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Thread: Let's Talk About Table Flatness

  1. #1
    Join Date
    Jan 2004
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    iBILD Solutions - Southern NJ
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    Default Let's Talk About Table Flatness

    So I thought I'd kick off some discussion about table flatness, consistency and versatility. I've learned a few things over the past month that I didn't know/encounter much before and thought I'd share some of them. Feel free to jump in and compare notes, ask questions etc. My company, among other things, is classified as a job shop. I machine just about any material in 2D and 3D, so versatility is important. By versatility I mean that sometimes I can get away with holding things down using some kind of clamps, screws or adhesive, while other times I need vacuum - and even less, but not out of the realm of possibility I have to bolt things down.

    My shop is between two rivers and weather-wise I encounter everything from hot & humid to dry and sub-freezing cold. As you probably know, every material you put your hands on has a coefficient of expansion, and wood-based materials also change size by humidity. Material also moves around while it is being machined because it is being heated up by the action of cutting and the surrounding shop air which is also heating up. This is why I never stop a long toolpath and go to sleep because it WILL change dimension overnight as things cool down. What I didn't know was how much an ultralight MDF bleeder board could change in size during a long cycle time (13hrs).

    My machine, 'Big Bertha', is a 5x16' machinable area PRT Alpha (7.2:1 all around). It started out as a custom tool with dual Zs, 5hp spindle on Z1 w/5x8' spoilboard and a Hypertherm plasma cutter on Z2 with 5x8' steel grid. I've since done away with the plasma and my current machinable area is 5x12 - if for no other reason than I can't get sheet goods larger than this. I've got a 2nd gantry parked down at the 16' end which I am going to bring back online. My current setup consists of 5x12' 3/4" MDO plywood as the support board, regular MDF for the vacuum grid (zoned out accordingly) and 5x12x 3/4" Trupan Ultralight MDF for the bleeder board. All layers are glued down including the bleeder (on top of grid squares AND zone perimeters)

    In days gone by, I used my own vacuum plenum design (BradyVac removable sheets) made out of Trupan UL and for a while a Fein Turbo III (@7Hg") as the vac source. When used properly, it worked pretty well, and still (via screws) gave me access to the MDF table underneath for screws, tape etc. It was OK when the vacuum was on in terms of being relatively flat and I could machine it flat with the tool. When I moved things to the new shop, I did the dual vac setup, which allows me to select between a 5hp FPz (9.5 Hg") and 10hp Becker (25.5 Hg") depending on what I am doing. Hey, why run 10hp when you can run 5?

    Ever since doing a fully dedicated vacuum system (which I really wrestled with - in terms of giving up some versatility) - I've had issues with the table not being as flat as I was accustomed to. My previous layup was MDO & Medex water resistant MDF - which I think they changed the formula on...and it was ROCK SOLID in terms of not moving around and staying flat. Now I'm finding myself flattening the bleeder more often than I think I should just to get things to be flat...for a little while (as in only hours). I recently did a 8'+ long filagree panel in 3D which took 13hrs to complete on the finishing pass alone. After doing a 3D roughing pass that left .03" allowance on it, I finish machined it with a 3/16" ball @ 8% because it had a lot of compound curves to reduce tool marks. About 1/2 way through the finishing pass, I was seeing that it was barely taking off that .03" of allowance - which meant that the entire table was sinking in...I found myself doing some black magic, stopping the tool every .05", nudging the Z down .003" each time until I was able to get that .03" back. When all was said and done, the table had sucked down a full .14" (YIKES!!!) across only 24". There's no way I could have put my name on that piece so it's hanging up on the wall of shame...

    I wound up machining off what was left of the bleeder board and machining the grid flat. I glued down another piece of Trupan Ultralight and then machined it flat. I did the entire job over only to find to my extreme displeasure that it was doing the EXACT SAME THING - albeit a little less at .05" creep. I was able to fudge things a bit and it all worked out - but the 'builder's curse' still remains where I know all that is wrong with it even if the customer doesn't. (They literally high-fived each other in the office - which is next door to the Ferarri dealership...when they saw it - which erased some of my shame.)

