Router spiral cutting bit question

microxlAugust 18, 2007

Can you use a router spiral cutting bit to do a cutout (for example, in a piece of 3/4" ply)? If so, I assume I would use it with a plunge base and a jig to guide it for the cutout shape that I want? This would basically be the way electricians use a rotozip to make cutouts in sheetrock.

It seems this would leave a much neater cut than with a cutout done with a hand saber/jig saw. This inevitably ends up quite wavy and irregular. I HAVE done this and then cleaned up the cut with a jig and a router trim bit but of course it's a two-step process.


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Yes, you can. Move the bit clockwise within the cutout to keep it from climb-cutting. I would probably do it in 3-4 passes.

The last time I did this was a 4" x 1" oval for a cable feed in the middle of a $4000 desk. Talk about measure twice, cut once -- I did not want to mess this up.

I used a sheet of 1/2" plywood as a jig, with a collar bit guide.

    Bookmark   August 18, 2007 at 6:32PM
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One of the woodworking mags to which I subscribe had an article about that very thing this month. It says that using a spiral downcut bit with a double bottom bearing and a pattern is the best way to finish a rough cut piece.

    Bookmark   August 18, 2007 at 6:36PM
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If I were to make the cutout from the unfinished side of what is 3/4" cherry ply, would there likely be any tear-out of the veneer on the finished side?

Fortunately my situation isn't quite that high-end but cherry ply isn't particularly cheap!

Thanks for your help.


I'll check out the Aug mags at Borders. Thanks!


    Bookmark   August 18, 2007 at 10:08PM
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The mag is ShopNotes---and it says there is zero tearout on the top face.

I've taken a tour of the publishers facility and workshop---they have three guys who build/test everything that is published in any of the three mags they put out.

    Bookmark   August 19, 2007 at 8:10AM
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Spiral bits come in up-cut and down cut patterns.
The direction is where the chips go, and also how force is applied to the material being cut.
An up cut removes chips better, but can lift the edges of the material (like veneer).
A down cut tends to pack the chips into the groove, but pushes down on the face veneers and minimizes chip-out.

    Bookmark   August 19, 2007 at 11:23AM
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You want your spiral to push the veneer down, not pull it up, that is when chipping is most likely. I think I put down a a layer of masking tape prior to cutting. It also helps to score the veneer with a utility knife prior to routing, if you can do that.

The other option is to finish the cut, then run a 1/8" round-over bit around the top edge. Or lightly dull the edges with sandpaper.

In my situation, it was solid wood and going to be covered with a brass grommet. Sharp edges are more prone to damage than rounded over ones.

    Bookmark   August 19, 2007 at 12:48PM
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When looking for pricing of spiral bits I noticed the up-down types and was convinced I needed the "Up" type because I was going to do through-cutting and would need quick chip removal. So, I suspect you saved me some grief with your advice. I assume the "Down" type WILL remove the material well enough, especially by doing the cutting in stages? The roundover suggestion is a good idea should I still end up with a little tearout. Regardless, I will do some trial runs and tests to see what kind of problems crop up.

Thanks again to all!

Bob W.

    Bookmark   August 19, 2007 at 8:36PM
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The down cut bit should be used with the good side of the ply up. If you need to have the good side down---to cut from the B side, you will need an upcut bit. According to the magazine article, by using the proper bit in relation to the A or good side of the plywood, you will have NO tearout. kmealy's explanation tells why that is so.

    Bookmark   August 19, 2007 at 9:56PM
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This was my first inclination because there would be a pulling-up action on the ply layers which would less likely result in tearout. If there was damage it would be the top layer which isn't the critical one for my situation. With the down-spiral action the bit would be actually pushing out on the bottom layer of ply and damage the finish layer. It seems fairly logical.

Hope to get a bit and try it out in the next couple of days.

Thanks again!

    Bookmark   August 20, 2007 at 10:45AM
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If you hunt around there are bits that are designed for though cutting of veneered surfaces that combine an up shear and a down shear.
They are not normally spirals, but use straight cutters angled both ways.
Amana used to have them, but I have not replaced one in a number of years.

    Bookmark   August 20, 2007 at 9:44PM
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I just wanted to thank the three of you for your thoughts, info and advice re my spiral bit question. Yesterday I had an opportunity to get a bit (one of HD's Porter Cable line)and it worked great. As reported by handymac, I encountered zero tearout on the finish side of the ply. Even on the top work surface there was only very tiny, minor splintering.

I'm a "once in a blue moon" woodworker so a lot of anxiety goes into every move I make. I must admit that not only was this the first time using a spiral bit for a through- cut but it was also the first time I had ever used my plunge router base! I did a test piece in old ply to get my procedures down.

I never exactly said what I needed this for... simply because it seemed a bit long and complicated to write up. So, here goes. I designed a 3-case entertainment center for a 32" LCD, audio/video components, cable box and lots of drawer space at the bottom of each case. (Graphed it all out and made up a parts list with sizes et al.) All three cases are to be screwed together as one unit so I can disassemble it should we move. I needed to vent the area over the LCD and also over the stereo component case to alleviate heat buildup. The top of each case consists of a double thickness of cherry ply... one finish face facing down, the other facing up. With the jig and spiral bit I did a straight cutout into the LCD cabinet. Then with a 3/4" straight bit and the jig I routed to a depth of 1/2" into the bottom of the upward facing ply member. Next I used the 1/4" spiral to cut through fully. This left a matching cutout in the top finish ply member but one with a shoulder in which I inserted a piece of decorative aluminum mesh held in place with some small wood strips. Since I routed the two members separately I had to make sure everything was precise and so the cutouts lined up exactly when put together. They did!! After repeating the process for a smaller cutout in the stereo component case tomorrow then I'll start milling down solid cherry for 1 1/2" x 3/4" facia strips that will finish off the ply edges. I have edge glued solid cherry for five drawer front panels and one pair of doors that will hide the stereo components. The finished cabinetry should be fairly contemporary and simple with the slab doors and drawer fronts all being flush with the facia. I have had a jointer/planer for years but finally got a Ridgid thickness planer. Nothing like a new tool!

All-in-all it has been a challenging project but I have just taken it all little by little and not rushed things. There will be some imperfections here and there but I'm learning a lot and enjoying the process. And thanks to the internet and people like you who like to help others you can get through the rough spots.

Bob W. Campton, NH

    Bookmark   August 22, 2007 at 9:55PM
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