Blackfoot CNC accuracy X-Y


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Thread: Blackfoot CNC accuracy X-Y

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    Default Blackfoot CNC accuracy X-Y

    I got a blackfoot 4x8 used, disassembled. I got it back together, wired up and working. (previous owner shortened the table to 4x6) Anyway, I've been doing some simple vcarve signs nothing terribly small or detailed. I wanted to do some more detailed work and figured I would need to set up X-Y zero better than eye balling it. I am using a makersmark, If I do the x-y probe and zero each axis, then run it up and down the Z, then do x-y probe again it's off. sometimes .01 or .02 some times as small as .0004. I'm thinking this isn't normal. I did motor tuning several times and that seems to be good. Just not sure why the x-y would change so much just running up and down Z, or is that normal for this machine?
    I was thinking I would need to upgrade to rails instead of wood and chain, but was hoping to get some mileage out of this one first.

    Thanks

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    Default Re: Blackfoot CNC accuracy X-Y

    I took a look at the blackfoot webpage as I've never seen one of the machines in the wood,so to speak.When you rebuilt the machine did you have any means of checking that the Z axis rails are truly square to the table in both planes?The other apparent discrepancies may be attributable to thermal expansion of the chains or shrinkage/expansion of the wooden components with humidity changes.



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    Default Re: Blackfoot CNC accuracy X-Y

    Chain drives are a terrible method for motion, as they give a cogging affect from tooth to tooth.
    Also, the way the blackfoot is designed, the wood can't support the chain assembly properly when tightened, so it'll always be looser than you'd like.

    Gerry

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    Default Re: Blackfoot CNC accuracy X-Y

    Quote Originally Posted by routalot View Post
    I took a look at the blackfoot webpage as I've never seen one of the machines in the wood,so to speak.When you rebuilt the machine did you have any means of checking that the Z axis rails are truly square to the table in both planes?The other apparent discrepancies may be attributable to thermal expansion of the chains or shrinkage/expansion of the wooden components with humidity changes.
    thanks for the reply, I agree there could be some thermal/humidity changes, but I'm talking repeating the X-Y probe touch 10 time in 5 minute span and not being able to hit the same zero each time. If the Z isn't square, I don't think that would affect the zero, if the router was at 30degree angle, zero should still be zero. I think I'll go through the whole Mach3 setup process clearing out limits and homes and resetting them.



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    Default Re: Blackfoot CNC accuracy X-Y

    Quote Originally Posted by ger21 View Post
    Chain drives are a terrible method for motion, as they give a cogging affect from tooth to tooth.
    Also, the way the blackfoot is designed, the wood can't support the chain assembly properly when tightened, so it'll always be looser than you'd like.
    as I've heard, but I got the whole thing for less that it would have cost me for the steppers. But I thought there would be more accuracy, I mean they even sell a laser kit for it. and it's not like I'm traversing the table and it's off. I'm simply pressing the probe touch off button repeatedly. I'm going to go through the whole setup process, maybe something is wrong there.



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    Default Re: Blackfoot CNC accuracy X-Y

    Quote Originally Posted by ger21 View Post
    Chain drives are a terrible method for motion, as they give a cogging affect from tooth to tooth.
    Also, the way the blackfoot is designed, the wood can't support the chain assembly properly when tightened, so it'll always be looser than you'd like.
    also on that note. I've seen plenty of belt drives, would that be worth while upgrade before I rebuild the gantry in aluminum?

    Thanks again.



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    Default Re: Blackfoot CNC accuracy X-Y

    Quote Originally Posted by _taz_ View Post
    thanks for the reply, I agree there could be some thermal/humidity changes, but I'm talking repeating the X-Y probe touch 10 time in 5 minute span and not being able to hit the same zero each time. If the Z isn't square, I don't think that would affect the zero, if the router was at 30degree angle, zero should still be zero. I think I'll go through the whole Mach3 setup process clearing out limits and homes and resetting them.
    Your posts are extremely confusing as you keep talking about X-Yprobe touches and then complain about the Z not repeating. So it might help to tell us exactly what axis is probing here and which axises are moving here.

    I went to the Blackfoot web site and frankly was to put off by the bull**** on that site that I just left. The little bit of visual information I got indicates an extremely sloppy Z axis arraingement. The Z apparently uses V rollers on aluminum. That right there indicates design issues that will have to be addressed. Your best bet is to make sure you have a good preload on the bearings and that the bearings and rails are in good order.

