MDF instead of Granite or Glass for Lapping Planes

[Dwain Lambrigger]

All of you plane refurbishment experts out there, here is a question.

I was in Woodcraft yesterday, talking about refurbing old planes, and one employee stated that he used MDF as the substrate for sandpaper when lapping instead of glass. I wanted to get your opinions. I mean, MDF is cheap, and it is pretty flat, but is it flat enough for this? I can't imagine using anything thinner than 3/4 inch, but then again, I am not sure this will work in the place of plate glass.

Please provide your opinions, and thanks.

[Gary Herrmann]

If you lap while dry, it may be flat enough, but if you create a slurry - the MDF won't be flat for long. Glass isn't that expensive, and you won't have any problems with it unless you drop it.

[Kurt Bird]

Dwain,
I am a trim carpenter by trade and work with MDF all the time. MDF is not nearly the flat surface you want to use for fine chisel and blade sharpening. My understanding is that the least expensive and flattest is plate glass (because it is made by floating across a surface, so that gravity levels it out). And since you typically don't need anything bigger than, say, 4x10, it is really reasonable in cost. BTW, don't forget your infeed and outfeed tables on your jointer, or the bandsaw table, or the main part of your table saw, as long as they were machined at the factory. As long as you don't mind a little cleanup.
Kurt Bird

[JayStPeter]

Go to a glass shop. If you don't need a finished edge they may give you some scrap or at least sell it to you cheap. If you prefer the finished edge, you'll pay.

[Brian Kent]

I found some 12" square marble tile at Home Depot that is totally flat according to my reference straight edge. It is polished smooth and I imagine it to be safer than glass if dropped. It was under $4.

[Dave Lehnert]

I found some 12" square marble tile at Home Depot that is totally flat according to my reference straight edge. It is polished smooth and I imagine it to be safer than glass if dropped. It was under $4.

I always had the idea I would try what you did but never got around to it. Thanks for the info on how flat they are. Sounds like the way to go IMHO.

[Pam Niedermayer]

Where mdf comes in handy is when sharpening gouges. In that case you can dig a mirror image of the gouge, throw in some sharpening powder, whether diamond paste or silicon carbide or whatever, then sharpen the gouge in the powder filled depression.

Pam

[Jon Toebbe]

I found some 12" square marble tile at Home Depot that is totally flat according to my reference straight edge.
I'm using a couple of marble squares I picked up for free from a kitchen countertop shop. They were samples for discontinued lines, and the guy I talked to offered to help me load up a ton of broken scrap and other pieces. I took a large, rectangular piece (~24x36) that'll make a dandy top for the sharpening bench I'm going to build "one of these days."

They're all nice and flat, and didn't cost any more than gas and being willing to ask for them.

[Mike Cutler]

Dwain

I wouldn't use it exclusively. Maybe in a pinch.
MDF can warp, curl, twist, and cup ( I've seen all of these). Glass can't. Easier to buy the glass once than keep cutting MDF panels.

Pam
That's a pretty cool tip for doing gouges.
Thanks

[Michael Schwartz]

All of you plane refurbishment experts out there, here is a question.

I was in Woodcraft yesterday, talking about refurbing old planes, and one employee stated that he used MDF as the substrate for sandpaper when lapping instead of glass. I wanted to get your opinions. I mean, MDF is cheap, and it is pretty flat, but is it flat enough for this? I can't imagine using anything thinner than 3/4 inch, but then again, I am not sure this will work in the place of plate glass.

Please provide your opinions, and thanks.


Which specific woodcraft was this, I think I might know the guy you are talking about.

Personaly I wouldn't reccomend MDF and would use either the bed on a jointer (make sure it is flat first), or float glass that has been verified to be flat.

Idealy you wan't the sole on a plane to be as flat as the thinest shaving you are trying to achieve is thick. This would give a tolerance of arround .001" or beter

[Eddie Darby]

MDF being a wood based product suffers from all the things that wood products in general do, and that is movement.

