Planing toolpath leaves lines each bit pass

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xoneeleven
Posts: 34
Joined: Fri Aug 19, 2016 3:41 pm

Planing toolpath leaves lines each bit pass

Post by xoneeleven »

Hello All,
We have our Shark HD3 back up and running after finding out it had a bad z-axis bearing assembly. (this was the actual cause of the issue in my previous posts about the round sign we were making)

We are now planing a 20inch wide by 4feet long board.
The wood is a single slab of Douglas Fir
The bit we are using is a 1 3/4inch wide Amana bit designed for large pockets.

The issue is, each time the bit passes, there is a line it creates on the Y- side of the bit as it makes each pass.
We have verified that the bit is digging from the edge of the bit as it makes each pass, thus creating the lines.

It looks as if the router is not level on the Y-axis.

My question is, is there a way to adjust the CNC Shark HD3 router mount?

We have looked all over, and there does not appear to be much in the adjust-ability of this machine due to the plastic parts.
Mark
CNC Shark HD3

Rando
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Joined: Tue Jan 06, 2015 3:24 pm
Location: Boise, ID
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Re: Planing toolpath leaves lines each bit pass

Post by Rando »

Yes, it can be achieved.

Possibly the easiest solution might be to just make the passes in a different direction. That can sometimes help.

One thing that's not entirely clear from your post is the "direction" of the "down" side of the bit. Is that "down" aspect caused by it moving too fast, with a slower feedrate (and < proportional RPM drop), and it's a flex issue? Or, is your flatification bit moving so slow this should NOT be a problem? If it's the first, then consider if the direction of skew (out of alignment) is along the gantry direction, or perpendicular to it (along the bed's t-slots). If it's along the gantry, it might just need to be loosened and re-tightened moreso than before. side-to-side offsets are much less likely to be a flex issue under normal circumstances. If, on the other hand, we're really talking about "sag" along the direction of the bed t-tracks, well that I might be able to help with a solution.

Sorry, on the Sharks, it's not an easy way, but it IS possible to get very close. I used a combination of these two methods:

1) Using the system in it's exact current configuration, I machined out a new mounting (clamp) plate for the router/spindle. Ignore that it's not easy and takes some doing ;-). This puts the motor's offset permanently into the clamp plate. When it's done, remove your router and current mount plate, then FLIP the new one over and install that. That should neutralize most of the error.

If that's not enough, I also did this:

2) I'm lucky that I'm using a spindle motor that has a continuinous, smooth case surface. I used an 0.0005" (5/10,000ths) dial indicator to get an approximation of the total top-to-bottom displacement of the "top" of the spindle/router. In my case, with the z-axis in the "max down" position, I set the indicator at the spindle body top, then jogged up slowly. Taking that offset, I extrapolated that to get a total offset value. I divided that value by two, and found/made a shim that (half) thickness. Then I took the whole darned thing apart, until I could loosen the four bolts holding the carriage onto the z-axis leadscrew follower block. I put the shim under the bottom part of the interface between the carriage and the follower block's aluminum plate (yes, and then reassembled/tightened it all). Why half? Because that's the approximate ratio of vertical to horizontal in this instance. If we used a shim equal to the spindle-top offset, it would be moved L/H*offset, which in our case would be just about double what we want....thus the half :D.

The first step go me within about 0.010" total top-to-bottom offset, and the second step got it under 0.001". But, that's the static position, and you'll still deal with flex in the system as the cuts are made.

To get rid of the ugliness entirely, I started doing two things: using radiused mills so the edges aren't perfectly sharp corners, and adding a finishing pass. Heck, even re-running the finishing pass exactly a second time can help with the issue. But in the end, it's fundamentally a flex problem :-(.

Hope that helps; my solution was a bit extreme, but it worked for me making those aluminum parts.

Okay, last one. It's also feasible that reducing the (vertical) height difference between the clamp-plate and the spindle's bottom bearing can help with this, by reducing the flex/compliance in that one spot. In my case I needed the positioning I'd chosen, so this wasn't something I tried. But, if your router is pushed way down, then (where feasible) moving it up might make the cuts have less flex and misalignment.

And, of course there's always the tried-and-true technique of slightly loosening the bolts, applying percussive maintenance with a deadblow hammer, and re-tightening. I use that sometimes to get rid of the last couple thousandths of offset, but it's evil like me ;-).

Regards,

Thom
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