Conventional pass .015 too small

[r_jones]

Now I know my conventional pass will run a touch smaller than my climb pass but I am a heavy 0.015 plus or minus on each side. This adds up to almost 1/32 which is unacceptable for the parts that I am cutting. When I created an offset line by this amount in part works and recut the part it was dead on. I have a 48x96 PRS Alpha with a 2.2 spindle. The spindle is new maybe a month or two old. Any help would be appreciated.

Thanks,
Ron

[dlcw]

Ron,

I've found that when cutting parts out I cut all but the last 64ths using a climb cut pass. This pushes the bit away from the cut line. I then make a second pass using a conventional cut to clear the last 64th and this gives me a finished part cut exactly on the line. I use this in both SBLink for cabinets and for cutting parts outside SBLink with Aspire or Partworks.

[r_jones]

Don,
Do make all your parts 1/64 larger? At this point this is essentailly what I will need to do. As it stands my last pass convential cut ends up being 1/64 inside my part line rather than right on my line.

[dlcw]

Let me try wording it a little different.

What I do is create two tool paths for the part cutout vector. The first tool path goes in a climb direction for vector but I don't go all the way through the material. I leave a 1/64" skin. The second pass goes in a conventional direction on the same vector and cuts through the 1/64" skin left from the first pass. What this does is - on the first pass the bit/machine is deflected away from the vector because of all the material that is being hogged out. On the second conventional cut, since the bit is removing only 1/64th" material there is very little, if any, bit/machine deflection. So on that second pass the bit will stay right on the vector and not deflect either way.

In other words you first cut the vector one direction and then cut it again in the opposite direction. This compensates for the flexing of the machine. I have a PRSAlpha 9660 with 2.2hp spindle and air drill and my cuts are dead on when I follow this approach. Is it ideal - no. Does it add time to your cutting - YES. But we don't have $500K machines to work with and have to come up with these little workarounds to get what we want. :rolleyes:

Hope this is a little clearer for you.

[r_jones]

That's exactly what I am doing and they way I understood the concept. However, on my second conventional pass I end up 1/64 too small.

[dlcw]

Out of curiosity, when you make the first climb cut, and no second conventional cut, is the part the correct size when finished? Has this always been a problem or has it just reared up after a long successful run of cutting parts accurately? Does the size discrepancy favor the X or Y axis or is it the same on both?

I'm beginning to wonder if there is some accumulated flex in the machine causing this offset to happen. I'm sure you've already done this, but make sure your the racks and pinions are tight and there is no slop there.

How many hours do you have cutting since your last comprehensive tuneup on the machine? I normally do a comprehensive tuneup weekly on my machine. Check all nuts and bolts, clean V rails and V-bearings. clean up all the old squeezed out grease and put on new grease, check for square and plumb, etc.

The only other thing I can think of is that your configuration values somehow got knocked out. Go into the subdirectory:

Program Files\ShopBot\ShopBot3\Settings\PRS ShopBots\PRS_ALPHA Tools and run the SBP file for your machine to reset these values.

[r_jones]

Great questions Don!
My first climb cut is that little bit over as you would expect and does not favor an axis from what I can tell. From what I can tell this started when I added the spindle which seems odd but...

It appears that the rack and pinions are still tight. The machine is only 6 months old and hasn't seen all that much use, maybe 300-400 sheets and 300-400 parts maybe a bit more. I am not sure I do a weekly tune up but check things out after every job or so.

I will try and reset the values and see what happens.

Thanks so much for the input!

Ron

[r_jones]

So I reset to factory defaults, no change. Same difference in both x and y directions:( Checked the rack and pinions and the best I could tell it was off 0.005 plus or minus. I am not sure if this is within specs... it could account for a difference in the x but not the y. I am using a brand new 0.375 DLC compression bit that is dead on the 0.375.

[dlcw]

300-400 hours I would have done about 8 to 10 comprehensive maintenance cycles. But I'm kinda booty retentive about maintenance.

Are you driving your machine faster since you moved to the spindle (from a PC router I assume)? If so, it might be that there are very small, but cumulative, problems that were not present when you were feeding at a slower speed.
What it sounds like it might be is some flex/slop in the Z car. The car is somehow flexing on the gantry causing the size shift. I would make sure there is no grease buildup anywhere in the Z car rails. This can cause problems. Tug on your spindle at the collet and see if there is any movement. If so, it needs to be corrected.

Since the problems began when you changed to a spindle, I would focus attention on the spindle mount and the Z car for flex. Secondary is very small, but cumulative movement, in other parts of the machine.

[r_jones]

Now that you mention it the problem have probably been there for awhile. When cutting with the router I did have pieces that were inconsistant but I figured it was the router. Now that I have the spindle my cutting is much more consistant and so is the error in cutting.

I am running the spindle a little faster but not that much. In fact the test cut I just did was only at 3ips.

The Z car is pretty clean and tight:)

Any chance this could be partworks related? I changed the amount of stepover to 0.1. What is that "Allowence offset" setting?

[dlcw]

I've not had cutting problems with Partworks or Aspire. They have both cut dead on for me since I started using them.

Out of the manual:

"An Allowance can be specified to either Overcut (-ve will cut smaller) or Undercut (+ve will cut larger) the selected shape. If the Allowance = 0 then the toolpaths will machine to the exact size."

So if you have an inherent problem with oversize or undersize cutting then you could use this to compensate.

Personally, I would go through the machine with a fine-tooth comb to figure out why it is not cutting right on the money. That would drive me nuts until I found the problem...:eek: But that's just me...:D

[r_jones]

Sounds like a great work around but yes, something doesn't appear to be correct. Hopefully I can find something and report back on my findings