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  1. #41
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    Apr 2007
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    Re: Jake's 4x8 CNC Build

    doing a large build like this on a Bridgeport takes forever and a day. i have been working on my build for the last 5 months and i'm still not done or even close for that matter. so i can relate to the problems of getting stuff to fit on a small mill for the task at hand my problem is i make chips 40 hours a week for a living and spend another 15 to 20 hours a week working on my stuff at home which makes for a long week in general looks like your Bridgeport is in pretty good shape so that's a plus at least.

  2. #42
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    Re: Jake's 4x8 CNC Build

    Quote Originally Posted by JakeInCharlotte View Post
    I tried to get some work done on the router this weekend but all the Thanksgiving family fun made it pretty hard to do. The plan right now is to mill out the back of this 1" plate to save on weight to .300 thick leaving contact areas thicker. The spindle plate mounting hole will act like stiffener ribs I hope. If this fails, I will add some ribs to the outer edge as Peter recommended.

    Anyone with a mill knows you spend half your time figuring out how to mount work to the table. I decided to build a two piece vice for the facing operation like this:



    The back of the "Z plate":



    Layout:



    Outside dimensioning:

    Isn't the relieving counter productive in the stiffness dept? Stiffness comes before weight in importance? You are better to go to a thicker aluminium plate to get the weight loss and the stiffness gain...Peter

  3. #43
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    Jan 2015
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    Re: Jake's 4x8 CNC Build

    Quote Originally Posted by machinedude View Post
    doing a large build like this on a Bridgeport takes forever and a day. i have been working on my build for the last 5 months and i'm still not done or even close for that matter. so i can relate to the problems of getting stuff to fit on a small mill for the task at hand my problem is i make chips 40 hours a week for a living and spend another 15 to 20 hours a week working on my stuff at home which makes for a long week in general looks like your Bridgeport is in pretty good shape so that's a plus at least.
    The Bridgeport is great, but you are right about the speed. I'm doing the best I can with what I have. This a long term project.....

    Any suggestions for a endmill to remove a lot of material?

    Quote Originally Posted by peteeng View Post
    Isn't the relieving counter productive in the stiffness dept? Stiffness comes before weight in importance? You are better to go to a thicker aluminium plate to get the weight loss and the stiffness gain...Peter
    I'm not going to second guess your advise in the structural dept. My question is, how much deflection is acceptable when the Z plate is fully extended down? How much force on the bottom edge to produce such deflection?

  4. #44
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    Re: Jake's 4x8 CNC Build

    Hi Jake - Thats a good question and can't be answered without some calculation. Machine stiffness is measured in two ways: static and dynamic stiffness. Dynamic is difficult to measure but very accurate machines do it on the fly. Simpler machines use static stiffness as a guide. Small benchtop mills have a static stiffness in the range of 1-5N/um at the tool. Modern VMC's are about 150N/um and some big machines are 500N/um. Routers are 0-10N/um minimum. (1um is 0.001mm)

    So if you are engineering your machine you have to decide on the stiffness and then work towards it. The maximus thread did a comparison of this plate and from memory it needed to be 32mm aluminium to be equal to 25mm steel and it was considerably lighter. Routers are designed with a tool load of about 100kgf and mills are 500kgf or sometimes 1000kgf as the design loads. Your machine has the potential to be very stiff. At 100kgf (1000N) at the tool it should deflect less then 10N/um say so this means 1000N/0.1mm or less. Once you build the machine you can measure this. A computer model is required to figure out all the part deflections it's a big number crunch.
    precision heavy mills are about 70-100N/um stiffness Peter

  5. #45
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    273

    Re: Jake's 4x8 CNC Build

    Quote Originally Posted by JakeInCharlotte View Post
    The Bridgeport is great, but you are right about the speed. I'm doing the best I can with what I have. This a long term project.....

    Any suggestions for a endmill to remove a lot of material?
    I've tried a few different types of tools but have not had great luck with any insert cutters to be honest. probably because none have been anything but low end and cheap ones. seems like the sharper the nose radius the better they do just because the Bridgeports don't have the HP to push anything big, old school tooling seems to work better than anything and seems to be the most cost effective. carbide end mills work ok but don't seem to last as long as they should just because carbide likes rigidity. HHS and Cobalt tooling is more forgiving if your cutter grabs in a corner or the table is not locked down.

    these machines are not meant for big parts. they can do them you just have to take your time with them. when i first got into machining i was always running them but back then it was for tool and die work and the small die sections were what they got used for mostly. in a commercial type setting nothing bigger than something that would fit in a 6"vise found it's way on the Bridgeports.

    i would say a cobalt rougher and a mister if you have one is probably the most cost effective approach. a lot has to do with the machine condition as well. my ways are not in the best shape on mine so maybe carbide works good on yours? mine was a rescue from the scrap yard only paid $250 for mine

  6. #46
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    Re: Jake's 4x8 CNC Build

    just noticed you got the mister covered. one other thing that helps with these mills is to keep your table up high and keep the quill up in the head as far as you can, they are not as rigid when you have the quill sticking out, it even helps to crank the stop up to help lock things in too. pain in the butt paying around with the stop nuts but it does help sometimes.

