Work Holding

[martin54]

I used closed cell foam tape for mine, the sort that is sold for draught excluding doors & windows. I just mitre cut it at the corners & because I am using acrylic for the base I can pull a full vacuum with something like plastic on top. 7-8 inches would be enough to hold the material initially but once you start cutting parts out that is going to drop if you are cutting all the way through so I would be looking for it to be much higher initially.

Meant to say, I do have a couple of MDF jigs that I made for doing certain types of work that is repeated occasionally, they do work but not as well. That may be because I didn't seal the MDF as well as I thought I had, if I am using them then I need to keep an eye on the vacuum as it will start to drop off if the pump isn't running. I could modify the system so the pump starts & stops as required but have never got round to it.
The acrylic pucks I can just position & leave the pump off unless I am cutting through the material over the top of a puck.

[jbolt]

Another suggestion: Instead of trying to get a secure seal underneath the plastic, the plastic sheet could be dropped into a snug fitting recess (see example). Then just apply a strip of clear plastic (poly) packing tape on top, around the edges. This way, the tape will be pulled downwards into the tiny edge gap, making the seal tighter.

With this method, there is no need to seal the wood surface under the work piece. If the design has holes cut through the plastic sheet, place a piece of poly tape of rubber sheet over any holes as they are cut.

Instead of cutting a frame as shown in this example, strips of wood could be fastened to an existing vacuum table, as long as they fit firmly around the plastic sheet. For temporary vacuum fixtures, wood outside the immediate work area can be sealed with poly packing tape. It seals as well or better than varnish, and is fast & easy.

Thanks Rick, those are great ideas.

Jay

[martin54]

Rick, what are you using in way of a vacuum pump ?
I never seemed to have a lot of joy using MDF unless I sealed it as best I could but then I am using a vacuum pump that doesn't have a particularly high flow rate & only a small degassing tank as a reservoir.

[ger21]

Martin,
Try removing the reservoir.
If you don't have a lot of flow, then your reservoir is hurting your hold down.
The vacuum not only has to pull the air out from under your part, but it also has to pull the air from the reservoir, which is a much greater volume. By eliminating the reservoir, the pump doesn't have to evacuate as much air, and should be able to pull higher vacuum levels.

[martin54]

Martin,
Try removing the reservoir.
If you don't have a lot of flow, then your reservoir is hurting your hold down.
The vacuum not only has to pull the air out from under your part, but it also has to pull the air from the reservoir, which is a much greater volume. By eliminating the reservoir, the pump doesn't have to evacuate as much air, and should be able to pull higher vacuum levels.

No need to Gerry, my system works fine for what I do using the acrylic pucks I made which I use for most things that require a vacuum hold down. The MDF jigs I have don't get much use but are all sealed & work ok.

Just curious as to what Rick was using as he said he didn't have to seal the MDF under the work piece

Never seemed to have a lot of luck with unsealed MDF personally
If I ever go back to using MDF I will just buy a Roots blower, won't matter how porous the MDF is then

[Rcnewcomb]

Randall, coffee sounds like a great idea. Would have to be on a weekend as I work in the east bay.

You can email me at randall at 222artisans dot com

[Rick O.]

Hi, Martin

I've used a large vacuum cleaner for this. Because the airflow also cools the motor, it isn't a good idea for long jobs. Tape is already holding the work's edges in place, so it doesn't hurt to occasionally open a vent to let the vacuum cleaner motor have some air.

The surface holding the work down can be porous. After all, that is the reason for drilling holes in it. If the MDF was sufficiently porous, no holes would be needed at all. Of course, edges and surfaces below the plenum might need to be sealed. Wrapping in plastic, or poly tape, seals well and there is no waiting for varnish to dry.

Holes seem to work better than grooves on my projects, because cutting through the work to a groove appears to release vacuum from the entire workpiece, instead of just one spot.

This is just for engraving thin plexi sheets, to keep the surface flat. The work is actually held in place by the taped edges. Very flexible thin sheets are a pain to deal with so I use thicker plastic whenever possible.

[Rick O.]

Rotary engraving of nameplates, control panels, etc., is quite often done with a spindle equipped with a pressure foot.

The pressure foot helps hold the work down and ensures a consistent cut depth for lettering & artwork. The downside is the possibility of scratching the work surface, so the plastic or metal work is generally covered with paper or poly film masking.

A few companies make pressure feet to fit CNC routers. Search Google images for "engraving pressure foot" https://www.google.com/search?biw=1200& ... Zk#imgrc=_

[martin54]

Thanks Rick, I think it's mainly a case of finding something that works for the type of project/work you normally do. I didn't seem to get on to well with MDF myself but that could be due to a number of reasons, I had some 8mm clear acrylic that was to scratched to be useful for much so made some small pucks which seem to work quite well for most things I do, attached picture also has most of an MDF jig I made & still use in the corner, painted green because that was what I grabbed from the paint locker first
The pucks wouldn't work that well for very thin substrates as the material would sag & have no support between the gaps so I would have to make a much larger version if I were doing that kind of work.

My biggest problem is the additional height added to the table by any fixture, only got 55mm clearance under the gantry to start with so anything added to the table reduces the amount of height available for the actual stock. Looked at raising the gantry a couple of years ago & still haven't done anything about it

[jbolt]

I think I have a plan. I found a local supplier that stocks HDF so I am going to remove the 3/4" MDF from the T-slot table and make a grid board that is the extents of my cutting area. For the thin engraving material I will use a 1/4" for 3/8" HDPE board the size of my part or over size for locating pins and use the cover sheet gasketing material from All Star on the HDPE and cut out where I have holes and need vacuum channels. I can put cord gaskets where I need for the HDPE fixture. This should provide me with full support for the thin engraving material plus I will have a dedicated fixture that easily placed or removed.

Jay

[martin54]

Jay, don't forget to post pictures of the finished job, always good to close off a thread with a result

[CosmosK]

Man, all this is making me feel very good about my double sided tape method. Simple, relatively quick. No onion skin required, no extra motor to listen to. I use double sided masking tape: http://www.mcmaster.com/#76405a12 .007" thick and not gummy like carpet tape. Getting the 1/16" engraving sheet flat within .010" is easy. Engraving letters with ball end (vs. v bit) gives you margin for error too, I mean with engraving plastic, either you cut through the color of not, who cares, if one letter is .005" deeper, you won't be able to tell.
Unless you're making a bunch of exactly the same thing, the vacuum seems like more hassle than it's worth. my 2 cents. I'm sure I'll eventually have to try it too though.

[jbolt]

I found some 1/4" closed cell backer rod for $5 per 100'. Works perfectly with the thin engraving plastic. Sucked it right down without putting any pressure on it. The grooves in the grid board I made are a little too deep. The backer rod seems to work best with about 30% compression.

[jbolt]

I did a test part today. I didn't want to make a fixture for a one off part so I used double stick tape to hold the plastic. Everything ran fine except there is an anomaly in one of the engraving runs. The M didn't cut properly. At first I thought I was cutting too shallow so I increased the depth and it didn't change. I checked the material and it is flat so I'm thinking it is in the programming. I re saved the tool path with just that word and ran it again but no change. I need to check the code for a Z move in an odd place.

[jbolt]

Looked at the code and realized the font became too narrow to keep the tool deep enough with a vcarve tool path for the bit size. I changed to a one line font and used an engraving path.