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MDF for table saw miter bar on sled projects?

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Forum topic by CodeDaddy posted 02-17-2019 10:19 PM 859 views 0 times favorited 10 replies Add to Favorites Watch
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CodeDaddy

6 posts in 492 days


02-17-2019 10:19 PM

Topic tags/keywords: table saw miter bar cross cut sled uhmw mdf jig question

I’m going to try building my first table saw sled. I purchased a 4”x48” slab of UHMW plastic for sled project miter bars. I cut a 3/8” piece for experimentation. Attached it temporarily with Gorilla CA gel glue to a pice of good plywood (it was attached quite well). Then screwed it to plywood with 5/8” pan-head wood screws. Works beautifully. The screws in the UHMW were counter sunk with flat bottom drill bit.

BUT.. during all that I noticed that a 3/4” scrap of MDF fit the miter slot perfectly and slid smoothly. So I wondered if there is any good reason why MDF couldn’t be used for miter bars – round over or light chamfer on the bottom corners to prevent damage… why not? Cheaper than UHMW. Has anyone tried this? Good or bad results?


10 replies so far

View bondogaposis's profile

bondogaposis

5256 posts in 2650 days


#1 posted 02-17-2019 11:02 PM

I can’t say that I’ve tried it, but I will say that MDF is not very tough, and will probably start to crumble with use. It is also sensitive to moisture and may swell under humid conditions.

-- Bondo Gaposis

View therealSteveN's profile

therealSteveN

2203 posts in 873 days


#2 posted 02-17-2019 11:12 PM

Before gluing any MDF to a sled, or anything needing runners try this first. Cut your nifty 3/4” MDF into a 3/8” slice, then take that slice, and just shoot a screw through it. Usually it will break in 2, or well around the screw like it just bulged out. Glue alone probably won’t hold it for long, and even if you do get it through the first few minutes, putting it into and out of those miter slots will bugger up the edges pretty quickly.

Not an MDF hater, it just has some sues where it doesn’t work well, and surely doesn’t excel. The best will be the manufacture/adjustable ones you can snug for tightness. Also good is a non adjustable metal, usually aluminum bar made for a miter slot. Next is a good QUARTER SAWN hardwood, cut like below, you want the grain going up and down inside your slot, not side to side.

Any of the piece shown would work, but the best is right in the center.

Beyond those I would even use BB plywood before MDF or Plastic for a miter bar.

-- Think safe, be safe

View Dwain's profile

Dwain

587 posts in 4158 days


#3 posted 02-17-2019 11:17 PM

I’ve used UHMW, aluminum and hardwood for runners on my sleds. I woudn’t suggest MDF for all the reasons mentioned above. I think you would be happy with any of the three I’ve used. I have been. Hardwood probably wears the best, as it is less likely to dent over time. Still, I can’t imagine it has much of an advantage over the the other two. I say use any one of those three, but stay away from MDF. I just don’t think it will last.

-- When you earnestly believe you can compensate for a lack of skill by doubling your efforts, there is no end to what you CAN'T do

View Rick  Dennington's profile

Rick Dennington

6429 posts in 3493 days


#4 posted 02-18-2019 12:23 AM

I would not use MDF for all the reasons stated above…..MDF has its place sometimes, but in this case….NOT.!

-- " At my age, happy hour is a 2 hour nap".....!!

View JAAune's profile

JAAune

1864 posts in 2615 days


#5 posted 02-18-2019 12:51 AM

MDF can be soaked with epoxy to get a hard, durable surface. But epoxy isn’t cheap either.

-- See my work at http://remmertstudios.com and http://altaredesign.com

View BlueRidgeDog's profile

BlueRidgeDog

301 posts in 78 days


#6 posted 02-18-2019 02:27 AM

You can mix oil based poly and spirits at 50/50 and apply it to the mdf to give it some toughness…will soak in. No harm in using what you have and experimenting. I use hardwood mostly because that is what I have heaps fo scraps of. Build it, use it, improve it, replace it. Sleds (IMHO) are consumable to a degree.

View Desert_Woodworker's profile

Desert_Woodworker

1725 posts in 1513 days


#7 posted 02-18-2019 03:55 AM

MDF from the big box stores is not the same as a professional MDF- seek out the good stuff -Medite or Medex
Regardless, MDF or particle board, again no comparison- it is all about the rating i.e. A, B or C grades of your substrate. (density)

-- Desert_Woodworker

View CodeDaddy's profile

CodeDaddy

6 posts in 492 days


#8 posted 02-18-2019 05:03 PM

therealStevenN: I wouldn’t just “shoot a screw” through the MDF. I would attach it exactly the same way I did with the experimental UHMW I tried: drill a pilot hole, counter sink the screw head with a Forstner bit, and use a 5/8” pan-head wood screw. Seems to me that this would not break the MDF. But it would make it much weaker at the screw locations and maybe subject to future breakage at those points.

However, from other comments, the MDF seems like a bad idea. I did not realize there was a swelling issue with humidity. The MDF fits perfect now (winter, low humidity) but I don’t need trouble with swelling in higher humidity summer months. And treating it with some diluted finish just seems like too much trouble. The UHMW seems to be just perfect, at least for now, I guess time will tell. I do have some steel miter bar stock (I don’t remember where I got them – had ‘em for many years) that also fit the miter slot perfectly but I’m saving those for a big cross cut sled I plan to make after I finish the small-parts one that I’m starting now (using the UHMW). I’ve also thought about using hardwood but I’m leary of the humidity-swelling issue.

I’m not a total woodworking beginner, but maybe not quite up to “intermediate” yet. This is the 2nd time I’ve asked for suggestions on this forum and everybody has been so helpful.

Thank you.

View Andybb's profile

Andybb

1797 posts in 902 days


#9 posted 02-18-2019 05:17 PM

Didn’t read all of the above but….in my experience using MDF for that purpose, yes it always fits great in the slot but it swells depending on the weather so it gets a little tight when it gets humid out. Hardwood strips or stolen slices of my wife’s HDPE cutting board are my choices.

-- Andy - Seattle USA

View WoodenDreams's profile

WoodenDreams

505 posts in 210 days


#10 posted 02-18-2019 07:25 PM

The thickness of the MDF bar will change at the screw area when you tighten the MDF bar to the sled. Causing the need to file down that area. and more apt to have slop in the track.

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