# Jigsaw circle cutting jig fail - now need advice on spiral router bits



## mathguy1981 (Aug 30, 2018)

I wasn't happy with the amount of dust/noise/danger when using my router and 1/2" straight bit to cut plywood. 
So, I found a neat youTube video on a circle cutting jig/sled for a jigsaw, and went ahead and built it. Turns out, it won't cut 3/4 plywood and the blade is deflected so badly by the circular motion. Tried a larger and stiffer blade, same issue. It's making an angled edge - seems obvious now that it was never going to work on material over 1/2"

So I can get out the ol' Jasper circle cutting jig again, but what's a better bit to get? I've heard of "spiral cutting" up and down bits, and they appear to be more suited to this. But what direction to buy, up or down? And can anyone recommend a specific brand like Freud or Bosch or other? They get pricey, fast.
Thanks, all.


----------



## John Smith_inFL (Dec 15, 2017)

what are you trying to make and how many ?
I would rough cut the object just shy of touching the line of the circle
and finish it up with the upward spiral router bit.
but - I have had just as much success and clean cuts with a pattern bit.
you make a plexiglass or 1/4" plywood base for the router.
many examples on google on how to do that.
here is a simple jig that will cut a six foot circle down to a few inches.
taking small bites at a time will yield the best results.
[I have never shelled out $$.$$ for a circle jig when I could make it for free].










.


----------



## mathguy1981 (Aug 30, 2018)

I'm cutting circles and rings for my dust collector and Thien Baffle, so not too many…3 more at the most I think.
In 1/2 and 3/4 plywood. Thanks John it should of occurred to me to rough cut it close to the line then use the router to make it perfect and smooth. That will reduce wear and sawdust.


----------



## Andybb (Sep 30, 2016)

The Jasper jig is really designed to be used with the 1/4" bit it comes with. The angle is the issue. Two jigs come to mind. Some are here using a table saw. I used this to make a tapered circle and it worked just fine. The other would be to use a band saw circle jig with the table tilted but blade deflection is an issue with that setup too. (ask me how I know) The blade in the BS has got to be able to cut that radius without binding so that really onlt leaves you with a 1/4" or 1/8" blade. I'd try it with the table flat and if it works try tilting the table.


----------



## pintodeluxe (Sep 12, 2010)

1+ Jasper jig for the router (or a shop made version) and a 1/4" spiral bit.


----------



## Andybb (Sep 30, 2016)

> 1+ Jasper jig for the router (or a shop made version) and a 1/4" spiral bit.
> 
> - pintodeluxe


Actually I think a straight bit would work better than a spiral. The Jasper comes with a straight bit. Spirals don't seem to like that sideways cut as much if it can't use the tip in my very limited experience. Also, a 1/4" bit doesn't have to try and remove as much material. I find straight bits are better for cutting.


----------



## TheFridge (May 1, 2014)

Had to be careful with my 1/4" upcut. Too much pressure and it would slide down in the collet.


----------



## mathguy1981 (Aug 30, 2018)

Somehow I didn't get a bit with my Jasper template, just the acrylic and some mounting screws. I did buy it several years ago (don't cut circles often). I have a 3/8 spiral UP bit on the way now, with a 1/2 shank.


----------



## Redoak49 (Dec 15, 2012)

I cut circles for my dust collector gates with a up cut bit - 1/2".

I made a template out of 1/2" plywood with a scroll saw and spindle sander.


----------



## CaptainKlutz (Apr 23, 2014)

+1 Jasper jig.
Have used jasper circle jigs many, many times.

Trick to using them is proper router bit for material being cut. Hardwood, softwood and man made materials all need slightly different bit (speed and feed change too) for best overall performance. Plywood is especially hard on small router bits, as the glue gums up the bit quickly and needs to be cleaned often. 

Challenge is Jasper suggested 1/4 inch bit requires really slow feed rate on thick wood. For a plunge cut, will usually only cut 1/4 inch deep on each pass. That means 3/4 inch plywood requires 3 passes. If you attempt to cut too much in single pass, the bit overheats and breaks as human hands typically feed wood too fast. 

For hardwood; I like using a 2 flute bit with either a spiral or straight edge up cut. On man made materials like plywood, or MDF/OSB, I prefer to use single flute shear or spiral up cut, or 2 flute compression spiral bit. When milling man made materials, spiral bits will need cleaning less often than straight edge blade; but still need to be cleaned any time you see residue on edge or you risk overheating. If you do not need a perfectly smooth edge, I have used roughing 'end mill' bits for cutting circles, and they last longest of any bit I have used on plywood.

You are 100% correct that solid carbide spiral bits can be expensive. 
The only way to be cheap on spiral bit is buying HSS instead of solid carbide. I have used many HSS spiral bits, and as long as you avoid overheating the bit, they work very well for a project or two. HSS stays sharp and has decent longevity when cutting softwood. You will get put to sleep using one for hardwood or plywood due ridiculously slow feed rate required to avoid overheating. 
Another source is buying import carbide spiral milling bits from a tooling supply house when they go on sale. MSC send me a sale flyer with various type of carbide milling bits on sale every quarter. The best import sale prices are about same as retail HSS, and are 1/3 cost of spiral router bits from Whiteside or Amana. 
Before you ask, It is hard for me to compare bit life between various spiral bit brands on plywood. Common issue is that I forget to check, or delay the time between cleanings, and end up overheating bit and it breaks when it hits a knot or gravel (yes, I really find small stones!) inside the plywood.  
So I can not stress enough the need for keeping spiral bits clean and cool when machining plywood/MDF.

PS - if you need circles below 8 inch OD, highly suggest using a fly cutter on drill press. Works much faster, and lasts much longer than any router bit. Not safest contraption to chuck into a drill press, but it works. There are many existing threads on the topic if you search.

Best Luck.


----------



## lumbering_on (Jan 21, 2017)

> what are you trying to make and how many ?
> I would rough cut the object just shy of touching the line of the circle
> and finish it up with the upward spiral router bit.
> but - I have had just as much success and clean cuts with a pattern bit.
> ...


+1 to not buying a circle jig. I have an adjustable jig I made a few years back, and it works just fine. The only thing is I have a bit of a screw hole in the center of the circle, which can usually be hidden or just sanded out.


----------



## JAAune (Jan 22, 2012)

Upcut bits leave a clean lower edge and tend to cause fuzzing and chipping on the top surface of the cut. Downcut bits are the opposite. Be careful when using a downcut in a handheld router. They want to lift out of the cut. Upcut pulls down.

Southeast tools sells solid carbide spiral bits that are excellent and priced lower than most competitors. They aren't as good as a premium bit from Onsrud or Vortex but they're close enough that the difference only shows when production cutting with a commercial CNC router.

A curious thing about carbide tooling is that breaking most often happens when the feedrate is too slow. Overheating shortens the life and dulls the edge prematurely so it breaks under the load. The faster the feedrate, the cooler the cut. Taking shallow cuts allows for faster feedrates and better chip removal.

I'd recommend cutting about 1/16" outside the desired radius with the jigsaw then cleaning up the cut in a single pass with a sharp 3/4" straight bit. You don't need chip clearance capabilities for this type of cutting.


----------



## mathguy1981 (Aug 30, 2018)

Wow, thanks so much for all the replies! That was exactly what I needed to get back on track. Special thanks to CaptainKlutz for the in depth explanation


----------

