Drawer Dimensions for Blum Undermount Tandem Plus Blumotion

[TomInNC]

I'm taking my first crack at installing Blum undermount slides, and I'm more than a little confused about the tolerances around the directions Blum has in their manual. I am going to use overlay doors, and the manual says that the inset for overlay applications should be 3mm. I bought the undermount jig from Rockler to help hold the slides in place while I'm drilling, and the manual for the jig says to use the 2mm stop for the Blum slides. For an overlay application, is there really any difference between 2mm and 3mm? If so, Rockler owes me some money.

The second point of confusion stems from how the drawer width is calculated in the Blum manual, which says "Inside drawer width must equal opening width minus 1 21/32nd of an inch." Is this the minimum width to clear the slides, or am I supposed to hit that number dead on? I don't have anything that I would trust to measure the drawer box openings within a 32nd of an inch tolerance, so hitting that number dead on is highly unlikely. 

[Coop]

My brother, using these for the first time, has the same problem. After using these a year or so ago with ease, I made a one hour trip each way yesterday to show him how it’s done. I took pics of mine as a guide and ours are similar but different. I’ll be darned if I could figure his out. I won’t be using these again. 

[JohnG]

 

As far as getting the inside drawer width correct, it will probably be easier to go by their metric measurement if you have any rules with metric on them. It'll be an even number, 42mm. It probably needs to be pretty close to that number in order for it to work properly, but you may be able to get away with a slight variance with the side-to-side adjustments.

Might be good to make a mock up drawer from some scraps to give it a try before making a batch of drawers.

[Tom King]

Make a test drawer first and only use a couple of screws in the slots in the part that fastens inside the cabinet until you get it just right.  If it's been several years between my using them, I always do because I forget the little details.  I made a jig for the holes in the back of the drawer years ago. 

For inside the cabinet boxes, I have always just used a piece of plywood to mount the slides that fits under the slide, sitting on the cabinet bottom.  Mount the top ones first, then rip the plywood down for the next one down, and so forth.   I set a combination square for the setback from the front.   I never bought any specialized jigs.

[TomInNC]

I will definitely be making a test drawer, but even before doing that, I need to figure out how to get the dimensions in the ballpark. I've uploaded below the part of the "manual" for the slides. The point of confusion concerns the calculation of the inner drawer width.  The manual says: "Inside drawer width must equal opening width minus 1 21/32"." So the question is whether this means that the inside width must be at least the opening less 1 21/32 so that the drawer box clears the runner, or does this statement mean that the the inside width must be exactly the opening less 1 21/32? If you look at the "Opening width minus 42" part of the sketch, I believe the numbers the "opening width minus 1 21/32"" number is just to get the clearance from the runners, so that should be interpreted as a minimum interior diameter between the box sides. I'm sure the locking mechanism plays a role in all this as well, but there's nothing in the manual about tolerances, so I have no idea how close you need to be to that 1 21/32 difference to avoid issues. 

One thing that strikes me as very odd is that there's a false sense of precision in the manual itself since they are rounding the metric numbers when they convert to imperial. 

 

 

[BillyJack]

Last time I used Tandems were on the designers  cabinets 3 years ago. Just remember …. It doesn’t matter if you use 5/8 or 1/2 sides, it will jus5 change the gap along the sides and the drawers will have to be a specific depth..

[TomInNC]

Thanks. I forgot to mention that I went with the 1/2 sides and 1/4 bottom.

[Tom King]

I just figure all that out after I have the slides in hand.  I don't think I ever worried about figuring it all out before the process of building them. One good thing about half blind dovetails are the front and back of the drawer are the finished width before running the dovetails.  The lengths of the sides are whatever works with the slide that has to engage the holes in the back of the drawer.  Being full extension makes it easy to put a front and back in place and measure what length is needed for the sides.  I do very little figuring and measuring.

If I did it all the time it would probably be different, but I just built one house full of cabinets a year, so didn't bother to remember all the details.  For just one set of cabinets, I wouldn't do it any differently.  I'm sure I've built over a million dollars worth of cabinets with these simple minded methods though.

[TomInNC]

I'm going to be using box joints, so the box width is determined by the initial cross cut of the side and back plus any protrusions you get when cutting the fingers on the joint. I usually leave 1/16 or so to sand flush on box joints, and that is how I ended up going down this measurement rabbit hole. I just ran the cuts for my test batch, I believe my width is within 1/32 of what Blum specifies in the manual (provided I nail the 1/16 protrusion on the joint). I did ask Blum about this, and they said I should be fine as long as I'm within a 1/16. Apparently there are fancier locking devices than what I bought, and those allow for some kind of lateral adjustments that give you a larger margin of error. 

[Chestnut]

This is why i use hardwood runners..... butter smooth and a lot less confusing. Yep not helpful.

Tom i advice that you follow the blum instructions but Get a metric tape. Rest assured that if they are rounding numbers that means that there is some small amount of slop that is allowed. 2nd piece of advice, make a test drawer before you go for the big show.

If I understand from your last post, just measure from the bottom of your finger joints on the front and back pieces. Take the metric opening width and subtract 42mm, that's what the distance between the bottoms of the fingers should be. Blum is 100% of the reason why i have a metric tape in my shop. It's just easier. The best part o that method per their instructions is that the side thickness really doesn't matter as pointed out above the gap just changes. Also keep in mind the bottom needs to be recessed per instructions.

 

[Tom King]

They work best when all the clearances are just right.  Even though there is a range, there is a difference in how nicely they operate.

[Don Z.]

Looking at the diagram, what I see is if the inside dimension (based on the side thickness) is too large, it will foul the slides.  You might be OK if they were a bit more narrow, but the thicker you go, the more chance of fouling.  

Honestly, if it were me, I'd get as close as you can, and if the sides end up being too thick, you can plane/sand the outside to make it work.

 

[BillyJack]

Just follow the manufacturers specs..

[BillyJack]

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