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  1. #61
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    Apr 2007
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    39

    Re: Jake's 4x8 CNC Build

    Quote Originally Posted by peteeng View Post
    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
    i don't have these problems with someone else to clear things with but on a home build most don't probably get into splitting hairs on tolerance. i think if you end up with something in the +/- .005 range most are happy with that since most take these builds on with limited equipment to begin with.if you have a decent mill with some readouts you could probably get something in the +/- .002 to .003 range if you work at it. i guess a lot depends on budget and the quality of the linear motion parts. most rolled ball screws are not able to hold things tight because of lead accuracy to begin with.

    i'm not that far into my build yet and the last month or so i was working on my building on some much needed attention. this week it was the flu hopefully this is about kicked so i can get back to work on my own project so hopefully in another month or two i get some things moving around under power.

  2. #62
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    Jan 2015
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    36

    Re: Jake's 4x8 CNC Build

    Some really good info there.
    I started work on the Z and Y(?) baseplate. I tried out my fly cutter, with pretty good results. Not as good as the insert shell mill though. The insert mill is able to produce and almost mirror finish. Photos don’t really do it justice. Those dips are in the .001 range. With the fly cutter on my old machine, the dips are more like .005 to .015. But I feel more comfortable with the speed of the fly cutter, and I can cover the surface with three passes instead of five.

    The plan right now is to get all the parts fabricated, the gantry beam, the Z sled, Z&Y plate, and gantry legs. Hopefully in that time I can figure out the stress relieving process.

    How do you even talk to heat treaters? What specifications do you quote? Tolerances? How do you even know if they did it, let alone if it worked?

    And then the big question, what if I just leave it alone. How much distortion will I see? Will it occur slowly over time? Or when I crank it up for the first time?

  3. #63
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    Re: Jake's 4x8 CNC Build

    first off if you can mill most of your bow out i don't think you will see it come back over time. the only way it would cause problems is if it were introduced to some form of stress from machining or welding after it has been worked out.

    your shell cutter probably works better on a solid piece. since you have hollow sections depending on the placement of the ribs any wide span could be a chatter zone because of lack of support in that particular area. every piece is different and reacts to machining differently. you could have a piece that starts off machining well and as you take more and more material away the finish gets worse and worse as you go.in a case like this the shell mill might end up not working and you would have to finish with a different tool or change the amount of step over to adapt to the finished part. when your making the one of's this can be tricky if the part is complex.

    when you have something that is not solid where your vise jaws grip can make a difference to how much the part flexes under tension. you can add construction legs to parts or make a support that fits inside so as the vise is tightened the part can only flex so much. so a strategy for material removal can have all kids of solutions. the only thing that matters is you end up where you want to be no matter what path you take.

    a lot of the process of machining stuff manually when you making just one of something is to problem solve as you go at the machine so you can get to where you need to be. if the shell cutter is working better then go with it. i'm just trying to give you idea's for thing to try if you hit snags.

    something i do is mark my parts with a marker to know where the high and low spots are so i can tell if i am taking material off the right area first when i creep up on flatness or bow. knowing how much material needs to be removed for and exact area is helpful. if the material being removed matches the picture of what the surface measures then you can tell if you on the right track.

  4. #64
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    Jan 2008
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    1031

    Re: Jake's 4x8 CNC Build

    I'm not an engineer / machinist but my understanding is that at least the majority of movement will occur at the time of machining. I could be wrong. I spent quite a while trying to get a clear answer on whether a steel weldment will continue to move in weeks / months / years after machining but could not find a clear answer. It may also depend whether the piece is subject to certain conditions (mainly vibration) during it's use.

    If you can machine both sides multiple times (side A then side B then side A then side B and so on) you may get to a stable flat piece. The movement should get less each cycle as there should be less residual stress. (This is largely based on first principles, so take it with a grain of salt).

    When surface grinding it is common to take light passes then flip the part, and repeat the process a few times.
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  5. #65
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    Re: Jake's 4x8 CNC Build

    actually when surface grinding it is common to grind with the magnetic chuck off to work the bow out of something. a lot of the strategy of working bow from a piece stems back to principals in surface grinding. you need to know what the parts looks like in your head before you even start cutting so you can be precise about getting something flat.

    when surface grinding they use wedges to partially magnetize to hold the piece while you do the grinding. the magnet on a chuck with pull the bow down to the chuck and when released the bow comes back because stress was added to the part.

    it you hold a steel part in your hand to long the size will grow. and if your working with carbide the parts don't grow from the heat but the grinding wheel do so you have to know what your doing to split tenth no doubt.

