What Part To Start ? - Page 5

# Thread: What Part To Start ?

1. ## Re: What Part To Start ?

I'm simply dissecting the problem into bits so each bit can be understood. So we now know that we can build a light frame as long as it stiff enough and damp enough. We do not need to use mass only as a vibration minimisation approach. To follow the mass approach a bit further in the context of a transportable machine. High buildings use water tanks as a dynamic damper. So we could build a light frame, with provision for plastic water bottles (or built in tankage which would be structural) in the bottom which provide mass and dampening. Then these are easy to remove and the table becomes transportable. One of my machines had a light frame so I stacked my plywood under it to give it mass so it didn't skip occasionally.

The inertial loads certainly can flex a machine if high accelerations and masses are together. If the gantry, z axis and base flex synchronously then this does not matter. But there will be overlaying harmonics that make them asynchronous then your in trouble. For instance this will result in chatter. The Z Axis being a cantilever is the main culprit. I hope we are not confusing Jake!! I've designed many machines and I usually start at the bench but the latest machine I have decided to start at the Z Axis and work to the bench for this reason. Really need to sort a bombproof Z Axis and gantry connection before anything else. Peter

Hi Jake your original question is what part to start? My answer is a) buy one b) your a Maker so "The entire machine" in CAD, resolved, then start.

2. ## Re: What Part To Start ?

Originally Posted by davida1234
Pete, thank you for the calcs.

However, I think it is not about actual sliding of the machine on the floor but any flex or distortion on the base relative to the gantry, no?
As a general statement I don't think the bench is the main culprit when it comes to machine deflection. I think its mainly the Z axis and the gantry that contributes to the bulk of this issue. The X Axis is a cantilever so is a natural tuning fork. The longer the worser! If a moving gantry there are issues of walking ie the gantry cannot move smoothly but wobbles down its ways. This is a stick/slip, drive synchronization, friction and complex issue resulting in all sorts of micro harmonics. That's why mills are fixed gantries to stop all of those issues in their tracks so to speak. So in short I think the heavy bench concept should be sidestepped and put the effort into the Z Axis/Gantry design. Peter

3. ## Re: What Part To Start ?

Originally Posted by wmgeorge
Dan, Is that steel or aluminum? Thats down in Mactec54's home town?
I believe it's steel from the pictures they posted on Instagram of the table base construction.

The plasma tables are steel frame with aluminum gantry's. They have a huge mill that they machine everything on after welding. And do everything in house which is nice for QC. So far I haven't read any negative reviews on the plasma tables. Their main business is tube benders I'm sure all off road guys on here have heard of them.

Dan

4. ## Re: What Part To Start ?

If someone has a step file of a bench I can run a modal analysis on it.

I've attached two images of the first mode of vibration of two different machines. The first one is from a machine called Scoot. Its first mode is the Z Axis going back and forth like a pendulum at 46htz. This also shows up in reality on some jobs when I cut in this direction and say come to a corner it wobbles if done fast.. If you look at the machine and push and pull on it you would predict that this is the weakest link. Scoot has been under stiff but it is a development machine for various reasons.

Then there is maximus, the one I'm working on now. Its first vibration is 300htz and is the gantry lozenging due to the bearing loads. This will be improved by internal shear webs. Its really good that even though the Z axis is sticking out 200mm same as Scoot, it's not the first harmonic.

A bench being big is unlikely to vibrate at these frequencies. Would be good to compare two different benches if anyone has them? Peter

5. ## Re: What Part To Start ?

The frequency tells us part of the story,what is the amplitude at the tool tip?

6. ## Re: What Part To Start ?

