Help me choose Spiral Bit for plywood

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Forum topic by Calneva posted 09-27-2017 12:24 PM 855 views 0 times favorited 3 replies Add to Favorites Watch
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2 posts in 1048 days

09-27-2017 12:24 PM

I am cutting a substantial amount of 23/32” hardwood plywood with a router. Some of the cuts for example are 33”, 22”, 15”, 10” and 5” diameter circles, and curves with similar radii. I estimate that I cut about 500 lineal inches out of a 4’x8’ sheet, and up to two sheets per week. I use a Porter Cable 7539 with a radius jig and with templates I have made.

For cutting, I’ve been using a Whiteside spiral compression bit, but a little one, 1/4” with two flutes: part UD2102. I bought it because it was less expensive and could do what I needed. After cutting a few thousand inches with it, it has worked acceptably but I wonder if I could do better with a larger bit.

I cut the 23/32” in one pass. With the 1/4” bit, I’m getting about 36 inches per minute.

I’m wondering if I go to a larger 3/8” or 1/2” bit whether I will be able to get a higher feed rate. The wider bit will chew more wood and will require more torque from the router, but I think the 7539 will provide that. But will it feed faster?

With two flutes, the finish is acceptable. Will 3 flutes feed faster or just provide a better finish?

I think the up/down compression bit slows feed because chips get crammed between the up and down sections. A up cut or down cut bit might have better chip ejection and feed better but I’m concerned it will tear-out the veneer. I haven’t tried it so I don’t know.

Whiteside makes “ultimate compression” bits with extended up-cut flutes to improve chip ejection, but they are chamfered from a 1/2” shank to a 3/8” cutting width. That would make the depth setting very particular and it could complicate my use of templates. I would have to use a 1/2” guide but get a 3/8” cut, so my templates would have to be sized to compensate. I’ve got enough to think about that I don’t want that.

So I’m thinking either a 3/8” or 1/2” compression. Whiteside’s literature claims 3/8” is optimal for sheet goods because it takes less power than 1/2”, but they don’t compare it to 1/4” which should take even less power. So I’m wondering if there is any improvement to be had at all.

3 replies so far

View ArtMann's profile


1480 posts in 1622 days

#1 posted 09-27-2017 03:41 PM

I do this kind of cutting on a CNC router. An up-cut bit will certainly make the cut easier but as you already know, it will cause some splintering. You may want to try it anyway just to see whether you can tolerate it for your application. If only 1 side will show then you will be fine. An up-cut on the topside cuts like a down-cut on the bottom side. Sanding will fix it in some cases.

You will be able to cut faster with a 3/8” bit if you have a powerful enough router. A three flute bit will also permit you to cut faster if you have the horsepower to handle it. Google the term “chip loading” as it relates to CNC routing if you want to know more. If you are using a handheld router then you will run into power issues pretty quickly. Big CNC machines use 5, 10 or more horsepower spindles rather than a 120VAC plug in router.

View DS's profile


3518 posts in 3226 days

#2 posted 09-27-2017 04:01 PM

With 3 flutes, if you don’t get a high enough chip load, it will overheat.
Since you are doing this manually, I would think two flutes (or even one flute) is a good fit.

Larger diameter bits will allow a faster cut and have a higher velocity on the cutting face which usually means a cleaner cut.

A 1/4” two flute spiral should do better than 36ipm, (I would expect 80ipm, or so), but, your router, (or even your arms), may not handle the larger work loads that a higher ipm may present. (Same is true with larger bits)

A 1/2” compression cutter should work fine in the 100ipm to 800ipm range and you could consider a chipper/finisher bit which clears the waste faster and would allow feed rates upwards of 1000ipm, however unachievable this would be by hand.

I would say a bit of experimentation is in order to characterize the work you are trying to do – Especially if this is an ongoing matter and not just a one time project.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

View Calneva's profile


2 posts in 1048 days

#3 posted 09-27-2017 05:51 PM

Whiteside replied to my inquiry indicating the UD2102 will feed 300 ipm in 3/4 ply with 3.25hp on a CNC machine. From this I conclude it is not the bit that’s slowing me down, but my ability to apply force while keeping it under control. They indicated the larger bits will feed faster, to 600 ipm, but will require a larger router to do so. I think this is because as DS pointed out the higher face velocity.

It’s still not clear whether I will see any advantage with larger bits when hand-holding because that obviously depends a lot on me. The more force I have to apply to push the bit, the more risk I have of jumping a template, tilting the router, or some other loss of control, and the more demand I put on my work-holding setup (typically a few Bessey clamps that already get in the way more than I’d like).

Whiteside suggested 16,000 to 18,000 rpm to let the bit get a better bite. I’ve been running 21,000. I’ll try that. I’ll also try two-passes to see if that works better. I also think I’m going to try a two-flute 3/8” compression bit and see how it goes. If nothing else, I could use the extra cutting length for thicker templates.

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