Vacuum Hold Down Help

[PNast]

I have been reading and reading about vacuum hold downs for the last week and yesterday I think my head almost exploded! I mostly cut pocket text signs 23 x 11 and have been using T-tracks and Rockler clamps, I have attached a picture of my setup. I really don't use the end clamps anymore as they tend to make the problem worse. My pocket cuts .15 deep. The problem I have been having is during the pocket clear cut (1/4 end mill) the middle of the board moves enough to cut deeper on the profile pass. This happens even when I clamp with the slight cup down. Tape is not an option because of the amount of signs I do per day. So I decided it was time for a vac hold down. I have attached the Aspire cut file I made. I thought I would be able to use my extra Delta dust collector since I only needed to hold down a 23 x 11 piece of wood. I did some reading on the forum to see if this would work before I cut into my table top, I quickly learned that wouldn't work. Is there a way I can just connect a vacuum pump to the bottom of my table? I read about Lighthouse vacuum motors, what size would I need to hold down 23 x 11? Would I still need to use inline filters? Would it have enough power to suck down a cupped board? Is there a certain type of motor? Will the cut file I made work? I really can't afford to spend a lot of money on a vac motor now.
Thanks in advance for any help and saving my sanity!

-Paul

[khaos]

This really sounds like an ideal setup for a puck. You cut the same size sign often and you dont penetrate the board.

Look at #16 here (http://www.talkshopbot.com/forum/showthread.php?t=18374&highlight=puck). BCondon has what you need.

[PNast]

Thanks for the reply Joe, I thought about using pucks but I would rather have it cut on the table. I do 8-10 of this size sign a day and really want to make sure the whole pocket area is secure to the table. I have a 1 inch border around the sign, the rest is pocket area. I use mostly pine and some poplar, and find there is no such thing as a straight board. The signs are text and 3-D carvings.

[Kyle Stapleton]

Why is there a cup?
Do you mill your own blanks?
Quick fix would be to put the cup down and clamp the outside edges and that will flatten out the cup.
But again if the blanks are make/milled right there should not be a cup.

[PNast]

I buy my blanks, sometimes there is a slight cup. I do clamp with the cup (if any) down. Even if the board is straight there is no clamping pressure in the middle of the board. As the material is cut from the pocket and stress in the grain is released the unclamped center of the board will move.

[PNast]

This can also cause a slight difference in Z zeroing bit changes when zeroing to the top of the board.

[bleeth]

Paul:

Odds are a LH vac motor will not hold the cup out of a board as it is getting cut. There may be a shot with 5 pucks-one for each corner and one in the middle, but if it didn't hold all vacuum would be lost and you would probably lose the part.

An all smooth large pocket cut in solid wood is definitely problematic. If it is possible to change the designed background to a texture, one can be done as a vector cut that cuts fairly quickly rather than a relief cut that cuts slowly and it will then disguise the problem.

If your blanks are made up of 1 piece or only 2 pieces edgeglued, then I would reduce the width of the pieces to a few inches each, make sure they are glued up with alternating ring, and try that.

Zero to the table bed instead.

[PNast]

Hi Dave
Thanks for the info. I buy 1 x 12 x 3/4 x 12 foot boards and cut them to size. I stack and sticker them so the cup is usually not that bad. I use Yellow Pine, White Pine and Poplar. The Poplar isn't as bad as the Pine but is very fuzzy when carving and takes a bit more sanding mop time than the Pine. I do some with Red Oak and have no issues at all but it is so expensive and eats up my tooling. This movement issue has been driving me insane for quite some time now. I do have a few signs that have texture (attached pics) but my biggest selling signs do not. Having a sign business in the Smoky Mountains I keep quite busy with the tourists, anything with a bear on it sells! After much trial and error I have figured out that it is the clamps causing problems. I find sometimes using less clamps toward the center of the board works better but each blank is different and always cuts deeper in a place that's real hard to sand. I sometimes wonder if the wood is swelling from the spindle and dust collector air movement. When it cuts deeper during the profile pass its always on the ends and center. I am going to try and reduce my pocket depth to .13 and see if that helps. I may also try some sort of long metal bar under the clamps to distribute the pressure a little more evenly rather then the little clamps here and there.
Thanks again

[bleeth]

Yep:
Darn hard to keep a 1 x 12 from changing shape while milling so much of the surface. I assume you are milling in a raster. You might try setting it up as an offset starting from the center. If you have it clamped on the border and the cup down as you cleared it the piece could simply put less pressure against the clamps.

