why do you want sprockets at the center? wouldn’t you want just a pivot point?
The sprockets are there for 2 reasons.
- To cancel out the shifting point at which the chain departs the motor pulley. As the sled lowers, engaging less of the motor sprocket, it engages more of the sled sprocket, and vice versa, thus eliminating that source of error without any math.
- To enable sled X position adjustment, quantify Y position error, and allow rotational alignment despite misplaced center of gravity. The sprockets have either a shaft clamp or set screw and are on a fixed axle(bolt).
During setup, the set screws are loose, sled sprockets can move.
Motors can pay out the amount of chain that should theoretically result in sled being centered on the workpiece.
Now you can manually move the sled to the actual X center of the workpiece and tighten the top setscrew so it can no longer turn. You have now aligned the software’s logical X center with the workpiece physical X center eliminating that initial error.
Now, note if the sled hangs plumb by dangling a plumb bob from the top shaft and see if it aligns with the bottom shaft. If not, add weight to the high side till plumb. Then tighten the lower set screw to lock in that alignment. If for some reason, you are not able or dont want to balance the cg, you can just manually set it plumb and lock it in.
Make sure X is still on the mark, if not, readjust.
Now you can physically measure the error in the Y direction and feed that info to the controller firmware.
This procedure probably takes longer to explain than to do. It may be unnecessary as software can guess at location as has been described in current documentation. But this provides an empirical means to eliminate or quantify static initial error. Something I have yet to see described elsewhere. But I’m new and might not have found that section yet.
One source of systematic error that remains is chain catenary. Chain sag. I believe there is a solution to that as well. Current documentation on calibration calls for cutting a sheet of plywood. While actual cutting is necessary to confirm that everything is staying aligned in the presence of load and vibration, it shouldn’t be a first step. We want a good indication that Maslow can hit any target on the workpiece before we make sawdust.
Since the sled no longer rotates and we can add multiple tools, I propose adding this line following sensor to the sled for systematic workpiece calibration. Carefully stretch black electrical tape in straight lines around the perimeter of a workpiece or backer board, and now Maslow has some ground truth features it can follow. A calibration routine can have the sled follow the line of black tape around the workpiece, noting intersections with white tape at measured intervals, and comparing the actual position with the calculated position based on motor encoder counts. This information can be used to build up a table of correction factors across the workpiece. Interpolation can fill in the blanks between plotted points. This eliminates the effects of catenary and various other possible error sources. It does not eliminate dynamic error such as chain whip, router bounce/tug, frame creep. However, as a nondestructive automated loggable test, we can compare old test results to new to identify specific areas of change which we might be able to trace back to a shift in a specific frame joint or other cause.
explain what you are talking about (diagrams please)
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No, if you lower the sled, the chains will wrap around the 4 sprockets on the corner, and the ‘virtual chain’ mid way between them will no longer point at the bit.
I don’t understand what you are saying here. As far as I can tell, regardless of sprocket number used, the same motor movement would result in equivalent sled movement. Please point out any situation where that is not so.
Edit: I see it now. It works in the picture with that fixed angle of chain, but while operating at different chain angles, the 4 colinear sprockets are no longer colinear. Good catch.
Another thing I was slow to see, this does look like a Maslow built entirely of chain with no frame pictured. I lost access to NX and have been too set in my ways to learn another CAD package, so you are stuck with incomplete bad drawings for now.
I see what you are talking about now, but I think it’s drastically over complicating things, you can do far better just nesting the chains will probably work better.
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Maybe I drew it poorly, it seems simple to me. I was not suggesting both the over and under configuration for a single machine. Just showing 2 different possible configurations on the same sheet of paper. What do you mean by nesting the chains?