Hi all - The amplitude is driven by the tool forces and the harmonics of the machine. If the tool force is in sync with the harmonic it can be large (chatter) if its out of sync or in a different direction then it will diminish. Dynamic affects are a difficult area to predict. This is getting easier with better software. Generally the plan is to make it as stiff as possible, use naturally damp materials and use lots of mass. With machine tools such as boring bars they use active damping, mass spring damping and other methods. Some sophisticated machines use piezio-elecric elements that push an out of phase force back to the vibrating element damping the vibration. In this forums arena I'm keen to look at water damping for some of the light machines. Dynamics is a really tough area of design. My software can apply the inertial loads to a structure and if the deflection mode agrees with the modal shape then you probably have a vibration issue on your hands. Or you run your machines slow and easy so inertia loads are very light. Do we have an acceleration figure out there yet? Peter

Out of interest I put 1g (9800mm/s2) on Maximus and it deflected 0.01mm. It does not have a spindle on it yet though. The stress at 1g is tiny which is good.

7. ## Re: What Part To Start ?

Originally Posted by peteeng
As a general statement I don't think the bench is the main culprit when it comes to machine deflection.
This is generally true and in many cases is obvious in some of the DIY designs.
I think its mainly the Z axis and the gantry that contributes to the bulk of this issue. The X Axis is a cantilever so is a natural tuning fork. The longer the worser! If a moving gantry there are issues of walking ie the gantry cannot move smoothly but wobbles down its ways. This is a stick/slip, drive synchronization, friction and complex issue resulting in all sorts of micro harmonics. That's why mills are fixed gantries to stop all of those issues in their tracks so to speak. So in short I think the heavy bench concept should be sidestepped and put the effort into the Z Axis/Gantry design. Peter
The importance of getting the gantry right, from my perspective anyways, is that there is no easy remediation for a bad gantry design. If for whatever reason the bench proves to be a problem there are often very simple solutions to improving its behavior. With the bench one can bolt in gussets, fill the beams with mass, bolt it down to a floor, or do a number of other things that one simply can't do with a moving gantyry machine. With the gantry and Z, they are literally hanging in air.

8. ## Re: What Part To Start ?

If you have ever ran a real router, a large machine with a gantry you will find the movement of the gantry at rapid speeds tends to want to move the entire machine. The inertia of the gantry, the up and down of the Z all contribute to the frame movement. Frame bracing and mass is important to hold the machine solid. Bolting to the floor base would be another. Placing water bottles in the frame is not going to help, filling the frame legs with concrete like they do on some surface grinders would however.

There is a reason why existing machines are built the way they are.

Since the gantry is constantly changing speed and direction, how can it have a frequency? If it was static yes.

Added: And the other reason why there are water tanks at the top of taller buildings....... Anyone else?

9. ## Re: What Part To Start ?

Hi WMG - Please don't say a big machine is a real machine. The issues with a benchtop machine are the same as a machine that cuts aircraft wings. I have a benchtop machine at the moment which is sliding my table in the garage. The faster something accelerates or decelerates the more inertial effects it has. Adding mass is the easiest solution not the smartest solution that's all I can say. All the machine structures vibrate especially when moving. If something exists it has a reason to be there, totally agree. In my early career I ran industrial robots and they are lightweight machines that move exceptionally fast. They have to be bolted to the ground or to a suitable structure which is bolted to the ground. They maintain very tight accuracy at very high speeds. I appreciate inertial forces quite well. I've punched through several steel safety barriers in my time with a robot. Sorry Jake we have moved along from your question, how are you travelling now? Peter

10. ## Re: What Part To Start ?

Friday was a rainy day, so several crews laid out. I took that opportunity to drive down to Belmont NC and visit with the guys at Colombo USA. I meet with Martin, Craig, and John for an hour and a half and went over my options for the spindle. I'm thinking I will design for a 110 frame size and maybe purchase a 90 frame for now. Those guys were incredible to talk to. I learned so much in that meeting (like don't crash the spindle, ever!), and I really appreciated the time they spent with me. During this meeting in the warehouse Martin was getting call after call about eBay purchased spindle problems. He was really nice to the callers and listened to them and requested pictures. This made an impression on me that these guys are willing to help even if they didn't sell you the spindle. I won't be purchasing any spindle until I get the machine up and moving so I can best use the 1 year warranty. For know, Im going to weld up a mock up spindle with the correct weight and dimensions.