[donek]

You can try this simple approach:
http://donektools.com/cnc-basics-tutorials/
It will typically pull a 1/2in board flat, but I don't know about a 3/4in. Usually if you want a stable wide board that doesn't cup, you'll laminate strips with the rings going in opposite directions. You could rip a 12in wide board into 4 3in strips, flip every other one over and glue them back together. It sounds like this is more work than you want to do. Perhaps you could have a local company to produce them for you or switch to a synthetic material.

[PNast]

Hi Sean
Thanks for the info, I have watched your videos many times. The cupping of a board is not really the issue, I was asking about that to judge the suction a Vacuum hold down has. If I have a board that is cupped bad I wont use it. Even if a board is 100% straight and lays perfectly flat on the table I still have the issues with the soft wood moving in the center during carving because I am only clamping the ends and not the middle. Like I said in my last post, I sometimes wonder if the wood is swelling from the spindle and dust collector air movement. So I wanted to get some feedback if a vac system would hold the complete bottom of the board to the table before I invested the money. I am assuming I can connect a LH6765-OD motor directly to a 2 inch flange under my table. Anything after that I am a little confused. I know you have to have some amount of filtered air leakage but I am not sure how to go about setting that up correctly. As far as cutting the pockets, I can still use some clamps. I just want to make sure the center of the board/pocket area stays secure to the table. I have been thinking about making several box vac hold downs like your video for the different size signs I make but would still need a vacuum system that could run 10 hrs a day 6 days a week.

[donek]

Paul,

I have some lighthouse type motors on one of our machines and still use the shopvacs on the other. I just cut adapters to fit those vacs that allow me to screw them directly to the underside of the table. You do want to be able to remove them easily as you can get debris falling into your holes and clean out is helpful.

I would make a table big enough for your biggest sign and keep a supply of masking tape on hand to tape off unused holes. There's no real need for multiple tables unless you are talking about zones. Our table size on our shopbots us 14in X 96in and 36inX 48in on the small machine. Masking tape from Walmart is pretty cheap and you can block off large sections with scrap materials easily.

Don't overthink things. Just build. You won't regret it.

[bcondon]

Good advice from Sean.... DOn't overthink this!!

Here is my vacuum solution. It is only about a week old but boy am I happy.

I built a bunch of pucks out of trex, it taps well 1/4" NPT and routes clean.

My first picture, you wee a puck in the middle. The puck in permanently on my table. The plywood fixture screw "on, around" the puck. I have several plywood fixtures of different sizes. The fixture is really only used to orient the workpiece to (0,0) the origin. Note the lower arrow with a 1/4" hole... This is (0,0). Notice at the top of the picture is a second arrow. That is (14, 0).

When it is time to make that size sign (I tend to make 20 at a time), I screw one corner of the fixture down (ok... I use the same screw holes), then align my 1/4" bit into the hole. I zero x and y. verify that (14,0) aligns and if not make adjustments until they are aligned...

Very simple and very quick. This leaves the rest of my table to whatever else is going on...

I use a 2 stage, 3CFM vacuum pump from Harbor Freight ($159) and you are not going to move your workpiece!!

Good luck

Bob COndon

[Bob Eustace]

Bob - how do you limit the vacuum? Does it need some sort of relief valve?

[jTr]

Bob -
His vacuum is generated with a pump, which is different than the lighthouse / shop vac devices for vacuum, as it will not work if there is an air relief.

Simply put:
Vacuum pump: very low volume of air movement (2-6cfm), very high vacuum achieved in the range of 20-25 inches of mercury.
Method of regulation: atmospheric conditions limit the amount of vac pulled - no relief needed as they operate in a way which relies on a 100% seal. They are made to run at max vac, zero air flow, which is what Bob C is utilizing, and is typically reserved for smaller parts.

