i also wonder if this kind of cast iron gantry weight around 200kg can be used for milling steel? it cost around 250usd at my area
i also wonder if this kind of cast iron gantry weight around 200kg can be used for milling steel? it cost around 250usd at my area
Hi TL1 - yes I'd expect the cast gantry to mill steel easily if everything else was up to it. Peter
Wow, a lot of interesting discussion in this thread. Thank you for the comments everyone.
One question I've been mulling over - why do internal stresses in the base frame matter? Obviously it's annoying when you're welding to get everything located correctly, but if you're smart about your weld order you can at least minimize the effects of most of that distortion. I can see why it would be a problem if you were machining a lot of material off of the rail mounting surfaces. If you're epoxy leveling though, would it have any noticeable effect? In that case you're only adding non-stressed material - not removing any metal from preloaded areas. I'd appreciate any insight on this, if for no other reason than to satisfy my own curiosity, as I often hear people cite internal stress as being a bad.
Progress Update:
I've continued to flesh out the CAD, though haven't had much free time recently. I'm planning to pick up the main steel tube on Friday - excited to finally break ground on this build. Trying get a trailer/not be at work/steel yard is open has been an interesting logistical experience. I won't get everything then, but hopefully all the large pieces that are too big to just go inside my car. Getting the remaining pieces at a later date shouldn't be difficult. I think progress will pick up once I have something to physically look at.
Hi Gadgetguy - 1) internal stresses in a frame do not matter as long as the frame stays straight... and this is the crux. Minimisation of distortion is not zero distortion and if you are trying to make a machine to a particular tolerance then you have to have a machine that is true within that tolerance. 2) It is impossible to weld anything without inducing stress into it due to the shrinkage of the parent metal, the shrinkage of the weld metal and the thermal expansion/shrinkage cycle of the process 3) After you fabricate the frame and then machine it, it will move due to the change of geometry of the structure and removing various bits of metal that are stressed or not stressed. Even if you get a piece of hot rolled section or nearly any piece of metal and machine it it will move. To machine something you have to hold it to a machine table so in some way you are restraining it against internal and external loads. So when you machine it, it will release or relax in some way. 4) if you get a big piece of metal for a mould for instance you rough machine it then release it and leave it for a week or 2 so it changes shape, then you do a little more roughing and release it again., It's quite amazing how much a billet can move if you remove a lot of material from it and this is without welding 5) to prevent all of this palava you should fabricate the frame and have it stressed relieved in this way you reduce the internal stresses to a minimal amount before you machine it. 6) If epoxy levelling was a good thing the commercial manufacturers of machines would be onto it pronto and they aren't, so I suggest this is not a good solution 7) The ultimate object of the frame structure is to provide an accurate surface to mount the rails on. If you can get a steel frame stress relieved then this is the best path 8) If you can't stress relieve I suggest you investigate a fully bolted frame so this issue is moot. Plus a bolted frame is damper then a welded frame. At the end of the day 99% of machines work with some effort, it's the quality of the cut and the accuracy of the cut that matters and hobby builders, semi professional builders and professional machine builders and users all have different expectations, objectives, solutions and results. Everyone wants a micron accurate machine for beer money. But that's not possible. Peter
http://www.meta-lax.com/Home/Literature/literature.html some info on TSR and VSR
Hi routalot, I totally get what you're saying here. I've been doing a lot of simulating in my head of various cutting situations, and the Z-Axis clearance issues will definitely be a limitation if I'm cutting a piece of material that's pushing the limits of height on my current design. I haven't nailed down exactly how much Z clearance and Z-Axis travel the machine will have yet - the CAD is still changing. I very well may have to move my spindle up and down on my Z plate/use multiple tool lengths if I want to do anything very tall, but that should be a rare enough occurrence that I'm willing live with the inconvenience.
If I'm doing smaller parts that require more stiffness, I'm planning to position the work piece high on the bed (spindle high, in it's stiffer position) and X-Axis all the way to one side near the main frame. While making the Z-Axis longer will make it more flexible in it's lowest position, it shouldn't change the stiffness higher up. It seems much easier to elevate the work piece in a stiff manner, since it doesn't have to move at all.
I don't have a huge amount of machining experience, but I've not yet used a machine that seemed like it had too much z-axis travel, so I'm trying to maximize that as much as possible.
The gadget guy - Hi - Something that you maybe able to do "at home" and others, is to use vibrational stress relief vs thermal stress relief. If you can make (or hire) a vibrator and attach it to the frame then run it up and find its natural freq it may relieve itself enough to make a difference. You can estimate its resonant freq by tapping it and measuring its ringing freq. The theory is that internal stress increases the structures natural freq. So if done properly you bring it up to ringing freq hold it for a while then back down and then bring it up again. If relieved it should ring at a lower freq. If you have access to FE you can do a model analysis and estimate its stress free vibration freq's.
The stress relief is achieved by the inertial forces moving bits of the structure past its plastic strength which stretches the material, reducing internal stress. One factor of this is called the Bauschinger effect. This effect is that if you stretch steel in one direction its compressive strength reduces in the other direction so when it vibrates the other way it relieves itself easier. Then you stretch it in the other direction and then it returns and relieves further in the original dirn. This is to do with how the internal grain boundaries and grain dislocations work. It's sort of a hysteresis effect.