    So...I made some calls to some good friends who also do CNC routing professionally and we talked about the issues. My one buddy's shop is in Miami and he routinely cut 40-60 sheets at a time. He told me that it wasn't uncommon for him to replace his bleeder every 3 days (!!!) because the bleeder would either swell up, compress under the vacuum suction or dry out and change dimension in the winter - or a combination thereof. He was running a few Lighthouse vacs (9Hg")and was having this happen...it could only be worse with higher Hg". So what do the $500k+ CNC router machining centers use then? How do they get around this problem? Density...and more vacuum.

    The big machines use several Becker vacuums (or similar) that pull 25 Hg"+ to get the CFM up AND they use regular old MDF. When they have all vac zones open, they pull 12 Hg" or so with nothing on top of the table - for a delta/useable vacuum range of about 13 Hg" out of their max 25 Hg". 13 Hg" is usually plenty to hold down sheet goods through a bleeder. THis makes sense that the higher density MDF will be more stable - and if I really take a look at the Trupan UL, it is about as free-flowing as anything out there. It would probably make a good air filter! So I think I'm all done with ULMDF - which is OK for guys running the Lighthouse/Fein setups. It just crushes down so easily under the suction of the Becker. It also must absorb a lot of moisture and in turn shrink from getting sucked out.

    Just because I've been doing this for a while doesn't mean I don't get schooled from time to time The important part is learning and moving forward. One interesting thing is the little SB Desktop machine. I've had that for (I think) about 4 years now. It has the Bosch extrusion - same stuff used on the PRS table sides, as the 'support board' with a piece of 3/4" MDF bolted down every foot or so, with countersunk bolts. Now it's just a small machinable area (18x24") but that spoilboard is totally rock solid. When I flatten it, I only have to take off like .01-.015" and it's dead flat again with all kerf traces gone. It's simply amazing compared to what I go through with the big tool.

    So I am strongly considering doing a similar setup on the 5x16. The MDO support board goes away & gets replaced with some aluminum bar that adapts the steel crossmembers to 1030 8020 aluminum extrusions - all the way across the 60" bed (20pcs). This alone will function as a torsion box, and I can get it as dead flat as possible using a dial indicator on the gantry. Parts to be machined can be clamped or bolted down using any number of clamps/fixures including gasketed high-Hg" vacuum pods.

    On top of the extrusions, a regular MDF table, consisting of 1-man manageable sheets/tiles can be bolted down just like the little DT machine. Parts can be screwed down with a nice soft MDF board to protect the cutter. There's no more need to mess with big 5x12' sheets - they can be all put together and made into one big sheet as needed and replaced individually.

    To keep the versatility of a universal vacuum system, the same system can be used. The MDF tiles can be removed and replaced with a BradyVac (BradyVac3 anyone?) I plan to use regular 3/4" MDF as the bleeder - machined BradyVac style about 1/4" deep for the grid. Then I'll glue on a 1/4" MDF backer with 2" hole for the vacuum port and flip the whole thing over. The same countersunk bolt scheme will be used (bolt holes coincident with 1" squares every 12" or so) along with o-rings under the fastener heads. The MDF backer can be sealed where it meets the extrusion and edges. Of course there will need to be a 3x3" piece of extrusion missing where the vacuum ports are...but no big deal.

    See attached pics below...tell me what you think...what you'd change/add/do differently. It's not a cheap endeavor @ around $2k for the extrusions, but confidence in the machine is paramount and I don't think I'd rest easy until I've done my best to get positive control over the situation.

    -B

    1x3ExtrudedBase.jpgCountersunkMDF_BoltedToExtrusion.jpgInvertedBradyVacGridwithQuarterInBackerBoltsDown2.jpg
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  2. #2
    Join Date
    Dec 2008
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    Diamond Lake, WA
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    Brady,

    I can say I don't have the weather/humidity extremes that south NJ has but I do have temp extremes. Hundreds during the summer and below zero for stretches at a time in winter. BUT, the humidity is pretty consistent. So with that said.....

