[rogue-insight]

What I thought was a "brilliant" design of mine has disappointed me! I've made an aluminum extrusion frame router/mill with Hiwin 20mm rails and 1605 ballscrews. My goal was to be able to machine aluminum. It can but it's really wimpy. I have to take really small depths of cut. If I try to use the side of the end mill to machine the edge of a 1/2" thick plate, I get this terrible high-pitched squealing resonance that develops; when I take a cut of more than 0.5 mm into the part.
Here's a video showing the machine moving in all axes: https://www.youtube.com/watch?v=nWbaQHA50cI

Can anyone see an obvious weak point that causes low stiffness? My estimates suggested this would be quite a capable machine. There are several resonant modes right in the wrong spots down at low frequencies.

Here are some pics:

This is the front of the machine. It has dual y-axis ballscrews, a single x-axis ballscrew, and the entire gantry moves up and down for the z-axis, so the Z has two ballscrews


Here is a side view of the machine, showing the gantry plate, and one of the y-axis ballscrews


Here's a view isometric showing the rear. One can start to see the dual z-axis ballscrews


Here's a straight-on view of the rear of the machine


A closeup of the x-axis, showing the headstock and its drive motor. That's a hobby brushless motor. I use a regualr helicopter ESC to drive it and an 18 V power supply. The pulleys from motor to spindle have a 1:2 ratio.


Here's a closeup of one of the z-axis ballscrews attachment points to the gantry.

[louieatienza]

Hmmm.... possibly the somewhat thin-walled extrusions? Those two Z towers that are not tied together, relying only on the bearing balls to provide structure? Those stepper motors only on stand-offs, not on proper stepper plates? Aside from that, what are your parameters for aluminum cutting? I'd say .5mm (.02") rDoC at 1/2" aDoC might be a bit too aggressive for your spindle? I'd say a cross beam atop the two gantry towers, that also act as stepper mounts, along with gussets, would help. Maybe heavier extrusions or even flat bar or steel tube for the long rails.

[jfong]

I think this was the same machine that was on Reddit many months ago when the op had asked about the design. I had mentioned the dual z on profiles linear rails might be a issue. Too many points relying on just the ball bearing carriages for rigidity. It would be better to use a single z and just move the spindle up/down. Solidly attaching the uprights with the cross beam would be more rigid.

[rogue-insight]

/r/hobbycnc right? Ya that was me. Since then I've made the z-axis a lot taller because I needed more room. With this I can fit almost 10 inches under the spindle! Oh and BTW, the reason I have the whole gantry move is so that the spindle is always supported with something behind it. My idea here was that it would maintain rigidity throughout the z-axis range. If just the spindle moved up and down, the range would be quite restricted and there would be a large moment arm that would evolve when the spindle is deployed all the way down.

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I was thinking of putting a torsion plate across the back of the gantry plates, but your idea of spanning across the top of them is even simpler. Thanks!

[jfong]

I think it is a great looking machine and it was thinking out of the box for sure. I don't think I've seen a DIY gantry with that much z travel. Adding the plate would add rigidity.

If you lean on one of the uprights, how much movement do you get right now?

[pippin88]

Interesting design.

I think the gantry moving up and down has great potential, but I think the uprights must be fixed rigidly. That means a moving table machine.

The uprights also need to be tubes rather than plates.

In modifying your existing design without huge changes, I agree with adding a cross beam between the top of the Z uprights.

[rogue-insight]

If you lean on one of the uprights, how much movement do you get right now?

Good question. I would be nervous about doing that, suggesting that it is indeed a weak point!

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The uprights also need to be tubes rather than plates.

This part interests me, can you tell me a little more why tubes are superior to plates?

[jfong]

For a non scientific test. If I place a dial indicator against one of the uprights on my gantry, about 6" above the table top (center line height of the y axis). I get about 20thou deflection if I push my full weight against as hard as I can. I weight 190lbs. I don't consider my gantry as rigid as some of the builds here.

One time I saw one of those rockler CNC's at a woodworking show. Cnc made mostly of high density plastic. While they were not looking I push against a gantry slide. Seemed like I could make it move 1/4" without trying to hard. Very flimsy.

[fretman_2]

Dual Z would be my guess...but I'm no engineer. When I saw that it was the first thing that came to mind.

[G59]

Basically tying together the uprights on the back with an aluminum plate, would increase rigidity 10x. As they are right now, both are free standing and are totally dependent on the connection of the gantry via linear rail and blocks which is no good. You have the room to do it so not a problem to implement. A 1/2" by 8" piece of aluminum bolted to the back of your uprights will be out of the way as it would be behind the gantry when you are at the top of Z.
Just my thoughts.

[Fyddler]

I'm no engineer, but to me it would seem a plate on top, and one on back would be best that way you are covered in both directions.

Sent from my SM-G935V using Tapatalk

[manofmanyhats]

I don't have anything to add to the suggestions so far. Just kudos for trying something different.

[rogue-insight]

I don't have anything to add to the suggestions so far. Just kudos for trying something different.

Thanks. It was a gamble on my part. I fronted a large chunk of money to build this; it made me nervous. I really want it to work well so I feel good about it in the end.

[fretman_2]

Yes...very original!!! Nice machine...

I don't have anything to add to the suggestions so far. Just kudos for trying something different.