If you want to build a system that will tackle any job, then I would get 3/4" thick Float glass. Make sure it is cut big enough for whatever you will be tackling.
I've seen some shops get a piece of glass that has several pieces of different grit abrasive paper on it all at once, and can flatten even a #8 plane.

The amount that it will cost, versus the valuable work that it will do, makes this a good buy in the long run.

[Dwain Lambrigger]

I don't remember the employee's name. He was new, someone I hadn't seen before. Sounds like plate glass is the way to go.


Which specific woodcraft was this, I think I might know the guy you are talking about.

Personaly I wouldn't reccomend MDF and would use either the bed on a jointer (make sure it is flat first), or float glass that has been verified to be flat.

Idealy you wan't the sole on a plane to be as flat as the thinest shaving you are trying to achieve is thick. This would give a tolerance of arround .001" or beter

[Ken Werner]

I have used glass [1/4 inch thick] and MDF [3/4 inch]. I don't see a difference. I think it depends on what you're trying to do. If it's initial work at say 80 or 120 grit on a plane bottom or side, MDF works fine. I feel safer using it than glass. I know the glass should ideally be 3/8 or more thick, but I used what I had.

[Craig Che]

I second stopping by a local granite dealer, you can probably get a sink cutout from a counter for free. I got a couple of pieces from a local place free, on large enough to do a bathroom counter with.

[Tony Zaffuto]

As I answered on another forum, whatever used, be it MDF, granite cutout from a countertop, marble floor tile, glass, machine top, should be verified with a known straight edge and feeler gage.

[Randal Stevenson]

MDF is used to make a Mr. Sawdust table, that is flat, but they require a flat surface to build to begin with. (needs a bench to build a bench)

If one were going to use MDF, Consider a torsion box assembly table design, not just a single sheet.

[Robert Rozaieski]

I haven't lapped a plane sole in awhile but when I did, I found that getting a flat enough reference substrate wasn't the problem, it was whether that substrate was stiff enough. Anything will work, mdf, glass, granite, as long as it is stiff enough. And if you don't think granite or glass will flex, think again. If it is not thick enough it will flex.

When I first started out, I had a very roughly built workbench with a top that had a hollow over it's length. I tried to flatten a jack plane on a marble threshold, the kind you find at the BORG for making the transition from a tile floor to another type of flooring at a door jamb. This was about 3/4" thick and 36" long by maybe 4" wide. Much thicker than a tile or your typical piece of float glass. However, because my workbench wasn't flat enough to support the marble over it's entire length, the marble flexed and I made the sole of the plane worse by sanding it more convex over it's length. When I got a 3" thick granite surface plate, that solved the flexing problems.

Now I'm not saying you need a granite surface plate but if you want to lap your planes, that's my recommendation. You don't need "A" grade for woodworking tools and the prices of these have come way down. You can get them now at some suppliers for about $20 for one big enough to do a smoother or jack plane.

However, there are many other ways to flatten a plane sole that will work without getting a 3" thick granite plate. I haven't used mine for lapping soles in years. The most important thing to make sure of when lapping, whether you use a marble tile, mdf or glass is to make sure it's on a flat surface or that it is adequately supported along it's length to prevent it from flexing when under the weight of the tool plus the pressure of lapping, otherwise it will not remain flat no matter how flat it was originally.

These days, if a plane really needs flattening (which they rarely do) I prefer a mil file and straight edge instead of lapping. You don't need to worry about a super flat reference surface except for your straightedge and even then, a typical steel ruler from a combination square is typically straight and stiff enough. It's much faster than lapping and works as good if not better since there's no chance of dubbing the edges. And you can do any length plane, even long jointers, without needing long sandpaper or a really long reference surface. And, there's no sandpaper to wear out and constantly change, which can get expensive when lapping since you need to change the paper very frequently. With a 24" ruler and a mill file, I can flatten any iron plane sole plenty flat enough for working wood.