Increasing the X-axis Plate Height

discussion of design changes / improvements / suggestions
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Sorwrith
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Increasing the X-axis Plate Height

Post by Sorwrith » Wed Jun 03, 2015 6:28 am

So I've been running simulations on some projects, and the biggest problem I've really come across so far is the Z height. Would there be too much twist if I was to go ahead and make new plates with an increase of say 2 inches of extra height?
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Hans
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Re: Increasing the X-axis Plate Height

Post by Hans » Wed Jun 03, 2015 7:27 pm

I think the first issue you run into increasing the plate height is parallelogram movement of the x-axis beam along the x axis. Some of this is from the plate bending (which you can mitigate with a thicker plate), and some of it is from the increased v-wheel loads due to the longer moment arm (which you can't do anything about). Are you planning to use the machine to make the new plates? It's not advertised for milling steel, but you could probably at least do a decent job of spot drilling all the hole locations and scribing the new outline using the stock machine. Or, you could make new plates out of aluminum and increase the thickness to get the same plate stiffness as stock, but you would have to do a lot of pocketing to get all of the mounting the same as stock.

Increased twisting in the YZ plane is the next concern, though I think it will be insignificant. The wheel spacing could be increased to compensate if you're worried about it, but you lose an inch of working area for every inch you increase the spacing by.
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Sorwrith
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Re: Increasing the X-axis Plate Height

Post by Sorwrith » Thu Jun 04, 2015 2:03 am

I was going to have a friend at the local state university do the milling for me. Was thinking of using 3/4 plate aluminium, I remember that some one on here said it would be comparable to the strength of the current plates. Losing an inch or so on the X would be ok if I could gain 2 inches in height, as a lot of the work I want to complete is in the +3 inch Z.
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Gadgetman!
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Re: Increasing the X-axis Plate Height

Post by Gadgetman! » Thu Jun 04, 2015 7:24 am

3/4" plate?

No...
Generally, 'structural' aluminium such as 6061 and 6082 require a 50% extra thickness to have the same stiffness as steel.
So going to 6mm or 1/4" plate will increase the stiffness somewhat. Going to 8mm (5/16") will add a lot more stiffness, but will add a bit of weight to the system.
Going to 3/8" will be overkill...
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cvoinescu
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Re: Increasing the X-axis Plate Height

Post by cvoinescu » Thu Jun 04, 2015 10:41 am

What Gadgetman said. Stiffness is proportional to the elastic modulus and to the the cube of the thickness.

Mild steel has an elastic modulus of 210 GPa, and 6061-T6 aluminium 70 GPa (common alloys like 6063, 6001, 6005/6005A/6105A, 6082 are all in the 60-80 GPa range; the 1000 series tend to be softer, but they can be hardly described as "structural"; and there are some very expensive alloys that are as stiff as steel). This means that aluminium is three times softer than steel, so it needs to be the cube root of three times thicker: 1.44, that is, 44% thicker.

So going from 1/4" (which is as stiff as 4.4 mm of steel, or four times stiffer than 12 gauge steel) to 3/4" will make it 27 times stiffer (more than a hundred times stiffer than 12 gauge steel). Way, way, way overkill, and unnecessarily heavy.
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Sorwrith
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Re: Increasing the X-axis Plate Height

Post by Sorwrith » Fri Jun 05, 2015 12:44 am

So with this knowledge what would be the max height increase while still keeping the machine rigid? My end game is to have it high enough and with enough clearance to add in a 4th axis. Part of the items would be working with are 5in diameter "logs" for bust making. if the machine isnt currently rigid enough to support that, it can wait till I expand it to full plywood sheet size.
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WillAdams
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Re: Increasing the X-axis Plate Height

Post by WillAdams » Fri Jun 05, 2015 1:37 am

Would you need to be able to cut past the center?

If not, you could make do w/ 2.5" and arrange the 4th axis off the front, which is what I have sketched up somewhere.

Waiting on the plans to be published so I can formally work it up.

A machine was just posted to reddit which has rather a tall gantry: http://www.reddit.com/r/CNC/comments/38 ... 3axis_cnc/
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Hans
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Re: Increasing the X-axis Plate Height

Post by Hans » Fri Jun 05, 2015 12:35 pm

I don't think you can add enough height to fit a fourth with 5" capacity and still be able to reach the wasteboard with the spindle. WillAdams's idea is probably the way to go. You may have to build the fourth axis into a hole in the wasteboard, but a new wasteboard is cheaper and easier by far than new side plates.

If you do still want to increase the plate height, I don't see any need to go thicker than about 1/4" in aluminum. While the geometry won't be as stiff as the stock plates, so a stiffer plate is called for, the Z travel of the machine limits the extra height to 1 or 2 inches and I just can't see that doing much to the overall machine rigidity.
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Sorwrith
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Re: Increasing the X-axis Plate Height

Post by Sorwrith » Fri Jun 05, 2015 1:50 pm

Thanks guys, I'll just wait on the 4th axis until I enlarge my machine to 10 x 5, or create a standalone rotary mill. So when I get more money to upgrade I'll have the plates made, and post back on its ability.
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Hans
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Re: Increasing the X-axis Plate Height

Post by Hans » Fri Jun 05, 2015 2:38 pm

When extrusions become available to expand the machine in x and y, the z extrusions will probably also become available. You could get all the z travel you need in that case.
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