Vacuum blowers: High volume (150-500+ cfm), Low vacuum achieved, in the range of 5-10 inches of mercury. More commonly used for supplying vac to the whole table when processing sheet goods. As parts are being cut through, more bleeding is occurring, hence the need for much larger volume of air flow.
Method of regulation: Bleeder valve (see instructions at lighthouse vac supply company under heading: vac motors for cnc). These systems are expecting to see air flow - a 100% seal is kiss of death, as they rely on a continual air flow to cool the bearings. The trick here is to pull as much vac as possible without premature motor failure.
How, exactly? This is a balancing act - thermometer to read vac motor temp, vacuum gauges to indicate amount of pull at the table, bleeder valve to fine tune the balance. I'm utilizing Gary Campbell's black box vac source, and attached a remote sensor from an outdoor thermometer directly to the primary motor and am able to monitor motor temp.
Average temp: 110-119 degrees, and my sheet goods are well behaved for cutting while gauges read 5-7 inches of mercury. (This is a little low, but I'm a couple thousand feet above sea level here in the Midwest)

jeff

[Bob Eustace]

Thanks Jeff - beautifully explained. Thank you.

[bcondon]

Jeff, Excellent explanation!!

I would be curious to set of 4 pucks on the corner of a sheet with a vacuum pump...

I think my puck can handle a 2x4 piece of sheet product...

Gotta play when I have time. I did cut some 3/4 plywood today which was 24x10 with no issue

[Bob Eustace]

To stop your head exploding Paul heres what we do in Oz where we get just about the crappiest wood in the world bendy wise. Using these Shopfox cam clamps really pushes it down on the table. I hate screws as they bite my cutters! Sorry the pic is inverted!

[scottp55]

That looks nice Bob. Door panels?

[hh_woodworking]

I though you posted the picture that way because you live down under:rolleyes:

[dana_swift]

Addendum on Jeffs rundown of vacuum sources:

Vacuum sources come in all sizes and capabilities, and as you would expect you pay for what you get.

Vacuum blowers can be one stage or several stage, resulting in low to medium vacuum levels and high cfm. The higher the vacuum level the more $, the more HP required and the louder they get. They can be extremely noisy.

Vacuum pumps can be any of several designs, resulting in the possibility of good vacuum levels, around 27+ inches of mercury. They can have low CFM or high CFM. The more you pay the higher CFM you can get.

Pumps will not deliver the same CFM as a blower at the lower vacuum levels, but can come close. Blowers cannot achieve the higher vacuum levels at any CFM.

Pumps often do require a vacuum relief valve. One is built into my pump, as very high vacuum levels 28"Hg and above make short life of expensive parts. Better to regulate it for better live expectancy.

I am running a Becker rotary vane pump, and I know Brady uses one also. Both of us have several vacuum sources, but these include high CFM pumps.

If you are even thinking of the Becker type pumps hold on to your wallet. They cost extra, but they are wonderful to use. Medium level noise. About the same as a PC router with my pump.

Lighthouse motors suggest a bleeder air hole for cooling or they can overheat. Its not an automatic relieve valve, its just an easy way to get a guaranteed air flow. The Lighthouse motors are a good compromise between cost, cfm, and vacuum. But they never get above 15"Hg or so. 10" is very good performance for them, but that is adequate for a lot of production work.

Small parts need high vacuum, or something like Bradys vacuum skin technique to increase the surface area and keep the cfm's down. Or both.

Its a great technology, try something you can afford and see how it works. You can always buy a higher performance blower/pump and use it in conjunction with your first setup.

:)

D

[Bob Eustace]

Dana would you need a relief valve with the el cheapo Harbour Freight units mentioned in this thread? If so where does one get these please. So far we have only used venturi vacuum. Looking for less noise and lower power costs (we have a doopey carbon tax kicking in over here and costs are going through the roof)!

[dana_swift]

Bob the only thing I know about these units is what was posted:

I use a 2 stage, 3CFM vacuum pump from Harbor Freight ($159) and you are not going to move your workpiece!!

Beyond that, I would speculate that pump like that is for air conditioning purge use and is intended to pull a very high vacuum. As such it does not need a relief valve, assuming I have identified the type correctly!

If you are needing to lower your carbon tax, use a passive hold down like clamps! Make these from trees, which can be obtained without paying any tax at all!

The carbon tax thing is getting pushed all around the world, and sooner or later we will have it here. I wonder how they will tax my wood stove? Those of us who use wood heat cut down the trees without paying for anything more than chain saw gas, and hauling expenses. Yet wood stoves are not clean energy unfortunately. So the thing they are trying to discourage is exactly what they would end up promoting, for the same reason I heat with wood now. Natural gas is way over taxed, yet it is cleaner burning, if they really wanted greener they would revoke the taxes on natural gas.

