Dados with a router - straight or spiral bit???

[Bob Elliott]

I am getting ready to start making cabinets for our kitchen. I plan on making a dado jig for use with a router to cut dadoes in the plywood for cabinet construction. Do you suggest using a straight bit or a spiral bit and why?

[Brian Holcombe]

I like a downcut mortising bit, the cutting pressure is down against the material face and it has clearance to evacuate chips.

[Simon MacGowen]

I am getting ready to start making cabinets for our kitchen. I plan on making a dado jig for use with a router to cut dadoes in the plywood for cabinet construction. Do you suggest using a straight bit or a spiral bit and why?
A spiral upcut or downcut bit depending on what you want. Upcut gives better bottom edges, downcut better top edges.

A compression spiral bit gives you both better top and bottom edges.

Simon

[Stan Calow]

Are you taking note of the actual plywood shelves' thickness? I have better luck with the straight bits myself, and it seems easier to find them in the "undersize plywood" sizes.

[Jim Becker]

Are you taking note of the actual plywood shelves' thickness? I have better luck with the straight bits myself, and it seems easier to find them in the "undersize plywood" sizes.

That absolute best results for this kind of work will come from using a jig that permits you to use a much smaller cutter than the material with multiple passes to get the total groove/dado/rabbit width that exactly matches the actual material you are using. Unfortunately, sheet goods are variable in thickness, even in the same stack and sometimes even on the same sheet...including with the highest quality "domestic" product. The undersized bits are certainly closer to reality, but they often still leave gaps or do not provide the best fit.

I agree that downcut spiral or a compression bit that's designed for the limited penetration one uses for dados/grooves/rabbits will provide the best quality edges. It may be easiest and simplest to acquire the downcut spiral than to find a compression bit that is "short" enough for the job...many compression cutters are designed for full thickness cutting out of sheet goods on CNC rather than non-through cutting jobs. Straight bits or those with slightly skewed straight edges can cut pretty cleanly if they are sharp...like with a dado stack, running masking tape along the line can also help with keeping things clean.

[Mark Hennebury]

A sharp straight bit will cut fine.

A spiral up-cut will tend to lift the top fibers and depending on the plywood / feedspeed/ sharpness, leave a damaged edge. It can also creep out of the collet making progressively deeper dados and eventually cut right through to the other side, if you are not paying attention, so make sure that the collet is tight, and check the depth of cut occasionally.

A down-cut will leave a clean top edge.

A compression, up-cut / down-cut bit is for full depth cuts, for cutting out parts, not for dados. The lower section of the bit (tip end) is up-cut, the upper portion ( shank end) of the bit is for down-cut, it is designed for pulling the bottom face up and pushing the top face down to leave a good surface on both faces without lifting the surface fibers or chipping the surface finish such as melamine. Used to cut a shallow dado of say 1/8" -1/4"you will be using on the bottom up-cut portion of the bit and get the same results and concerns as an upcut bit.

[glenn bradley]

I do 99% of my dados with the router. I get my best result with a spiral bit. As a disclaimer I use an undersized bit with a jig so my use case is a little different.

I use an up-cut spiral for hardwoods (great chip ejection) and a down-cut spiral for veneers (nice clean edge). Straight bits work fine, I just adjust my feed rate a little. Any of these bits will start to leave fuzz behind as they lose their edge. It is worth noting that I can touch up a straight bit and get more work out of it as it starts to fade; the spirals have to go out for sharpening.

[Charles Lent]

+1.

Charley

[Bill Adamsen]

That absolute best results for this kind of work will come from using a jig that permits you to use a much smaller cutter than the material with multiple passes to get the total groove/dado/rabbit width that exactly matches the actual material you are using.
Same as my approach when using a router for dado/rabbet. Build the slot to the requisite width by adjusting the tracks that guide the router. Maybe smaller rather than much smaller.

[Peter Christensen]

Bob have you considered making the cases with a plate joiner? Once I bought my Lamello ages ago I found biscuits to be faster and easier.

[Brian Holcombe]

If you tenon the edges the board, you can use whatever bit you plan to. Example being; 18mm plywood with .500” tenons. Or, just cut a rabbet on the bottom of the plywood to gain the same effect.

This likely to result in an accurate joint, rather than picking a bit that matches the plywood. I buy really nice plywood and there is still .005” inconsistency within the sheet. That’s enough to show gaps.

Ive never had an upcut bit make a nice looking groove in plywood, it would normally look like it was chewed to size. I’d rather use a downcut and take multiple passes if need be.

[Bruce Wrenn]

For plywood, I use a reverse helix down shear bit. It is a screw on cutter designed for panel routers.

[Tom Bain]

If you tenon the edges the board, you can use whatever bit you plan to. Example being; 18mm plywood with .500” tenons. Or, just cut a rabbet on the bottom of the plywood to gain the same effect.

This likely to result in an accurate joint, rather than picking a bit that matches the plywood. I buy really nice plywood and there is still .005” inconsistency within the sheet. That’s enough to show gaps.

Ive never had an upcut bit make a nice looking groove in plywood, it would normally look like it was chewed to size. I’d rather use a downcut and take multiple passes if need be.

Brian — What is your process for tenoning the edges of the plywood? I’m wondering if there is variability in the thickness of the plywood, how do you get consistent tenons to match a fixed dado width?

[Brian Holcombe]

Tom, use one face as a reference, rather than working both sides toward the center.

[Mike Wilkins]

The suggestion Jim offers is a great way to cut dadoes; make a jig using the actual plywood to get the proper spacing, then use a narrower bit (1/2" for 3/4" dadoes) and you will get spot-on results. There are suggestions for such a jig in numerous router books or online.

[Bob Elliott]

That absolute best results for this kind of work will come from using a jig that permits you to use a much smaller cutter than the material with multiple passes to get the total groove/dado/rabbit width that exactly matches the actual material you are using. Unfortunately, sheet goods are variable in thickness, even in the same stack and sometimes even on the same sheet...including with the highest quality "domestic" product. The undersized bits are certainly closer to reality, but they often still leave gaps or do not provide the best fit.

I agree that downcut spiral or a compression bit that's designed for the limited penetration one uses for dados/grooves/rabbits will provide the best quality edges. It may be easiest and simplest to acquire the downcut spiral than to find a compression bit that is "short" enough for the job...many compression cutters are designed for full thickness cutting out of sheet goods on CNC rather than non-through cutting jobs. Straight bits or those with slightly skewed straight edges can cut pretty cleanly if they are sharp...like with a dado stack, running masking tape along the line can also help with keeping things clean.

I should have been clear with what i wanted to do.

I am doing exactly what Jim suggested. I am using 3/4" plywood for the cabinet boxes. I made a dado jig that I clamp the plywood in to match the size. I clamped the jig to the plywood that I want to cut the dado in and used a straight bit with a bearing that rides along the jig to cut the dado. I used a 1/2" bit when I made the cabinets for the bathroom.

I am getting some new tools for my kitchen cabinets. If I can get a downcut spiral bit with a bearing I would like to because I want to have the cleanest edges possible on the plywood dados. Is a bit like this available?

[Jim Becker]

Spirals tend to not have bearings, especially top bearings. What you might need to do is "re-jig" your jig to work with a guide bushing instead of a bearing toward the same end.