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  1. #1
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    May 2019
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    Default Tearout on edge face of blackbutt hardwood timber

    Hey Guys,

    I've been having to take some material off the width of some DAR recycled blackbutt. I've been doing it by first removing the bulk with a handheld circular saw and straight guide, and then following it up by taking the last 1 to 2mm off with a 20mm straight router bit (using the same straight guide). A problem I'm having is that even on a relatively shallow depth of cut (1 to 2mm as mentioned), with what's still a relatively new carbide bit, I still often get tearout on the edge face. It hasn't been a disaster in this case as I'm gluing these boards together into a benchtop and so the tearout won't be visisble in the glue joint, but I'll be doing something similar on the outside edges of the benchtop once complete, and so want to avoid tearout at all costs when it comes time to do that.

    I've read that spiral bits can produce cleaner cuts and reduce the risk of tearout due to the fact that have more of a shearing action; like sliding a knife through something vs plunging it straight down. Would investing in a spiral bit help me do you think, and if so, what diameter should I be looking at, and up cut or down cut? I've always assumed bigger bits would produce a cleaner edge, but I've also heard that sometimes you get a cleaner edge by running a bigger bit at a lower RPM. My router isn't speed-adjustable and runs at about 20,000 rpm, so I can't do that.

    Any tips?

  2. #2
    Join Date
    May 2011
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    Albury
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    You don't say how thick the material you're cutting is. A spiral bit would certainly help. For that type of finish machining a compression bit would be best, but you are likely to find that either an upcut or downcut would provide a very acceptable finish. If you intend to use the bit for any full width cutting or any plunge cutting an upcut is the way to go. Downcut bits can't be beaten for cutting dadoes across the grain. For general routing upcut bits are much more versatile than downcut. 1/2" upcut bits are readily obtainable and there is no problem running them at full speed.
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  3. #3
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    Quote Originally Posted by Wardrop View Post
    with what's still a relatively new carbide bit
    How "new" are we talking here? Carbide can feel sharp on your finger and look sharp to the naked eye, but under magnification show that it's actually due for a regrind.

    A spiral bit will definitely help, although I think a compression bit is, at best, an unnecessary extra expense and potentially worse than a single direction spiral because the broken cutting edge could leave a line where the up and down sections meet. At my old work we exclusively ran single direction spirals in timber and a sharp bit left a finish that was nearly good enough to polish.

  4. #4
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    Quote Originally Posted by elanjacobs View Post
    A spiral bit will definitely help, although I think a compression bit is, at best, an unnecessary extra expense and potentially worse than a single direction spiral because the broken cutting edge could leave a line where the up and down sections meet.
    A compression bit doesn't have to be an 'extra expense' and a brand name bit can actually be bought for less than the cost of a generic brand upcut. The up and down sections don't meet at 'a line', they overlap each other and I've never seen a 'broken cutting edge' when using a quality bit. As already stated unless there is a compelling reason to purchase anything other than an upcut then upcut is the most sensible way to go.

    The fact that the straight bit being used is 19mm in diameter wouldn't be helping the finish.
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  5. #5
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    Oct 2014
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    Climb cut with the router. It's not dangerous, even if it begins to self-feed. "If" it self-feeds, it will push away from the timber and stop cutting anything and therefore stop self-feeding. (for those not actually reading the posts, we're not talking about a router table here....). By climb cutting, the cutter will cut down into the wood fibers rather than getting under the fibers and then cutting as the cutter pulls away from the surface. You may notice that the edge surface looks lumpy after a first climb cut. Once you have removed 99% of the material, do a final conventional cut to clean up any hills left behind due to the cutting action pushing away from the timber.

  6. #6
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    I have had lines in MDF at the overlap when nesting with a compression bit (Carbitool), maybe it was just a dud bit.

    Quote Originally Posted by aldav View Post
    The fact that the straight bit being used is 19mm in diameter wouldn't be helping the finish.
    It wouldn't be hurting it either. I've run 6, 8, 10, 16 and 20mm spiral bits and moulder heads (straight, shear and profiled, carbide and HSS) from 50-150mm and the primary cause of a poor finish with all of them was edge wear, a new/freshly ground cutter produced a silky smooth finish regardless of diameter.

  7. #7
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    Not quite sure where the router table comment came from.
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  8. #8
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    Quote Originally Posted by aldav View Post
    Not quite sure where the router table comment came from.
    Just for those that read two words "climb cut" and begin to sweat. I've found that climb cut is a trigger word nowadays. I probably sound like an angry grump that has lost faith in the internet. Nup, but I do expect the expected

  9. #9
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    May 2019
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    Queensland, Australia
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    Thanks for the replies guys. The material is 32mm thick. The bit 20mm straight bit I'm using is a new Carb-i-tool bit that I've only used to plung cut MDF to a depth of about 5mm, so if there's an wear it would be on the bottom 5 to 6mm. I've got about 3mm protruding from the bottom of the face of the timber anyway, so I'm mainly cutting with a "virgin" edge.

    @Kuffy, I thought about climb cutting as logically it would seem almost impossible to get tearout from a climb cut, unless it runs away from you and starts ripping chunks out rather than cutting. What about climb cutting with a up-cut spiral bit. Is that any more or less risky than with a straight-cut bit?

    I think I'll invest in a spiral up-cut bit around 1/2" as suggested.

    Cheers

  10. #10
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    May 2019
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    Not much difference in price from Carb-i-tool between the 2-flute up-cut and 2-flute compression bits. Thinking I may as well get the compression bit for a little more versatility: 2 Flute - Compression Bit - Carbitool (bottom option) vs 2 Flute - Finishing Spiral Bit - Up Cut - Solid Carbide - Carbitool (3rd bottom). I also looked at CMT but that'd be $169. Not sure if it's worth a 50% increase in price over carb-i-tool?

  11. #11
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    The versatility is with the upcut, but if you want a compression I'll send you a PM.
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  12. #12
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    Most obvious is to use a climbing cut- the opposite to how they say to use a router in fact they advise NOT to attempt it however with small cuts and the knowlage that the router will want to runaway on you and precautions made for IE keep body parts clear of what might happen. With fine cuts is normally good. The way the blade cuts tells you a lot. best left for the final - rather than bulk cuts and hang on hard.

  13. #13
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    Quote Originally Posted by Wardrop View Post
    @Kuffy, I thought about climb cutting as logically it would seem almost impossible to get tearout from a climb cut, unless it runs away from you and starts ripping chunks out rather than cutting. What about climb cutting with a up-cut spiral bit. Is that any more or less risky than with a straight-cut bit?

    I think I'll invest in a spiral up-cut bit around 1/2" as suggested.

    Cheers
    The risk is probably about the same, and fairly negligible either way. With a climb cut, the router will want to push away from the timber rather than pulling itself into the timber. This gives you a lot of control over the cut itself. You may have your straight edge setup to take 2mm off, and with a conventional cut you will likely need to do that in one pass. But with a climb cut and the natural pushing away from the timber, you can take several lighter passes freehanding away from the straight edge until you get down to full depth. That may be hard to understand, but you will see as soon as you give it a go. Just make sure you position yourself so that you have the router directly in front you at a comfortable height, and push the router away from you (better yet, walk it forwards with locked arms and shoulders). That will allow you the full use of your upper and lower body to control the beast. If you setup so the router is in front of you but moving from your right shoulder to your left shoulder, you will have to overpower the beast with only your upper body as it tries to twist you into a corkscrew.

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