Baffle Issues

[meb46]

Arrgghh... I have stumbled across and issue with my front baffle and I need peoples wise wisdom...

When I designed my current speaker project I constructed the entire cabinet out of 25mm MDF with curved 25mm Plywood sides. The internal barces are all 25mm MDF, however... I overlooked the front baffle thickness and blindly went with 25mm. I should have doubled this up to 50mm, but I didnt... sill me. Now when it comes to mounting drivers and flush mounting them, I significantly reduce this baffle thickness to enable flush front baffle.

My questions are as follows;

1. My main concern is flush mounting the 10" drivers. If I do this, I would need to route in about 5mm for the mounting and hence the baffle thickness at the mount would reduce to only 20mm. Is it essential to flush mount the woofers? My understanding is that it is less important to flush mount the low frequency drivers than the tweeters?

2. For the Mid base, the flush mount process only reduces the front baffle by a few millimeters, so less of a problem. Further to this, the Mid/Tweeter section of the cabinet has individual enclosures for the mids, so they are exceptionally rigid. But... my tweeter holes are very small and access to them using a rabeting bit is quite tricky. Anyone have any easy solutions for flush mounting cuts in small holes? Bear in mind, I am planning on making these flush mounts after the veneer is applied.

Any help always appreciated,

Thanks chaps,

Mike

[---k---]

25mm is approximately 1" for those of us metrically challenged. I think worrying too much. I've made speakers with 3/4" ply and flush mounted an 8" driver and then chamfered the backside (to allow the driver to breath), further reducing the thickness. The wood thickness increases rapidly and so there is sufficient strength. Yes, more thickness the better, but what you got isn't the end of the world.


Oh and yes, surface mounting woofers isn't as critical, but it does create other issues - such as diffraction for the mid. I would say this would be worse than surface mounting and thinner wood section.

If you're worried, you can always add thickness inside the box and trim using a router bit with the bearing on top instead of bottom.

[meb46]

Thanks for the feedback... now for the next trick question...

When my front baffles were CNC machined, I only cut the driver holes... but no recess for the flush mounting. Now, when I come to routing the flush mounts on all the drivers I find that my standard size adjustible rabetting bits dont perfectly route out the desired diameter. I have a few options but are generally scared of trying them and ruining my fresh veneer job Aside from a rabetting bit or a circle template are there any other methods I should consider for accurate flush mount routing? The flush mounting is required for 10", 6.5", 2" and 1" holes, or will varying diameters.

I suspect I just need to bite the bullet and get testing with the circle templates or my oversized circle guide... fingers crossed...

[meb46]

One other thought, I could use the standard accessory with my Router (Makita 3612) called a trimmer guide. This enables me to manually set the flush mouting route diameter... but how accurate is this method? Anyone with experience on this method?

Cheers

[Not2Evil]

I see your dilema.

I believe I would screw a temporary backer board in from the rear. It wouldn't have to be a solid piece, a couple of strips would be fine.

I would then use your router and your circle jig to make three circles to fill your driver cut outs. This will reestablish the center pin of the router jig with the center of your driver cut out as long as you lay everything out accurately.

Carpet tape the cutouts to the backer board making sure you have the same gap from the temporary circles to your baffle. Set your jig and route your flush mount recesses.

I'm hoping that your router is a plunge model and your circle jig is a Jasper or equivalent.

Oh, and use some painters tape to protect that veneer.

[knifeinthesink]

when you say circle template I'm not sure if you mean the same thing Im about to suggest but here's what I did a few days ago face with a similar situation.

My router has a 1/4" hardboard plate on the bottom that is larger than the router base. This allows me to put a screw through it sticking out the bottom just far enough (a couple millimeters) that it can anchor the base of the router to the centre point of the intended cut out and I can cut the correct diameter hole.

Once this screw is in the right place on the router base I cut a hole the diameter of the tweeter face plate in another piece of 1/4" and use this as a template for a pattern bit. Key thing here is that now I can check the diameter of the hole against the tweeter face plate to ensure its perfect. It took a few attempts to get the screw in the perfect position.

I had to build up the depth of my template with a couple pieces of plywood because my only pattern bit is 1 inch.

I tried it out on another piece of wood first to make sure it was perfect because I really didn't want to have to do another baffle.

Using this method I was able to get a very tight fit. And as a bonus, you have a template you can use later if you ever use that tweeter again.

[Bill Schneider]

I've split the difference between two available sizes of an adjustable rabbet bit by wrapping electrical tape around the bearing. It looks very crude, but it works fine.



Each wrap of 3M electrical tape reduces the cut diameter by 0.007", and I start with more wraps than I need, and then unwrap to get the proper diameter.

One thing to watch is to place the wrapped bearing in contact with the side soon after starting the motor so that the bearing doesn't begin to rotate too quickly. Once the wrapped bearing is contacting your round cutout, it won't spin. If it spins up to router speed, it will cause noticeable vibration which probably isn't good for router bearings.

Other than starting to cut quickly after turn-on, it's business as usual.

[meb46]

Guys,

Many thanks...

Bill - My problem is that my rabetting bit isnt a deep enough cut. The perfect solution would be a 1/2" shaft, and a rabet cut diameter/depth of up to 1"... but I can't find anyone that makes them.

The backing boards is an interesting one that I think would work quite well... but with the cabinets fully assembled, getting the boards properly screwed in from the back might be difficult. Will try it over the weekend.

Cheers

[Not2Evil]

One could glue the backer boards from the front using hurricane nuts and screws, or clamp in place until the glue is dry. If using the hurricane nuts, if properly placed, they can be used for mounting your drivers later.

Once the routing process above is complete, cut the backer boards flush with your driver cut out, leaving the glued tabs of the backer boards in place beyond the diamater of your driver cutouts. (trying to break them out seems like a REALLY BAD idea.)

I can't picture a way to bevel the four left over glued strip pieces of the inside baffle from the outside, so I guess I'd use backer board strips that weren't too wide so it wouldn't make any difference in the long run.

If you do use the hurricane or T-nuts, make sure you run a screw through each and every one. One burr in the threads can screw up your day. I also use a little bit of 5 minute epoxy to make sure the nuts don't spin.

I've had more than one "Oh, crap! Why did I do that?" momment. I hope I've helped.



1 more post until Senior Member Nirvannaship! If that's not a word, it should be. :P

[dlr]

Oh and yes, surface mounting woofers isn't as critical, but it does create other issues - such as diffraction for the mid. I would say this would be worse than surface mounting and thinner wood section.

For a woofer, surface mounting shouldn't pose any significant problems for nearby drivers, even a tweeter. The round frame is such that the quickly and widely varying distance effectively disperses any small amount of diffraction from the frame. I prefer to surface mount woofers as long as the back side is adequately open, either due to deep frame or chamfering.

The driver diaphragms, no matter the mounting, will cause diffraction, especially to a tweeter response, but also for a small midrange. Larger midranges and midwoofers are too directional for there to be much of a diffraction problem from a woofer.

Dave

[meb46]

By some miracle, the selection of router rabetting bits and bearings I had are perfect for all 4 types of drivers. My mid driver required me to add a fraction of masking tape to the bearing, but after a careful evening and a large amount of MDF and veneer dusk... I have 6 drivers perfectly flush mounted drivers in my mid cabinets. It was one of those nights where everything went perfectly and my enthusiasim for such a large build was instantly revived All set and eagerly awaiting another session tonight.

Will get some photo's posted soon