    I put a 3/4" baltic birch plenum on my machine when I first set it up. I then carved the air channels out of the plenum and sealed it with about 5 coats of shellac. Also, before I bolted it to the cross-arm supports, I sealed the underside with 5 coats of shellac. I then put the 3/4" LDF spoilboard on and glued it down in specific areas. I have 7 zones in my table and each zone is its own piece of LDF. Around the edges of each piece of LDF I glued on PVC edgebanding. Of course I skinned the back side before gluing it down. My system is hooked to Gary Campbell's Blackbox vacuum system with 4 lighthouse motors. I don't mount anything directly on my spoilboard. When cutting plywood I use a 1/8" piece of MDF, skinned both sides, and put my materials on top of that. This serves me two purposes. One, I don't cut into my spoilboard thus creating vacuum leaks. Second, when cutting is done, the sacrificial board and material are slide off the CNC to an outfeed table for part sorting. I then put a new 1/8" sacrifical board on the CNC, put the next piece of plywood on and start cutting while sorting parts on the outfeed table. I cut plywood in two passes, first pass is climb cut, second cut is conventional. If the parts are small I turn on the 2 pair of vacuum motors to hold things tight.

    When cutting material that needs to be held using clamps, I mounted T-tracks to 1" MDO and mount the material to the MDO with clamps. The MDO is in turn held on the table using the vacuum system. With the MDO base being held by the vacuum, I've never had a problem with the MDO or the MDF spoilboard compressing. Granted I don't have enough vacuum to launch a jet fighter like you do...

    I have made it a habit that before the start of each plywood cutting job, I skin the spoilboard to make sure it is dead flat, relative to the gantry. I also skin the spoilboard prior to doing a long carving job. I've done several large mantel faces with 15 to 20 hours of carving time and haven't had a problem. But I think a lot has to do with the fact that I don't have the humidity variations like you do, nor do I have the vacuum system you have. But at 2500 ft elevation, I couldn't pull the Hg's that you pull. Even from day to night and back to day, there is very little fluctuation in humidity.

    Your table looks really high tech and quite honestly if you can compress that with your vacuum system you should market the vacuum system to the Navy to hold airplanes to the decks of aircraft carriers. I just wonder if your still going to have the spoilboard compression problem even with the great support underneath. Just my .02 cents.
    Don
    Diamond Lake Custom Woodworks, LLC
    www.dlwoodworks.com
    ***********************************
    Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece; But to skid in broadside, thoroughly used up, worn out, bank accounts empty, credit cards maxed out, defiantly shouting "Geronimo"!

    If you make something idiot proof, all they do is create a better idiot.

  3. #3
    Join Date
    Jan 2004
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    Don,
    Thanks for the feedback. I didn't seal anything other than the edges of the plenum and bleeder this last round because time was ticking to meet the deadline. In terms of the current layup leaking...I can put a 4x8 sheet of acrylic on top of the 4x8 zones and pull 22 Hg" - a few more if I put some plastic sheeting around the perimeter edges because there is always some bleed thru at the edges of the active zones. So I don't think I have any major air leakage, but that does nothing in terms of moisture absorption.

    I like the disposable 1/8" MDF trick. Not sure if I can even get 1/8" MDF from my supplier. I think 1/4 is the thinnest. I think I'd fill up the dumpster if I adopted that method though...I remember visiting a busy sign shop years ago and the operator was pretty well on top of things. He used a sheet of 1/2" Homosote of all things the same way you use the 1/8" material - except I don't think he flattened it at all. It pulls vacuum nicely and is cheap.

    Dividing the machining surface into tiles or pods of sorts makes sense for a number of reasons. First, I can move them around without difficulty by myself & without fancy contraptions...unlike a 5x12 footer! Another reason it makes sense, at least in my mind's eye/theory is, the break up of the large surface with smaller sheets allows some room for expansion/contraction in the XY plane. Furthermore, it'll be much easier to really seal everything off on all sides on say a 2x4' panel opposed to a big sheet.