Unintended consequences I suppose :) That or its not really their agenda.. hmm

Good luck with the pump! Sounds like just hook it up and go!

D

[curtiss]

Dana,

Anything that would save money in some manner,

would first have to be approved by those who are ...making lots of money... off of doing it the wrong way.

See a few natural gas stations around the area, would seem to convert a vehicle might make some sense.

I believe France is about ready to build a small fusion reactor. Trees in Tulsa are a bit scarce aren't they ?

[Burkhardt]

A while ago I built a pretty high performance vacuum pump from an M62 automotive roots blower (supercharger) and a 3 hp motor for reasonable money. Works very well with moderate 10-13" vacuum and 80 cfm.

However it was a foolish thing to do. Like all such blowers it is noisy as an air raid siren and even with a muffler I can not use for more than a few minutes in my residential neighborhood. Also I am usually not machining large sheets and the high air flow is not necessary.

Instead I am using now an industrial 3/4hp oil-less dual piston vacuum pump that I found on eBay for $70. It can make 26-27" vacuum and can handle four well sealed 5x5" pods easily. It is built to run for hours and I am not sure if the Harbor freight A/C pumps are up to continuous duty.

I may keep the roots blower gadget however for a some vacuum forming experiments I want to do.

[Brian Harnett]

This vac pump is made from a lawnmower engine I pulled out the camshaft put a softer spring on the intake valve and made a check valve on the spark plug hole it works surprisingly well pulls 25 inches on average.

Added an oil splasher to the connecting rod since the motor was a vertical shaft originally

I use it mostly with my pin router for holding patterns to parts I am shaping its been working for almost two years now. Looks hilbilly but works much better than the Gast pump I used to use. It pulls a whole lot more volume to make up for leakage


Dana I usually enjoy your posts but I think you went off the rails on that last one.
http://i98.photobucket.com/albums/l249/brianharnett/Modified%20and%20Made%20Machines/th_vacuumpump2.jpg (http://i98.photobucket.com/albums/l249/brianharnett/Modified%20and%20Made%20Machines/vacuumpump2.mp4)

[dana_swift]

Brian- I will agree tax issues are not the purpose of the thread of the forum. Sorry if it became a rant- I was just responding to the concerns of Bob Eustace who had posted "(we have a doopey carbon tax kicking in over here and costs are going through the roof)!". I have also been reading articles where the US should be "punished" for not passing carbon taxes, and as somebody who works with wood (99% carbon), those issues are relevant to me, and perhaps other forum readers. End of rant.

Also- I love the recycled lawnmower engine as a vacuum pump! Very resourceful! I have thought about using a Briggs and Stratten and adapting it to run on steam or compressed air. Never considered using it as a vacuum pump! Well done!

Curtis- wood is plentiful in eastern Oklahoma. The area is highly wooded, and trees are treated as weeds to be removed. Tragic because some of the "weeds" have economic value, maple, walnut, pecan, oak, etc. Along the rivers the hardwoods easy to find, perhaps harder to access. There are several sawmills in the area. Every spring the Twisters provide many "free" trees, people pay good money to have them hauled off, it makes me want to cry. Western Oklahoma doesn't grow enough tumbleweeds to make a decent fire. But on the eastern side of the state we receive a lot of rain and the plants love it. The city of Tulsa runs an "organic dump" for the trees, just show up with a truck and you can have all you can take, free. I suspect that will come to a cold stop if the Washington wizards tax it, at least now it gets used mostly for mulch, etc.

Side note for the general forum, if you live in a wooded area and want the wood, just let people know. Around here the price of a tree is the price of hauling it off. (Well, include aspirin and spa costs.. lots of work!)

D

[scottp55]

Off topic, but had to cry last week when visiting a friend and saw quilted and burl maple in his woodpile waiting to be stacked. I said it was beautiful wood and he said "Aw that stuff. Two whole tree lengths were like that! Had to go down to the hardware store and rent their gas splitter for that". Took a few pieces of burl that weren't split to bad home.:(

[Bob Eustace]

That looks nice Bob. Door panels?

No just more of Mikes Paradise Boxes. We make them to support Breast Cancer.