How To Flatten A Plate

If it is not hardened then I would try to scrape it by hand and see how it goes. You're not going to take enough off by scraping to mess it up any worse. You may even start with some wet/dry sandpaper on a surface plate or other flat surface, which may get you close enough.
 
Thanks for the input. I ended up supporting it along its edges on the mill and using the will to apply pressure to bend it back. Using as metal rule and observing for light under the rule I was able to get it within 0.1mm among most of its surface which is adequate for this implementation. It was 0.4mm before.
 
For similar future projects, and for others who stumble upon this thread... this would have been a good place to hone your lapping skills. Lap any three surfaces against each other, and they'll all wind up as flat as they can be within your shop's limits - flatter than either milling or scraping can make them.
 
One thing to consider is thermal expansion. The heater is mounted on the plate, but I'm guessing it does not cover the entire plate. So, despite aluminum's good heat conduction, some parts of the plate will be cooler than others especially around the edges where it is fastened to the frame which is a heat sink tending to keep the edges cool. While I can't think of a mechanism that would work, it is possible that the dome in the original part was there to accommodate this problem. In any case, I'd suggest mounting the plate to the frame and heating it up and see what you get for flatness. It takes an amazingly small amount of material growth on a constrained plate to generate a "bubble" in the center. Good luck with your project and the mail strike.
 
Thought of that Ed T, I even assist the plates levelness with it hot as well as the extruder head (since this is what I need to zero the Z Axis against. This was how I noticed the plate was bowed. It only hearts up to about 90 degrees C anyway.
 
I know you already solved your problem but wanted to add this idea if needed in the future. Finding the flatest surface you can like a table top & sit a piece of thick glass on it. Then gule a grit that best suits your needs of sand paper to it & lap the surface to the glass. I've never tried it myself but seems like it would make a good poor mans granit surface.
 
atunguyd said:
Hi All, can any of the experts among you give me a hand with something?

I am working on a 3D printer that I have built up, the bed that things are built on is made from aluminum plate with a heater built in under it, the plate is about 210 x 210mm and 3mm thick.

For the printer this bed needs to be a flat as possible and it is mounted in the machine with bolts at each corner, the bolts attach through springs which allow one to level the bed by adjusting the mounting bolts.

Problem is that being thin aluminum the bed has been manufactured with a bow in it leading up from each corner toward the centre.

I mounted it flat on my milling table and took a light surface cut with a 50mm index face mill but I was not thinking and all I achieved doing this was to flatten it with the clamps, face mill it and then reintroduce the bow when the clamps were removed (I think it is even worse now).

I am looking at ideas on how to now flatten it more correctly.

What I have come up with so far is the following:

Taking two sturdy pieces of aluminum bars longer than the plate I tapped holes in each bar and mounted the plate to this bar using its mounting holes. The bars in turn were clamped to the milling table. This now in mind has the plate secured to the table but still allows it to keep it's natural bow.

I then plan on face milling it again like this (very lite cut, almost like a lathe spring cut), which I am hoping with make the top flat even though the bottom may be slightly bowed (when in use it does not get much pressure on it, only light plastic parts are built on it).

I started this tonight but the chatter was terrible and amplifier so much by the plate that I decided to try again tomorrow as making such a racket at midnight is very neighbor unfriendly.

In the mean time maybe some of you may have a better idea on how to go about this?

Thanks

Anthony
Click to expand...
 
take the plate off and on a solid flat surface lightly tap around the edges of the plate with a 1 lb hammer. What has happened and happens with lghtplates is that they expand in the center of the plate and dont exactly contract at the edges but the edges are cooler. As contraction is greater than expansion - after use the plate will always have a bow. Now to avoid hammer marks in the plate, grind a slight curve on the hammer face - practice on a bit of scrap till you get the desired result.
 
3mm thick is only 0.118 inches so you can't remove much material. You really only want to polish away the high spots. I'd get several sheets of emery cloth in varying grit sizes and place those on a plate of glass. Rub the part on each grit, starting with the most coarse. Rotate the part 90 degrees with each successive grit. Your part will be very close to dead flat and polished in no time.
 
For what it's worth,we flatten sheets of silver by clamping them between 1/4" thick flat hot rolled steel plates with heavy duty C clamps and heating the plates red hot. Then,let the whole thing cool overnight slowly in air. This relieves the stress in the silver sheet and makes it flat.
You would not heat aluminum that hot,of course. I'd suggest putting saw dust on the top of the uppermost steel plate. Then,heat from beneath until the sawdust turns black and catches fire. Dump it and let the assembly cool overnight.
 
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