blumotion tandem drawer slides drawer depth and dovetail design issues

[Prashun Patel]

Have you installed Blumotion runners before?

I am using Blumotion drawer slides on a desk for my daughter. The slides are 21 9/16", and the installation chart specifies a 21" drawer length, but I would prefer to make them a tad smaller. My instinct is that it will be fine, but the manual specifies 21" as the drawer depth - not the "maximum" drawer depth.

Next, the back of the drawer needs to be notched at the bottom to allow the runner to pass through. However, when combined with a slot for the drawer bottom, it makes for a very weak pin on a dovetailed drawer. How have you dealt with this?

[Michael Weber]

Prashun, if you make the drawers less than 21 inches, the tabs on the slide won't engage the drawer back and the drawer will tilt down when opened. Guess you could jury rig something, but might be visible with the drawer fully extended. I believe there won't be a problem with notching the back. There should be more than enough strength. While nor particularly attractive, it won't be visible.

[mreza Salav]

You have to follow the sizes and tolerances they specify exactly; as mentioned above you cannot make the drawer shorter (or wider/narrower).
Yes the notch will make a weak pin that I have broken on a few occasions but you can glue it back.
If you haven't done this before, I suggest you make up a model drawer to test how it works and fits before making the actual one.

[Mark W Pugh]

Have you installed Blumotion runners before?

Next, the back of the drawer needs to be notched at the bottom to allow the runner to pass through. However, when combined with a slot for the drawer bottom, it makes for a very weak pin on a dovetailed drawer. How have you dealt with this?

Just cut it as it needs to be. There is no weak point, the bottom of the drawer is resting on the slide.

[Mark W Pugh]

Yes the notch will make a weak pin that I have broken on a few occasions but you can glue it back.
If you haven't done this before, I suggest you make up a model drawer to test how it works and fits before making the actual one.

I'm not getting this comment.. I cut the notch after the the drawer is constructed. What pin are you having to glue back in? The cut is on the back of the drawer.

[Jamie Buxton]

The safe thing to do is to make the drawer box exactly as Tandem specifies. However, there is a "tad" available, depending on what you mean by a tad. The standard Tandem works for 21" boxes that have a wall thickness from 1/2" to 5/8" thick. There's your tad: 1/8".

My usual drawer box construction with Tandems is to make the bottom slide in from the back after it is varnished. I do use through dovetails at the front and the rear. The bottom half-pin at the rear is completely disconnected from the back. It stays in place because it is glued to the tail above it. There's plenty of glue surface, so it isn't going anywhere.

There's a different way to handle the dovetails at the back, which I've considered using with Tandems. I've seen this approach on old furniture, so I'm not making it up. In this approach, the rear dovetails are half-blind, the drawer back gets the tails, and the drawer sides get the pins. That is, when you pull the drawer out of the casework, you don't see any joinery at the rear -- it is half-blind. So what's going on here? The back holds the sides together, which is all it needs to do. And because the joinery is not seen from the side, there's no flakey half-pin visible at the rear.

[Andy Giddings]

Jamie, regarding your box construction method - if you're sliding the bottom in after I guess you are making the back shorter in height such that you're fixing the bottom to the underside of the back? Therefore you don't need a cutout for the slide as such? Or am I missing something

[Justin Ludwig]

I'm not getting this comment.. I cut the notch after the the drawer is constructed. What pin are you having to glue back in? The cut is on the back of the drawer.

He's talking about the bottom "pin" of the dovetail. When you notch the back of the drawer it leaves the bottom dovetail without any structural support. One way to prevent this is to design the drawer with a shorter back and then saw off the last dovetail. It may not be aesthetically pleasing to purists and the like, but the drawer remains functional and you only see it when it's fully extended.

Prashun,

If you make the box shorter, you'll have to use a 18" guide and make the box 18" deep. You could build a cross rail under the box that the locking devices could screw into if your box is between 18-21" deep, but then you'd lose the full extension aspect. Best bet is to just build them 12, 15, 18, 21, 24 per Blum's instruction.

[Bill Orbine]

Don't think you will see those dovetails in the back once you open drawer fully.... so the point is kinda mute. You could reinforce the bottom pins (glue it back with a nail pin in the pin) or alter the size of the pins (make'em them bottom pins larger as if to adjust sizes of the rest of tail & pins) . Whatever you do......the drawer needs to be 21" deep to operate effectively. You could make the drawer shorter, but you still need the 21"as you mentioned.

[Jamie Buxton]

Jamie, regarding your box construction method - if you're sliding the bottom in after I guess you are making the back shorter in height such that you're fixing the bottom to the underside of the back? Therefore you don't need a cutout for the slide as such? Or am I missing something

Yes, that's exactly how. I get two things from this design. Mostly I get to finish the drawer bottom separately from the rest of the box. That's much easier than trying to work finish into the inside corners of a full drawer box. Also, I eliminate one construction step: notching the back.