I have turned the corner on Fusion and must say I love it! I went back to the tutorial book I purchased so I can get better for designing the gantry drive, and Z-axis.

Originally Posted by routalot
Have you thought of looking on ebay for an older machine to retrofit a modern controller to?Even if you have to replace some worn components,the prices are often startlingly low and you have the basis of a machine that can be customised to your requirements.I find the discussions on this board about massive frames for people engaged in hobby activities tend to lead to massive and overbuilt frames,with correspondingly massive costs of materials in pursuit of levels of accuracy that would be in order for components in chronometers.If your ambition extends to making a dozen kitchen cabinets a month,what is a realistic amount of deviation from utter perfection?A solid table is a necessity and the legs are there to keep it off the floor,adjustable feet are a help if the workshop has a slightly bumpy floor.
@routalot I plan to put 3/4" machine feet under the lower tube sections, 6 in total.

Originally Posted by wmgeorge
Big question of the day. Is the end result going to justify all the expense?
A hobby user will not have room for it, and the standard 4 inch thick concrete floor is going to crack under that weight. So that means a new reinforced base floor / foundation.
My rough numbers right now have my machine between 4500 - 5500lbs. Spreading the weight out over 6 contact points should be ok for whatever I build.

Originally Posted by peteeng
Hi All- To look at the heavy bench argument a little further. The only reason to have a heavy bench is to stop it from moving around. Weight and stiffness are not connected so lets just assume that we need a heavy bench to stop it from sliding around. I looked for some acceleration figures on commercial machines and they range from 0.5g to 2g so lets pick 1g as a nice number that can be scaled easily. If someone out there can give us a real number that Jakes sort of machine would run at that would be excellent. So the calculation attached is to find out the ratio of gantry weight to table weight needed to achieve no slip on the floor. I've also assumed a friction co-efficient of 0.2 but it could be 0.1 (slippery steel to steel) or 0.3 rough. Its a simple calc neglecting moments and some other factors. So upshot is the bench and gantry have to weigh 48x the gantry weight to not slid. So if the gantry assembly weighs 50kg then the entire machine has to weigh 2400kg. Looking at some of these machines they weigh just over the tonne so perhaps 0.5g or less is more like it? But if you bolt the machine down then it weighs what the earth is so bench weigh is irrelevant. So what accelerations are out there? Peter
@peteeng wow, just wow! You must be some sort of a rocket surgeon. I am more like the guy you see on NASCAR races drinking a beer in the infield yelling, "Put'em in the wall Darrell!".
Nevertheless, thanks for putting in the time to calculate that. You and guys like you are the biggest reason I feel confident I can do this. You have a real passion for theses machines and it shows in your curiosity and willingness to help someone like me just starting the adventure. Thank you!

11. ## Re: What Part To Start ?

Hi Jake - Nothing to do with rockets. Currently I design medical equipment and analyse very big mining equipment. I'm working thru a trailer that carries a 400 tonne truck at the moment. So get your ideas into the CAD world and share them as soon as you can. We (the forum) are happy to pull it apart. Peter

12. ## Re: What Part To Start ?

Originally Posted by Ntl
I believe it's steel from the pictures they posted on Instagram of the table base construction.

The plasma tables are steel frame with aluminum gantry's. They have a huge mill that they machine everything on after welding. And do everything in house which is nice for QC. So far I haven't read any negative reviews on the plasma tables. Their main business is tube benders I'm sure all off road guys on here have heard of them.

Dan
I looked at there plasma tables, you posted about, do you really think they are good for that price no Plasma or torch is in that price and are not even using half decent linear rails, that's a step back to hobby level for most plasma machines using a rail system like they are

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•