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Downdraft Table Troubleshooting

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Forum topic by ~Julie~ posted 08-11-2020 11:02 PM 546 views 1 time favorited 15 replies Add to Favorites Watch
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~Julie~

623 posts in 3882 days


08-11-2020 11:02 PM

Topic tags/keywords: dust collection downdraft sanding table question

Can I get some help with my sanding table, I am trying to get maximum suction, here are the details and questions.
I have a ClearVue Cyclone with 6” piping.

The downdraft table top is about 25” x 43” and is pegboard sitting in a rabbet, not glued down or attached. I have cut some larger holes in the pegboard. The inner box has a slanted floor leading to 6” pipe which then heads up to a gate and continues in 6” piping to the Cyclone. This should all be clear in the attached photos.
Questions: should I cut more larger holes in the pegboard? Would blocking off some of the pegboard or the table itself at the right end help with suction?

Thanks for any advice,
Julie

-- ~Julie~ followyourheartwoodworking.ca


15 replies so far

View kajunkraft's profile

kajunkraft

190 posts in 3058 days


#1 posted 08-11-2020 11:29 PM

My thought is larger holes/more of them. Can’t send a picture because I no longer have/use it, but for years used a table similar to yours. The top was plastic lattice, holes about 1.5” x 1.5” (?). Started out with pegboard but found the holes to be way too small. I suppose that if you enlarge a lot more of the holes it may work better.

Another difference in mine which may or may not be applicable for you is that it was not picked up by my regular DC system. I used an A/C squirrel cage blower with a few filters between the top and the suction fan. It was easy enough to take the filters out occasionally and shake the dust out, shop vac out the rest of the box.

View MrBob54's profile

MrBob54

9 posts in 1233 days


#2 posted 08-12-2020 12:26 AM

Julie
Here is a link to Bill Pentz’s downdraft table design. He is the original designer of the ClearVue. I was a HVAC Mechanic for 25 years and I like the way this guy thinks when it comes to dust collection.
http://billpentz.com/woodworking/cyclone/downdraft.php

-- Bob, Washington State

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~Julie~

623 posts in 3882 days


#3 posted 08-12-2020 12:12 PM

Thank you kajunkraft. I do think I need some larger holes.

Bob- Thank you I previously have read all of Bill Pentz’s online information. This is why I used a corner because two walls are the same as the side baffles he suggests. Perhaps a high side on the third side on the right where the pipe comes in will help? I can’t follow his sizing information though, this is why I wonder if I should remove some of the holes on the top right side. The pegboard just sits on top and I could make a solid top on the right third. I just am not sure if my table is too big, or what size the holes should be, and Pentz’s writings don’t make that clear to me.

-- ~Julie~ followyourheartwoodworking.ca

View Redoak49's profile

Redoak49

4869 posts in 2836 days


#4 posted 08-12-2020 12:15 PM

I have a down draft table hooked to a 5 HP DC. I cover up parts of it that are not close to my workpiece. I use pieces of thin mdf.

View ~Julie~'s profile

~Julie~

623 posts in 3882 days


#5 posted 08-12-2020 01:06 PM

Thanks Redoak49, I will try that!

-- ~Julie~ followyourheartwoodworking.ca

View justoneofme's profile

justoneofme

835 posts in 3327 days


#6 posted 08-12-2020 02:25 PM

Hi Julie! Good to hear you’re keeping busy!! I have no input to share regarding your downdraft table … other than it looks mighty impressive. But I do give thought to you on PEI every now and then, hoping you and yours are safely avoiding Covid. Good luck with suction!

-- Elaine in Duncan

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therealSteveN

6241 posts in 1422 days


#7 posted 08-12-2020 02:31 PM

It really isn’t a straight forward path to the correct mix of holes to allow air in, and how much to keep it from that.

On a contractor saw, you need to seal it up to get good dust collection, but they are wide open. On a router table with an enclosed box, you usually need to create an air draft to allow for good collection.

In all ways your box looks workable. SO when you do start making holes, drill ONE at the extreme range of where you think your “normal” project will sit. Try it out. Proceed one hole at a time, until happy.

My personal thought is looking at your projects to date, your table itself is huge for the sanding of what you have been doing, so I think Redoak’s point of covering up what you aren’t using has a lot of merit.

-- Think safe, be safe

View theart's profile

theart

224 posts in 1402 days


#8 posted 08-12-2020 04:07 PM



It really isn t a straight forward path to the correct mix of holes to allow air in, and how much to keep it from that.

