Lapping PEI

Hey I am trying to get my print surface really dialed in. I have run a dial indicator across the thing cold and hot and dialed in the corners to within ± .5 thou. I know the center of my bed is ~7 thou higher and was wondering if anyone has tried or had any luck lapping their PEI. I searched the forum and the googles and got no hits.

The apparent hump in the middle is actually not a result of a surface variance in the PEI. It’s downward rod deflection on the X and Y axis rods, but the X axis weighs less so it shows up as an apparent upward bed deflection. That variance is going to change with temperature, with mass on the print bed, etc. You may want to look at the Openbuilds extrusion modifications first before worrying about lapping the PEI surface, as that may resolve the issue for you.

If you decide to lap the surface anyways, PEI is sandable and somewhat millable. it will tend to be a little brittle, and if you try to use a power method like a dremel on the X carriage, chances are you will loose surface chunks. If you have access to a large honeing stone with a side profile bigger than 12", you could probably pop the bed off and do it that way, You would have to account for the glass and the adhesive parts, so it would not be easy,

Your starting layer is generally going to be thicker than the variance you are looking at, so any surface variance will generally just be absorbed by the part. If you are having adhesion issues, the Aluminum bed mod and the X and Y openbuilds mods will resolve that entirely.

Thanks for the reply,
That’s interesting that its sag in the x and y axis. But if the x weighs less wouldn’t it show up as dip instead of a hump?

As far as modifying the PEI goes, I think I would just do it by hand with some really fine grit paper as I am looking to take off a couple thou and I don’t trust myself to do it on any other machine. Let alone taking it off the printer as that introduces a whole new set of variables.

I don’t believe I am having adhesion issues, although I am trying to track down the source of some less than desirable effects on the first layer. I will check out those mods, thanks for the tip.

The X axis dips due to gravity, the bed dips due to gravity but at a different rate, and with different bearing spacing, so they don’t dip the same amount. The x carriage is narrower, so even though it is lighter it may deflect slightly more just due to being more of a point load? The rate of deflection goes down the closer you get to the rod mounts.

So I am looking at bed mods and a little unclear about the aluminum bed mod you were referring to. Are you referencing this
or this

This one actually:

You will need a PEI sheet or Buildtak sheet, a 3/16" thick 12" x 12" Aluminum plate, and another adhesive heater core. Or reuse the existing ones. Print otu the larger corners, insert the heat set inserts, and then enjoy smooth even heat to the edge of the bed rather than just the middle-ish area.

I use this plate:

if you look for the “fun with Infrared sensors” thread you can see a comparison.

Those platform leveling screws should be standard on the TAZ. I made the mod and can’t believe I went this long doing it the old way.


I use this plate: > > … xyLchRx4NT

I bought 2 plates from that guy on eBay and both of them are warped. Using them would cause more problems than they are supposed to solve.

I then bought some cast plates from Midwest Steel and they are 100% flat. Problem is, they weigh about a pound more than the ones from eBay.


Huh. That’s annoying. Mine was flat. Maybe see if someone locally can mill the cast plate down to 3/16"?

Do you think it would be too much weight to use for the bed using the Openrails mod?

The cast plates are 1/4inch and about 3 pounds. 1 pound more than the ones from eBay.


I actually bought one of those cast plates as well. I think it will work, but I was worried it would cut down my overall speed. it does feel pretty heavy. And then I got the flat 3/16" plate and didn’t need to test it out. I have a Y belt tensioner in mind that might help offset the risks of a layer shift there. It might also be worth getting a wider pulley and going with a wider belt.

Its a 3D printer folks, what are you making that you need to be within +/- .001 inch? Your printing with plastic and it expands and contracts… just like any material. If you need that kind of precision you need to buy a CNC milling machine.

There is no reason a 3d printer can’t have the precision of a good cnc machine. Mine does…

It depends what your needs are. I need a consistent good looking first layer across the entire bed. Others may not care if one area of the bed is smashed out and smooth while other areas are loose and rough.

And as I recall you have greatly modified your machine.

I too am not as concerned with part precision as part aesthetics. I have used other printers that use bed mapping to level the bed and know what my parts can look like. A flat, note I said flat not level, bed also helps when you are printing multiple copies, of a single part on the bed at the same time, have a consistent print quality.

I ordered one of those plates from ebay and it was full of scratches (deep scratches) and defects all over. So I’m returning the plate and buying one from somewhere else.

So I was wondering:

Does the grade have to be 5052 or can it be different?
Does it have to be .1875 thickness or can it be different?

The harder the grade the better. I found that anything less than 3/16" thick was prone to deflection, and those 1/4" cast tooling plates are milled flawlessly, but they are heavy, to the point I would say “too heavy”. If I were going to use one of those (and I did buy one) I would probably have it ground down to 3/16" by a local machine shop.