I have a number of jigs I built using MDF. Is this stuff moisture absorbent. The material does a good job and the results will let you complete some pretty precision work.
Do I need to coat and seal this material or is it stable as is.
Thanks in advance for your replies.
Replies
couple of coats of shellac will hold up for a while yes they machine well
MDF is not nearly as stable as plywood, it will expand and contract with changes in relative humidity. I have found from experience that it will move enough to cause problems with jigs that need to be precise.
Even the best of finishes will only slow down the absorption or loss of moisture, so there isn't anything you can apply to the jigs to make them entirely stable.
John White, Shop Manager, Fine Woodworking Magazine
so if i made a jig to do a 100 piece and sealed it with shellac do you think it would be worth it
Worth it?How much time would it take? 10 minutes?I seal every jig I make, with whatever odds and ends I have laying around -- varnish, shellac, et al.I don't really buy what John White has said.********************************************************
"It is what we learn after we think we know it all, that counts."
John Wooden 1910-
I seal mdf with two or three coats of west systems epoxy works great!
Alan Goldberg
http://www.alan-goldberg.com
I have made countless number of jigs out of MDF and have never had a problem. They will only absorb moisture if there is alot around or if you sit it directly in water. The only problem I have ever had with MDF is delaminations when used as a substraight with veneer. Plywood on the other hand doesn't maching like MDF, as in if you are making a shaper of router table jig, your edges won't be smooth. Plus plywood is wood, I know that it is meant to be stable, but like all wood it does move.
I have found that lipping the MDF or sealing it with shellac or I have used super glue works great. It also helps to preserve the edge.
http://www.kalafinefurniture.blogspot.com
Since I've been challenged on this I went to technical references about MDF.
In thickness MDF swells more than most hardwoods and softwoods gaining on average about 4% in thickness when going from 6% moisture content to 12% MC. This would occur in typical climate conditions in much of the world.
If exposed to high humidity, a panel can gain an average of 12 to 14% in thickness with a 4% permanent increase in thickness that won't disappear when the panel dries out.
In length and width, an MDF panel moves some but not as much as solid wood. For a change in relative humidity from 50% to 80% a typical MDF panel will expand an average of .002 " per inch of width. In practical terms this means that in an area like New England a 12 inch wide panel will expand about 1/16" between a dry winter and damp summer. This isn't a lot, but it is enough to cause problems with large jigs.
With the exception of encapsulating a panel in a boat builder's epoxy, no finish stops moisture gain and loss, a finish's only effect is that the jig will gain or lose moisture more slowly, but given a couple of months of dry or damp weather, the material will eventually move about as much as if it had no finish at all.
John White, Shop Manager, Fine Woodworking Magazine
im only asking john
would it be ok if i created the jig sealed with shellac and ran my 100 pieces the next day
i dont really think it would be that noticable
My response was addressed to "All" not directly to you, and it was Kaleo and Nikkiwood who questiond my assertion that MDF will move with changes in relative humidity.
In any case, the expansion and contraction of the jig only matters if you are doing work that needs a lot of accuracy, such as fancy joints, and you live in a climate where the humidity changes a lot seasonally.
The shellac probably won't make the jig sturdier, and won't make any significant difference in whether or not the jig will expand and contract with seasonal changes in humidity, but it won't do any harm either.
John W.
Edited 4/10/2007 6:21 pm ET by JohnWW
i apologize john your comments have always been constructive
I never question that MDF would move. But with those facts I didn't know it would move that much. With that said I would still use MDF for all my jigs, it just machines much nicer than plywood, you can fare out all the little dumps on a curved jig nicer and easier.But to be honest I lived in Virginia and right now in Tasmania Australia, and I never saw MDF that was taken care of ( meaning that it wasn't left in a overly moist place ) like I would assume most of our shops are a relatively dry enviroment, move. But to be honest I have seen it swell, but only when it had a water source real close to it.Kaleo
http://www.kalafinefurniture.blogspot.com
I made a cut off sled for my tablesaw out of mdf. Half the time it wouldn't work in the miter slots because of swelling due to moisture ( I had it completely sealed ). Finally made a new one using Plexiglass, works great. I NEVER, NEVER, NEVER make any of my jigs out of mdf. If I'm going to take the time to make an accurate jig I use the best materials I can . In my opinion, mdf is not that material.
Paul
MDF is the ideal material if your jig is to be used in a production run then discarded. Or, if your plan is to use the jig for production work that does not require accuracy over the long haul.I wouldn't (and don't) use it as the primary stock when fabricating "shop jigs" - any jig that is used for joinery - but I do use it regularly for templates, one-off production runs (usually during the milling process) and as secondary stock for production jigs. Never have had a problem. One coat of shellac then steel wool provides a smooth running surface.I toured a *major* shop this year that had MDF as their secondary stock for production run jigs. The primary stocks were (mostly) plastics of varying sorts.
tommy,
If you are making the jig to do one run in a day you shouldn't have to worry to much about it changing size or moisture content.
Thanks for the info John. I thought that MDF was more stable than that.
Rob
Using “technical references” concerning MDF - John that’s not fair. What are you trying to do, make sense on this sight? Stop it right now!
I make jigs for "lawn art", and short term use out of MDF. The 1/4-inch with white vinyl on it is great, because you can draw right on the surface.
For semi accurate long term jigs; Acurra ply, Euro ply, or any ply wood with lots of thin laminations is usually adequate. Occasionally you will need to sand and seal the edges to get them really smooth.
Things that are long term or high accuracy, are some form of plastic. Frequently UHMW-Poly as we have scraps at work, and it machines well.
In many towns you can get good buys on small pieces of acrylic, (lexan) from sign shops. They use it for back lit panels on strip malls etc., and end up with quite a bit of scrap.
Watch the bargain store for plastic cutting boards on sale. The white poly ones aren't too expensive if you catch them on sale. And, if you give the wife the pretty new clean one, and use the old grungy one your'e a hero.
Just about any plastic you can think of shows up on eBay in small sheets. I have gotten some very good buys on paper and linen phenolics. It is strong stable, hard on tools and a pain to work with but the best in some applications.
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