I have a cabinet under construction that's built like this. I'll post a pic tonight.

[Andy Giddings]

Thanks Jamie. makes a lot of sense. Getting rid of the notch at the back is helpful when you have a lot of drawers to make - how are you fixing the bottom panel to the rear - brads and glue or some other method?

[Prashun Patel]

Thank you all for this. I will make the drawer 21" long exactly. It doesn't seem likely that the tiny little spur at the back of the runner will keep the drawer from tipping. Then again, the drawer is resting completely on the runner, so I can't understand why it would tip anyway. It appears to me that if the drawer is slightly shorter in depth, then the softclose travel would be reduced. At full length, that softclose travel would be, say 2". Even if that were reduced by 1/2" or 3/4", big deal. The false-front would contact the case before the runner could reach final position. I suppose that's not a good thing, as it would eventually compromise the finish on the contacting pieces.

I'm planning to use Jamie's method. My bottoms are veneered MDF, so I will just glue the rear panel to the top of the drawer bottom. Haven't decided if I'll prefinish. If yes, then I may just screw the bottom to the back panel.

I'll do through dovetails on the front and a sliding dovetail for the rear. If my calculations are a little strong, then I can shoot off a smidge from the rear edges easier this way.

Last year I ordered RTA cabinets from Barker and I see now that they have brad nailed all the bottom, rear pins to the tails above them. Now I know why.

[Jamie Buxton]

Thanks Jamie. makes a lot of sense. Getting rid of the notch at the back is helpful when you have a lot of drawers to make - how are you fixing the bottom panel to the rear - brads and glue or some other method?

I use screws.

[John Lankers]

You could build the drawer in a shorter version and glue some blocking on the back to get to the required length. The slides don't have to be installed right against the drawer side, you could glue a 1" or 2" spacer between the drawer side and the blumotion slide so that it won't be seen when fully opened.
I hope this is making sense :rolleyes:

[mreza Salav]

I did the notches on the dado (table saw) after the box was glued up. I broke a few of those lower pins but glue them back (with a pin nail); it is only for the appearance.
If you haven't noticed there is a pin at the back of the slide that goes into a hole in the back (that you drill) preventing the drawer box from lifting up at the back. You have a bit of wiggle room
to make the drawer box shorter but not a whole lot. It's best to make a mock up to see how things work. If you deviate from the tolerances, the soft close mechanism might not work properly....

[Andy Giddings]

Prashun, just a heads up - the little pegs at the rear of the slides also allows the drawer to be adjusted for fit (the peg and its mount rotates). If it's not engaged in the drawer somehow, you lose that feature.

[Prashun Patel]

Thanks, Andy. I see that now. Boy, these are great little slides!!!

[Glenn de Souza]

Another vote here for following Blum's instructions precisely. The one exception I make is that I increase the depth of the drawer by 1/32" and I find it allows the tab at the back to engage better. The only call back I have ever had with Blum Tandem drawers is the somewhat rare occasion when rail is becoming easily disengaged from the locking mechanism. Most of the time this doesn't happen, but if I make the drawer 1/32" longer for the better engagement, this never happens.

Personally for some intuitive reason I like the standard way of making the notches better than the idea of the drawer back sitting on top of the rails. There's something satisfying about setting the drawer on the rails and having it drop securely into the notches. Think of setting a notched shelf onto the shelf pins in a cabinet as opposed to setting an un-notched shelf right on top of the pins.

These are the best slides around IMO, and they are so highly engineered, that if you start deviating from Blum's instructions, it can begin to cause a cascading set of complications. Just like modifying Ikea products.

[Andy Giddings]

No problem, takes a while to figure out how the adjustments work! Same also applies to the locking devices - depending on how much adjustment you want, you can order the basic type or the all singing and dancing type that adjust for every axis

[Andy Giddings]

Glenn, regardless of whether the notch is there or not, the drawer will still not move around at the back due to the drawer sides that rest against the slides. The benefits of not using a notch are useful - not really any drawbacks (pun intended) apart from the cosmetic issue around the rear joints

[Kevin Jenness]

If you are careful, the short-grained pins post-notching will stay on, but they are weak. You could secure them with a micro-pin. At my last job, I installed hundreds of factory drawer boxes (many custom shops buy in drawers for economy and scheduling reasons, and you might want to also) and about 1 in 10 had a pin broken off in shipping. If I could find the missing piece it was glued back on, but not a few went out gap-toothed. I don't recall hearing about any complaints from the end users, but still...

One labor-saving way to avoid the problem is to biscuit the back rather than dovetailing it, ripping the back to meet the top of the side grooves, thus avoiding notching and allowing for finishing the bottom separately. In fact, you can biscuit the front as well if you are not set on visible dovetails, and still have an adequately strong drawer. (Purists, avert your eyes.)