  7. #47
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    Jan 2015
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    39

    Re: Jake's 4x8 CNC Build

    The holidays slowed me down a bit but I got a little work done on the Z Sled. My original plan was to mill it out of a solid piece of 1" which steel. After realizing how incredibly slow that would be on a Bridgeport, I decided to fabricate it out of .5" x 1.5" sides and 1/2" thick top. I have about .045" of weld distortion when done. I clamped it to my welding and put 220 LBS (me) of weight on the edge. The deflection was .0011". I'm pretty happy with that?




  8. #48
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    Re: Jake's 4x8 CNC Build

    if you would have made that from solid it would have most likely moved some after you machined the bulk of the material away. something like that you would have to be pretty careful machining to keep it from moving. you would have to rough most of the material release everything and finish machining it by holding on to the piece in a manor that would not throw stress back into the part.

    since you welded the piece and have some movement from welding you can work it back to being flat the same as you would after roughing from solid you would have just needed to have some thicker stock than needed so you have some material to straighten things back out after welding. stress relief after welding would make this process easier but that is not always needed if your careful in your approach machining things.

  9. #49
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    Jan 2015
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    Re: Jake's 4x8 CNC Build

    Quote Originally Posted by machinedude View Post
    since you welded the piece and have some movement from welding you can work it back to being flat the same as you would after roughing from solid you would have just needed to have some thicker stock than needed so you have some material to straighten things back out after welding. stress relief after welding would make this process easier but that is not always needed if your careful in your approach machining things.
    I left an extra .125" on the linear guide blocks for rough milling and scraping flat. Hopefully I won't have to take too much off the front and compromise the stiffness. The upside is I get to use my homemade table vise again. Do you have any opinions on soft jaws? Will they help grip the part better?


  10. #50
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    Re: Jake's 4x8 CNC Build

    soft jaws are not needed here. they work if you want to hold onto something irregular in shape or not mark a part up if it cosmetic. since this is just a basic rectangle steel jaws will work just fine.

    most of the time soft jaws are used in a secondary operation on a contoured shape done on a CNC in the first operation. flat stock would be over sized mostly on thickness so you have a work holding hub to grip onto and the contour work is done above the vise jaws. once the first operation is done then a set of soft jaws are machined to match the contour so the work holding hub can be machines off and any additional work can be machined on that surface. this would be something typical where soft jaws come into play. for what your doing they would not offer any benefit.

    when you flatten your part by resurfacing the faces watch your vise does not flex the part when you tighten it. you want just enough force to hold things but not so much it distorts the part. small light cuts to work the bow out is what your after on the first side and you don't want to hit the part down on this side. once you get one side flat in a free state then you can tap the part down on the second side but you still what just enough force on the vise to hold the part but not so much it flexes the part here too.

  11. #51
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    Re: Jake's 4x8 CNC Build

    i would probably approach the flatness something like this. take small light cuts with light force on the vise to take a little of the high spot out of the center. use a long straight edge so you can see where the bow is on the part. the bow is most likely to be across the long side and have a convex and concave side to it. i would put the convex side up and the concave side down since this will be the most stable position to hold the part. the first material to get removed will be in the center area, once you get the first side flat then you can flip the part and tap it down so you can hold the thickness close. i made a quick sketch for reference nothing fancy Attachment 434150

  12. #52
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    Re: Jake's 4x8 CNC Build

    That drawing was a big help. I started milling the front last night and took of about .020".

  13. #53
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    Re: Jake's 4x8 CNC Build

    .020 is not bad. when you flip it over you should end up taking material off the two ends first if the bow was how i thought it was. you will probably end up taking another .020 to .025 off the second side. if you still have bow after this it should be a lot less than what you started with and you can repeat the process. light tension on the vise and small light cuts is what you want to work the bow from the piece.

  14. #54
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    Re: Jake's 4x8 CNC Build

    Hi JIC - If you have an oxy torch or big propane torch you can heat the piece up as hot (and evenly) as possible then let cool in air. (Maybe a BBQ burner??) This will stress relieve the piece before you finish machine it. Ideally if you took it to a heat treater for SR they would heat it to cherry red in an oven then slow air cool. Peter

  15. #55
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    Re: Jake's 4x8 CNC Build

    Peter, I have thought about that. I just have not been able to figure out what the actual temperature needs to be. It sounds like you need to get it up to temperature, hold the temperature for one hour per 1 inch of thickness and then cool down. That is about as far as I have got. Heat treating sounds extremely complicated, and sort of like black magic. I completely agree I need it, I just don’t know the temperatures and time that would be beneficial. I just don’t want to waste my efforts on some backyard process that has zero benefit.I even thought about vibratory stress relief, but that sounds like snake oil. I read in a post from a few years ago that you can get some of the stress out by simply leaving the frame outside for a couple years. What are your thoughts on that?

