down draft cart

[Bob Riefer]

Hi,
In my shop, I use a 3x4 cart on swivel casters for many steps of my projects. I stage lumber and parts on the cart, use the cart to move those items from station to station, I do handheld work (e.g. routing, sanding, jig sawing) there etc.

I recently upgraded my shop dust collection, so all my stations are pretty darn efficient now, but the last portion of my setup statement above is a mess.

AND, I need a cart in my finishing room, so my thought is that my old faithful cart retires to the good life in the finishing room. Nice and clean and bright in there. It will be so happy after years of abuse. And, I build a new, better cart...

My neighbor recently gave me very nice plywood panels that his company was throwing away. Picture really nice, perfectly 3/4", dead flat and very stable, peg board panels at 2x2 each.

I would build the cart so that the top connects can connect to a nearby 6" DC port and act as a down draft table. Inside the cart, I would also install my spare shop vac with bag filter. So I could connect to back end of tool AND work on a downdraft table at the same time.

As this project is basically free (I have everything on hand already from other projects etc.) it's in the "I have nothing to lose" category... But I'd be curious to know if you all think it's a sensible approach, or any suggestions etc.

Thanks!

Bob R.

[Patrick Morris IV]

Great Idea, only problem is I didn't think of it first and do that in my shop

Pat

[Doug Garson]

Sounds like a plan, go for it.

[David Utterback]

Air volume increases with hole diameter to a great degree. If you have 1/4" diameter holes that are 3/4" long, you might not be happy with the results. If you really want to try it, you can smooth the inlet entries and outlet edges to reduce turbulence. I would think that 3/4" holes would be much better - again with smoothed entries and outlets. Good luck!

[Jim Becker]

Bob, I've been watching a bunch of vids on YouTube lately to try and settle on a design for a similar construction. I have an additional adjustable height bench setup available and my plan is to create a surface that can be used as a "real" downdraft sanding station as I'm doing a lot more hand sanding with the guitar projects than I normally do with other things as well as double as additional assembly space for smaller projects, etc.

I think that the key to success here is a combination of good calculation of ventilation area on the surface combined with appropriate "direction" under it to insure air flow is relatively even across the board, as it were. I haven't gotten that far yet, but I'm getting close to starting those calculations since I've somewhat arrived at the potential size of the surface...which will be "about" 4' long and about 2' deep, give or take. I may or may not make the whole surface downdraft, depending on the math.

[Bob Riefer]

What’s this “math” thing you speak of

I’ll be done with my table this week and can give you some practical feedback. Consider mine the prototype

[Dan Bundy]

Several years back, Woodsmith Magazine had a plan for one, which I built. It's sized to use two 20x20x1 furnace filters in the top (it's a downdraft table). It works pretty well. Not sure of the issue it was in, maybe somewhere in the 50s, that long ago.

[Jim Becker]

What’s this “math” thing you speak of

I’ll be done with my table this week and can give you some practical feedback. Consider mine the prototype

LOL. But seriously, ideally, the area of the "input" holes should be compatible with the "output" (DC port) area. Sadly, that requires math...which I personally don't enjoy, but hey...sometimes you just have to do what you have to do. They make adult beverages to make up for the effort required after it's expended.

[Bob Riefer]

Each hole is about 1/8” or so, and the DC port is 6”. My quick iPhone math (I think) shows
that it would take the area of lots of little holes to add up to the area of the 6” hole. Like the opposite of an air hockey table.

[Jim Becker]

Exactly. If you have too few holes or holes that are undersized, you're not going to get the maximum air flow if the area of those holes isn't at least close to the area of your port.

[Derek Meyer]

A 6" port has an area of about 28 1/4". To get that much area with 1/8" holes would require around 3200 of them. That's a lot of drilling.

[Jim Becker]

The solution is larger holes...

[Bob Riefer]

I measured the predrilled holes and found them to be 1/4” each. There’s roughly 1000 of them so that’s a good start. Additional holes can always be added later if necessary

[David Utterback]

The solution is larger holes...

Spot on and for many reasons. The most important reason is to reduce turbulence and increase laminar flow. A set of holes 1/4" diameter that are 3/4" long will create a lot of turbulence and frictional wall losses and greatly restrict air flow. If they are 1/8" long, the effect will be substantially less. Even at 3/4" diameter, a 3/4" length of the perforations will create noticeably greater frictional losses than a 1/4" length. Most source control exhaust ventilation systems use slots instead of round holes to greatly reduce the inlet pressure losses.

The capture velocity needs to be sufficient at a couple of inches above the entry slots (holes) in order to control the dust. The box below the perforated surface, if rectangular and sufficiently large for a 6" outlet duct, will act as a plenum and distribute the static pressure across the holes in the surface and improve capture of particulate. The plenum may have dust settling on its bottom surface and in corners so you need to be able to clean it out occasionally.

I have a concern that the use of the suggested design could create excessive static pressure in the plenum and the fan performance will be greatly impaired. You may check for this be trying to raise the surface panel from the plenum in initial testing. If you find it impossible to lift it, your static pressure may be too great. A manometer connected to the plenum, though, would be the best way to check it.

I hope this helps.

[Doug Garson]

Another consideration is that in use some of the holes will be covered by the piece you are sanding so your flow area thru the holes will depend on the size of the workpiece. Maybe design with substantially more flow area and a method of blocking some of the flow area when sanding small pieces. Could be as simple as laying a piece of plywood or MDF over part of the top.