Taking some slop out of the Bot

[chunkstyle]

Hi all,

I'm finding I need to dive back into my PRS and straighten my X rails. I've contacted SB to see if they had any suggestions for this recurring issue. This will be my 4th time at it since I've had machine running (somewhere in mid 2011).
It becomes noticeable at edge bander when the Bot starts cutting a slight arc (from slight arced V rails) and the middle of the panel's edge gets skipped by the glue roller. Checking the panels edge will reveal that curve getting cut by the Bot. It has been found to be a creep out of alignment from the aluminum extrusion that the rack and V rail is attached to. It creeps out of alignment.
Has anyone dealt with this problem and found a more or less permanent solution for their machine? Production has slowed down some so I will be going after this problem in the coming week. but I thought I'd ask and open up a conversation on how to keep the PRS running tight. SB support has asked me to send pictures of how I align my rails (stretched fish line and feeler gauges is how I've done it and then tighten them bolts down). This has been good for about a year.
Attached is a picture of what I'm getting after cutting some panels apx. 92" long. One panel is flipped and edges butted up to magnify the introduced error of the Bot. Showing at the butt is a heavy 1/8". That would imply the X rail is out apx. .0625" in 92". This, what we used to say, blows.
I've been toying with the idea of using Loctite between the mating surfaces of the Bosch extrusion and pillow block. It would be reversible (applied heat) but hopefully would not be needed. I've been running belt drives on my machine and the 20t aluminum timing gears have been mounted to the motor shafts using red loctite and show no signs of letting go. This has made me wonder about using it on X rail/ pillow block joint. Anyone else find a reasonable solution for this particular problem?

[chunkstyle]

I will say that one thing I've encountered after upgrading the Steppers is I've experienced no more slipping of the pinions out of the rack that I would often get in the standard .25" steel motor mount plates.
My thinking is the fly cut pattern in the belt drive end plate that contacts the painted Y car end plate 'bites' in. You can see the mill marks in the plate steel in the attached photo.
I would have thought that the pillow block finish would have given me some 'bite' into the extrusion similar to the steel mill marks in the Y car end plate. But it doesn't appear to be the case. In any case I would like to get a more permanent solution. At least get it under .0625" in 12 months. I think it should be doable. Leaning towards the Loctite at the moment. Some kind of reversible chemical bond seems favorable. I'll post up more pictures as I go after my rails. Might loosen up an idea from somewhere. I'm open to ideas.

Thanks again,
Tim

[Brady Watson]

You cannot take anything for granted. No mental responses like, "Oh I know that can't be it." Question & check everything.

The machine must be absolutely level.

The table should be checked for square using the 3-4-5 method.

Loosen all of the rail screws down both sides just to where they allow the rail to be nudged in or out. If you take them all the way out, watch your feet :eek:

Slowly roll the gantry down with your eye in line with the rail as you go. Do this several times for each side taking note if wheels lift or are not contacting the rail properly. Select the straighter of the two rails and lightly snug down the bolts starting where the gantry is parked at one end & let the gantry weight be near the next bolt you snug down...repeat until you get all the way down.

Go to the loose side and repeat. Adjust the rail in or out as needed. When you get to the end, go to the other side. Eyeball the v-roller to rail contact with scrutiny. Back and forth from one side to another roll & check - repeat. Make small adjustments as needed - no 'forced' adjustments whatsoever - EVER - it should be nice and smooth all the way down. If it isn't - STOP - and observe. If you look hard enough the problem will be revealed.

It is possible for one wheel to be sitting up with the other 3 down...that's a different adjustment - get them parallel first.

I can't comment on your drive system since it is non-factory & I have no idea what you have going on in there. I would get it sorted out sooner than later because that can really affect your cut quality. It may sound over-cautious but a dragging belt stretches...and causes positional deviation...and slop.

-B

[chunkstyle]

Thanks Brady,

The advice you have outlined is very similar to what I've been doing. I normally jack up the gantry, getting it off the rails entirely. I check that the rails are co-planer by stretching fishing line, corner to corner, as you would to check door framing. I get the lines to touch, no matter whicj one is crossing over the other.

