SO3 Gantry Stiffness in Y direction

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AnonymousPerson
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Re: SO3 Gantry Stiffness in Y direction

Post by AnonymousPerson » Tue Sep 15, 2015 1:56 pm

Thanks Will. :)
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TomDChi
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Re: SO3 Gantry Stiffness in Y direction

Post by TomDChi » Thu Sep 17, 2015 5:41 am

I took the router out of the mount and grabbed the mount and flexed it (basically pushed down) with the motors on. Once I dealt with a loose eccentric nut/V wheel, I could see that the bottom quarter of the moving Z plate was definitely flexing a noticeable amount relative to the aluminum extrusions behind it. It would help if that plate was stiffer. In addition the overall gantry flexed some also, and presumably, the X aluminum extrusion was deflecting somewhat in torsion. But of all of that, stiffening the Z plate seems like the easiest thing to do to reduce flex.

I've been thinking about steel ribs on either side of the router mount, which would be screwed in from the back. Basically, what LTPEM did with the aluminum track. (Steel is about 3x stiffer than aluminum.) Counter sink holes on the back face of the mounting plate, then use flat head machine screws to screw the plate and the ribs together. (But LTPEM's approach of just tapping holes in the plate is certainly more simple.) Holes/screws could also come in from the side to screw the router mount to the new rails, reducing the opportunity for flex at that connection also.

I just did a moment of inertia (I) calculation ( "b * h^3 / 12") and in inches, the existing plate (5.111"(130mm)) wide by 0.135"(3.4mm) thick) has an I of 0.001 in^4

Thickening the overall plate would also be a good choice: Going from the existing 0.135"(approx.) plate to a 1/4" plate has an I of 0.007, 1/2" thick gives you an I of 0.053 and 3/4"(19mm) plate would have an I of 0.180 in^4. The increase is to the power of 3 based on thickness, so once you're out beyond 1/4"(mm) you're getting massive increases in stiffness and decreases in flex from that element. It might be worth pestering Edward to release at least that one part as a CAD file, though you'd also have to deal with finding eccentric nuts that work with the thicker plate.

In terms of mounting stiffeners to the moving Z plate, my thought was 2 bars each 3/8" (9.5mm) wide and 3/4"(19mm) deep which would have an I value of 0.026 in^4 - about 26 times more stiff than the existing plate. In steel, they would add about 1lb (454g) to the moving Z system.

I'm treating these as sort of isolated elements. Tying bars to the plate as LTPEM has done moves the existing plate off the axis of bending, which lets it contribute more to resisting bending, like the top and bottom flanges of an "I" beam. The math is harder to write out here, but it's a good thing for stiffness for the same reasons we use I beams (technically W shapes today) and precast concrete T shapes.

LTEPM
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Re: SO3 Gantry Stiffness in Y direction

Post by LTEPM » Thu Sep 17, 2015 1:56 pm

TomDChi wrote:
I've been thinking about steel ribs on either side of the router mount, which would be screwed in from the back. Basically, what LTPEM did with the aluminum track. (Steel is about 3x stiffer than aluminum.) Counter sink holes on the back face of the mounting plate, then use flat head machine screws to screw the plate and the ribs together. (But LTPEM's approach of just tapping holes in the plate is certainly more simple.) Holes/screws could also come in from the side to screw the router mount to the new rails, reducing the opportunity for flex at that connection also.
I opted for aluminum to keep the weight down on the Z axis. I need to upgrade the springs on the Z axis as it is, with the weight of the router, dust collection hose, and dust boot. I felt the T track extrusion offered more resistance to bending than aluminum angle since it is effectively two webs.

I chose to tap the Z plate rather than thru bolt for two reasons.
1) screw heads would be over the Z axis rails, did not want to risk one working loose and jamming and could not be tightened without disassembling the Z axis.

2) I wanted to maintain the functionality of the t-track and not obstruct the channel. It accepts 1/4" bolts/nuts or 1/4" t bolts.

Gussets can be added onto the t-track to spindle mount or additional angle can be bolted on top of the track if needed, plus it allows additional attachment options to the Z axis.

It would be interesting to see what the difference in deflection is between steel angle/bars compared to the T track extrusion, as well as comparisons of steel plates vs. aluminum plates for the Z plate.

Hans
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Re: SO3 Gantry Stiffness in Y direction

Post by Hans » Thu Sep 17, 2015 2:37 pm

When making a thicker replacement plate, it's possible to pocket down to original thickness for any components that would not fit with the added size. The effective stiffness of the plate will still be much improved. Wrench clearance for the eccentric nuts may be an issue, but I'm sure there are ways around that problem.
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AnonymousPerson
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Re: SO3 Gantry Stiffness in Y direction

Post by AnonymousPerson » Thu Sep 17, 2015 3:45 pm

Hans wrote:When making a thicker replacement plate, it's possible to pocket down to original thickness for any components that would not fit with the added size.
Good thinking. :)
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Fablicator
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Re: SO3 Gantry Stiffness in Y direction

Post by Fablicator » Fri Sep 18, 2015 1:47 am

Added (2) .625 x 1.25 x 7" Aluminum stiffeners to mine. Testing to commence tomorrow.
IMG_20150917_212705.jpg
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TomDChi
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Re: SO3 Gantry Stiffness in Y direction

Post by TomDChi » Fri Sep 18, 2015 2:35 am

Fablicator - that's exactly what I was imagining! It will be interesting to see how much it improves things.

LTEPM - excellent point about running the fasteners in from the front where they're accessible and won't risk binding. (Also, I just noticed I typed your username wrong a couple times!)

Overall, there are several sources of flex. I suspect it won't take much more than what LTEPM has done to get the flex at that plate down to where it's effectively zero, and all the other sources overwhelm any additional stiffness, if it isn't already there.

I'd be curious to know what kind of lateral forces we'd expect the end of the mill to be generating doing this kind of cutting in aluminum. From there we could ballpark the amount of bending the plate would undergo, and get an order of magnitude estimate of how stiff it needs to be to get its bending/deflection down to be insignificant.

AnonymousPerson
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Re: SO3 Gantry Stiffness in Y direction

Post by AnonymousPerson » Fri Sep 18, 2015 3:00 am

As a concept, would it be useful to suspend/mount the router above the shapeoko (eg in an framework above it), and use a flex cable from the router down to the Z-axis plate?

Thinking that would take the weight off the Z-axis plate, which might help. Also thinking it might allow positioning the end mill nearer to the Z-axis plate (compared to having the router mounted on it), for less torqueing affect. Unsure though.
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WillAdams
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Re: SO3 Gantry Stiffness in Y direction

Post by WillAdams » Fri Sep 18, 2015 3:09 am

Yes, moving the spindle off the gantry is a good idea:

http://www.shapeoko.com/wiki/index.php/Spindle_Options
(mounting the majority of the weight) off the machine, gives some great advantages. One that wasn't mentioned is the fact that the tool itself is closer to the z-axis, creating less of a lever arm.
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TomDChi
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Re: SO3 Gantry Stiffness in Y direction

Post by TomDChi » Sun Sep 20, 2015 4:03 pm

WillAdams wrote:Yes, moving the spindle off the gantry is a good idea:

http://www.shapeoko.com/wiki/index.php/Spindle_Options
(mounting the majority of the weight) off the machine, gives some great advantages. One that wasn't mentioned is the fact that the tool itself is closer to the z-axis, creating less of a lever arm.
So there's stiffening the moving Z axis plate, and the potential for getting the "spindle" off the gantry. What are some additional practical things that can be done to stiffen the system and get better precision?

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