Thin plates bend when clamped

[Carbuilder]

There doesn't seem to be a section on general machining questions, so I'll ask here. I make a lot of parts from 1/8" thick aluminum plates. I've notice that the plates tend to bend when they are clamped down. I don't think I'm doing anything different than I used to, but it is more noticeable now. Especially when I have to space it above the table a bit to allow for cutting through in places. Here is a 4" x 3" x .125" plate clamped down at the ends.



I have 1/16" thick aluminum pieces at the ends, under the plate, where it is clamped, and one in the middle. The plate deflects up in the centre so the ends are about .010" lower than the centre. Not a huge amount, but when you are machining features .015" deep, it basically is no good. I could clamp it at the top and bottom too, but that seems excessive for such a small piece. I've resorted to pre-bending the plate up at the ends to try and minimize this, but that's not a very good solution.

Any thoughts on why this happens and what I should be doing different?

thank you,

Rick

 

[kwoodhands]

Try clamping over a solid scrap. I use MDF often because it is flat and cheaper than metal. I do not know why the ends are down .010
Is it possible the shims are not the same thickness? If the center shim is thicker than the ends then this may cause the problem.
Other than that I don't have a clue.
mike

 

[Jasonb]

Is it actually cut from plate or from 3" or 4" flat bar.? I have found extruded bar and section can be a bit thinner towards the edges which would account for your lift.

If you are engraving 0.015" deep then worth skimming across the top first

 

[timo_gross]

Is it actually cut from plate or from 3" or 4" flat bar.? I have found extruded bar and section can be a bit thinner towards the edges which would account for your lift.

I made a sketch to explain to myself what Jasonb explains. If you can afford it (in terms of size) You could try to cut the step in the clamp longer to keep the clamp more away from the edge. Maybe you can add a little recess in the clamp to make sure it clamps on its tip not on the inside corner?

p.s. I do not want MDF in the coolant tank, so I would not use it as sacrificial plate.

 

[Carbuilder]

The material is cut from a piece of stock, 4" wide by several feet long. I didn't think to check how flat it is, but I can see the centre going up as I tighten the clamps. As I release the clamping pressure it goes back down. Some simple physics going on here. I did remove the centre piece of 1/16" aluminum that was under it since it wasn't doing anything.

I just got the posting by timo_gross as I was writing this. I did re-clamp the piece, raising the ends of the clamps like this:



That did help. So I sketched up how I should modify the ends of the clamps. I goes along with what you just showed. I was thinking of something like this:


Same idea as what you show. Clamping a little distance in from the ends should actually push the material down in the centre. But you may have a point with the material being a little thinner on the ends. A couple thou thinner on the bottom causing it to lever upward in the centre.

I will often put a scrap piece of MDF under a part. I use mist cooling so not an issue with the wood deteriorating.

Thank guys, that helps a lot.

And I'm in Canada so my autocorrect keeps changing center to centre.

Rick

 

[timo_gross]

Made a test with a piece of hot rolled plate I found, revealing a predictable outcome. The taper on the edge is quite visible on those plates. I overdid the angles of the clamp on purpose.
Packing on both ends is same height if the rear of the clamp is lower than the workpiece I can see it lift the opposite side. When the clamp is lower on the workpiece side it does not lift it visible.

p.s. Haha now we beat ourselves, while I was making my test you did too, with the same outcome.... :- )

 

[Jasonb]

As it is from 4" flat bar, try rotating the part 90 degrees and clamping on the two long edges and see if it makes a difference. There should not be any change in thickness doing it that way.

 

[Carbuilder]

It's interesting what you say about rotating the piece of stock. These are custom car badges I make, and most of them are from strip stock anywhere from 3/4" to 2 1/2" wide material, so they are naturally clamped on the cut ends (the ends I cut) when I put them on the mill table. The 4" wide one I sometimes clamp on the ends I cut, if I am making a number of badges at the same time and maybe have piece of stock about 4" high and 8" wide. But sometimes I clamp on the sides of the stock as it comes. Maybe that's why sometimes I get lifting more than other times. Interesting....I'll have to keep an eye on that.

Anyways, I did machine the end of the clamps to put a slight undercut, and raised them up a bit. No lifting, but of course that was with a piece clamped on the ends I cut:


And then:



Damn! My newly machined clamp! Oh, well, that's why they are aluminum. I was trying to minimize the size of the blank and forgot that these are not rectangles, but have angled sides. I run the cutter around the outline about .010" deep, then bandsaw it out after.

Thanks again for all the help and suggestions.

Rick

 

[STEVEJ]

The material is cut from a piece of stock, 4" wide by several feet long. I didn't think to check how flat it is, but I can see the centre going up as I tighten the clamps. As I release the clamping pressure it goes back down. Some simple physics going on here. I did remove the centre piece of 1/16" aluminum that was under it since it wasn't doing anything.

