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.

[Jim Becker]

Doug, one solution for what you suggest (which is real) is that many down-draft sanding solutions have little rubber bumpers that lift the workpiece off the surface slightly. Depending on the design, that may or may not affect the downdraft hold-down effect on the workpiece. Sometimes that's good to have; sometimes it's not.

[Doug Garson]

Jim, certainly a valid solution but prevents the suction from holding the workpiece in place so depending on how sticky the rubber feet are you may need another type of hold down.

[Jim Becker]

A rubberized surface would aid in preventing wonton lateral movement of the component being sanded. I'm not really sure it's necessary for a downdraft table to physically hold the workpiece down via vacuum in most cases, but like many things, that's certainly subjective. The rubberized pads many of us have used to aid in sanding and routing use friction to prevent movement...the downdraft table with the right kind of surface can do the same with the added benefit of capturing a lot of the fines resulting from the sanding process.

[Jon Nuckles]

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.

I get 2,304 holes. Still a lot.


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

576 1/4" holes will give you the same area as a 6" port. If your workpieces covers almost half of them, you'll be right on.

[Bob Riefer]

Smart people here :)

Cart is done and works so so. With the shop vac added it’s worlds better than before. When I cover some holes it’s also dramatic change. So, worth living with for awhile before further improving

[Bob Riefer]

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.


:-) I think it helps, let me try to answer a few and ask a few...

I think concern #1 is that the thick (3/4") "peg board" will create holes that are essentially mini-tunnels where each tunnel is a 1/4" wide by 3/4" long... and the air doesn't want to travel as easily through 3/4" long tunnels as it would through, say, a 1/4" long tunnel. I do have a large countersink / taper bit that I got when buying my drill press years ago.. I could reasonably easily taper the underside of each tunnel so that they are less than 3/4" length each. So the top would still be a strong 3/4" thick "in general" (important because this is a general purpose cart in addition to being downdraft), but each hole would be thinned out. Worth the effort?

Around point 2... The middle 10" section of the top is slots instead of pegboard holes.

So far, what I've found is that there's enough suction through the top to pull a sheet of cardboard down to the top. Or, if I put a flat board on the top, it is lightly "clamped" to the surface. Some of the mess I make on the top gets pulled in, and the rest I can either shop vac (since the car holds a vac too) or dust brush into the holes to help it out.

I don't think I have too few holes... and here's my unscientific method of feeling that way. On other tools (e.g. my drill press) in my shop that step down to 4" from 6", the flex hose kind of "jumps" and contracts when I turn the DC on with just that port open. Or, for my 6" flex hose areas, if I cover most of the port with my hand, the hose similarly jumps/contracts. But the hose to the downdraft cart is slack as if it's completely open without any restriction. Does that make sense?

I'd like the performance to be a bit better than this, so my ideas are:

- Taper the underside of each hole as noted above
- Use a rubber mat to block some holes so that the other holes get more suction each

[Jon Nuckles]

I'd like the performance to be a bit better than this, so my ideas are:

- Taper the underside of each hole as noted above
- Use a rubber mat to block some holes so that the other holes get more suction each

Your second option is certainly a lot easier to try, so I'd do that first. ;) And, if my calculations above are correct, you do want to cover almost half the holes with your workpiece and a mat to even out the inflow and outflow. Maybe covering more holes will increase flow at the holes near your work? That would also be easy to test. Turbulence is way above my pay grade, so no opinion on that!

[John Makar]

I built a downdraft box a couple of years ago, modifying the free plan from Rockler, while using cheaper but the same rubber rimmed holes plate from Amazon. That went okay, but then I decided to add a set of stainless finely perforated plates to use as a breathable finishing station. Those also work okay. Then I added some simple 1/4" MDF plates finished with wipe-on poly, to use as the base for a sharpening station. Then I added the nice silicon mat that Rockler had on sale to cover the MDF plates. All that stuff needed to be stored somewhere, so now I have a nice, strongly built 1/2" BB box with a vacuum port that is full of stuff. I think there is a real potential to over-engineer this downdraft thing.

[Jim Becker]

I'm actually considering alternative top panels for my design, John...while down draft will be the primary purpose for the surface, I expect I'll be doing some small-project assembly and or small parts finishing work on it so my primary bench stays free for client work.

