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  1. #11
    Can you post a picture of a part which has rounded off corners, which you propose are caused by deflection? The reason I ask is if you have the acceleration set too low for the X and/or Y axis, that will consistently cause the machine to cut noticeable curves in corners if it's in constant velocity mode. Search 'constant velocity' or 'G64' on the forum and you'll find plenty about it. If your acceleration for the Y axis is set too low, then the corners you indicated will be rounded, so I'd start by checking that.

    I agree that the list of parts you have are not bad except for the rails, so if you replaced all the rails and make a rigid gantry you'd have a good machine. The 2.2kW spindle would have been nice, due to the larger collets, but that's minor compared to the rails.

    With 100 you're struggling, as that's not enough to get rails for both the X and Y axes. Without enough funds to replace every 'weak' part, we need to work out which is the least stiff and replace that as it will have the biggest effect overall. The deflection of unsupported rails for a force applied at the center is proportional to the length cubed, and inversely proportional to the diameter raised to the power 4. That means your X rails will deflect (1200/700)^3*(20/25)^4=2.1 times more the Y rails, so I'd be inclined to replace those first. Having said that, the difference is not that great so you would still notice some gain if you replaced either.

    Bear in mind that even if you replaced the rails with just 12mm supported rails, this would still be stronger than your current setup as it is the strength of the supporting structure that matters most when using supported rails, not the rail itself. I would advise using at least 20mm though, especially if wanting to cut harder materials like aluminium, since the extra stiffness of the bearings does help.

    Another option would be to invest the 100 in getting the open bearings that the supported rails use (SBR25 & SBR20) and adding your own supports to the existing rails by drilling and tapping holes in them. That could be a courageous decision though...

  2. #12
    Quote Originally Posted by cncJim View Post
    Would you mind taking a look at the parts of my machine and give me your opinion? I don't anticipate that I can economically improve the machine but I would like to learn more on what to look for in the future.
    Agree with Jonathan and Pretty sure this will be tuning and CV problem. Post the feed rate in G-code or better still the G-code file it's self so we can check.

    Regards the rest of machine then agree you have a half decent kit of parts so I'd just look to upgrading the Frame. . BUT. . Don't try to do it on the Cheap because there's very little gain for the Money and time invested.!! . . . Best to Save up and buy decent rails for all axis and Beef up the frame where needed.

    Post a picture and lets see what's what.!!

  3. #13
    I had similar problems with corners cutting as curves. I changed from CV mode to 'Exact Stop' mode in Mach3 and it solved the problem. I know there are probably better ways by tuning CV mode etc, but it might be a quick way of proving whether it's mainly a software or hardware issue.

    Last edited by firetrappe; 24-10-2013 at 05:45 PM.

  4. #14
    Hi Jim,

    Glad to hear you are enjoying the machine despite the problems. The rounded corners sound like the software setting already mentioned, but on the subject of unsupported rails - my first CNC machine used 900mm unsupported 20mm rails for the Y axis (it was a 'landscape' machine rather than the more usual 'portrait' machine) and 600mm unsupported 20mm rails on the X axis. The Z axis was around 300mm unsupported round rail.

    I paid about 50 for all the rails and bearings (from a colleague) just to have a play with CNC and see if it was just a passing phase. Well the machine did work, but it flexed badly. The cutter was almost dragged through the work (plywood) at a slight angle so feed rates had to be low. However, I was hooked so I sold the mechanical bits on ebay and designed the machine I have now which is much better (although about to upgrade again!).

    I remember playing around with temporary supports clamped to the rail at intervals (if the part being cut was small) but this didn't help alot. As Jazz as mentioned I think you might go the same way - trying bits and pieces but not making too much difference. Jonathan's suggestion might work (change bearings for half-bearings and try to support the rear side of the rail), and I had the same idea with my original machine but didn't do it in the end for fear of grinding the toughened surface to be able to drill into the softer core (to tap) and releasing in-built stresses and distorting the rail. I might have been worrying over nothing there - but it would also have made them more difficult to sell on e-bay after use so I didn't pursue.

    About 2 years ago I bought a pair of 16mm supported rails around 1300mm long for ~100 from Chai on LinearMotionBearings2008 (ebay), so this is probably the best sort of upgrade for your 100 and you could use it on your next (!) machine, but the unsupported Y axis will hold you back.

    By the way what rails are on your Z axis because this is also part of the chain?
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  5. #15
    The term "Mission creep" springs to mind.

    Edit: Jim your original 'rough diagram' did not do your machine justice, it's quite a bit better than that.
    Last edited by EddyCurrent; 25-10-2013 at 09:32 AM.

  6. #16
    Thank you everyone.

    Mach 3 config:-

    Machine pics:-

    Love the idea to diy support my existing rails! Not sure I have the tools or skills to pull it off though!

    Z axis rails are also 20mm.


  7. #17
    Missed a couple of questions...

    I cant post a pic of the part with the rounded corners as it is now installed in my fireplace chamber! (observing the correct distances to combustables, of course!). I can post the g-code used to create it later tonight though.


  8. #18
    Your acelleration on X and Y is quite low - you should be fine with about 1000mm/s^2. Try increasing them, or at least set them both to the same thing.

  9. #19
    Thanks Jonathan, I wil try changing the values as suggested.

    BTW was that just a general comment on my config or do you think it would be part of my problem?

  10. #20
    Ok well Agree most of this is motor tuning and like Jonathan suggest you balance the tuning up.!! . . . But looking at the frame it has some serious issues.?

    Quote Originally Posted by cncJim View Post
    Love the idea to diy support my existing rails! Not sure I have the tools or skills to pull it off though
    Following on from above statement regards serious issues then I wouldn't suggest you do because there are other problems that will make any attempt pointless without addressing these problems also has they are quite big ones.

    The Gantry and the way it fastens to the bearings is a major weakness. It's only mounted to the X axis bearings with 2 bolts per bearings plus the mounting plate area is woe fully inadequate. Also the fastening of the Y axis rails into the puny narrow gantry ends is very weak and the whole thing must flex quite badly and could be some of the problem on direction changes with deep Cuts.

    Like said before given the parts you have already and spending a few hundred quid building a new frame will be far far better solution that will give a very good machine that will do every thing you want and to good standard for years, and still be worth more money than you invested when time to sell.!!

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