Harmonic SR is not done with welded fatigue structures because its known that this is a severe loading and cracking of the weld toes does/can occur so fatigue performance can be compromised. I deal with very large mining trucks and trailers and they are way too big to put in ovens so VSR is used a lot. We have had welds crack for instance in ears for rams that have not been captivated before VSR and they have cracked and then put in service and cracked further. In a static structure such as a machine frame this would not matter. Its harder to do sub harmonic stress relief unless you have accurate vibration measuring and excitation equipment. But harmonic SR is easier as you clearly know the structure is active!! Use a welding rod with high plasticity and a wide freezing range to make the welds as plastic as possible. To do the VSR you hang it from the roof on ropes or wires or sit it on rubber pads so it can vibrate freely. Unfortunately it only SR's areas that vibrate well eg it will have preferential SR areas. Thermal stress relief will do the entire structure. Once VSR is done then finish machine and fingers crossed it doesn't potato chip on you too much when machined. Peter
I learnt about the Bauschinger effect when I was involved in drawing aluminium tube. We found in say a 2 pass or three pass draw that sometimes the metal would snap say at pass three. But if we turned the tube around from pass to pass it rarely snapped. This is is because of this effect, it's more plastic if you turn it around between passes. Peter
https://www.advancedvsr.com/
the vibrator required is not very big as the process takes advantage of the natural vib freq.... Been reading a bit more and this is also called vibration softening behavior. Hi Gadget guy - If you publish the step file of your bench I can run a modal analysis on it and tell you the freqs required. cheers
Mactec54
Hi Mactec - I kind of like the nostalgic aesthetics of rivets but yes hot rivetting is a chore. High strength bolts outperform rivets these days. The red hot rivet is annealed steel and alloyed bolts are far stronger than rivets. This allows the bolt to be done up very tight, tighter than a shrinking rivet to produce the required friction joint. There are friction joints and snug joints. A hot rivet is more of a snug joint as it expands slightly to fill the hole. Snug joints usually are reamed to snug fit the bolt, typically aircraft connections are like this. But on a machine frame a friction connection would be fine. Pins could be used for future alignment if needed. If you want a true snug joint then once the frame is aligned you can undo each bolt and use epoxy putty to set the joint and fill the hole thus becoming a snug fit. Peter
A typical structural steel hot rivet has a UTS=450Mpa whereas a Class8.8 bolt has a 800Mpa UTS and is done up to 65% of this. A class 10.9 bolt has a UTS of 1000Mpa so is better again, then a capscrew is 1200MPa
Hi TL1 - This comes down to the tolerance you want in your machine. The steel is made to a tolerance so you can check against that. But I expect you still have to machine it flat for the purposes of a good CNC machine spec. There are many comments on this in the forum that std steel sections are not flat or true. Peter
yeah thanks well need to stick with extrusion then since there no place here to do that
The cold rolled steel has a lot of stress in it, so it would need stress relieving and machining to mount rails on, cold rolled is normally .005" to .010" per foot surface flatness, so not that great for mounting linear rails on without being machined / Ground after mounting, the square / rectangle tube that most use for the frame is most likely just as flat as the cold rolled steel
Ideal you weld your frame together add mounting plates for all you precision part mounts, stress relieve and machine
Even those that use aluminum extrusion the mounting surface should be machined where precision parts are mounted this company offers extrusion with machined faces for better linear rail mounting not saying you should use aluminum extrusion just giving an example of what is available
https://us.misumi-ec.com/vona2/mech/...isplay=mc-list
Mactec54
Hi Teamlok1 - If by "extrusion" you mean construction or T slot extrusions they are not ideal. They are convenient but the t slots are thin so the load transfer is not good at the bolts. They are not globally stiff vs their outer geometry, they are expensive and the ends are rarely square. Std heavy channel or square extrusions are much better value, you can do bolts up correctly, the critical surfaces will still need machining. Steel channel is cheaper and very stiff. If its a hobby grade machine then std aluminium extrusions are quite straight over short distances, just as good as T slot extrusions as they are made to the same spec. If you are near an extrusion outlet you can hand pick good bits. You can use the construction system brackets for connections to make 90deg connections easier.
Cheers Peter S
yes they offering milled surface here the extrusion im going to use it got 5.7mm thickness is it good?
Hi Teamloks1 - You have to make that decision. Personally I wouldn't use T slot to construct a machine. See attached article. If you buy a std 100x100x5 square section it will be less cost and much stiffer. Aluminium is sold by the kg so you pay a lot for the T slot vs its performance. To make attachments for the rail you get a steel flat bar and drill it and thread it on the right pitch, slid it into the tube which is drilled at the right pitch and then assemble. Much cheaper much stiffer and much more efficient.
The issue of flatness and straightness has to be resolved against the tolerance of the machine you are aiming at. If a hobby machine then no machining is required it will be as good as the extrusion, same as any other extrusion based machine. If you are aiming at a precision machine then the surface will need to be machined (or shimmed and scraped) and the section checked against your target tolerance and the rail makers tolerance. Peter
yes thanks for that just my problem is there no place to machining it flat here so pretty limited which material to use
Hi Teamloks1 - No you are only limited by your lack of imagination. Stonemasons and carpenters for millennia have made exquisitely accurate structures with hand tools and some effort. You maybe expecting too much from your first machine. A first machine will have some bad habits and quirks. So you then use that machine to build a better machine and then that to move forward etc. Build a plywood machine and learn about machines. Then use that machine to build an aluminium machine etc. Your 3rd or 4th machine will be about what you are hoping your 1st machine will be... T slot extrusion machines are fast to build but automatically limited by the system. A plywood machine you can develop and design cycle and test very quickly. All depends on what you want it to do. Cheers Peter
looking back on posts you want to mill steel! If you do not have access to machinery suitable for this task then do not do it. This is a hopeful and hopeless exercise and you will be very disappointed and out of pocket, time and enjoyment. Go find a mill and buy it. Peter
Then I highly recommend a plywood machine - cheap quick big, very stiff and easy to develop. There are many in this forum...Peter