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  1. #1
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    Dec 2011
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    Default Number of Teeth in the Cut

    Today, we had a new replacement cold cut saw installed at our school’s Engineering Dept., a MACC brand one with single vice.

    I overheard the teacher telling the students how to cut a flat bar 75mm x 10mm. He told them not to stand it on its edge but lay it flat in the vice which means more saw teeth are engaged in the cut thus saving saw blade sharpness.

    To me it sounded like an old wives tale. Has his method of cutting been scientifically proven to aid saw blade sharpness?

  2. #2
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    Rule of thumb is 2 to 3 teeth in the cut so that the saw is not plunging into the work when the tooth gullet is on the work (breaks teeth). Depending on the pitch of the teeth on the blade, there may be something in what has been said, but he should be explaining his rational to the students so they are not blindly carrying on a tradition they know nothing of.

    Michael

  3. #3
    BobL is online now Member: Blue and white apron brigade
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    The 3 teeth in the cut recommendation is a good one but is really meant for situations where there is no proper feed control of stock/blade. If the cold cut saw is half decent it will have a controlled feed of the blade and this is what determines the rate at which the blade descends so you can have fewer teeth in the cut than 3. Fancier machines even have constant force cutting controlling the rate of cut.

    The same applies to bandsaws - once I put a hydraulic descent controlling pistol on my bandsaw I found I was able to cut even thin walled (1mm thick) with a a 6 TPI where the teeth are ~4.2 mm apart.
    Of course if you get the down feed wrong you can serious munt the band.

    Have a look in your saws manual.
    Here's an extract from a Brobo Wldown manual showing how stock should be stacked for cutting.
    The holding methods are designed for safety and then to try and maximise the number of teeth in a cut
    This is reflected by the arrangements in the top row eg the C and H sections are arranged so that there are 2 section of vertical to be cut rather than one.
    Watching the guys at the local metal merchant cutting with cold saws they don't seem to pay much attention to how they arrange things.

    Screen Shot 2019-08-13 at 5.10.57 pm.png

  4. #4
    Join Date
    Jan 2004
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    I can mirror BobL's comments.

    When I worked in NDT the boss bought a Macc bandsaw for cutting up the weld coupons for destructive testing.

    We got the best results from paying careful attention to the feed rate and running in the new saw blades.

    These blades cut the coupons in the width vertical position. I was away one day and some one did a cut width width horizontal and promptly buggered the blade.

    It overheated the blade on 75 mm -well actually 150mm W -2 x75mm bar welded together.

    I can remember the saying -No less than 3 and no more than 24- meaning teeth engaged in the cut.

    I'll move this post back to general Metalworking as it does not belong in Projects.

    Grahame

  5. #5
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    Jan 2004
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    Default

    One of the things I do when setting up and running in a new bandsaw blade is to check the shape of the chips.
    If you read the offerings by Morse,Lennox and other bandsaw makers this is mentioned as being a very useful indicator towards BS blade performance.

    Using the illustration below as a guide it is easy to see that a build up of chips will start to affect the blade performance.
    As the blade is asked to cut a excessive width what occurs is shown on the right hand side of the view.

    To my way of thinking there is an optimum point ( in thickness being cut ) beyond where the blade is unable to discharge the contents of the gullet.

    Beyond that point, decreases the optimum cutting ability and wear characteristics of the blade teeth.

    Yes! I know the illustration below (from the Lennox guide) refers to gullet size in respect of the tooth size. However ,when you thing about it,the further the tooth travels in the cut the more chip, it picks up a ever increasing length of chip. Logic says that excessive chip load is harmful at some point.


    Also the guides I have found don't show a single flat bar as a recommended cut . On the other hand, nowhere in any of the guides I read, does it mention anything re not cutting a single width flatbar on edge vertically.

    I would surmise that most bandsaw owners ( such as myself) are not constantly swapping blades to gain the correct TPI for thickness cut but opt do a "oncer" when needed.
    That's my view on it and I hope it makes sense to you.

