downcut router bit

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I would probably use a larger bit, and have the part cut with a jig saw close to the finished dimensions.
The last time I used a router was to make some oak flooring in huge arcs to go around a fireplace, and the adjacent flooring as well. Huge smoky mess, destroyed a good bit, but got it done...
 
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That seems strange. I cut a lot of circles in plywood, and don't have those problems. 1/2" I cut in two passes. The only troubles I've had is when using a flush trim bit with 3/4" solid walnut. following a template. I've gotten better at it, but have broken 2 spiral flush trim bits. I now use a 2 flute straight bit.

Are you going too slowly with your circle cut?
 
Hm, i cant imagine im going too slowly, if anything i fear i was going too fast, definitely no sign of burnt wood or smoke.. I think i couldnt believe i needed to go so slowly with 3-4 passes.. Im used to jigsawing things. Seems routing is rather more time consuming?

Anyway now im on the market for a straight bit to finish my second sub cutout as well.. I could buy a 3/8 one, as i have a collet for that size too, i just didnt want to stress my little palm router too much.

Ah, and i measured the bits that broke. All 6.45 mm, not 6mm, and were a snug fit in the collet even when it was open. So there was certainly some force on the bit to make it pull down into the work.. Could going too fast cause that?
 
It is really curious that you say that you can even see jaw marks on the bit shafts. What means the chuck is working fine but there is a good amount of force pulling the bit out.

If they're straight cut I'm baffled about what could be pulling them out.
 
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I realized that my spiral buts are UPcut, not down. So they pull toward the router base. Don't know if that's making the difference from your downcut bits.

As for speed, I certainly go faster with the 1/4" router bit than I do with a jigsaw, much faster. Of course the router isn't doing it in one pass, like the jigsaw, and no need to carefully follow a line with the router circle jig. One pass works just fine and plenty fast in 1/4" MDF, but anything thicker is done in two passes, at least.

Before I learned about the spiral bits I just used 1/4" straight bits for circles. Seemed to work OK. Let us know what your results are.
 
Yeah im mystified. It wasnt a smooth experience either, maybe im not used to the noise of a router, but it screams. My ears are still ringing. (well i have tinitus so maybe thats normal) also i had assumed it would run more smoothly. The level of resistance to me pushing varied quite a lot as i rotated through the grain directions. Made keeping a steady speed a bit tricky.

I went to a couple of carpenters shops this morning to ask for a price for them to cut my translam holes for me. Both took one look at my irreplaceable cnc cut translam cabinets and said they didnt want the responsibility of messing them up. Yay! So im kinda on my own (apart from you lot obviously ;)

Here is a pic of the plunge base in my circle jig and half finished hole for sub:
 

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How wide is the groove that's cut? Does it look dovetailed? I'm worried about whether everything is square and you're side loading the bit.

Groove varies between about 6.1mm and 7. 5mm (i did change bit 3 times, removed router from base to do it, and its 2 passes, and i had bits sliding in collet, so some variation expected). Other than that, the base is solid, and all looks square
 

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Those cabinets look awesome.
Maybe the bits are not happy plunge cutting. Try just going down 1mm deep and see how that feels.
I did get a 6mm 2 flute carbide end mill off eBay meant for CNC use and it was cheap, can't remember if it was up or down cut though. Might be worth a look for you.

Thanks :). Hopefully i can finish them to a suitable standard.

Once ive got a new straight bit (probably a 3/8 inch one). Ill try again, and start with a 1mm pass. The idea of doing 22 passes doesnt thrill me though :D
 
One variable could be my cheap *** router? I got it due to tons of great reviews, but its certainly at the budget end of the scale! Having said that ive seen no sign of it struggling or slowing, and it seems well made.

101748 6MM 1/4" Electric Hand Trimmer Wood Laminator Router Joiners Tool 220V: Amazon.co.uk: DIY & Tools

I think you're on to something. When you say it wasn't a smooth experience it just doesn't sound right. I have a full size router, so that might be it, but other than the noise I don't even know it's turned on. There is absolutely no vibration, it is totally balanced.

I'm wondering if maybe your router has a bent shaft, a bad bearing or something that is throwing the bit instead of just spinning it. You can clearly see on your pictures that the cut is not smooth. It is a little "toothed"... My cuts are totally smooth, with maybe some fraying on the top, like yours.
 
When i say not smooth, i dont mean vibration, i have zero notiveable vibration, just it feels "lumpy" as i cut the wood, like there are patches of different density im cutting through. I assumed this was due to grain direction changing. One observation, the "stepping" is all on the inside of the circle. The outside edge is pretty clean (its shown in other photo)
 
When i say not smooth, i dont mean vibration, i have zero notiveable vibration, just it feels "lumpy" as i cut the wood, like there are patches of different density im cutting through. I assumed this was due to grain direction changing. One observation, the "stepping" is all on the inside of the circle. The outside edge is pretty clean (its shown in other photo)

Definitely not unbalanced then. But FWIW I've never seen that stepping on my cuts...
 
I think you could be onto something with the "cheap ***" router. I've bought two plunge routers over the years and when checking them out in the shop some models had slight wobble of the plunge and baseplate arrangement in relation to the body and cutter. Funnily in my case though the expensive Bosch did it but the cheaper JCB and B&Q own didn't!

Obvious but worth ruling out, in the photo are you going clockwise round the board to cut the groove? And your cutter spins clockwise too?
 
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