[ihavenofish]

obvious weak points:

- those long unsupported screws. those will vibrate and that will be reflected in the cut.
- the lack of a base. that thin table is all that's holding your uprights stable. not ideal.
- the spindle. thats a play toy, not a cnc spindle for heavy cuts in aluminium. check that the bearings are preloaded enough, but you may be better to just buy a proper spindle (yes, i know they cost lots of money and it sucks, but they are the most important part of the machine).

the rest actually looks rather cool and i dont see any obvious fundamental flaws.

things to note though, any play in the ball screws and linear bearings (and spindle bearings as mentioned) will cause chatter in a cut like you are trying. if you didnt get everything preloaded (ebay bargain finds often arent) you may not get much improvements.

[wizard]

There is nothing wrong with trying something different, however I suspect a good part of your problem is strength of materials not so much the concept itself. That is technically there is nothing wrong with moving the gantry beam up in down The fact is a lot of machines use that approach such as planer mills, some bridge mills and other tools. The trick is the implementation which isn't that easy as you need to build very rigid structures.

/r/hobbycnc right? Ya that was me. Since then I've made the z-axis a lot taller because I needed more room.

First off I hope we have the same idea when it comes to axis naming, your Z should be the axis moving vertically on a router, the Y axis moves laterally on the gantry. I know people will object to this wanting to put the X axis on the longest axis but it simplifies communications greatly if we are all on the same page.

In any event I hate to say this but I see this design as pretty hopeless, it will need a major over haul to get to a point of good rigidity. I'd start right at the bottom and redo the X axis and work upwards from there. Here is the thing like a good foundation supporting a house you need a good foundation for a router targeting aluminum and bolting your X axis rials to thing T-Slot extrusions is starting off on the wrong foot.

From there you can improve the vertical by making it a π shaped upright out of steel tubing. The idea here is a shape that is structurally solid even without in mounted on the linear bearing saddles. Right now i can imagine pretty significant lateral movement a the top of those upright slabs. The gantry itself should be heavy walled tubing also, ideally all tubing is internally braced and thick enough for bolting of the linear rails.

With this I can fit almost 10 inches under the spindle! Oh and BTW, the reason I have the whole gantry move is so that the spindle is always supported with something behind it.

This isn't a bad idea at all so don't let that stop you. The problem is your uprights need to be stiff enough to support the Y axis gantry. The uprights columns can't be free standing either, they need to be joined to make that π shape. Putting this all together though is not trivial. Frankly I'd want access to a machine shop to pull it off. Distort you will want the linear rails for vertical mounted on the face of the π. The mounting pads for the rails will have to be square to the mounting feet where ether X axis saddles attach. These feet also need to be in the same plane. You would likely need access to a horizontal mill and possibly a vertical mill to get this done to any precision.

My idea here was that it would maintain rigidity throughout the z-axis range. If just the spindle moved up and down, the range would be quite restricted and there would be a large moment arm that would evolve when the spindle is deployed all the way down.

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I was thinking of putting a torsion plate across the back of the gantry plates, but your idea of spanning across the top of them is even simpler. Thanks!

You can certainly try these "easy fixes" but in the end I'm not to certain you will get what you want.

On another note where did your spindle come from? To be perfectly honest it looks to be a bit pathetic on that machine. Some of your problems could very well be the spindle complaining about the load it is under. Machining half inch thick aluminum by side cutting with a long cutter requires that that bit be stiff which also implies a large diameter, frankly a half inch diameter bit is a bit small in my mind. If you are doing this regularly I should think that you would want a spindle with a capacity that exceeds a half inch in the collet.

[wizard]

Thanks. It was a gamble on my part. I fronted a large chunk of money to build this; it made me nervous. I really want it to work well so I feel good about it in the end.

This is where asking questions first can save a lot of grief. Your basic idea here isn't bad at all, like I said before there are lots of examples in the machine tool industry. what is a problem though is the arraignment of the components and their cross sections. You might get some trivial improvements out of this machine following some suggestions here but to get to the point of really machining aluminum you will likely need a total over haul basically a new machine.

[rogue-insight]

On another note where did your spindle come from? To be perfectly honest it looks to be a bit pathetic on that machine. Some of your problems could very well be the spindle complaining about the load it is under. Machining half inch thick aluminum by side cutting with a long cutter requires that that bit be stiff which also implies a large diameter, frankly a half inch diameter bit is a bit small in my mind. If you are doing this regularly I should think that you would want a spindle with a capacity that exceeds a half inch in the collet.

It's a Sherline Industrial Headstock. I modified it a bit to be able to mount it flange style. spindles

I've seen some pretty badass router/mill builds that use the aluminum extrusion parts as frame members. They all seem to be more capable than mine though. Here's one that inspired me:
http://www.cnczone.com/forums/diy-cn...82294-cnc.html

His can tear through aluminum parts, if you look at the videos he's posted. His other youtube videos show it blasting through no problem. I think I should be able to get that kind of performance out of this machine.

[ihavenofish]

It's a Sherline Industrial Headstock. I modified it a bit to be able to mount it flange style. spindles

I've seen some pretty badass router/mill builds that use the aluminum extrusion parts as frame members. They all seem to be more capable than mine though. Here's one that inspired me:
http://www.cnczone.com/forums/diy-cn...82294-cnc.html

His can tear through aluminum parts, if you look at the videos he's posted. His other youtube videos show it blasting through no problem. I think I should be able to get that kind of performance out of this machine.

hes got a 1-2kw professional hihh speed spindle on there. his machine also has a base structure which yours lacks. his machine is also fairly low in height, making it stiffer.

[rogue-insight]

hes got a 1-2kw professional hihh speed spindle on there. his machine also has a base structure which yours lacks. his machine is also fairly low in height, making it stiffer.

I'm not sure spindles need to be that powerful. In my case I was able to maintain RPM while cutting, and the motor only drew 100 W. That was a surprise to me, but the data is there.