Matrix Table Upgrade?

[Scrit]

Does anyone have experience of replacing pod bar type tables with matrix beds? In particular, what is the ideal size for the "islands" in a matrix bed and how much vacuum draw down is required? We curently have a Biesse Rover 335 PTP router (3 x 7.5HP HSDs, 24 vertical and 8 horizontal boring units) and we'd like to replace the pod bars with a system which could draw down small plastic or wood compononens, preferably only a sacrificial bed of MDF or similar. Any thoughts gratefully received.

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[cncadmin]

One totally different thought, is to place a "spoil board" and fix it to the machine. If your looking to nest wood, you can place a full sheet down, than nail in place using fiberglass nails. Than program your file to make a 1/8" score trace of the parts on the full sheet, this is so you can see where you need to shoot additional nails to hold the part down, than once that is done you can run the full program.

The other way is to use double sided carpet tape to tape the sheet to a spoil board. I done done and have worked out very well.

[Scrit]

Thanks for the suggestion. I've tried the double-sided trick on an auxilliary spoilboard, but it doesn't seem to work brilliantly in that you've got to machine at much lower speeds, reduce the depth of cut per pass, and so on. In a WW environment it tends to be a one-shot solution, whereas I'm normally cutting small batches (I also do plastics, mainly HDPE and nylons).

Using pins to hold work marks the surface, and that isn't acceptable on either veneered panels or melamine coated MDF/chipboard used in kitchens.

This machine has a 120 cubic metre/hour vacuum pump capable of generating 60 to 65 cm of vacuum so ideally I'd like a solution which doesn't just throw that away.

[HomeCNC]

Take a look at www.allstaradhesives.com

[Scrit]

Thanks HomeCNC - really useful information - I've just emailed AllStar to find out if they have a distributor in England. I'll post the results when I know.

Still leaves me with the question of pod island sizing and spacing though. Anyone got any suggestions?

[Scrit]

Well I've now built a test piece of matrix bed. It comprises a 32mm (1-1/4in) thick harwdood plywood underside support for ridgidity with a 25mm (1in) high density polyethylene top screwed on. I surfaced everything both sides with a trepanning cutter before and after assembly then machined 6.35mm (1/4in) round-bottom grooves in a chequer-board pattern on the top surface some 7.2mm deep. The whole thing has a gutter round the outside and the islands are 28 x 42 mm (same size as the Biesse auxilliary pods off newer machines). These islands have a 0.5mm deep x 12mm groove milled across them from side to side. This is a small bed some 605 x 310mm in size (approx.) with a 6 x 17 island grid and has a ten vacuum draw holes of 4mm diameter drilled in the middle of selected islands and with the exhaust connections made by filing a notch in the thread of G1/4in threaded pneumatic hose connectors which are then screwed into the underside of the bed. From there hard polythene tubing connects everything back to a couple of brass manifolds 6-into-1) and thence to the vacuum pump. The vacuum draw holes are deactivated by screwing a self-tapping screw into them. The vacuum area is "defined" by inserting 8mm foam rubber cord into the grooves around the outside of the area to be put under vacuum. The bed certainmly seems to work well with timber and composite timber materials although a smaller matric, say 25 x 25mm, might be more effective.

Can anyone suggest a material to sit between the matrix table and the workpiece which will allow me to machine through but still retain vacuum or is it a case of machining tabs onto the workpiece and trimming off after removal from the machine? I'd particularly like to machine 2 to 3mm thick ABS for a product.

Regards

Scrit

[cncadmin]

MDF will allow air flow or they do make a speical board I don't remember the name. We always just used MDF.

[ger21]

They make a low density MDF that vacuum flows though easier.

Gerry

[cncadmin]

I would seal the edges of the board to reduce leakage.

[Zephrant]

Wouldn't a low density MDF have to be called LDF?

It looks like your setup is for high vacuum, low volume. That is the method I currently use on my mill, and it works fine until you open a single hole to the vacuum chamber. With your small fittings, you won't get much volume. If you are careful, you can make custom vacuum chucks for each part you build, that maintains the vacuum inside the part boundaries. I often use O-rings (BUNA) or 1/4" closed-cell foam weather-stripping tape for sealing.

If you are expecting to be able to mill around a general matrix pull-down table, I think you might be disappointed. The first time you open a hole, vacuum will be lost, and the part will move.

My plan is to use 1.5" feeders to each block of four square feet of table space, manifolded back to a pair of 6" ducts, that will go to a two or three horse blower (for a 5x10 table).

The problem with the blowers is that they rarely get over 5psi of pressure (according to the published curves) where my small "pucka-pucka" vacuum pump will pull 12 psi or better, but for extremely low volumes.

The research I did indicated that the big production houses used about a HP per square foot for high-volume hold-down. I was recommended a 50hp as a "good place to be", and a 100hp if I never wanted to have any issues with it.