[John Makar]

Oh, mine is useful. But I still end up slapping things on my old B&D Workmate and sanding them outside on the driveway. It's just a lot quicker than clearing someplace to pile things, and then unpacking and repacking my purpose built sanding box. For a lot of activity where I'll be setup for a day, or when it's raining and can't work outside, I actually sand things on it. Mine is a small shop, no box goes empty for long. But that doesn't actually solve a problem, it just changes it to a different one.

I suspect if you could set one up to fully dedicated to sanding it could be quick. Mine is 5" tall, 27" long, 17" wide, and has a 2 1/2" port that connects to a 1,700 CFM stage one dust system. That is barely adequate to create real flow to move dust into the system. What happens instead is dust gets drawn into the box, and I vacuum it out with my shopvac hose when I'm done. Combined with the dustport on my Festool ROS things stay mostly clean.

[Jim Becker]

...and has a 2 1/2" port that connects to a 1,700 CFM stage one dust system. That is barely adequate to create real flow to move dust into the system.

Yup...you can't get more than about 150 or so CFM through a 2.5" port and dust collectors work on the principle of moving large amounts of air. You'll get a better result just using a shop vac with that port because of its much higher static pressure which doesn't rely as much on "moving large amounts of air".

[Bob Riefer]

An addition to the downdraft cart last night. A couple of "top savers"... basically some runners that have little nubs of dowels sticking out that mate/index into holes drilled into the cart top - I can install the runners on the flat when cutting plywood on top of the cart or routing etc., or on edge when using the jig saw. This enables me to work above the downdraft surface, while also attached to shop vac, and keep the top from being damaged. Not ground breaking idea I'm sure, but for 10 minutes of effort and $0 spent, sure works well.

[Jim Becker]

That's a good solution, Bob, and similar to what I'm planning for an alternative top for small assembly, small finishing, etc..

[Bob Riefer]

I tested covering a portion of the top with a rubber mat tonight. Helped a LOT!

[Bob Riefer]

A bunch more shop time with the new cart, so it's worth one final report here...

Using thin rubber mats to cover a portion of the holes has resulted in much better suction, so this has been my usual process whenever possible. At times, the piece that I'm working on covers many holes itself, so the mats aren't needed in those situations. I have the mats just sitting on top, held down by suction, so it's easy to include / exclude them based on situation.

The shop vac with dust topper setup, along with outfitting many of my handheld power tools with dust ports also helps a lot. It makes certain cuts/actions more cumbersome, so I take this on a case by case basis, but in most instances have found it's worth dealing with the clumsy to enjoy a clean shop while I'm working. Also, lining the top of the 5 gallon bucket that the dust topper sits upon with weather stripping helped that component a lot.

Note that 2" J-hooks (used to hang plumbing pipe) are perfect pressure fit to hold 2.5" shop vac hose when not in use.

Since my original configuration, I have moved the power strip from the left side to the right side as I found that I was competing between hose and power cord (whereas now they both originate in the same place).

Good 4" locking casters (locking movement and swivel) are the only thing I'll ever use again. They roll over minor clutter easily and lock so solidly. Well worth the $$, and I have since outfit several other aspects of my shop with the same.

The stand offs that allow me to work on the cart without damaging the surface are a low-tech, zero price, amazingly useful add-on. Highly recommend.

That said... I love the cart in it's location so much, that I haven't really moved it out of its general location a lot lately. I do all my handheld power tool work here now, and I really only moved it while doing a bunch of planing recently. Was handy to be able to move it for that task, but I guess my point is that even if it was a non-mobile station, it would still be a very useful to have a easy-to-keep-clean setup like this..

[Doug Garson]

Good to hear it's working well for you and thanks for the follow up post. One way to manage the shop vac hose (and power cord) is to hang it from a hook on the ceiling. The screw in hooks sold for shop organizing work well. https://www.homedepot.com/p/Everbilt-Screw-in-Steel-Bicycle-Hook-with-Vinyl-Coating-25-lbs-21407/206585761

[David Utterback]

Thanks for the post about success with your table. Many good suggestions from all on improvements to consider. Your observation of the hose contraction is right on.