Speculation wanted

[JoshK]

I have been pondering about roundover bits for my OB project. I already own a 1.25" roundover bit. However, I have had my eye on this bit for a long while. I think the baffle would look seriously cool with such a profile.

I was wondering if anyone would have a guesstimation on how the different roundovers would affect things? Would it be enough to measure a difference? To hear a difference? Would the handrail bit cause a weird diffraction affect that would be harder to accomodate in the xo? Or would it be a push?

I will probably just buy it and try it, but was wondering if anyone wanted to speculate.

P.S. I have a 3HP Hitachi M12 and a router table, so no worries about using them. However, I do wonder if anyone has had experience, good or bad, with routing baltic birch?

[Hdale85]

Wow thats huge...

[ThomasW]

P.S. I have a 3HP Hitachi M12 and a router table, so no worries about using them. However, I do wonder if anyone has had experience, good or bad, with routing baltic birch?

JonMarsh uses a similar bit with his M12. And last time anyone checked he still had all his fingers and toes..

The issue problem routing any plywood is the grain of the laminations being a 90 degrees to each other, so tear out can be a problem. And I think this problem would be exacerbated using such a huge round over.

[benchtester]

Big Bits

BTW: Ebay is very cost effective place to buy carbide bits. :P

[A9X]

I'd like one. I didn't see a price on the page linked; what are they worth?

[EdL]

Josh,

My limited experience is from EDGE.

I ran simulations for a speaker that I have built.



I compared the edge diffraction for a 9" wide cabinet with a 3/4" radius with that of a 10" cabinet with the 1 1/4" radius. The slope and magnitude of baffle step were ~equivalent for both conditions. The greatest difference was in the ripple within the baffle step area. The larger radius edge had ripple with fewer cycles and less amplitude.

I think the table top edge will provide a softer "launch zone" from which diffraction will occur.

On a side note, there is a shop close by with a shaper and 2" radius roundover cutter. Also, I picked up a pair of B&C 250's from PE's tent sale. :T

[JoshK]

Wow, those are beauts Ed! You really are a whiz woodworker.

[JoshK]

The issue problem routing any plywood is the grain of the laminations being a 90 degrees to each other, so tear out can be a problem. And I think this problem would be exacerbated using such a huge round over.

Yeah, I think I'll probably have to laminate some MDF to the BB, leaving the BB only for the core. Hopefully that will defray tendency to tear out.

[---k---]

I have a 1" and a 1.5" roundover bits. I've been stupid and used them with a hand held (and modified base) router, but luckily I still have all my fingers.

I've had problems with tear-out on BB ply. But, I also have problems with small diameter roundovers, so I think some of it is my technique.

According to the few sims I've done with BDS, bigger is always better. I can't comment on the difference in profiles. I think that is a small difference. JonW did several simulations and actual build tests with different roundovers and chamfers. He found chamfers were better than roundovers, and I believe chamfers with softened edges even better. Search for his thread - I think you'll enjoy it.

[Paul W]

My speculation is the change will be very slight but with the large radius nearer the drivers followed by the tighter radius 'round the bend, diffraction ripple will be reduced over a simple roundover. Enough to measure? With careful on and off-axis slices, quite possibly. Enough to hear? Sorta doubt it.

My experience with the large router bits is to work up from smaller sizes and then, with the largest bit, remove only a small amount of material in each of several passes. Takes a little more time, but much lower stress level.

[Eton]

This is the profile you get using bit that size.

[JonW]

JonW did several simulations and actual build tests with different roundovers and chamfers.

Here's the link:

https://www.htguide.com/forum/showthread.php?t=25031&page=10

It starts around post Firmware news... and goes on for quite a while. :-)

Given that it's real testing of different cabinet configurations, rather than just simulations, I've wondered if it would be worth writing up a small paper on it for a speaker building magazine or something. But I don't think that such things still exist (in the US). But it could be info that would be helpful to others. Or, like most of my work, it could just be misleading.

[---k---]

AudioXpress would probably print it in a heartbeat. I subscribed for a year, but it seemed to be too focused on electronics. The few speaker articles they did publish were entry level and not new info. I think it would be good.

[JonW]

AudioXpress would probably print it in a heartbeat. I subscribed for a year, but it seemed to be too focused on electronics. The few speaker articles they did publish were entry level and not new info. I think it would be good.

Thanks Ryan. I didn't know AudioXpress was still published. I wonder about that Journal of the Audio Engineering Society or whatever it's called. That sounds more serious. I could put it on my resume and count it in my job publications. I'd imagine the contents in there are much more serious than a simple diffraction study. But I've not thumbed through it to know. Papers in there are mentioned on this board now and then.

[Dennis H]

I'd imagine the contents in there are much more serious than a simple diffraction study.

Yup. Lots of theory and higher math required (brush up on your LaPlace transforms). Kinda like a redneck who built a still in his backyard trying to publish his findings in JACS.

[JonW]

Yup. Lots of theory and higher math required (brush up on your LaPlace transforms).

Sounds like it would be interesting to thumb through a few issues. The one we're referring to is called the "Journal of the Audio Engineering Society," correct? Looks like my library here has it. :B

Kinda like a redneck who built a still in his backyard trying to publish his findings in JACS.

Hey, I do that all the time. :P

[Dennis H]

Yeah, that's the one. It's cool that you have free access! :T You can even look up the papers by our own JonMarsh (Jon Hancock). Just don't let Evil Twin catch you reading that subversive trash.

[ThomasW]

Here's Jon's first article, I'm not sure it's going to be very helpful to people building DIY speakers....

http://www.aes.org/e-lib/browse.cfm?elib=5970

Sounds like it would be interesting to thumb through a few issues. The one we're referring to is called the "Journal of the Audio Engineering Society," correct? Looks like my library here has it.

The JAES loudspeaker articles are published as separate volumes, I think there are 4 volumes currently available.

[NEO Dan]

This is the profile you get using bit that size.

the V-groove on the glue line is a nice touch



I also use the 1.25" and 1.5" bits:

The 1.5" is about as big as you can go in the porter cable:



1.25" on the right:




I'd definitely recommend a large baseplate for the router, say 12" x 12".

[JonW]

Yeah, that's the one. It's cool that you have free access! :T You can even look up the papers by our own JonMarsh (Jon Hancock). Just don't let Evil Twin catch you reading that subversive trash.

Here's Jon's first article, I'm not sure it's going to be very helpful to people building DIY speakers....

http://www.aes.org/e-lib/browse.cfm?elib=5970

The JAES loudspeaker articles are published as separate volumes, I think there are 4 volumes currently available.

Cool. Thanks guys. It'll probably be a while before I find the time to amble over to that library. But it promises to be intersting.