Zeph

[Scrit]

Yep, pretty much. My set-up pulls 100/120 cubic metres per hour at present (depends on whether or not the phase inverter is in-line) from a 3kW(4HP) Becker unit. The point about going to a matrix is to try and get away for the need to make a new spoilboard every time I cut a part. The pods are also very fragile and every time I nick a rubber seal it costs me about $40 (thanks, Biesse) so there is a financial need to get away from the disadvantage of paying through the nose and sometimes having to wait 4 to 6 weeks to get parts from Italy. As I do a fair few small volume parts the setup time for jig making is also killing me.

It seems that based on your rule of thumb I'd need to be looking for a 30HP unit to cover my work envelope - whew!

Thanks for the input

Scrit

[HuFlungDung]

Originally posted by Zephrant

The research I did indicated that the big production houses used about a HP per square foot for high-volume hold-down. I was recommended a 50hp as a "good place to be", and a 100hp if I never wanted to have any issues with it.

Zeph

100hp vacuum pump

Should hold down anything as open as a square yard of chicken fence, but hang onto your hat

[HuFlungDung]

Could a person make a table with a large number of small pockets cut in, say 3/8 material (or thicker), with one vacuum hole in the center of each pocket. I suppose the pocket could be what, 2" diameter? Then, countersink a seat at each vacuum hole location, and provide a single plastic ball that is captured within each cavity, but is just rolling around loose to start with.

When cutting the part opens a pocket to the atmosphere, then air flow tends to suck the ball over to the hole, where it plunks down into the seat and blocks it off. Wait a minute, this is giving me an idea for a household appliance........

[Scrit]

I can imagine this the other way round, sort-of, i.e. a cavity with a hole in the top where the ball is held against the opening by a weak spring. The spring would need to be strong enough to counteract the vacuum. When a work piece is placed on top the ball is depressed and the vacuum is "switched on". If there is nothing on top of a ball, then it is forced upwards by the spring, blocking the vent and the vacuum is "switched off". There is already a commercially-made system called the MiniMach / MaxiMach (by a firm called VTech, I think) which does this and you can buy kits of the parts in the UK. How would you stop the balls from being sucked into the hole, though?

I am beginning to wonder if the technique used by some of the plastics boys might be the best approach. Acrylics are supplied with a thickish self-adhesive paper coating on both sides and the guys I've watched have the matrix set-up to draw down over a number of cells (divided up by foam rubber coird) over the maximum area they can. They set-up their cuts so that they skim the paper with the final pass, but don't pierce it. It believe that this requires the matrix bed to be regularly reskimmed, though

[HuFlungDung]

The problem with the spring and plunger though, is that the cutter would have to totally clear the material overlying the exact plunger position, in order to free it so it would spring up and allow the ball to seal the hole. You also might have the danger of cutting the plunger.

The vacuum hole does not need to be large enough to "swallow" the ball, this is what the countersunk seat provides, is a place for the ball to sit, blocking a small hole in the center of the seat where vacuum is applied.

A bit of wind from the cutter bit might cause the ball to fly around, or even get out of the cavity, so maybe a small "ball cage" (screen cover) would have to be provided to prevent this possibility, and to keep the ball down safely out of the cutter tip zone.

Another approach might be to use a "reed flapper" over top of the vacuum hole. The trick with this system is that you would need perhaps a secondary, but completely isolated vacuum grid system to each pocket to perform the initial evacuation, from another hole in the pocket, on the "top or air side" of the reed valve. This would then allow the reed valves to stay open. At that point, then the secondary system would be disconnected, and the vacuum redirected to the initial vacuum grid. Then, because no air is left in the pocket at this stage, the reed valves would not move until the integrity of the chamber was breached.

The reed valve would have to have enough natural stiffness that it would lift off the vacuum hole when the state of vacuum exists in the pocket.

[ger21]

The pods on our Masterwood have the ball and spring. Although it's only half a ball with a pin on the bottom to go in the center of the spring. very soft plastic, or hard rubber to seal good. Remember that you'll have to seal each of your pockets, and if you cut through this seal or the ball, you'll lose vacuum.


Gerry

[HuFlungDung]

Would it be possible to improve the efficiency of a small vacuum pump system by zoning off areas that have been exposed to the atmosphere? This would take a series of solenoid valves that could be called by M functions, to disconnect or reconnect certain zones back to the pump. The connection to the various zones could even be cycled on and off as necessary to support the work whenever the cutter is in that zone.

[skippy]

I'm an Australian living in Belgium and last time I went back home to Oz I was amazed to find someone using MDF as the sacrificial board (vac system on a cnc router). The MDF there was much lower density and therefore allowed the air to pass through it. The MDF here in Europe wouldn't let any air through at all I'd imagine.
Phil

[ger21]

You can get regular MDF and low density MDF, which is probably what you saw. But even regular MDF will let some air through, as long as you seal the edges.

[nemi]

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