I just got the posting by timo_gross as I was writing this. I did re-clamp the piece, raising the ends of the clamps like this:

View attachment 136113

That did help. So I sketched up how I should modify the ends of the clamps. I goes along with what you just showed. I was thinking of something like this:

View attachment 136114
Same idea as what you show. Clamping a little distance in from the ends should actually push the material down in the centre. But you may have a point with the material being a little thinner on the ends. A couple thou thinner on the bottom causing it to lever upward in the centre.

I will often put a scrap piece of MDF under a part. I use mist cooling so not an issue with the wood deteriorating.

Thank guys, that helps a lot.

And I'm in Canada so my autocorrect keeps changing center to centre.

Rick

Center to Centre............Correct!

 

[Badhippie]

Try clamping it 90 degrees from your spacers and make sure your clamp goes from end to end.

 

[Badhippie]

That or use mite bite clamps they will apply side pressure to the plate by the way of an eccentric

 

[Badhippie]

Miter-bite clamps sorry for the mis-spelling

 

[Badhippie]

Damn spell check it’s mitee-bite clamps

 

[ShopShoe]

Food for thought: Covered in posts above, but interesting none-the-less. Skip this post if you want.

The un-even-ness issue of stock and whether it is more, or less on different stock due to extrusion or other forming is a problem sometimes.

A few years ago I suspected the same problem from time to time and spent some time testing.

Took the stock and blued one side (face) all over. Put abrasive paper on sheet of plate glass and lightly sanded stock piece. Used different holding techniques to try to avoid more pressure in local areas. Result showed definite high and low areas. On the project, I modified clamping to correct and sometimes continued sanding the stock flat before marking and milling. (BTW, my granite surface plate would possibly be better, but I don't use it for sanding.)

Took the stock and clamped it in the mill, trying some of the things as mentioned above. Carefully ran it past a fly cutter, keeping feed as constant as possible. Just light cut. Result showed irregularities as well. Ultimate result was to think through clamping and order of operations more carefully.

All of this was done as a special series of tests as part of my learning experience. The end result is that sometimes more thought that might slow down individual steps can same more time in the total project.

--

Some other things I have tried or that have been mentioned:

Double-sided tape either to the table or to stiffener material, either MDF or aluminum.

Hot-Melt Glue to augment clamping or add stiffeners.

Various adhesives as used by watch and jewelry makers.

--ShopShoe

 

[Carbuilder]

Not quite related, but I have found that the 3/4" wide strip stock I buy does vary in thickness. Sometimes I'll buy a length of it from Home Depot and sometimes from Princess Auto (a Canadian company that sells all sorts of weird stuff). I've measured it and the Home Depot material can be about .002-.003" undersize while the Princess Auto is usually right on. Makes me think they are trying to save money like food products these days.....reduce the size of the product but don't reduce the price! Anyways, no doubt it may also vary in thickness a bit along the length.

I've never used mitee-bite clamps, but they do seem interesting. Just had a quick look at their website and they do have a lot of different designs. Might be an issue with thin plate, pushing in on the sides might cause more bending? Of course there are times, like others I'm sure, when trying to clamp something thicker when you don't have an edge that won't be machined. Have to look into those a bit more.

Rick

 

[Badhippie]

There are also vacuum plates on the market that work very well also mag plates that work very well. But they won’t work well for going through the material. The mitee-bites work very very well. you do not need a machined edge for them to work but again they will not work for raised material. Can you mount your material on a scrap piece of aluminum plate. Or as mentioned above a piece of MDF. I have also used two face 3m tape as mentioned above and it works but again not for raised material
Thanks
Tom

 

[HMEL]

There doesn't seem to be a section on general machining questions, so I'll ask here. I make a lot of parts from 1/8" thick aluminum plates. I've notice that the plates tend to bend when they are clamped down. I don't think I'm doing anything different than I used to, but it is more noticeable now. Especially when I have to space it above the table a bit to allow for cutting through in places. Here is a 4" x 3" x .125" plate clamped down at the ends.

View attachment 136103

I have 1/16" thick aluminum pieces at the ends, under the plate, where it is clamped, and one in the middle. The plate deflects up in the centre so the ends are about .010" lower than the centre. Not a huge amount, but when you are machining features .015" deep, it basically is no good. I could clamp it at the top and bottom too, but that seems excessive for such a small piece. I've resorted to pre-bending the plate up at the ends to try and minimize this, but that's not a very good solution.

Any thoughts on why this happens and what I should be doing different?

thank you,

Rick

Yes, I have a thought but I can not prove it. If a shear cut the metal the blade will have compressed the metal distorting the shape enough so that a clamp will force that area down against the table until it is flat this will force the thicker section up. The question is will a plate cut by a band saw do the same thing. Making a clamp reach further to the metal of the same dimension as the center would definitely help. If this was critical issue I would oversize the plate and trim the sides with a band saw but I doubt the time and cost would be worth it given a change in clamping procedure is effective.

 

[kstrauss]

I assume that you are using a CNC mill for your work. If so, an effective approach is to probe the material surface at various points and apply the necessary correction to your gcode to cut a uniform depth beneath the surface. See for an extreme example of engraving on a non-flat surface.