    Recently I did a job where they spec'd out 5/8" MRDF - I never heard of the stuff before. I was able to get some - they called it 'double refined' MDF and say that it is easier to cut and paint with less fuzzies. I am wondering if it is similar to the Plum Creek stuff or other high quality 'cabinet grade' MDF. Anybody know more info on it? I may try making a sample pod out of that stuff to see how well it flows and resists crush down. When it was up on the table I could feel it pulling vac through it on top of the ULDF...so it should be good.

    In pursuit of getting good, consistent material for a vacuum bleeder I called several manufacturers around Thanksgiving...Sierra Pine, Arauco, Roseburg - and explaining my application, issues and requirements were dicey at best. I might as well have mentioned that I was building a space ship and needed MDF to insulate the flux capacitor from the turbo encabulator - with so many CNC routers running vacuum in commercial applications, you'd think that at least somebody (MDF application engineer) would know at least a little something about our industry....Nope. So as with most things, it's doing my own R&D - even if it's only trial and error (which gets expensive sometimes!)

    Just the other day I was wishing I had the T-track/extrusion installed. Customer needed these 1.75" thick x 6" wide poplar balusters first profile cut, then flipped up on end & sharpen the internal corners. I would have been nice to bolt down a fence and use cam clamps. I think I am going to pull the trigger.

    Working on some quick disconnects for vacuum. @$60 ea, these are a lot more than I want to spend, so I am going to see if I can 'borrow' the design and 3D print something similar with a bolt on flange. That way I can just pull off the vacuum pod without too much fiddling...in theory anyway.

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  4. #4
    Join Date
    Feb 2008
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    Miller Marine Products, Ridgefield Washington
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    I use vacuum to hold most of my parts cut from aluminum and plastic. I built my vacuum table from info I gained on here much of it from you Brady I never went to the boxes though. My vacuum motors are light house I run 2 of them most of the time I only run one motor to cut 1/2" Starboard I make tons of parts from that stuff I make the outside profiles to a skin of .050" so only the drilled or bored holes go through to leak air and a few profile slots.

    The .250" aluminum has created more problems for me to cut due to the thickness not letting me cut to a skin because there is not enough material to effectively use a laminate trimmer to trim out the parts like I do with the plastic. I cut the aluminum parts and leave tabs the through cuts are making to much vacuum loss for my vacuum setup as built. I plan to add 2 more light house vacuum motors to the system. One thing I do now is I installed a series of aluminum dowel pins this helps a bunch now the vacuum only has to hold down and the dowel pins to stop any lateral movement. I turn off each zone after those parts are cut to keep as much vacuum as possible I program to do it that way by selecting which parts to cut in the order of my choosing. I may also look into a real vacuum pump.

    I really like using vacuum my vacuum plenum has 5 zones and I use Valtera knife valves to control the vacuum zones. My spoilboard is Trupan glued down to the plenum in the high spots. I cut my plenum from counter top particle board I then sealed it with some shellac. The addition of the dowel pins has really helped with cutting the aluminum parts.
    WWW.MillerMarineProducts.com
    Proto Trak DPM CNC Bed Mill
    Brand X Industrial router

  5. #5
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    Thanks Mike. Vacuum certainly isn't the end all be all of hold down. It's really a nice convenience for production, but there are times where I just can't use vacuum alone to hold things down if for no other reason than the parts don't have enough surface area to stay stuck - and as you point out, the kerf area can get quite large.

    For times like these, I use my 'Vacuum Film' technique where I adhere the substrate to a sheet of .040 styrene or 3/16" coroplast. To mitigate some of the cleanup on some materials, like AL, I first put masking tape on the back of the substrate to be machined - then I put mounting tape (or fiberglass carpet tape) to the styrene/coroplast and laminate the two together (by hand). This gives you some buffer in Z, allowing you to cut all the way thru the AL etc and slightly into the plastic. You never lose an ounce of vacuum.