    Grahame

    B:S tooth gullet capacity copy.jpg

  6. #6
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    I spent a lot of time working cold cut saws. The number of teeth in the cut is important, but blades can actually come to the blade supplier without teeth and they can cut teeth to suit your requirements, or for that matter, they can grind the teeth of a blade and recut it in a different configuration for a different task if required. As an example, for our 315mm blades, our tooth count varied from 180-220 teeth depending on the wear in the blade for blades for 1.6mm wall SS and MS tubing through to 60-80 teeth for bar stock.
    There is a lot of value in sourcing high quality blades from a reputable saw doctor and having them do your sharpening etc. Cobalt blades are a definite plus unless you are only cutting soft materials like copper or ali, as they need much less sharpening than standard HSS ones.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  7. #7
    BobL is online now Member: Blue and white apron brigade
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    @ 315 mm with a 200 tooth count that's a spacing of about 5mm between teeth. If the "minimum of 3 teeth in cut" is applied this means that blade should not be cutting anything thinner than 15mm thick stock. Stacking a half dozen 1.6mm tube on top or alongside each other may not increase the number of teeth cutting at some points in time as the teeth spacing could be synchronised with the tube width - ie there might even be zero teeth cutting at some points in time.

    This is why these things usually have a controlled feed.

  8. #8
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    Dec 2011
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    Thanks for the replies to my post. The teacher did not talk about the number of teeth in the cut but rather cutting the 75x10mm flat bar by having it on edge rather than laying flat would blunt the teeth faster. So that is possibly not true.

    At home, I need to cut 75mm aluminium coving on an angle which is about 4mm thick using a Makita drop saw, but after reading the posts I am a bit nervous about the process. It is a normal woodworking blade and the teeth are quite spaced.

  9. #9
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by Kidbee View Post
    Thanks for the replies to my post. The teacher did not talk about the number of teeth in the cut but rather cutting the 75x10mm flat bar by having it on edge rather than laying flat would blunt the teeth faster. So that is possibly not true.

    At home, I need to cut 75mm aluminium coving on an angle which is about 4mm thick using a Makita drop saw, but after reading the posts I am a bit nervous about the process. It is a normal woodworking blade and the teeth are quite spaced.
    It should be find provided you use a slow feed.

    Woody circular blades have positively raked teeth so they dig into the wood and so sort of self feed.

    Metal cutting circular blades tend to have negatively raked teeth so they scrape away the metal - this means they won't "grab" the material and won't self feed.
    The same applies to cutting plastics.

    This is safer as it can literals rip the teeth of a woody blade like this.
    sawblade.jpg

    This was cutting 1/2" thick ally plate - I was just using too fast a feed speed. When the teeth came off they flew around the shed like bullets. A couple went straight through the Al sawdust hopper under the saw. With the unbalanced blade the table saw started jumping around like a demented Collingwood supporter. I ran for the door and hit the shed power switch on the way out

    Initially I though it was the hardness of the Al plate but I subsequently cut it successfully with another woody blade using a slow feed.

    I then bought a negatively raked toothed blade (its called a composite blade mines is a Bosch that cost me $70) and it lives on the TS as I cut more plastic/Al than wood. It cuts wood very nicely - just slowly.
    TSBLADE.jpg

    If you are worried about it clamp a piece of wood alongside the Al.

  10. #10
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    I'm interested in the comments relating to "controlled feed" for cold saws. The only ones having this feature that I am aware of are automated saws. The standard "Brobo" saw is simply pulled into the work by the handle and returned by a spring. I cannot see a school installing an automated saw.

  11. #11
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    Sep 2012
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    Hi Guys,

    My chop saw uses a 10 " HSS or TCT one. The HSS blade has 220 teeth and was originally for cutting thin wall aluminium sections with wax lube, which it does very well. The other blade that has TCT is intended for plastic and has 80 teeth. This is the blade I use for most things alloy and wood. The saw uses a direct of the end of the shaft motor at 3000 rpm. As long as you are careful to control the feed and use lube on aluminium there is no problem. I've cut four inch solid aluminium bar several times without any drama. The major problem with cutting big solids, is that it gets very hot and will seize the blade if you push it.
    17-08-2019-01.jpg 17-08-2019-05.jpg 17-08-2019-02.jpg 17-08-2019-03.JPG 17-08-2019-04.jpg
    This is my chop saw, I've had it a fair while. Italian made 2.5 Hp motor 2800 rpm, 10 inch blade. There is very little steel in this saw, its almost entirely made from aluminium castings. The bulk of its weight is in the motor. I've removed the blade guard for the picture.

    Edited to add pictures.
    Best Regards:
    Baron J.

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