Next, stretch the fish line down the better looking V-rail, lower the gantry onto the rails and follow your prescription with regards to the rest of the rolling and checking the opposite rail. Shimming up and down is not needed. Only side to side for the last two times I've had to repeat this 'adjustment'.

I'll finish with a milled straight edge held against the V rails side and shining a flashlight behind it looking for any Gaps to tweak. Then I proceed to sighting down the V rollers while moving up and down the length. Finally, I do a test cut, flip and observe since it is the final result here that matters. Are you figuring this re-adjustment as a yearly routine as I have found it to be (edgebander lets me know if I don't check it in time) or are you maintaining tolerances longer than I have found them to hold? I'm wondering if I'm falling in the realm of normal or do I have something else I need to address?

Belt drives are the garden variety 3-1 XL timing belt and pulley style that you see all over the intertubes. A worthwhile project if inclined.

Good point on the belt dragging. I have spent some time soda can shimming the stepper motors to get the belts tracking true but there is one that still squeeks and I have it on my list to fix.

What I have noticed since running them is the lack of slipping I had gotten from OEM motor mount plates. I have run about a unit and a half of sheet stock thru it and there is still no slipping of the pinion out of the rack. That was a happy side benefit to the mod and much improved from the 10-15 sheet durations, making it necessary to go in and re-tighten X,Y pinions.

Had a waitaminnut moment thinking about the upcoming X rail readjust. I'm acquainted with getting the rails straight but curious about how to extend the level of tolerance I'm able to achieve longer. I think I will try spotting some locktight around the mating surfaces of the pillow block and extrusion at a few select bolt locations. Don't see what it could hurt. Who knows, it might even help. Very curious to know how often others have had to make this adjustment.

Thanks again Brady, your advice is always appreciated.

Tim

[Brady Watson]

No problem Tim. I'm sure I have set up at least 100 or more new PRS machines over the years. I always get one straight & let the weight of the gantry pull the other side in. You have to 'true' it like spokes on a wheel until it is perfect. You really want the weight of the gantry on the rails when you do it because there's no other practical way to get it right.

This is pretty much a 'set it and forget it' thing. Rails don't typically go out of parallel or adjustment. This is of course not barring people running forklifts into the machine or slapping material on the rail all day long. I don't think that Loctite'ing the rail bolts is necessary & would look into the reasons why those rails could be moving.

I'd take a close look at the center legs of the machine (if equipped) - it's real easy to have the machine 'high sided' where it imparts some stress on the rest of the chassis. This falls under the levelling operation...Just level the outer 4 feet to get machine level, then let the center feet touch the floor and turn it about 1/2 turn to take the bow out of the rail/extrusion. Eyeball the sag...If you don't have center legs on a 4x8 or larger PRS - that may be your problem right there. The early ones didn't have them...

How's table flatness in the center of the table, say X48? Do you notice that this spot is high or low? How thick is your spoilboard at X0, X48, X96? (assuming you have flattened it recently)

-B

[adrianm]

Very curious to know how often others have had to make this adjustment.

Since putting the machine together in 2009 I've never had to adjust the rails.

I check them as part of my monthly maintenance but they're always fine. I do have the centre legs on my bot though.

[chunkstyle]

Thanks Brady,
I will get the one rail straight with the fishline/feeler guage then drop the gantry down. Generally speaking I put a clamp on the straightened rail side of the gantry and work the other sides' rail using the v rollers. I will photo my steps this week and if you spot anything I would really appreciate any observations you may have.
I will recheck the middle legs. Thanks.
Ditto on the table flatness. I have to borrow a friends milled straight edge to allow me to see any humps or dips. Im encouraged from what you and Adrianm have experienced. There should be an answer to this reoccurring problem. I just have to find it.

Much appreciated.

Tim

[Brady Watson]

No problem Tim. Keep us posted.

-B

[chunkstyle]

Well it's only been, what..., a year?!

Anyways, just ignore the curve, there's too much to do. Open the gate on the glue pot a little more to compensate for it at the edge bander.....
I finally couldn't ignore it and, although I don't really have the time I was finding that ignoring this problem until 'I can get to it' was costing even more and I decided to finally tackle it and here's what I did. If anyone should have any comments or concerns of my approach please don't hesitate to say so. I'm all ears.