    When all is said and done, you just peel the parts off. I have to do this method for adhesive-backed laminates (that must retain its own 3M adhesive/paper as part of the product) - there's just no other way. The two downsides are A) time/labor/materials laminating and B) getting residue off the profile/edges from the adhesive riding up the bit. Other than this, it works pretty well - especially if vacuum is marginal. Carb cleaner makes short work of cleanup on aluminum. You won't need tabs or have to worry about using a trimmer anymore...

    --

    I took some measurements out at the table rather than relying on the SB table drawing for crossmember locations. The PRT uses 2x2x0.25" angle (and 3x5x0.25) - Looking at it and imagining 431 lbs of AL extrusion on top...It would be bolted, but I am still concerned about deflection/sag at the center. I'm toying around with either sistering up another 2x2x0.25 angle to each existing one, or switching over to like 2x3x0.25 wall rectangle. Don't think I want to do center legs (like the BIG SB custom tools I've installed...PRS96-336 !!) - because that invoves making a strongback etc and it gets complicated. But it would be wicked stout!

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  6. #6
    Join Date
    Feb 2011
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    Duluth, MN
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    Hey all, I am delighted to see this discussion as I am experiencing table top stability issues with my 96x60 PRT Alpha that is otherwise still going strong. I bought it used and always have to work around the issue. One theory I have that may contribute to issues is this: The heat generated by the two lighthouse vac motors under the table cause the steel supports to warm up a little and expand causing the surface to bow downward in the middle of the table which is what I see though not 100% of the time. Another theory is getting material that is not flat enough and so when holding thicker and less flat sheet with the vacuum, the table top is being sucked to the sheet and they meet half way. Thoughts?
    Marcel

  7. #7
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    Jun 2013
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    Pasadena, CA
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    As for the vacuum quick disconnect of vac pods, I use a short piece of 3/8” O.D. hard tubing and slip it into 3/8” ID pressure hose that supplies the vacuum. Seals tight enough and costs next to nothing.
    Box Joint, Dovetail, Guilloche and MazeMaker Software Here

  8. #8
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    Gert - I'd love to get away with 3/8" hose and couplings, but the plumbing has to be sized for the pump CFM. In my case about 200 CFM, where 2" is what the engineering charts point to for free air flow.

    For small type 1 PVC pods...yeah - the 3/8" stuff is perfect for that.

    Just got my hands on a Sharkbite XL 2" elbow...says it fits CPVC...NOT PVC Sch40 etc. So...admiring their handiwork/engineering. Will make something similar for 2" PVC. @ $60/ea, I'm sure I could improve on that. More later...

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  9. #9
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    Sep 2014
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    Poughkeepsie, NY
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    Just a thought,you have a lot of wood products right now, (base, plenum, spoil) that's a lot room for movement. Instead of using 8020, why not use a sheet of 3/4" 6061 Aluminum plate? With the 8020 you have a lot of smaller pieces that have to be aligned and secured to each other or they can move independent of each other. A sheet of 6061 weighs around 514 lbs. A bit heaver than 8020, yes, but a lot more stable. A few extra cross members will handle the weight. A heaver machine is a more stable machine. The expansion coefficient is minimal. You could machine your grid right in to it and use gasketing or use a structural bonder and attach another plenum board.
    With regards to the MDF shrinking, could it be with a 13 hour run, the moisture content is going down in the board with that much air flow going through it? That time of the year, dry, dry,dry. With the big guys running heavy iron, there changing there spoil boards so frequently they don't have a problem with it shrinking.
    Double refined MDF. Try Ranger Board by West Fraser. Awesome product, finishes out after machining like it has been sanded to 150. Also no extra edge sealing, the edges absorb the same as the flats.

  10. #10
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    Sep 2014
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    Poughkeepsie, NY
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    6061 3/4" Aluminum plate is $1905. Shipping is $267. Price is from Discount Steel .com Shipping from Minneapolis, MN to New York

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