  6. #66

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    Oct 2019
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    24

    Re: Jake's 4x8 CNC Build

    It's a very interesting topic. Before I started doing any welding on my components I went back and had to refresh my self on how I could limit distortion in welding components, as the distortion can also introduce stress into the welded components and cause me to do a lot of extra work to offset/fix the distortion.

    Lincoln Electric has some good information and there are a number of "older" YouTube videos on this as well. But I also have 2 very good friends that have been welding almost their whole career with Stainless Steel and Aluminum in poultry processing plants and I listened to there knowledge of limiting distortion while welding. I applied these principles as I began welding up my frame. gantry uprights, etc. and I think it helped tremendously. In short, clamping pieces to a fixed item and letting it act as a heat sink, welding in segments on one side and quickly moving to the opposite side, direction of welding, etc are all different technics that help.

  7. #67
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    Jul 2018
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    1022

    Re: Jake's 4x8 CNC Build

    Hi Pippin and others - Mild Steel will not change shape over time unless some other factor is brought into play (red heat, more machining, considerable harmonic vibration, sand blasting, mechanical overloads ). The welded structures internal stresses change with change in geometry (machining) with a bit of luck and slowly working each side it may come into tolerance. Or it may not. JIC and others, there is no reason to 100% weld the bits together in a machine part application. So stitching it together is a much better strategy to minimise distortion if stress relief is not available. Hard soldering or brazing are much better alternatives if thermal SR is not available. Peter

  8. #68
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    Re: Jake's 4x8 CNC Build

    everything on the gantry is the easier part to keep flat and straight. the table and all the linear motion parts that bolt onto it are the hardest part from what i have seen in my build. but i cheated a little and had the bearing blocks and a short set of rails to determine the needed width of my table. so all i really needed was one cross section to fit like a glove and make the rest of them exactly the same. bolted frames are easy to control this way. welded frames are a different story so it will be interesting to see how Jake pulls that part of the build together.

  9. #69
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    Re: Jake's 4x8 CNC Build

    Quote Originally Posted by peteeng View Post
    Hi Pippin and others - Mild Steel will not change shape over time unless some other factor is brought into play (red heat, more machining, considerable harmonic vibration, sand blasting, mechanical overloads ). The welded structures internal stresses change with change in geometry (machining) with a bit of luck and slowly working each side it may come into tolerance. Or it may not. JIC and others, there is no reason to 100% weld the bits together in a machine part application. So stitching it together is a much better strategy to minimise distortion if stress relief is not available. Hard soldering or brazing are much better alternatives if thermal SR is not available. Peter
    Good to have an answer.

    So machine the weldment, check the geometry is satisfactory, further machining as necessary and then get on with the build.
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  10. #70
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    Jan 2015
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    Re: Jake's 4x8 CNC Build

    Quote Originally Posted by beartooth View Post
    It's a very interesting topic. Before I started doing any welding on my components I went back and had to refresh my self on how I could limit distortion in welding components, as the distortion can also introduce stress into the welded components and cause me to do a lot of extra work to offset/fix the distortion.
    I put a lot of thought into the welding. Weld distortion is real and spectacular! When I welded up the frame, I staged my welding so one weld would pull against the other. It seemed to work pretty well as I don't have a banana. I also choose to pull the bead in a single line to minimize heat instead of stitching. I looks kind of ugly, but worked out OK.

    Quote Originally Posted by peteeng View Post
    Or it may not. JIC and others, there is no reason to 100% weld the bits together in a machine part application. So stitching it together is a much better strategy to minimise distortion if stress relief is not available. Hard soldering or brazing are much better alternatives if thermal SR is not available. Peter
    Fingers crossed!

    Quote Originally Posted by machinedude View Post
    everything on the gantry is the easier part to keep flat and straight. the table and all the linear motion parts that bolt onto it are the hardest part from what i have seen in my build. but i cheated a little and had the bearing blocks and a short set of rails to determine the needed width of my table. so all i really needed was one cross section to fit like a glove and make the rest of them exactly the same. bolted frames are easy to control this way. welded frames are a different story so it will be interesting to see how Jake pulls that part of the build together.
    I'm putting a lot of faith in the 3 x 6 surface plate and my homemade power scraper. Also, I'm the lookout for an autocollimator.


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