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  1. #61
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 27 times, giving thanks to others 24 times.
    Sorry for rambling post...

    A few people on cnczone seem to swear by it, I've only ever used the stuff before for patching up rust holes in cars not for anything structural but watching this vid impressed me enough to seriously consider it:

    DP-420 3M EPOXY overlap shear test by Doctorbass PART 1 - YouTube

    (In part two he adds a load more weight and the join holds perfectly)

    The thing with welding properly is it does take a lot of practice, if you don't prepare the joint carefully, don't make sure you have good penetration and don't make a consisitent pool in the weld than the strength of the join drops off dramatically, that is why people doing mission critical welding like nuclear power plants or submarines insist on x-ray inspection of every single weld. A weld can look and will look absolutely perfect but still be structurally weak, especially with arc welding where bits of slag can slip into the weld and be completely invisble to you when you have a welding goggle on.

    If the people recommending repeated tack welds ever took an x-ray of the joints or sawed through them they would be shocked at how weak the joint they ended up with is in real life, fortunately the amount of steel that gets used for CNC machines is not there for strength, it is there to provided rigidity. (I know nothing about CNC but I can weld...)

    Metal epoxy seems to provide more than enough strength for the job of holding a machine together but introduces another problem because the young's modulus of the various epoxy mixes seems to vary between 2-20 Gpa (compared to about 70 for ally or 200 for steel, higher means more rigid). I'm satisfied that any epoxy joint will structurally hold together on a CNC machine but I still haven't worked out what effect joining two pieces of steel together with what is by comparison a bit of rubber will have on the overall performance of the structure.

    If I can't determine that my current plan is to build a frame from mild steel box section that is held together using brackets made from steel angle section welded to plate that is bolted to the box section. Enlarging the holes for the bolts mean I can perform accurate positioning before tightening the bolts and secure the joins using metal epoxy. That way I get the benefits of steel on steel rigidity and the epoxy makes sure that it will never move out position but if I welded a 'perfect aligned bolted structure' the thermal shock of the welding would distort the alignment as metal expands and then contracts. Bolting alone would mean loosening of the bolts over time.

    My current thoughts anyway!
    *Subject to radical change when I get a clue about what I'm doing. :)

  2. Have you considered Ally box section and ally welding like this: Durafix UK Ltd - Pages

  3. #63
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 27 times, giving thanks to others 24 times.
    Sorry about the thread hijack Joe, you seem to be more of a wood guy and not an engineer so make a cup of tea and read this thread on cnczone:

    My First Router, Built in Steel - CNCzone.com-The Largest Machinist Community on the net!

    A few years ago this english guy Haydn that builds custom guitars built his cnc machine from 80x40x4 steel box section, bolts and epoxy. It is a really great thread because he posts on it over several years and gives lots of details and photos not just of the machine but the work he does on it.

    If you check out the mop/abalone inlay work his machine is producing on page 9 I'm pretty sure you will be stunned by what his machine is capable of, exactly what I am aiming for.

    It is quite inspiring that a guy with basic wood tools and a drill press can put together a machine like that on a realistic budget. I don't have access to an engineer's flat table but I can work round that by using a few car jacks to get my box section roughly level and then putting a layer of self leveling epoxy on top of them. After reading that thread I'm about 90% sure I can build a similar machine without going over £1500.

    If I'm not happy with the rigidity of the machine I will still have the option of pouring some fibre concrete into the gantry and just have to live with a slower machine.

    Edit to add: The guy is using the machine to make a living and the epoxy joints are holding up fine after a couple of years of regular use that is good enough for me, long live the epoxy! :)
    Last edited by D.C.; 22-10-2012 at 07:35 AM.

  4. #64
    Quote Originally Posted by D.C. View Post
    Sorry for rambling post...