Started with getting all the squirreled wood out from under the bottom of the machine.

Checked to see if the near rail was level. It was.

Turned in both middle legs.

Set up 2 monofilament fish lines and ran each kitty corner, forming an 'X' over the middle of the table.
Cranked on the far rail legs to get the two lines to just touch. Line is .010" D.

Once that was done I cranked middle leg's feet down to make contact with floor and added 1/4 turn as per Brady's advice.

[chunkstyle]

Checked the near side X rail's straightness running front to back by placing a long level next to the side of the rail (resting on the top edge of the Bishop Wisecarver rail's screw heads) and checked for any gaps between the rail and the level. Results are shown

The end of the rail (looking down towards 0,0) is bending away from the level. This rail has been the usual one found out of whack.

I proceeded to loosen the button head bolts clamping the X rail extrusion to the X table rail. Moved the X rail in and out to get any gaps to disappear by checking the gap with a .004" feeler guage (also known as a post it note).

Did did the same for the up and down on the rail and found I had to shim in front of and behind the middle leg area of the rail using the blue plastic SB shims. Think they were .005" thick shims.

Jumped over to the opposite rail and found I had to shim it similar for up and down with the same shim thickness.

Finally, I moved the gantry up and down, tightening the far X rail down when I could see the V rollers riding evenly on it, taking it one button head bolt at a time, tapping the rail around to center it.

[chunkstyle]

Tested the result by running the 2 long rectangle pieces again from some 5/8" thick particle board cover sheet on my next cut jobs load.

Pieces were numbered 1 & 2 and marked to tell how they were oriented together. They were laid up to each other with the second piece turned over. This resulted in a noticeable gap at the far end of the seam as shown.
About a skinny 1/16". So I'm guessing there's still a 1/32" bow in one of the rails? I'm kinda disappointed with the whole time consuming result and wonder what else I could do to try and find this Very annoying inaccuracy. What are others able to achieve with their machines and would anyone have a suggetion on what I missed in my alignment.

[bleeth]

" What are others able to achieve with their machines and would anyone have a suggetion on what I missed in my alignment."

After getting to where you are now micro adjust further to improve.
In order to get cutting results I was comfortable with I use a straight-edges for the y that I paid to have a mill shop mill a dead straight edge on and for the x adapted a "guaranteed" straight edge of a custom extrusion made for wallpaper hangers (and yes-they are dead straight) and set my trusty old level aside for a 4 foot digital of reputable manufacturer.
I've learned to plan on dedicating a good 4-12 hours of work to tweaking in level, square, and parallel in my frame when it needed it. Part of owning a tool put together with bolts and few welded parts. Facing reality: There are more nuts and bolts holding together a SB or SB style tool than in any of the decently made similar but welded and milled tools. At the same time they cost 2-4 times as much. Most botters don't need the kind of accuracy a typical anal cabinet maker wants. Getting that accuracy is pushing the limits of capability of the design. But pushing it is not exceeding it. It will do it. It is just a real pain in the ass to dial it in so you get it.

[curtiss]

Years ago when setting up, I made a simple hardwood T square for squaring up /with a digital caliper taped to the far end.

A saw groove across the top of the T sat on one rail.... the "depth guage" worked as a "feeler guage" to get the opposite rail exactly parallel.

[Brady Watson]

How's the router mount/mounted?

There's prenty of room for slop above the X rails...

Chuck a long rod in the router and put an indicator on it. Power up the machine. Now push on it in X. Move the indicator to measure in Y and push on it in that direction. Does the indicator move?

What about the Z axis? Move it all the way down. Wiggle it (leave power on to provide holding). Move it up 1". Wiggle. Another inch. Wiggle. so on. Feel any slop? Measure any movement/slop with an indicator? Those Z axes don't last forever...they are NOT hardened and can get galled and wear out. On PRS tools the lower Y rollers are too loose on most machines...but I think you've checked that already.

Sanity check - there's slop somewhere...you are not crazy....you just haven't found the culprit yet. You've got the X rails tip-top - bravo - keep investigating. You WILL find it and have a tight machine.

