need advice on project please

1018 Views 7 Replies 6 Participants Last post by MikeMa, Mar 6, 2019

lking

Discussion Starter · Mar 5, 2019

i appreciate your thoughts --

i volunteer at a state park in SW VA and have made several display cases for instruments that have been donated to the park -- all the wood for these projects has been wormy chestnut which came from buildings that were on the property when the park was formed -- this project is a shadow box for a dulcimer -- the biggest constraint is that the supply of chestnut is exhausted and i have tried to make the best with what i have --

there are two issues that i wanted to run by somebody who actually knew what they were doing before i commit to implementing them --

the attached pic shows the big picture -- overall size is 26" x 43" -- 1) sides are 3/4", dovetails at the corners -- 2) face frame is 2" wide, 9/16" thick -- mitered half lap joints at the corners -- planning 3 hinges down the left side -- mortised in and 2 magnetic latches on the rt -- 3) back plane is 9/16" thick -- basically made up of whatever was left planed down to the best thickness i could get -- there are 12 elements of different sizes -- some pieces were constructed by putting sliding dovetails between 2 smaller pieces --

planning on cutting a rabbet in the bottom of the sides 9/16" x 3/8" to hold the back plane -- i didn't see a problem with that --

issue 1 -- plexiglas for the front cover -- thinking of .22" thickness -- will a 1/4" groove in the 9/16" thick frame be a problem ? is 3/8" deep adequate ? the alternative is to use 1/8" plexiglas and i confess i haven't been to lowes to look at that thickness and whether it would be OK -- would like to stick with 1/4" if possible --

issue 2 -- can't decide if simply edge gluing up the back is enough or would it be advisable to add T/G (1/4" wide, 1/4" ?, 3/8" ? deep) for strength due to the size of the piece -- right now i don't have enough wood to add the T/G -- there is just enough to get the size needed -- they are looking to see if they can find any more -- typically i use a frame to lay the individual pieces of a glue up on -- F clamps across the top to pull it together -- have recently acquired Bowclamps to provide the "flatness" -- have 2 pair of 24" and 2 pair of 36" -- would glue up the left half of the back using 3 pair of bowclamps, then the rt half -- and the final glue would be the middle joint -- using the 2 36" bowclamps in the center and adding 2 pair of my homemade cauls at the ends -- they work OK but pale in comparison to the bowclamps --

apologize if i have wandered into the TMI area -- bottom line is what size groove -- which will drive what thickness plexiglas and T/G or not the back plane --

many thanks -- i'm sure you don't want more words -- be happy to provide more pics if helpful --

larry

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MikeMa

· #2 · Mar 5, 2019

Issue 1: A groove would leave a little over 1/8" on either side of the plexiglass. I wouldn't go the full 3/8" deep on that, as it probably isn't needed since it is plexiglass, and will also make that wall material a little less brittle. It is still a little thin, but as long as it doesn't get abused, will probably be okay. That said, if it were me, instead of a groove, I would set the plexiglass in a rabbet so that it can be installed/uninstalled after the project has been assembled and finished.

Issue 2: The only reason to do a T&G for the back panel is to help make the boards line up. Since it is a long grain to long grain glue up, you will have plenty of glue strength without the T&G. You may want to consider having that back 'float' in grooves in the back to allow for wood movement.

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Herb Stoops

· #3 · Mar 5, 2019

if you are worried about the glue giving way ,maybe some small keepers screwed to the back of the frame might assure the plexiglass doesn't come loose.

https://www.rockler.com/glass-retainer-clips

https://www.amazon.com/mirror-retainer-clips/s?k=mirror+retainer+clips
Herb

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lking

Discussion Starter · #4 · Mar 5, 2019

many thanks --

mike -- the rabbet for the plexiglas is a great alternative -- how would you secure the plexiglas ?

and your last thought re: issue 2 -- would be to switch to a dado -- 1/4" above the back edge ?

larry

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DesertRatTom

· #5 · Mar 6, 2019

I think I see a face frame on top. You could cut a 3/8ths rabbit on the bak, maybe half an inch deep. Cut the rabbet on a table saw and keep the pieces to hold the .22 in plexiglass in place. Or if it's already assembled, cut the rabbet with a router and find some closely matching wood to cut into retainer strips and use those strips to hold the plexiglass in place.

