Is it ever OK to put screws in your bleeder board?

[blackhawk]

I am helping a friend of mine setup a vacuum system on a Shopbot that he is in charge of at a local college. We are going to make the plenum out of standard MDF with 6 zones. Then glue down a 3/4" thick piece of Trupan for the bleeder board.

I've had my own Shopbot many years without a vacuum table, so I am new to vacuum systems and I am accustomed to screwing everything down. Is it heresy to use screws to hold down small irregular pieces that don't go all the way through the bleeder board?

I know you can use a sacrificial piece of plywood and screw to that and hold with the vacuum. But, I just wondered if your could screw into your bleeder board without ill effects in order to save turning on the vacuum for small jobs.

[Kyle Stapleton]

Yes, that is fine we do it when small parts just will not hold.

[EricSchimel]

You can also put a "carrier" board on top of your spoilboard and vacuum it down. Then you can fire screws into that carrier board. I've done that before to avoid putting screws in my bleeder board, especially when the bleeder board gets real thin and there's not much meat left for screws.

I've even routed a little channel into the underside of a carrier board to direct a lot of suction at a smaller part.

[Martin Reid]

Dear Kyle

We have a series of thin laminated boards that we place next to the 'part' to (a) help with the vacuum and (b) help hold the 'part' from moving see attached picture.

31709

Also we use a 12mm thick MDF sacrificial board. We don't glue it down as we change it every two or three months. There are some location blocks around the edges then use the vacuum to hold it 'flat' while it is surfaced also while it is being surfaced we place masking tape round the edges to stop it moving.

Any way that's my two penn'orth.

Sincerely and in good faith
Martin

[Martin Reid]

Dear Kyle

I forgot.

To maximise the 'vacuum' don't bleed air through the plenum. When you have machined the plenum zones, seal it with lots of coats of sealer and when you think you have finished sealing it - give it another coat. When we bought our first machine (second hand) the guy said his best decision was a vacuum bed and gave us the 'more sealer' advice. Of course we didn't follow his advice and gave it two coats of sealer, it looked fine. For months we couldn't understand why everyone raved about 'vacuum' we were putting screws in all over the place. Then while changing the sacrificial bed I decided to give the plenum another couple of coats of sealer. We couldn't believe the difference. We gave it more coats the next time we changed the bed.

Remember; vacuums don't 'suck' down, they evacuate the air from underneath and atmospheric pressure pushes down. If you could evacuate all the air then the maximum hold down pressure (at sea level) would be 14.7 pounds per square inch. As soon as the air bleeds out from the vacuum you start to reduce the hold down. I've even taped over behind the kerf to help prevent bleeding the vacuum.

Now I've had more than my two penn'orth.

Again sincerely and in good faith.
Martin

[Kyle Stapleton]

Martin,
Thanks, great tips for others.
Your picture does not show a small part and the forces of a 3d cut are normally not the same as a 2d profile.

[jerry_stanek]

I split my spoil board into 4 sections with a 6mm piece of sintra between each. My plenum is 1 inch thick sintra also. The reason is I found out that when I was only using 1 zone I had a piece of material all the way over to the other side and at the other end of my table and I went to push it it was held down. I now get better vac hold down

[Martin Reid]

Dear Kyle

It is the only photo I have showing how we can 'zone out' each part of the table with laminated boards. Laminated because they don't bleed air. I don't have photos but we use this technique with the 19mm roto tip cutter when cutting vision panels in 54mm fire doors. That really tests the hold-down.

Again sincerely and in good faith.
Martin

[blackhawk]

Martin - Can you give specifics on what you used as your plenum "sealer"?

[dlcw]

Here's what I do. 1" MDO with T-tracks and clamps. Vacuum system holds the MDO firmly in place. Eliminates having to put screws into your spoil board. I've got two of these to provide 4' x 8' T-track hold down.

http://www.talkshopbot.com/forum/attachment.php?attachmentid=31710&stc=1

[Martin Reid]

Blackhawk

We have a two pack lacquer we use in the spray shop. I mix that really thin and apply that with a brush. Soaks in really well, it dried quick too.


Don

The Tee track looks interesting.

I can see a 'shadow mark' of a cut-out in the baseboard. How much below the surface is the track? Or did you take the track out when you were cutting those?

Those oak cramping pieces - do you have those in different lengths?

That piece of softwood down the X axis is the edge of that the X Zero or is that something to do with those beech blocks? Now that I'm thinking of the X Axis, how do you know the work piece with the pockets is parallel to the axis - are you measuring that, or do you have some sort of spacer from those beech blocks?

It is really interesting, I've often thought that if I put one of these in my garage, that I wouldn't be able have a noisy vacuum and would need something different.

Thanks for the picture

Yours sincerely and in good faith
Martin

[dlcw]

Blackhawk

We have a two pack lacquer we use in the spray shop. I mix that really thin and apply that with a brush. Soaks in really well, it dried quick too.


Don

The Tee track looks interesting.

I can see a 'shadow mark' of a cut-out in the baseboard. How much below the surface is the track? Or did you take the track out when you were cutting those?
----> The tracks are about 1/8" below the surface of the MDO. My normal cut thru depth is 1/32". Yes, you will have cut out tracks all over your MDO board. But your spoilboard has no cut tracks in it.

