newbie thoughts

[chunkstyle]

Hi all,

Just finished running my first big file on my Bot. Cutting the plenum'

One thing I noticed was the machines shake and vibration while it was cutting. It struck me that it might benefit from some additional ballast much like a lath can be improved by adding weight (mass) to it's framing to soak up vibration. Has anyone tried this and, if so, was there any improvements?

Only reason I ask is that i notice that the plenum had a lot of chatter on the round "pucks" that I was cutting out. My plenum pattern consisted of 1" diameter pucks that are 3/8" high (tool cutting depth was 3/8").

I realize that it may have a lot more to do with gaining tool pathing experience and optimization. I was using a 1/2", 2 flute straight bit cutter. travel speed was 2"/sec. Spindle was 12k. I expect that my chatter was more a result of bit size, etc.. and I really wasn't that concerned about it as it was only a plenum. Couldn't help but wonder if a "lathe" upgrade may help minimize vibrations?
After I get my vac plumbing in I will look into this some. Didn't want to re-invent the wheel if someone else has tried it already.

On another issue, I was wondering if there will be any signifigant difference in running the pvc plumbing from a 4"manifold to the table plenum with either 3" or 2" pipe. It was reccommended that 3" pipe should be run from a 4" or larger manifold. However, after reading as many posts as I could it appears that cfm capacity of the plumbing is not as crirical as the vac pressure. Cfm requirements may only be an issue after alot of the cabinet parts coming out of a sheet have been cut thru. There may never be a point that the total cfm requirement needed for hold down may never be undermined by the restrictions of a 2" pipe as opposed to 3".

If I could source 3" pvc valves locally I wouldn't bother to worry about it but I can get 2" pvc valves at my local no problem. I didn't find any posts that discussed the plumbing diameter directly but I could easily have missed them.

Sorry to blend the questions into one post. Seems like I'm at a point where I am generating more questions than I can find answers for.

Thanks,
Tim

[myxpykalix]

That shake and vibration is not a naturally occuring consequence of running a bot. It sounds to me like you may not have things tightened down and that in fact would cause shaking and jittery cuts like you describe.
Also it sounds like your router speed is too slow and your feed is too fast. decrease your feed speed and increase your router speed and see if that helps. Its always better to start out slow and as you get more comfortable with the bot you can increase your feed/speeds. Plus the fact that you are using a large bit, it just can't cut that much material at 12K at 2ips 3/8th deep.

[chunkstyle]

Thanks Jack,

I'll slow down the feed speed and increase the RPM's as you suggested.
Spotted a bolt and washer on the floor. Came from a cross member bolt backing out from the t-nut in the side rail. Went around and checked them all. Looks like I just missed the one. The rest are all tight. Ditto on the rest of the machines frame.

I don't want to give the impression that the machine is jumping around. I just noticed, while leaning against the plenum as it was being cut, that you could fel the change in motion, direction of the gantry. This is probably a result of the incorrect feed speed as you pointed out.

I used the chip load calculator and left the chip load alone at .005. Used a cutting speed of 2"/sec and it came out at 12k rpm. I realize that the calculator is a rough guesstimate. I checked the bit out online (starter set Onsrud 1/2" cutting diameter part# 48-072) but couldn't find any guidance, from the web site, about the chipload of this bit. I will do as you suggested and try and "feel" the difference of the machines operation. I have a sheet of PB that I can burn up getting a feel for the controls.

Thanks for your suggestions and time, it's appreciated.

[myxpykalix]

also take note that while the toolpath is running you can slow it down or speed it up manually by holding down the shift key and hitting the left arrow (to slow) or right arrow (to speed up).

[chunkstyle]

Thanks Jack,

I'll try that. It will surely help. Guess it's sorta like your truck. You have to get used to what it's telling you when things are going right. Makes it easier to tell when things are going wrong.

Tim

[fredtoo]

Do a VR command and reset all the settings to default. I had the same problem as you and found that some of the values were way too high. A reset to default did it for me.

[dana_swift]

Tim what you are describing doesn't sound like my experience either. The only time my bot has any shake is when I am pocketing very tiny holes, just a few thousandths larger than the current bit.

Can you send pictures of the cuts you are getting? Then we can see what you are getting. If you can video the "shake" and post it somewhere where we can see it then we can compare our experience to yours.

"A picture is worth a thousand words".. "video can be worth a thousand pictures"..

Hope you get it figured out!

