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  1. #1
    Join Date
    Nov 2006
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    NSW
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    153

    Default Cutting accurate angles in RHS

    Hi everyone,

    I need to make some repairs to my boat trailer. A section of the undercarriage has some rust stopping me from getting a blue slip, so I've decided I'm going to repair it myself.

    I have a mig welder and a metal cut off saw, but I find them to not be real accurate when either cutting a straight 90 degree or an angle cut. Not sure if it's just mine, but I have used a few and had the same with all of them.

    So, just wondering if there are any pointers or tricks any of the experienced​ guys can pass on for cutting clean straight cuts either angled or 90 degree with an angle grinder or perhaps something else for a guy on a budget. I'd love to own a cold saw or porta band etc but just haven't and can't.

    I can supply some pics of the sorts of angles I have to cut tomorrow, but for now and advice or tricks much appreciated.

    Thanks
    Geoff

    Sent from my Moto G (4) using Tapatalk

  2. #2
    Join Date
    Jun 2007
    Location
    sydney ( st marys )
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    64
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    4,887

    Default

    If you can follow a scribed line or chalk mark just mark your angles with a protractor and use the angle grinder with a thin disc.

  3. #3
    Join Date
    Nov 2006
    Location
    NSW
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    Default

    Thanks for the reply pipeclay

    Yeah thought of that mate, but would really like something that will give me a precise straight cut line ever time.

    Sent from my Moto G (4) using Tapatalk

  4. #4
    Join Date
    Jun 2007
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    sydney ( st marys )
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    Default

    If cutting by hand I can't think of anything apart from a cutting guide.

    If your using a friction saw set the angle with a protractor,you may have trouble finding a straight or square surface to set it off as most have a pressed metal base,another thing to try if using a friction saw is don't force the blade through the job,let light pressure cut your material.

    Too much load may/will cause the blade to flex and may change your cut setting.

  5. #5
    Join Date
    Nov 2004
    Location
    Bungama SA
    Age
    52
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    960

    Default

    If its only a couple cuts a local fabricator mob might be able help you out
    ....................................................................

  6. #6
    Join Date
    May 2011
    Location
    Murray Bridge S Aust.
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    71
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    5,942

    Default

    I used to have them, and found them a MPiTB.(Major Pain in The Butt). Got rid of them and bought a 6 X 4 bandsaw, yes it's expensive, but the blades last a long time, no mess or noise, probably telling you something you already know.
    I've found that if too much pressure is put on the saw, they will have a tendency to want to run off to one side, just using a light downward pressure should help.
    Also check to see if the disc is square to the base (vertically) using a new disc, as I've found this to be a problem on some saws.
    If it's still doing the same, I'd cut the top, rotate, cut , rotate, cut, making sure that the disc is square horizontally first.
    Hope this helps,
    Kryn
    To grow old is mandatory, growing up is optional.

  7. #7
    Join Date
    Dec 2005
    Location
    South Australia
    Posts
    1,656

    Default

    Have a good look at your cut off saw and discover why it does not cut square and see if it can be repaired, Idid this to mine years ago and found it just needed shims under one hinge although as said above buying a band saw was the best thing since sliced bread

  8. #8
    Join Date
    Aug 2008
    Location
    Adelaide
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    68
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    1,373

    Default

    I have used friction cutoff saws for more years than I care to remember, currently on my 3rd or 4th unit. You need to take the flimsy base into account though. If you are cutting longer lengths it pays to support the over hanging end(s) so the weight doesn't warp the base. In my case the saw is bolted into a bench frame which reinforces the saw base and provides a 8 foot lead-in and 3 foot lead-out support, set up like that it cuts extremely accurately.

    When it comes to cutting angles you need to go lightly on the cut initially until the blade has established the line of cut (especially when the blade is new and has a large degree of flexibility) and then apply more pressure.

  9. #9
    Join Date
    Nov 2006
    Location
    NSW
    Posts
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    Default

    Thanks for all the replies guys.