-B

[chunkstyle]

Thanks Guys for the fresh eyes. I will continue to get at the slop, wherever it's living, and correct it. At the moment I have stalled off a cutting job that NEEDS to get out the door so I will run it as is. It has improved from where it was by checking the 5/8" test cut rectangles against a coupla cab parts run off before I jumped into this. I feel like the machine is battling for my soul at times :P
First thing I want to make is a coupla jigs that can stretch a fine fish line down the whole length of the X rail and confirm it's straightness without having to rely so much on the 7' level.
I will also check the Z. I have a nagging suspicion that you are on to something here Brady. What would be a realistic target of slop here? The "adjusting the PRS Z-axis Bearings" documentation from SB is vague about it. They mention observing any lateral inconsistencies..... whatever that means for a tolerance guide.

This is all about the dollar. If the panels are getting skipped in the middle of the edge, due to a subtle curve, and the edge banding is 'dry' then rework has to be done. That's money as you guys know already. My edgebander is setting the tolerance for straightness here,
Thanks for the advice. I particularly like the reference to sanity. Had several 'SERENITY NOW!!!' moments along the way.
I'll report my findings on the Z.

[Brady Watson]

The Z on those things consists of a profiled C-channel on each side of the Z and 3 (or 5 I forget) V-rollers that run inside the channel. There is an adjustment for it, just the same as the lower Y bearings, as they are on cams. They should be snug but not too tight as to not roll forge the channel wider.

Realistic slop should be low - I'd say under .008", but closer to .005" at the spindle with mild force. The focus should be on clunking, jiggling and how it feels as well to how it sounds when you muscle it all around. The indicator of course will tell the true tale.

-B

[chunkstyle]

Thanks Brady,

Rolled thru the next cutting job then last weekend cold snap settled in. Perfect time to look at the Z slide.
Chucked a 1/2" peice of drill rod stock into spindle with about 3 1/2" extending out of collet. Brought out the dial indicator and measured any play and it coincided with your numbers if my 'force' at .006-.008" is about the same as yours. Hard to say at this level of manual checking for play so I went ahead and applies a little 'snugging to the lower and upper eccentric bearings in the Z slide. Jut a tad more on the bottoms than the top bearings.
Checked indicator again and there wasn't much change in the dial indicator when play was checked again. Z slide was a bit stiffer going up and down from before adjusting the eccentric bearings but still smooth and even thru travel.
Checked X rails again with 7' level and post it note. Still good.
Decided I needed to SEE that the rails were straight so I threw together a coupla quick fish line jigs. Nothing fancy but they did show me what the rails were doing by eye.

[chunkstyle]

Though I didn't take a picture of it, the first thing that caught my eye was that the rail sagged in the middle a heavy 1/32".

SAY WHUH?!

I checked and rechecked up/down, left/right deviations of those X rails. All with the 7' level and post its though.
Hmmmm.....

Rechecked other, near side, X rail and it had the same sag. Readjusted both X rails until they were within a fishline's diameter along the entire length. Some shims were taken out and a few relocated.
Waydumminit.... If the level's got a slight bend that will mean the X rail adjustment for left right straightness will be off, no?
Made sure the line was directly over and barely above the top of the X rails and visually inspected the line the entire way down the line. Some curve was present in the middle of the rail. It was Tapped out and re-tightenedhttp://www.talkshopbot.com/forum/attachment.php?attachmentid=27326&stc=1

I had to run out and find the reading magnifier but it sure helped increase accuracy of inspection.

After I assured myself that the rail was parallel to the line I pulled the Z car over to that side, keeping the spindle weight on the rails that were going to serve as reference for adjusting the other side.

I loosened all the mounting button head bolts on the opposite side X rail from the straightened one and worked out from the middle as the SB manual describes and Brady recommended earlier in this thread.
Ran car up and down rail after adjusting 2nd. X rail to the first using the Gantry. Almost rolled my nose over with a V whel several times but I saw no uneven riding of any of the f Y car V rollers. All 4 made even contact along the entire length of X rails.

