next »

[Scott Peterson]

A vacuum hold-down has been filtering through my mind for long enough that I finally put it to use.  I thought I would post a few photos and maybe spurr a few of you onto ideas of your own.

I have been working on a new puzzle (not my design, but a great opportunity for me to make it) which requires a very small chip to be cut off of a rhombic pyramid block.  The chip is really too small to be held down by the toggle clamps that I normally use - and the RP block also needs to be cut off at about a 22 deg incline.  I finally worked out a venturi vacuum hold down method - which worked out really well. 

The venturi device is called a Ventuir Vacuum Generator and it really works pretty simple.  The devise uses a compressed stream of air flowing through a nozzle - inside the aluminum enclosure shown in the first photo.  As the air goes form the nozzle - to a larger diameter exit tube, the air expands and creates a pressure lower than atmospheric pressure (vacuum).  The vacuum port is located at the correct location of internal pressure drop, and a tube can be used to draw the vacuum through a stop block as shown in the second photo.

  I got this one from eBay for about $25 - but I have seen them for cheaper. 

[Canuck]

Very ingenious, not sure I would have even thought of the 'vacuum' hold down idea, way to go Scott! I've only ever seen 'vacuum chucks' on lathes, but never would have made the connection.  I think a pdf tutorial is in order(hint,hint ) Seriously though, can't you just throw some sawdust on your tablesaw (for ME at least) when you take pics, your giving me a complex:o
By the way your RP block tutorial was awesome thanks for taking the time to put that together

[Johan Heyns]

Now I know why I make only square stuff.  I agree on the dust bit John. 

Is the RP Block tutorial available somewhere?

[Canuck]

Hi Johan, I sent you the file by email let me know if it came through okay
By the way Scott has also done a pdf tutorial on making 'square sticks' eventually we'll have all these 'gems' together on the forum, but we're all a little busy at the moment
Have a look under: 'Knowledge Base' everything is there so far

[Robert Yarger]

What a wild idea.  To make pieces that small I have always had to cut small angle slices off of a larger bar that could be properly stabelized.  Without any way to hold down the small end piece I wanted, it would always fly off as the blade finished cutting through the wood.  I would have to toss away 4 or 5 garbage pieces for every piece that actually survived, and even then you had to sand a small residual nubb off.

  This method would save a lot of time and a lot of wood.  It is hard to believe that such a small vaccume could hold down a piece so firmly.

[Mark McCallum]

I have often thought it would be nice to have a vacuum hold down.  I just always assumed it was a high $ device.  Thanks for the tip.

Do you have any problems with sawdust entering into the vacuum tube and clogging the system?

What amount of vacuum are you able to pull with your system? I assume it is enough that your RP is not moving while you cut it, right?  How would it work with slightly larger pieces where the vacuum point is further away from the blade (ie a larger moment arm)?

[Canuck]

Sorry Scott I don't mean to steal your post but here's a link to Lee Valley's 'vacuum clamp device' it's a new item from their mail order catalog:
http://www.leevalley.com/wood/page.aspx?c=1&p=59743&cat=1,43838
This clamp boasts 330lb of clamping force

[RKB]

This clamp boasts 330lb of clamping force

Note that the clamping force is determined solely by the 'vacuumized' surface of the piece to be held in place.

In normal circumstances there is air pressure on every face of the object. The calmping force then is simply that air pressure taken away from a single surface.

[Scott Peterson]

Thanks for the comments, guys - glad this might prove useful to you.

The venturi vacuum generator is a really simple device, no internal moving parts.  It simply takes compressed air which blows straight through the enclosure, with the vacuum tube placed at the correct location of pressure drop just pas the nozzle inside.  If a piece of dust get sucked up by the vacuum port, I assume it would just get blown through.  This is why I didn't go with a vacuum pump type assembly - similar to the sets used for veneering.

Most of the venturi vacuum generators can produce a vacuum of about 25 inches Hg.  The thing to watch is the air consumption if you have a small compressor.  I don't run mine continuously, just when I'm making the cut.  I think mine consumes about 4-6 cfm at 90 psi from the compressor.  Vacuum is measured in atmospheres or inches of mercury - not psi (pressure).  I guess pressure implies pressing on an area...  I think Lee Valley uses pressure to put their description in terms we normally work with.  I was corrected on the phone while researching this!   

It works pretty well, but due to the inclined surface of the stop block - also where the vacuum is applied to the piece - the chip tends to walk up the inclined surface until the blade no longer contacts the chip.  This is the reason I put the other toggle clamp above the location of the chip - just to keep it from sliding up along the inclined fact of the stop block.

I will get a PDF together and send it along.

Thanks for the link John - by all means, any ideas are definately worth it!  I welcome all ideas since that gives me new ideas too, which is the best part.  I'm curious if the Lever Valve of the Lee Valley system is the venturi device, or if the venturi is located in the vacuum base they show.  This is the same idea though...  Thanks!

[Eric Fuller]

John, I think you may have solved my "How to keep stock flat in the laser cutter" problem...which is very important since the stock has to be at an equal distance from the head at all times in order to maintain proper focal length of the beam.

Thanks!  It was giving me fits...