# upcut vs downcut spiral



## Skyewalker (Aug 30, 2012)

what is the difference in use between an upcut spiral bit and a downcut spiral bit using a plunge router to make a stopped slot?
I have both bits but both seem to chatter severely when trying to cut a 3 inch slot in a piece of 1 in pine.
Am I trying to take too much at once or just don't have the right bit speed or....or..
Help, please
David


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## Mike (Nov 22, 2004)

David, are you removing about 1/4" of material in each pass? Please read the sticky threads, they are filled with good information. Start with this one:
http://www.routerforums.com/router-bits-types-usage/28685-spiral-vs-straight-bits.html#post237388


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## keith franks (Aug 12, 2012)

*cutters*

think about the way the cutters revolve downward would be left to right cut
upward would be right to left as both cutters cut clockwise slots i just use streaght hss cutter should do 25mm cut if sharp and slow feed rate without chatter


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## StevePixley (Aug 10, 2012)

I also have a query , I am cutting two sheets stuck together of polycarbonate totaling 0.134" thick. I have run the job before with an upcut and while it worked out ok, the poly was lifting throughout any of the 2d profiling and was pretty close to turning catastrophic.


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## Quillman (Aug 16, 2010)

Some notes on spiral cutters. See No. 12.


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## malb (Sep 15, 2008)

An up spiral will clear chips from a non through hole, but might cause surface fibres to tearout at the router face of the surface. Should leave a good surface at he tip end of the job once it penetrates through.

A down spiral will push chips into into the recess and pack them in there during a non through cut, soneeds shallow cuts and frequent clearing of waste. Will clear them on the final through cut. Should leave a clear surface at the router face, but may cause surface tearout once it penetrates through.

If you need 2 perfect faces, a compression bit combines a short (5/16 to 3/8 long) up spiral tip with a down spiral bit, so all cutting forces are directed to the centre of the work so both faces are supported for absolute minimum tearout.

As previous suggested, to cut a slot, take it in a number of steps, and clear te recess of waste if it is not being forcibly removed by vacuum or bit.


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## Phil P (Jul 25, 2010)

malb said:


> If you need 2 perfect faces, a compression bit combines a short (5/16 to 3/8 long) up spiral tip with a down spiral bit, so all cutting forces are directed to the centre of the work so both faces are supported for absolute minimum tearout.


Hi Mal

Sort of.... Compression spirals always leave a witness line in the transition area between upcut and downcut sections of the spiral which may or may not be acceptable

I have to concur with you on the need for vacuum extraction of waste - keeps the cutter cooler and makes for faster, cleaner cutting

Regards

Phil


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## malb (Sep 15, 2008)

Phil P said:


> Compression spirals always leave a witness line in the transition area between upcut and downcut sections of the spiral which may or may not be acceptable


Have spent a few years running a commercial CNC in a cabinet shop, and gone through maybe 50 or so 3/8 compression bits from 4 or 5 suppliers. With the exception of one much vaunted European brand, the 'witness line' has always been nothing more than a part line dead flush with the rest of the edge surface. With the European brand, it is often a raised land about .5mm high and wide, caused by the way this manufacturer grinds their bits for minimum overlap/maximum clearance and extraction at the transition point. Unfortunately the boss bought six months supply of these bits without trying them, and we had to run with them. Net effect, we had to spend about 30 seconds sanding critical edges on about 5% of the to be painted work cut with these bits, in our situation 95% of edges would either be hidden or edged on an edge bander so the land was not critical. The solution here is to understand what is happening in the cutting operation and select bits accordingly.


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