Put machine together and made test cut in some scrap 3/4" MDF.http://www.talkshopbot.com/forum/attachment.php?attachmentid=27327&stc=1

[chunkstyle]

Between Putting machine together and making a test cut as shown in previous attached picture Ma nature had a big laugh. After I got machine together and broke for lunch the power went out and didn't return until yesterday afternoon.
After getting the test cuts made I spun one board and laid it up next to the first to seehttp://www.talkshopbot.com/forum/attachment.php?attachmentid=27328&stc=1 what I managed to achieve by all this wrenching.
Here are the results:http://www.talkshopbot.com/forum/attachment.php?attachmentid=27329&stc=1
This is in the middle of the two boards and shows a skinny 1/32" to my watering eyes.
That's what I was able to get after ditching the level for a reference edge and going with the taught fish line instead. An improvement over where it started:http://www.talkshopbot.com/forum/attachment.php?attachmentid=27331&stc=1
The only questions I have are;
Is it possible to get it even closer to a 1/64" deviation in 94"?

If so, is it worth it or will the point of diminishing returns come into play here as it relates to what would be required?

It remains to be seen how long it will hold this straightness. The edge bander has a way of letting me know when it isn't.

[Brady Watson]

Yes it is worth getting it as dialed in as much you can. That tool is advertised to be good for +/- 0.015", which is about 1/64".

Regarding 'rail dip' this should be attacked via the center leg adjustment feet & not shims. Shims should be a last resort. The sag doesn't directly affect the XY deviation, so I am wondering what is going on there. It is completely possible to camber/bow the length the table sides out if the machine wasn't put together and trued up properly. In other words, the center legs could be too far +/- Y in comparison to the outer legs. This would account for a bow in the rails.

Eyeball the table sides and put your straightedge on it. See if they are straight or bowed and which way the English needs to be applied.

Be sure to LOCK DOWN the nut on your feet...

[chunkstyle]

Thanks for the input Brady,

IF the machine's specked at +- 1/64" over length then have I reached it's point of diminishing returns?
By spinning one of the boards around and placing it next to the other one, isnt any resulting gap between the boards a doubling of any deviation from a straight line? If so, then it would seem that I have gotten it as far as factory tolerance? The pic of the MDF cut results show a gap in the middle of the two boards (one of which was spun 180) that is apx -1/32". Divide in half and were at the 1/64" deviation from straight. Would this mean then that I'm out of the rain and on the porch as far as the house of shopbot PRS is concerned? Do you think it could get better than this?
Also, with respect to the rail dip, I had to take out shims after it was found that the middle sagged and the middle feet had to be lowered. One side needed none whereas the other side needed a couple of shims still between the middle and end legs. I think the MDF test cut board comparison picture show what had to be done to that one rail, shim wise. Does that look like typical shimming?
As far as the X table rails being bowed, even if that were the case (and I will check on it now that the thought is in my head) wouldn't the amount of 'slop' in the mounting of the X rails to the top of the table rail allow for any deviation here. If the X rail is straight to the fish line then would that take any 'bow' of the x rail out of the equation?


Machine does run improved now and you can hear it in long X jogs. Noticeably less rumbling and more evenness. Still, if it can get better than this and stay put after getting it there then I'm game to keep going.

Thanks for any insights into this dialing effort from this board. It's greatly appreciated.

Tim

[bleeth]

IMHO the SB tolerance has to do with distance and not trueness of cut.
A straight line should be just that and "micro tweaking" should solve it. Since you are looking at a bare 64th of deviance (I agree with your math) you will likely have some difficulty seeing where the bow is on the rail itself but may have an easier time by using the cut boards as a guide. Since your rollers are now tracking so much better it would seem that you may have a consistent bow in both sides. Easily explained by one rail having a slight bow and the other made so well parallel to it.

The fly in the ointment is that a slight dip or rise in one of the rails can have the same affect, as raising or lowering of one side slightly will cause the gantry to tilt a bit and lead to the bit moving towards a high spot or away from a low. You might run your mdf test near y,o; y,24, and y,48 to see if the gap remains consistant. If it does than it is more likely a slight bow and if it is different than more likely a slight hump or sag.


FYI: Although the all extruded side rails are inherently more true than the old steel C-Channel the sheer strength of the C-channel helps keep things aligned once adjusted IMHO.