    As mentioned elsewhere chain drives are terrible solutions for motion control. A proper timing belt drive is a far better solution and often cheaper. Now the question is are they a problem here with respect to repeatability? It is hard to say because of the other glaring faults in the design. However one can say with absolute certainty they will be a problem if they are not now. By the way chains don’t stretch per say, what happens is that the pins and bushings wear real fast producing considerable clearance in each link. So if you happen to have 0.020” clearance per link over 100 links that becomes 0.2” of additional length. This causes all sorts of problems including really odd motion around the sprockets. Of course to combat this chains are tightened but then you get into a situation where the chain will not lay in the sprocket correctly leading to even more issues. Basically with respect to motion control, with chain, you start off brand new with problems and it goes down hill from there as the chain quickly wears.

    Sadly the thing that bothers me the most about the Blackfoot is that it give wood framed machines a bad name. This is totally unjustified as wood framed machine can make for very good machines that can dry process just fine. Considering the bargain basement price you paid on this machine it very well may be worth upgrading. I wouldn’t throw a lot of money into it but wouldlikelystart with supported round rails on the gantry with a completely new Z on the new gantry saddle. You may be able to refactor much of the existing hardware into the rebuild. Ultimately leadscrews for the axis drives are in order but a well implemented belt drive may work also. Timing belts in ithis sort of application need a drive arraingement like us see in figure 5 here: http://www.gatesmectrol.com/mectrol/...older=brochure. Timing belts are not ideal for routers, especially long axis but they can bea huge upgrade over a chain drive. Also any axis that uses them must be structurally able to handle the belt tension.

    In any event it looks like you got a good deal here in that you can modify the machine into something g far better without a huge outlay in cash.



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    Default Re: Blackfoot CNC accuracy X-Y

    Quote Originally Posted by wizard View Post
    Your posts are extremely confusing as you keep talking about X-Yprobe touches and then complain about the Z not repeating. So it might help to tell us exactly what axis is probing here and which axises are moving here.

    I went to the Blackfoot web site and frankly was to put off by the bull**** on that site that I just left. The little bit of visual information I got indicates an extremely sloppy Z axis arraingement. The Z apparently uses V rollers on aluminum. That right there indicates design issues that will have to be addressed. Your best bet is to make sure you have a good preload on the bearings and that the bearings and rails are in good order.

    As mentioned elsewhere chain drives are terrible solutions for motion control. A proper timing belt drive is a far better solution and often cheaper. Now the question is are they a problem here with respect to repeatability? It is hard to say because of the other glaring faults in the design. However one can say with absolute certainty they will be a problem if they are not now. By the way chains don’t stretch per say, what happens is that the pins and bushings wear real fast producing considerable clearance in each link. So if you happen to have 0.020” clearance per link over 100 links that becomes 0.2” of additional length. This causes all sorts of problems including really odd motion around the sprockets. Of course to combat this chains are tightened but then you get into a situation where the chain will not lay in the sprocket correctly leading to even more issues. Basically with respect to motion control, with chain, you start off brand new with problems and it goes down hill from there as the chain quickly wears.

    Sadly the thing that bothers me the most about the Blackfoot is that it give wood framed machines a bad name. This is totally unjustified as wood framed machine can make for very good machines that can dry process just fine. Considering the bargain basement price you paid on this machine it very well may be worth upgrading. I wouldn’t throw a lot of money into it but wouldlikelystart with supported round rails on the gantry with a completely new Z on the new gantry saddle. You may be able to refactor much of the existing hardware into the rebuild. Ultimately leadscrews for the axis drives are in order but a well implemented belt drive may work also. Timing belts in ithis sort of application need a drive arraingement like us see in figure 5 here: http://www.gatesmectrol.com/mectrol/...older=brochure. Timing belts are not ideal for routers, especially long axis but they can bea huge upgrade over a chain drive. Also any axis that uses them must be structurally able to handle the belt tension.

    In any event it looks like you got a good deal here in that you can modify the machine into something g far better without a huge outlay in cash.
    sorry if confusing, but I haven't even gotten to checking if Z zero is repeatable. I was purely zeroing the X-Y with touch plate, running up on Z, then back down and doing touch test again. I got an email from their support saving .06 is acceptable error and most likely due to electronics/material error. Said I would need light, magnetic, or optical sensors to get closer.

    Yeah in the end run I hope to build a better setup, since it was worth the money for the electronics and computer. I'll stick with carving simple stuff to get the hang of it, then upgrade carvings as i upgrade the machine.
    Thanks again.



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