D

[chunkstyle]

Thanks Dana And Greg,

The machine isn't moving around so that you could see it. I'm referring to the vibrations that you can feel while the machine is cutting. I don't want to give an impression that it's a problem since I really havn't done any money making cutting with it yet and plan to build some shop fixtures with it first to help me get my head "dialed in".

I was doing the described slack jawed yokel pose while standing at the machine, watching it cut out all those little round pucks that the thought occurred to me. Leaning against the machine I could feel the gantry mass being telegraphed thru the frame as it was doing it's cutting. Same to for the bit's vibrations being carried thru the machine. It struck me as a similiar situation with wood turning. You have a hunk of wood chuched uo in the lathe head and you want to spin it around to take hunks of wood off.
I realize that by adding weight to the lath frame you are trying to dampen the centrifugal forces being applied to it from the out of balance wood. I thought the extra mass is also a benefit to soaking up the vibrations of the drive train and moving parts that are in play to help give your turnings a better cut.
With that said and still standing there slack jawed at the machine my mind had time to drift and I wondered if there would be any improvements made by adding ballast to the Shopbot. I'll admit I'm getting way ahead of myself as I have lots to do yet on the vacuum hold down. I'll be spraying the newly cut plenum with a sealer today and building more vacuum hold down components.
It was just a thought that, although the Shopbot is not a "big iron" machine, perhaps it may benefit by some of the mass of a big iron by adding some ballast weight much like a lathe benefits.
I'm still way to green to be able to say there is a cause for concern with my Bot's cutting performance until I have taken all the appropriate steps and trials that you all have, with respect to tooling, and have to look elsewhere.
I'm realizing that once you start getting into your bot the whole open source nature of the machine, control software and community starts getting your mind working.

Thanks for the help,
Tim

[dlcw]

Tim,

Something that you can look into is adding diagonal bracing along the X-axis like is on the Y-axis. There are the triangle braces that keep the machine from racking in the Y direction but nothing in the X direction - at least on my machine. I've been kicking this around myself. All the vibration and slight movement is a result of the machine making quick direction changes and this being broadcast through the framing.

Having cut hundreds of sheets of plywood into cabinet parts and dozens of small and large 3D carvings, you can be rest assured there will be movement in the machine. Because it is bolted together (not welded) and relatively light weight compared to full-grown CNC's you can count on flexing and movement. The challenge is figuring out how to compensate for all this through creative cutting and carving methods.

For example, when cutting cabinet parts from plywood I do it in two passes. The first pass is in a climb cut direction cutting into the material but leaving a 1/64" skin. This direction pushes the bit away from your final cut line. Second pass cuts through the material (1/64") in a conventional direction. This pulls the bit towards the material - but since it is cutting so very little material, the bit deflection is negligible. The result is super clean cuts exactly on the line.

When 3D carving I rough pass with a 1/2" or 1/4" ball nose bit. I make two finishing passes with a 1/8" ball nose. I first make a pass cross grain with a 13% stepover. My second pass is with the grain using a 6% stepover. This creates carvings that need virtually no sanding. A little hand chisel work for adding undercut detail and away you go.

[blackhawk]

Tim - I have a 48x96 and I built a lumber rack underneath my table. I attached uni-strut to each leg and then ran two 8 feet uni-struts down the length. I then put a couple of 4x4 timbers under the long uni-struts to support the middle. The uni-strut makes the frame that more rigid and then the 300 or 400 hundred pounds of lumber on there makes a useful ballast. I also put some rubber pads under the feet.

[chunkstyle]

Thanks Don and Brad,

It has occurred to me that there will be a lot of space under that machine. Even after all the plumbing gets installed, I will have to think of ways to utilize the remaining space. I'm thinking of all my cabinet hardware supply storage needs. Something heavy.
here's a pic of the plenum that got cut out yesterday. It has a couple of coats of sealer on it and I'll get a top coat on it after all that sanding! It's just a simple four zone with the 1 zone nearest the o,o position divided into two.

I'm wondering if I should run a Y under each of the other three larger zones and plumb two lines into them as opposed to one. I'm thinking that it may distribute the vacuum more evenly and avoid the offset hole that would be necessary if I only ran one pipe/zone.

If the goal is to keep cfm down so as to achieve max pressure, would it be as critical to plumb your zones with 3" diameter pvc as opposed to 2"? I've got to make up my mind soon and get on with spoil board glue down.

Thanks again for all the responses and advice. It's appreciated.

Tim

[myxpykalix]

Although you will have lots of space under there you need to keep in mind that sawdust is going to accumulate and you need to be able to get at it to get rid of it.