Upgrade or scrap old machine

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Hi,
This machine was bought of ebay and got me started on CNC.
However its tolerances are to large to be of much use, maybe 1mm off in some cases and is now in a state of disassembly to try and determine where the faults lie. Some of the bearings need replacing and I believe the design needs modifying and am hoping some users here may give suggestions.
I would ideally like it to be accurate enough to engrave small signs.
To me the linear rail for the x-axis which is 20mm supported rail 1000mm is orientated wrongly?
The Y Axis is the same 20mm supported rail @ 800mm wide.
The Z axis is smaller and seems OK.
I've recently updated the controller and have it working on Mach 3 via network, all good.
Appreciate any comments and suggestions.
Thanks
Keith, Cambridge UK

Accuracy is the result of everything working together correctly; inaccuracies stack up to create larger errors. If the linear bearings are shot, that can be contributing a lot of the error you're noticing. Some might be backlash, some might be due to tool deflection. If the frame is flexing, that can also cause some of it. You need to go over it one part at a time, fixing things or beefing them up, retesting after each fix to see if you've improved matters (or made them worse). From looking at it, it seems it should be able to hold better tolerances, but you might have to do some work to get there. It's hard to tell what's going on at this point since it seems partially disassembled - that bearing truck on the X axis isn't apparently attached to anything.

It looks like a fairly balanced machine design. Supported round rails and aluminum extrusions. So there's no easy thing to upgrade to help it out a lot. If you upgrade to profile rails, the rigidity of the frame is going to become the weak link. If you beef up the frame, the rails will become the weak link. If you do both.. well, you've basically built a new machine.

I'd try to find the source of your 1mm error though. Fixing that may be much less expensive than trying to improve the machine in general. Is there any backlash or free play? How are you measuring the error?

Id agree with Skrubol
Machine looks ok, where is the 0.1mm error appearing as may just be a bearing
Where abouts in the uk are you

That looks like a similar design to my old machine. It was a MTC (Machine Tool Camp) does your gantry run under the table & is powered with 1 stepper motor? I had a 4'x8' version of this table & it was a nightmare. I couldn't run it over 25-30 ipm without it chattering & making very bad cuts. I purchased a CNCRP (CNC Router Parts) Pro 4896 & love it.

Good luck
Gary

Quote:

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Originally Posted by Gary-Wiant

That looks like a similar design to my old machine. It was a MTC (Machine Tool Camp) does your gantry run under the table & is powered with 1 stepper motor? I had a 4'x8' version of this table & it was a nightmare. I couldn't run it over 25-30 ipm without it chattering & making very bad cuts. I purchased a CNCRP (CNC Router Parts) Pro 4896 & love it.

Good luck
Gary

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If that's the case then an obvious upgrade is to use 2 motors/screws, one on each side. A moving gantry with a single screw in the center is going to be susceptible to racking.

To reduce the racking from the center screw, add an 'H' cable configuration to the setup. The V-bearings are double stacked on both sides. Works great on my setup.

Hi FK - Firstly a clean up and good rebuild to determine how good the machine could be. Then decide on replacement of things that you have identified in the rebuild, considering cost. Then decide on new vs old. Cheers Peter

I'm seeing a tall,skinny gantry that might usefully have the uprights triangulated.I totally agree about the central leadscrew being sub-optimal and that would be a good place to do an upgrade.One other thing I would do is add an additional clamp for the upper end of the spindle body as it looks a bit unsupported.Otherwise it looks like the basis for a useful machine.

I also agree with everything that's been said so far... with a little TLC (Troubleshooting, Looking, and Correction ;) ) and an upgrade or two in a few strategic spots; I think that machine can be made to perform well.

Quote:

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Originally Posted by WayneHill

To reduce the racking from the center screw, add an 'H' cable configuration to the setup. The V-bearings are double stacked on both sides. Works great on my setup.

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Interesting. I see how that can equalize the two sides. What do you use for the cable - stainless steel or similar? Given that it's a relative easy and cheap approach, might be worth a try.

Though, I wonder how much resistance it adds. A benefit of the twin motor/screw design is more power to move the gantry.

Hi FB - the gantry column bearings are quite close, if it is racking then separating them is an easier approach then adding the cabling. Your first aim is to rebuild and establish how good the correctly assembled machine is. For instance the gantry column bearings are through bolted, through the extrusion. This means they are tightening on "air" if you torque those up correctly they will distort the extrusion and come loose easily as they will not have the correct preload. Issues like that have to be sorted on every connection. This type of machine has many many bolts and many non square connections as cutting an extrusion square is not a simple matter. So it has to be assembled carefully and every joint observed and corrected as you go...Peter