically inclined there's alot of moving parts of these things and alot to consider!
 

markba633csi

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I have read that certain areas of the Smithy-style machines are a bit marginal. The half-nut mechanism for one. This may be another.
By all means post some pictures when you get it apart
 

korecoa

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Mark, I've read the same, and i'm willing to give up a little consistency to save space for now, plus it was affordable. I've been happy with it so far, just trying to make sure I can correct all that I can before I accept it is what it is.
 

korecoa

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Got into it again tonight. Here’s my situation for you more experienced guys. Suggestions would be helpful. My first thought is, add two more adjuster screws in the feed screw mechanism, along with springs and perhaps small lock washers on all 4. Then figure out a way to secure the feed nut to the carriage. I should clarify, no movement left to right, no movement in feed screw itself.
 

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bill70j

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If once you get the machine back together and find the same issues, you might consider manuually checking tightness of all three sets of gibs using an indicator if you haven't done so already.

For example on your table, locate the indicator on the top, set the reading to zero and forcefully pull up and push down, while watching the indicator readings. Relocate the indicator and repeat the exercise side to side, then front to back. Once you have finished the table, move to the carriage then to the compound.

At lease this will eliminate one variable, if you haven't done so already.

Good luck, and HTH, Bill
 

Latinrascalrg1

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It that a picture of an "Anti-Backlash" lead screw nut in the 3 pictures you posted above? If yes did you make it or buy it? If you made it could you share any details?
 

markba633csi

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OK Bob was correct, the nut is causing the problem. Specifically the spring-loaded part is moving under load. Many lathes dispense with that and have a simple one piece nut, you have some backlash but you just learn to compensate for it. It's a constant amount that doesn't change.
Try this: re-install the nut in the opposide direction if possible, in other words, if the spring loaded portion was toward you before, install it with that portion away from you and see if the problem is reduced.
Alternately, remove the spring-loaded portion completely
 
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korecoa

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Mark, I will try that. The reason I added springs is because the screws were coming lose and removing the "backlash compensation" in the cross slide. I added the springs (per other designs I saw online) to alleviate the loosening of the screws. I think If i can get the screws to stay put and get the feed nut secured to the carriage I'll be in good shape. Perhaps just simple lock washers on the small screws?
 

Bob Korves

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The lead screw and nut should be set up so that it is solid in the direction that holds the tool to the work, no springs or adjustments in that direction. The adjustable side of the nut or any springs should be on the reverse side, not taking the heavy cutting loads. It would be better to make a setup that can be adjusted to a solid and robust nut to screw fit. Springs are a weak attempt at a solution.
 

markba633csi

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Yes you really should dispense with the springs and shim or trim the nut pieces such that it makes one solid assembly, leaving a tiny amount of backlash- Also what Bob said above; install in the direction where the main body of the nut takes the cutting forces
 
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rzw0wr

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Go to Smithy and leave Dave a note.
Dave very very knowledgeable about all of the Smithy machines.

Very helpful fellow.
 

Grasshopper

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I just went thru this recently. First and foremost I should say that I am new to lathes so listen at your own risk. I had the same issue with my Midas 1220LTD. I ordered a new nut which removed most of the lash. IMHO, the small or adjustable end of that nut is the weak point. It only has a few threads and wears more quickly than the larger end. After you have the new nut and lead screw in place adjust the 2 screws until you feel the slightest bit of drag when turning the handle. Reassemble the table and tighten the nut behind the wheel, adjust the gibb screws and oil it. There is a certain amount of lash that remains but it will be much better. You'll notice that the new nut has a smaller gap between the halves than the old nut when adjusted. IIRC, they are <$15. DRO is a big help on my machine. Especially now that I've corrected the the DRO's installation done by the PO, but that is another story. Every day is a learning experience but I'm loving it so far.
 

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