    I think the metal cut off saw would perform better if you could get the nice thin blades like you can for the angle grinder. Sometimes when you try and cut a piece of flat bar with the cut off saw it takes forever to get a cut started, like the blade is too thick or something.

    Haven't got the funds at the moment, nor do I probably do enough metal work to justify to the wife spending big money on a saw for cutting metal, that's why I was trying to get out of it with and angle grinder, but need some tips on getting a straight cut lines for the angles I need to but as I know it's hard to freehand with a grinder when cutting.

    Sent from my Moto G (4) using Tapatalk

  10. #10
    Join Date
    Feb 2009
    Location
    moonbi nsw Aus
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    69
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    364

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    Experience with any machine/tool will give you accuracy every time. With your RHS job I think I would mark the angled cuts you need and use an angle grinder. The way to get it accurate would be maybe wrap where you need to cut with masking tape and put your cut lines on it so it is really easy to see. Then when you are happy orient the tube around so you can easily see the cut. Clamp down to a sold something (bench?) Pick up the grinder and put you the grinder and the line inline and away you go. Oh. Cut the RHS long so when you are cutting the angle you are only cutting off say 25mm. This method may be a bit slow but you will have a very high degree of accuracy
    Just do it!

    Kind regards Rod

  11. #11
    Join Date
    Feb 2015
    Location
    Oz
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    Drop saws all seem to scew off in the vertical plain, particularly when cutting angles. I used one for many years before moving on to a bandsaw. Cutting the angle is easy enough but getting them to cut vertically accurate is near on impossible due to blade flex. After the cut I used to release the vice enough so you could slide the work back and forth, then I'd start the saw and feed the work onto the blade very gently until the work was vertically accurate. I also used a belt sander at times. You can get metal blades for drop saws but I think the revs might be a little high. There are cold saws available similar to the drop saws that spin a tad slower and use a metal blade, Makita make one, around $400 from memory. I've seen people use circular saws with fibre disks on occasion, they seem to cut pretty straight.

  12. #12
    Join Date
    Jan 2011
    Location
    Far West Wimmera
    Age
    63
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    4,049

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    Better drop saws like mine (GMC ) have a double hinge with spring loading which enables the saw head to be moved forward and backward a small distance. This reduces the tendency for the wheel to glaze and improves the cutting action. My saw is now getting rather lonely tho as it sits on a shelf looking down on the bandsaw getting all the attention.

    I would have thought the average drop saw would be capable of enough accuracy for welding purposes. Just how accurate do you want to be?

    Dean

  13. #13
    Join Date
    Nov 2006
    Location
    NSW
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    153

    Default

    I'd like it to be as accurate as the undercarriage of my boat trailer the way it is now.

    I'm not most experienced welder as it is, so don't want to have to try and weld gaps from the 2 pieces not closing together etc, or have to tack and try and clamp or hold pieces at the correct angle while I try and fill voids...

    Sent from my Moto G (4) using Tapatalk

  14. #14
    Join Date
    Feb 2015
    Location
    Oz
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    73
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    459

    Default

    I'd try one of these in your chop saw:

    14" x 3/32" x 1" Diamond Edge Chop Saw Wheel - DW8500 | DEWALT

    They're rate to 4300rpm and I believe some abrasive disk chop saws operate at around 3800 rpm, so it should work well.

    I've noticed a lot of metal suppliers use the metal blade chop says, cuts are always vertical and they're pretty darn quick.

  15. #15
    Join Date
    Jan 2011
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    I'd like it to be as accurate as the undercarriage of my boat trailer the way it is now.
    That would be the welding, not the cutting. You need to clamp pieces in place and tack it before welding or it will bend when you weld. You should be able to get a cut accurate enough with a chop saw to do a weld, and if this is not the case perhaps you need to practice welding a bit more or fix the saw. I would say a 2mm gap would be acceptable although others may have a different idea.

    Dean

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