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Upcut or upshear bits will pull the chips up and out of the cut. Downshear leaves a crisper, cleaner edge on the top surface of melamine and plywood for dadoes and mortises. It cuts with a downward pressure and tends to pack the chips down. There are also compression bits that combine the two for through cutting of plywood or melamine or any two sided material so it elimiates chipping on both sides. The bottom portion is upshear and the top portion is downshear. These are usually used more on CNC machinery than with the hand held router.

The cutting edges are either spiraling down or spiraling up or both (compression).
 

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It's all about which way you want the bit to pull the chips.

Upcut bit pulls them "up" - towards the router.
Downcut pushes them away from the router.

As Masrol said, if you're cutting a mortise, you want the chips being pulled out of your hole, hence upcut.

Downcut bits exert more downwards pressure (in addition to forcing chips away from the router), so that might be better for smaller workpieces. They are also good for reducing tearout on the upper edge as your vid mentioned.
 

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I use a compression bit in my router with a straight edge to rough cut plywood down to a manageable size. It works better than a circ saw and leaves a much cleaner edge.
 

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At times it comes down to personal preference but there are some basics to making gthe choice. I have a story on this at the lin below that shows some examples if that would help. When you remember the direction of the spiral cut you will find other uses in the shop for cleaning up cuts that can cause chipping.

http://www.newwoodworker.com/updowncutbits.html
 

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Please note that if you are using an upcut bit on a router in a router normal router table the chips are actually being pulled down (towards the router). For me, it gets confusing.

Nonetheless, I use an upcut bit frequently. I just used it today with a handheld router to cut a groove for an inlay. It clears the chips away very nicely. I also use it frequently in the router in my router table.

I virtually never use a router on a piece of plywood. I understand that, with plywood a downcut bit can be useful. However, from my perspective, I see no need for a downcut bit.
 

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Im a little late on this topic ..but was just in the shop using a 1/4 " down cut bit in my new little router table with a 1 horse motor cutting slots in 3/4' poplar and the bit seemed to devolop a slight wobble . Was a little concerned with the bit breaking ? Should i be ? Was really looking forward to making some much needed jigs with that bit …But the bit also created a high pitch whine ..didn"t sound too happy..Any feedback would be great
thanks
 

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Down cut is probably a poor choice in a router table, as it'll want to lift the wood off the table.

1/4" bits will frequently scream, unless you're taking very light cuts. Very common.

If the bit is carbide, it won't bend, it'll just snap. If it's wobbling, check the collet.
 
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