[daski]

If the work you do primarily involves melamine and particle board then stop fussing because the tension in the board is more of a factor than the straightness of your rails on the final straightness of the cut. Your bander is designed to band board that has been cut using a saw that will always have a far less straight cut that what you have already achieved on the Bot. If you aren't getting good banding results with the cut like that of the Bot... fix the bander it's the one with the issue! Remember you're building cabinets not doing eye surgery with the Shopbot.

[chunkstyle]

Hmmmm......
A bander that has been designed to take table saw cuts.... I'm gunna have my doubts on that. Even my friends 14" lagazzi wasn't tight enough for his holtz-her. I'm running an scm 201. What brands of edge banders are you referring to here daski? It's always been my understanding that edge banders require a straight cut. Less than that and the edge of the panel will wander away from the glue applicator. The only remedy will be to open the glue metering gate to provide more glue to the edge of the application roller in order for the glue to get picked up by the wandering panel edge, correct? More glue than what is necessary makes for a lot of glue slinging under the hood, not to mention the extra work the end, top and bottom trimmers have to do. Then all that clean up after a run of parts.
I believe the sweet spot of the amount of glue application is that you apply no more than is needed to get full coverage and a good bond. Having the pressure feed rollers adjusted correctly is also important. To little and you will get dry spots, to much and you get what I have.
Here's an example of what I'm talking about for anyone interested
My bander, a k201 that has far more machining with tighter tolerances than my bot:
http://www.talkshopbot.com/forum/attachment.php?attachmentid=27421&stc=1

Parts run yesterday and banded at the end of day. Longest parts are saved for last as those are the ones that have the worst deviation from the Shopbot. Try running at the glue gates current setting for the shorter cab parts (all of which banded fine):
http://www.talkshopbot.com/forum/attachment.php?attachmentid=27422&stc=1

Well that's not so good! Time to 'open er up' and start the glue slinging! As you can see in following photo, there is proper glue application on the leading and trailing ends of a panel. Notice how the banding has chunks of the core material stuck to those areas whereas the middle of the banding has very little.
http://www.talkshopbot.com/forum/attachment.php?attachmentid=27423&stc=1

The front and rear of the banding are getting proper glue amount without any unnecessary glue.
That bow in the panel is similar to the bow In my test cutting while I struggle to bring my off the shelf Bot parts, packed in a box to be later assembled, into tolerance of my bander.
Lastly I will say that for the first 5 yrs of my cabinet existence, I was having my cab parts milled on a Thermwood. I did not see ANY deviation from straight with two long parts butted up to one another. I'm not kidding myself into thinking that a bunch of off the shelf parts bolted together in my shop is going to even come close to the accuracy of a Thermwood. But I will say that any melamine cut on my Bot that has a bow in it is being introduced by the Bot and not from any stress introduced from the panel cutting.
I'm just trying to get my machine to run as 'true' as a Shopbot can be reasonably made to run in an effort to get consistent results at the bander without having to dive under the hood or pour on the glue.

[bobmoore]

You may want to have a machine shop dowel pin the v rail to a more substantial plate of aluminum. Machinists use dowels for accurate placement of things. I wonder if you aren't getting flex from the difference of expansion and contraction between the steel bed and the aluminum support. You can't tighten bolts enough to stop that movement, something will always give.

[bleeth]

Although I sold it when I moved to a smaller shop and miss it dearly I ran a Holzher Sprint for years.
Parts were cut on everything from a Unisaw to a 10' slider to the bot. As long as bander was adjusted correctly we never had an adhesion problem, including on long panels.

I totally am with you in wanting to tweak the "curve" out of it but you may want to play a bit more with adjusting the tape feed pressure roller as well.
I don't know that the "little" SCMI is more or less challenging to dial in but I do know a lot of folks using it and swearing by it, particularly the plus and k-203 versions.