Those oak cramping pieces - do you have those in different lengths?
-----> You can make the oak clamping pieces any size you want. They use a standard T-bolt with a knob. Very simple. And they can easily be modeled in VCarve or Aspire and made on the CNC.

That piece of softwood down the X axis is the edge of that the X Zero or is that something to do with those beech blocks? Now that I'm thinking of the X Axis, how do you know the work piece with the pockets is parallel to the axis - are you measuring that, or do you have some sort of spacer from those beech blocks?
----> The blocks along the edge of the table are used as reference blocks for anything I put on the CNC table. Those reference blocks are mainly for plywood when making cabinet parts, but they work well for zeroing the MDO board to the X axis too. They are EXACTLY in line with the X axis of the machine. When I cut the tracks slots in the MDO, I used those reference blocks to ensure that the track slots are EXACTLY parallel to the X axis. When putting a piece of material on the MDO hold down board, reference the edge of the material with any of the tracks and you can be assured that the material is parallel to the X axis. I use a tape measure to set the edge of the material an exact distance from any of the tracks.

It is really interesting, I've often thought that if I put one of these in my garage, that I wouldn't be able have a noisy vacuum and would need something different.

Thanks for the picture

Yours sincerely and in good faith
Martin

Martin,

Let me try and answer the questions one at a time. They are inline in blue above.

[curtiss]

I laid out a grid of 17/ 64 holes on 5-1/2 centers through the “high spots” in the vacuum plenum. The hole goes all the way through Trupan top and out of the bottom of the 3/4” plywood base.

At the bottom of the hole is a 1/4 inch threaded T- nut which can be used to secure the hold down clamps.

Sometimes I will use a lag screw coming up from the bottom to hold the work-piece using “pull down.” ... you have to miss the lag screws however...

Works great, I cover the bottom holes with heavy tape when not in use, vacuum still works fine.

[Gary Campbell]

I don't think enough can be said about adequate hold down. Along with destruction of material, edge quality and accuracy all are VERY dependent on good hold down. Most users don't experiment enough with alternate methods once they find one or two that work. I like curtiss' bolt method. When you can use them, you can really rock. Look at 4:15 into this video and see the force you can withstand with a few well placed bolts:

https://youtu.be/fjk9FjHg4vg

[coryatjohn]

The art of hold down is a lot like making concrete forms. You can go way overboard and nothing bad happens except for wasting a lot of time to set it up and trouble in taking it apart. You can go minimalist and end up with a huge pile of worthless mess. The art is finding the exact balance between excessive and minimalist.

[joe]

Lots of good information about hold downs. A few years ago there were fellows using the Raptor gun which shoots plastic nails to solve this problem.
https://raptornails.com/product-catalog/nailers.php

[coryatjohn]

>> A few years ago there were fellows using the Raptor gun which shoots plastic nails to solve this problem.

I tried going that route but it was essentially a waste of time. They would help in maybe 5% of the situations. Usually, they didn't hold well enough or they wouldn't penetrate the material.

There is no magic bullet for hold down. Having a variety of strategies, a large collection of clamps and long pieces of scrap wood work best for me. The primary thing one must know is what forces are involved. That's the key to success with hold down. If you don't know how much force the machine and bit are going to impart to the work, the most efficient solution can't be applied. Getting familiar with this aspect of using the CNC is probably the best road to success.

[joe]

I'm in full agreement with John. There's not a single answer to these questions. Below are some of the hold downs we used. I copied the design from piano keys. I liked the look and they worked out well.

http://www.talkshopbot.com/forum/attachment.php?attachmentid=31728&stc=1

[Bud Meade]

I know this is a very old thread, but is "bleeder board" the same as a "spoil board"

[dlcw]

Yes. It's purpose to provide a flat, true surface to mount material on. You can screw it down, but it is recommended to use caulk to attached it to your vacuum plenum. Putting screws (or any type of metal hardware) in could destroy bits in a split second if you touch them while running your machine.

[Bud Meade]

Thanks, I assumed they were the same, but you know what happens when one assumes.

I've had two previous machines. Prior to my Desktop Max, I had a Desktop 24X18. I populated the spoil board with T nuts and used clamps. Then I started using screws in lieu of clamps. I was happier with the screws. Of course, I had to resurface more often but it's not very time consuming once programmed.

My current Desktop Max has a factory vacuum hold down. Since I rarely use full width sheets, I'm finding that the hold down isn't very good on smaller (not small) boards w/o covering the remaining space with something else to increase the vacuum under the wood I'm working with.

I'm thinking about taking the vacuum set up off and putting on an 80/20 aluminum slat deck and fasten my spoil board to that.

[Martin Reid]

Dear Bud

Let me start by saying I'm quite envious of your DeskTop MAX. I thought that if I was ever lucky enough to get one (in my garage). I would go for the aluminium bed then build my own 'vacuum' on that if needed.

As for this thread, I wonder how many of the original 'contributors' are still 'users'. In the UK we have places you can go and declare; 'My name is Martin and I'm a Shopbot user'.

Good luck with your MAX
Always sincere
Martin