On a contractor saw, you need to seal it up to get good dust collection, but they are wide open. On a router table with an enclosed box, you usually need to create an air draft to allow for good collection.

In all ways your box looks workable. SO when you do start making holes, drill ONE at the extreme range of where you think your “normal” project will sit. Try it out. Proceed one hole at a time, until happy.

My personal thought is looking at your projects to date, your table itself is huge for the sanding of what you have been doing, so I think Redoak s point of covering up what you aren t using has a lot of merit.

- therealSteveN

I think that the best starting point is to have the total hole area be slightly larger than the cross-section of the duct. Too much bigger and you lose flow velocity (the un-sealed table saw), and too much larger you’ll lose pressure (the router table). Also, I would want a thick top with as much of the total area fraction to be holes as possible. Peg board sets up more of an orifice plate situation, which is not ideal for moving particulates.

View EarlS's profile

EarlS

3938 posts in 3196 days


#9 posted 08-12-2020 04:32 PM

I’d be VERY hesitant to start drilling big holes. Your DC line is 6” diameter, that means you have something like 28 in^2 of cross sectional area in the line. This is the number of holes it takes for different diameters to equal the cross sectional area of your DC line.

hole dia (in) # of holes
0.125 2304
0.25 576
0.75 64
1 36
1.5 16

From there you can figure out a grid and space them out accordingly. Personally, I’d go with pegboard since the holes are 3/16” and they are already drilled. Double it up to get some thickness and put a 3/4”support frame under it so it won’t sag.

I don’t think the orifice issue will result in a big pressure loss. Consider that random orbit sanders typically have 1/4” openings to pull dust away as it is sanded away. Covering up those holes not used wouldn’t hurt either.

FWIW – I’m an engineer so I deal with designing pneumatic transport systems and dust collectors as part of my job.

-- Earl "I'm a pessamist - generally that increases the chance that things will turn out better than expected"

View GrantA's profile

GrantA

2850 posts in 2255 days


#10 posted 08-12-2020 04:55 PM

I have seen lots of pegboard variants and it seems like the owners are rarely thrilled with performance. I’m going to make one along the lines of this one from Felder – I haven’t noticed any others like it, I just stumbled across this one a while back while looking for something else on their site.

As for what you’ve got, I’d play with covering up some of the small holes at the edges with tape and see what that does

View pintodeluxe's profile

pintodeluxe

6204 posts in 3661 days


#11 posted 08-12-2020 04:57 PM

My downdraft table is very similar to yours. I started with many small holes and the collection wasn’t very good. So I added a bunch of 3/4” holes and performance improved. If I had it to do over again, I would skip the small holes and only use 1/2” to 3/4” holes everywhere. Then just lay a scrap of plywood on some of the excess holes for maximum suction.

In practice, I usually have a stack of parts I’m sanding, and leave the finished parts on one side of the downdraft table to maximize suction.

So… experiment, but I bet you’ll like it best if you swap the pegboard for solid sheet stock with larger holes.

Good luck with it!

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View Holbs's profile

Holbs

2366 posts in 2877 days


#12 posted 08-12-2020 05:58 PM

For my downdraft table, I used some calculator (maybe the Pentz site?) that determined how many 3/8” (the most recommended size for downdraft hole diameter. I knew peg board holes were too dinky) for the CFM I was pulling. 800 holes!
https://www.lumberjocks.com/projects/312474

-- The Carpenter Bee is derived from the Ancient Greek word wood-cutter "xylokopos/ξυλοκὀπος"

View ~Julie~'s profile

~Julie~

623 posts in 3882 days


#13 posted 08-12-2020 10:19 PM

Thank you to all who took time to offer advice. I will experiment!

-- ~Julie~ followyourheartwoodworking.ca

View MrBob54's profile

MrBob54

9 posts in 1233 days


#14 posted 08-14-2020 02:55 AM

Using EarlS’ logic and Bill Penz recommendation of 3/8” holes you would need 254 of them to equal the same cross sectional area of a 6 inch duct. Good idea using the corner walls as sides. You could add a third removable 6 inch high side along the left side.

-- Bob, Washington State

View davezedlee's profile

davezedlee

59 posts in 1672 days


#15 posted 08-14-2020 11:01 AM

the slope looks to be 4 feet long, but the holes only extend halfway

might want to put a blocking panel 1/4” downwards from the right-most set of holes

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