[chunkstyle]

Perhaps my problem is running a glue pot instead of a cartridge glue system? Was the Holzher Sprint a cartridge bleeth?
My lil K201 does all right and will run all day without much fuss. Cleaning it after every run seems to do the trick. There's a sweet spot for the amount of glue applied to the applicator roller that produces minimal squeeze out after the pressure rollers and cleans up nicely from the trimmers. Unfortunately it's to light a coating on the roller for any pieces longer than, say, 42". That's why I roll the longer pieces at the end of a job with the glue gate opened up.
It's annoying to me to see the cab part getting pulled thru the bander with the leading edge and trailing edge making contact with the bander's guide fence but the middle bellies away. Opposite contact is made if the cab part's edge is on the convex side of a long part.
So getting realistic, confirmed expectation of 'trueness' from the PRS is a work in progress and will ultimately rely on the owner's resourcefulness to come up with whatever level of accuracy is needed. I wanted to share what I had done and will do for getting that last 64th dialed in.
Also, I wanted to get an idea of how long it could be expected to hold that level of accuracy from others that have gotten their machines as tight as what I'm trying to get to. I'll get into it some more, when things calm down a bit in the shop.
Bleeth, I think your suggestions will be helpful and I'm planning on bringing some more MDF in for test cutting in different spots in order to examine how the 'trueness' changes across the table, as you suggested.
Again, I don't know if this is pushing past the point of diminishing returns of what can realistically be achieved and maintained.
Gotta ruminate on it a bit, come up with a plan of attack....
If anyone has any good tips or techniques I'd be very interested and appreciative to learn about them.

Thanks again for all the help,
Tim

[Ajcoholic]

I dont know how it would be any different with the CNC, but when I cut 3/4" melamine on my panel saw, at least half of the sheets move (usually pinch after the riving knife) due to case hardening. I know if I again take a cut to straighten the edge, it stays put and doesnt move again. To my understanding this is pretty usual when processing melamine panels. If I try and edge band a concave edge, my bander too will not do a great job in the low point.

With the saw, I rip the sheets slightly oversize and recut. Since I dont cut parts bigger than 48" on my CNC I dont have any issues. But if you are cutting large gables on the CNC, do the parts stay straight or do they also move due to the case hardening?

[chunkstyle]

Hi Andrew,
I get the same curve in the same attitude of the parts wether they get cut from melamine, Mdf or ply.
curves always in the same orientation. It has always been present to a greater and lesser degree. Figuring it may be my approach to aligning the x rails is why I decided to start this thread. It has very helpful to have the advice of others more familiar with this kind if wrenching giving advice for where to look and do...
Its better than it was when I first started this down this road. I'd be very satisfied if it holds it where it is at present. Where it's at is probably good enough for most. Again, I may be going past the capabilities and expectations of what's realistic here.

[bleeth]

Yes-It was a cartridge system.

[chunkstyle]

O.K., it's been about 40 sheets of melamine, MDF and plywood since I finished adjusting the rails using stretched fish line. Finished cutting some mdf parts this morning and decided to check some 6' parts that were run side by side out of the same sheet to see how things are holding up. Here are the results:

Location of parts on machine
http://www.talkshopbot.com/forum/attachment.php?attachmentid=27437&stc=1

One part spun 180 degrees to the part and edges were butted up
http://www.talkshopbot.com/forum/attachment.php?attachmentid=27439&stc=1

Resulting gap
http://www.talkshopbot.com/forum/attachment.php?attachmentid=27438&stc=1

This looks to be about a skinny 64th of an inch between 5' long parts in MDF. Meaning I have 1/128" of deviation in 5' at that location on the table. Pretty good!
Also happy with the accuracy after pushing a unit of material thru it and getting these results. I'm cautiously optimistic about the long term accuracy. For whatever reasons and limitations I have, the stretched fish line jigging was easier and quicker for me getting one rail straight. Should the rails need a 'tune up' down the road I think it will also be quicker than using a feeler guage and straight edge. Others might find the opposite true.
If it does maintain this level of accuracy over time, I see no reason to try and get rid of that last 1/64th over 94" deviation. It will give me some time to ruminate and devise a strategy that will be simple and, hopefully, effective.
Onward!

[chunkstyle]

Whoops!
I incorrectly stated a 1/128" of deviation in 5' in the above post. Should have wrote 1/128" in 6'.
Panels in the above pictures were 72" long.

I also meant to say 'I see no reason NOT to try and get rid of the 1/64" of deviation.."

Typing to fast to get to lunch break.