Those narrow strips will be on the underside of the top and not very visible. Or make the strips of contrasting material and make it look like you meant to do it. Leave a little extra room around the plexiglass. Glue and then p=Pin the retainer strips in place.

Clamps: Sounds about right. First time I've run across bowclamps. Sounds like a machined cawl. Cawls are always a good idea on longer pieces, that or lots of clamps.

No T&G needed.

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Stick486

· #6 · Mar 6, 2019

lking said:
the attached pic shows the big picture -- overall size is 26" x 43" -- 1) sides are 3/4", dovetails at the corners -- 2) face frame is 2" wide, 9/16" thick -- mitered half lap joints at the corners -- planning 3 hinges down the left side -- mortised in and 2 magnetic latches on the rt -- 3) back plane is 9/16" thick -- basically made up of whatever was left planed down to the best thickness i could get -- there are 12 elements of different sizes -- some pieces were constructed by putting sliding dovetails between 2 smaller pieces --

planning on cutting a rabbet in the bottom of the sides 9/16" x 3/8" to hold the back plane -- i didn't see a problem with that --

issue 1 -- plexiglas for the front cover -- thinking of .22" thickness -- will a 1/4" groove in the 9/16" thick frame be a problem ? is 3/8" deep adequate ? the alternative is to use 1/8" plexiglas and i confess i haven't been to lowes to look at that thickness and whether it would be OK -- would like to stick with 1/4" if possible --

issue 2 -- can't decide if simply edge gluing up the back is enough or would it be advisable to add T/G (1/4" wide, 1/4" ?, 3/8" ? deep) for strength due to the size of the piece -- right now i don't have enough wood to add the T/G -- there is just enough to get the size needed -- they are looking to see if they can find any more -- typically i use a frame to lay the individual pieces of a glue up on -- F clamps across the top to pull it together -- have recently acquired Bowclamps to provide the "flatness" -- have 2 pair of 24" and 2 pair of 36" -- would glue up the left half of the back using 3 pair of bowclamps, then the rt half -- and the final glue would be the middle joint -- using the 2 36" bowclamps in the center and adding 2 pair of my homemade cauls at the ends -- they work OK but pale in comparison to the bowclamps --

apologize if i have wandered into the TMI area -- bottom line is what size groove -- which will drive what thickness plexiglas and T/G or not the back plane --

many thanks -- i'm sure you don't want more words -- be happy to provide more pics if helpful --

larry

Issue 1.. plain thin plexi will craze w/ time and UV exposure (even if it's reflected) and become brittle... check w/ a plastics supplier for UV treated..
yes/ok on the groove...

Issue 2... use construction adhesive and modified truss headed screws w/ the back set in a rabbet... major strength this way even w/ a thin (ply) back..
if you are doing a planked back, rebate rabbet or double rabbet them to the frame and spline the plank edges to each other...
https://www.tmshardware.com/collections/phillips-truss-head-k-lath

for the T&G... spline instead... zero waste..

for the plexi grove width... thickness plus 1/16'' and use space balls...

.

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mgmine

· #7 · Mar 6, 2019

I wouldn't use plexaglass. First off it will look nasty after a few years and second it's too expensive. Just go with window glass either single or double strength would be fine. I would make the frame with spines or pocket hole screws. Once the frame is made run it through the router with a rabetting bit, square off the corners and hold the glass in with eiter glazing points or thin wood strips.

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MikeMa

· #8 · Mar 6, 2019

lking said:
many thanks --

mike -- the rabbet for the plexiglas is a great alternative -- how would you secure the plexiglas ?

and your last thought re: issue 2 -- would be to switch to a dado -- 1/4" above the back edge ?

larry

I would use clips to hold it in place.

Yeah, a dado would work.

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need advice on project please

lking

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