    A few people on cnczone seem to swear by it, I've only ever used the stuff before for patching up rust holes in cars not for anything structural but watching this vid impressed me enough to seriously consider it:

    DP-420 3M EPOXY overlap shear test by Doctorbass PART 1 - YouTube

    (In part two he adds a load more weight and the join holds perfectly)

    The thing with welding properly is it does take a lot of practice, if you don't prepare the joint carefully, don't make sure you have good penetration and don't make a consisitent pool in the weld than the strength of the join drops off dramatically, that is why people doing mission critical welding like nuclear power plants or submarines insist on x-ray inspection of every single weld. A weld can look and will look absolutely perfect but still be structurally weak, especially with arc welding where bits of slag can slip into the weld and be completely invisble to you when you have a welding goggle on.

    If the people recommending repeated tack welds ever took an x-ray of the joints or sawed through them they would be shocked at how weak the joint they ended up with is in real life, fortunately the amount of steel that gets used for CNC machines is not there for strength, it is there to provided rigidity. (I know nothing about CNC but I can weld...)

    Metal epoxy seems to provide more than enough strength for the job of holding a machine together but introduces another problem because the young's modulus of the various epoxy mixes seems to vary between 2-20 Gpa (compared to about 70 for ally or 200 for steel, higher means more rigid). I'm satisfied that any epoxy joint will structurally hold together on a CNC machine but I still haven't worked out what effect joining two pieces of steel together with what is by comparison a bit of rubber will have on the overall performance of the structure.

    If I can't determine that my current plan is to build a frame from mild steel box section that is held together using brackets made from steel angle section welded to plate that is bolted to the box section. Enlarging the holes for the bolts mean I can perform accurate positioning before tightening the bolts and secure the joins using metal epoxy. That way I get the benefits of steel on steel rigidity and the epoxy makes sure that it will never move out position but if I welded a 'perfect aligned bolted structure' the thermal shock of the welding would distort the alignment as metal expands and then contracts. Bolting alone would mean loosening of the bolts over time.

    My current thoughts anyway!
    *Subject to radical change when I get a clue about what I'm doing. :)
    Jesus! That stuff is awesome! I want some! I don't know what for, but I just want it lol

  5. Quote Originally Posted by irving2008 View Post
    Have you considered Ally box section and ally welding like this: Durafix UK Ltd - Pages
    Have you used this stuff ? Is it any good? - Ali box does seem pretty cost effective...

  6. Quote Originally Posted by JoeHarris View Post
    Have you used this stuff ? Is it any good? - Ali box does seem pretty cost effective...
    Yes I have and its pretty good. Easier than welding but like all things needs practice...

  7. So the design has developed (changed completely) (again!)

    Attached images of revised gantry supports and ball screw connection detail. The last image is of some hefty milling in 50mm ali, but should be solid as a rock and looks pretty cool...

    I've done this to check it will all assemble OK and to explain my thinking to those who can spot potential pitfalls I may have missed.... Cheers


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  8. #68
    That's looking great Joe... can definitely see the designer in you coming through with that chunky funky machined bit!

    The only thing I can raise is that I think Jazz commented that attaching supported rail to alu profile can be tricky depending on the spacing of the holes and the internal shape and thickness of the extrusion.... maybe you've already checked.... otherwise might be worthwhile checking with somebody who has seen both of these in the flesh so to speak.

    I wouldn't worry about changing things completely again.... I'm starting to think that it may be better forever just re-designing in sketchup than ever building the wee beastie... Lol

  9. I know! One of the benefits of not having any money to spend on it - lots of thinking time!!

  10. Quote Originally Posted by WandrinAndy View Post

    The only thing I can raise is that I think Jazz commented that attaching supported rail to alu profile can be tricky depending on the spacing of the holes and the internal shape and thickness of the extrusion.... maybe you've already checked.... otherwise might be worthwhile checking with somebody who has seen both of these in the flesh so to speak.
    From what I can see from drawings of the profile and measuring the sbr20 I've got it should work... Would appreciate it though if someone could tell me for sure otherwise with this combo..

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