    Getting closer on the Z sled.




  16. #56
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    Re: Jake's 4x8 CNC Build

    Hi JIC - The SR temp for steel is 650C above. Its "hot strength" is nearly zero when its red so the relief happens nearly instantly. The 1hr per 1" rule is that it takes about 1hr to soak 1" into the steel so for thick sections need lots of time to get the middle up to temp. In aircraft welding tubes you typically TIG weld them then use an oxy to bring to red then let cool to SR and normalise the metal. Vibratory SR is not snake oil. Its used in mnay applications where thermal SR is not practical, large structures, pressure vessels, aircraft parts. It uses an effect called the Bausinger Effect. Thermal SR is better in your case...

    ---------------------------------------------------------------------------------------------------------------------------------
    copied from other thread on similiar

    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
    -----------------------------------------------------------------------------------------------------------

    Now mild steel metallurgy does not change over time it only rusts so time will not help mild steel. Cast iron and some aluminiums "age" There are aging steels as well...This usually means compounds within the grains migrate to the grain boundaries this process means there is a change of volume. This change is also a thermo-mechanical process so to some extent the internal strain drives the migration. To keep it simple this process reduces strain as most things like to be in a less stressed condition then a stressed condition. Ageing metals can be done in an oven to speed up this effect. but your steel is not an aging type... Oxy is the go... Peter

    I've also attached a image of a shaft I designed coming out of tempering, its the same temp as SR so your metal should be this colour for full SR. But as long as the metal has some red colour it is relieving. Peter

  17. #57
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    Re: Jake's 4x8 CNC Build

    looks like the chatter from that big face mill is not helping.I think i might have said this before but bridgeport machines don't do so good with insert tools, for the purpose of getting things flat a single point tool like a fly cutter seems to work better for finishing on these machines. two flute end mills will work really well too. when you get to the point of working a piece flat it takes time and your not going to be taking more than .010 per pass. any more than this you run into the cuts causing stress in the part. big cutters are not good in this stage because the larger the diameter cutter the more it pushes and causes stress. i would probably not use something any bigger than 1 1/4 to 1 1/2 in diameter finishing here trying to make the part flat.

    i hate to say this but stress relieving is one of the processes that got tossed to the waste side over the years to keep up with china pricing. so working with a piece that is difficult has been something that i have learned to deal with over the years.

  18. #58
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    Re: Jake's 4x8 CNC Build

    Hi JIC - In regard to holding the piece for machining. If you can put some holes in the part that are either needed later or are just holes , then use standoffs to bolt the part to the machine bed this is a better less stress hold then in a vice. This part is assymetric and all the weld are on one side. Its unlikely it will settle out flat by machining it. Creeping up on "flat" from both sides you may never get to flat You could try cold setting it if you have a press that can bend it? Thermal SR is the go. Are there potters or kilns around your way? Its a small part and they will get to 650C easy....Peter

    if you do oxy or torch it use a soft flame and keep it moving and try to bring the whole thing up to temp. Last resort thinking about it. You may warp it worse need an oven or a kiln that can bring it up slow and down slow...

    Did a bit of reading and mild steel will relieve as low as 200C so you run your kitchen oven flat chat and cook it for a few hours and see how it goes!! Even grit blasting can relieve it a bit if there's a sand blaster around your way?

  19. #59
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    Re: Jake's 4x8 CNC Build

    if putting it in a oven for a few hours is all you need it could not hurt to try it. i'm not going to say Mrs JakeInCharlott is going to be happy in the process but something to try. after that point smaller cutters with no nose radius will help with the chatter issues.in a ideal situation the part would be stress relieved and worked in close on the mill and then surface ground taking about .010 to .015 off in total for finishing. but if your not surface grinding and you can keep the mill finish good and get it flat with in .001 or .002 your doing pretty good on the mill.

  20. #60
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    Re: Jake's 4x8 CNC Build

    Hi machinedud - In the kitchen oven is not a full SR. Its as good as JIC may have and worth a shot. My wife over the years has put up with me putting various metal, plastic and other substances in the oven. I clean them very well, put them in an oven bag and clear it with her before lift off. Once I got a shed I moved in an old oven that got a good workout making urethane scooter tyres and aging aluminium parts. I'm lucky here as there is a heat treater about 2hrs drive away that does most things and has large ovens for the important things. Peter

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