Expert: Mark H. Miller - 6/14/2011

Question
QUESTION: I'm building a turn of century (1900) chair and will install no sag springs.  The frame for the chair is basically 3/4 thick by 6 inches tenoned into each leg.  I know the springs exert quite a pull so in addition to corner blocks I plan on further letting in front to back supports below the level of the spring travel to resist the pull of the springs.  I don't want to nail the springs to the top of the mahogany frame for esthetic reasons.  Rather, I plan on screwing on an inside poplar frame (3/4 by maybe 4 wide) as a ledge or rabbet under the top of the mahogany.  How much of a ledge distance should I allow and what do I top the springs with, e.g,, cotton batting or foam or what?  I already have the cushion that I'll be using that are about 6 inches thick.  I'd like to have the springs and covering come up to the top of the seat bottom.

ANSWER: Hello Mark

You are correct in that the 3/4 X 4 rails will break under the pull of a no-sag spring.   I suggest that you build a complete frame to sit inside the current frame built of 1 1/4 X 3 7/8 Alder or poplar and attach your no-sag springs to that inner frame - rather than just the front and back rails.  I would glue and screw the inner frame to the existing frame.  Doing that will give you a frame strong enough to support the no-sag springs.  The 1/8 inch lip is all you need for your upholstery fabric, covered by braid where you tack or staple.  Corner blocks would be nice but not needed if you build this way.  Make sure that the front and back rails extend over the side rails so that the side rails act as support that prevent the springs from collapsing the inner frame.  

Doing the above will also provide the rabbet you need.

Be certain to attach the no-sag springs to each other using helical springs or rubber bands.  Over the springs you should have burlap followed by a thick hard pad such as a carpet pad or two layers of cotton batting,  don't use foam.    Heavy gauge no-sag springs spaced 4 inches apart is what I recommend.  Your last point that you would like to have the spring construction come to the top of the seat bottom is correct,  that way there will not be a gap between the cushion and the chair frame,  in order to achieve that you can not nail your no-sag springs on top of the inner frame but instead on the inside of the inner frame - otherwise the springs will be too high to achieve your goal - use wood screws on the clips or "no-sag" nails that have a serrated edge to prevent the nails from pulling out of the wood.  

From what you write I think you are on the right track and doing fine.

I hope this helps.

Mark Miller.  

---------- FOLLOW-UP ----------

QUESTION: Great info, I think that I'm close.  So if I build this inner frame and secure it 1/8 below the rails of the chair it will give me enough space for the cotton batting and burlap as well as the clips to hold the springs?  Seems like I need a bit deeper rabbet??  Thanks again!

ANSWER: Hi Mark,

No do not nail your no-sag springs on top of the inner frame but instead on the inside of the inner frame - otherwise the center of the springs will be too high to achieve a flat surface that does not extend above the wood frame - use wood screws on the clips or "no-sag" nails that have a serrated edge to prevent the nails from pulling out of the wood since they will be on the inside and the horizontal pull could pull the clips out of your inner frame. .

Since the inner frame will be 1 /14 in width you should nail your burlap on the inside 3/4 inch edge,  don't nail the padding keep that at least 1" away from the wood frame.  That will leave the outer 1/2 inch edge to staple your fabric and then cover that with braid trim - also called gimp.  For a perfect job staple your fabric on the outside edge of the inner frame, right up to your decorative wood -  have the staples at about a 45 degree angle - then trim your fabric and over your fabric,  again with the staples as an angle staple a strip of thin - very hard cardboard that is slightly less in width than your braid trim.  That will give you a flat surface for the trim,  then glue your trim over the cardboard and the job will be perfect.  

Yes,  the 1/8 inch drop or 3/16 maximum is enough for your fabric, cardboard and braid trim.  

I hope this helps,  have fun with your project.

Mark Miller.



---------- FOLLOW-UP ----------

QUESTION: Could you explain how the clips are nailed/screwed to the inside frame.  I understand the design logic of the clips when they are nailed to the TOP of the inner frame because they hold the spring until the metal tab can  be bent over and nailed down but I'm not sure how one stretches and attaches the spring to the clip if it has been fastened to the inner side.  Wouldn't it be easier to simply make the ledge/rabbet deeper to accommodate the clip thickness?  I have read that the spring should be 3 inches shorter that the depth of the frame and stretched to fit the clip.  Is this the dimension that you recommend?  Also, what length helical springs to you recommend for the no sag springs installed every 4 inches?  Four inches seems reasonable but they would not be preloaded and they, like the no sag springs, might be better if they were shorter??  Finally, how many helical springs should be used for a chair seat?  Thanks again!

Answer
Could you explain how the clips are nailed/screwed to the inside frame.  I understand the design logic of the clips when they are nailed to the TOP of the inner frame because they hold the spring until the metal tab can  be bent over and nailed down but I'm not sure how one stretches and attaches the spring to the clip if it has been fastened to the inner side.  Wouldn't it be easier to simply make the ledge/rabbet deeper to accommodate the clip thickness?  

    Yes, nailing the clips on top of the frame is easier and is the prefered method.  But doing that will raise the center of the springs too high - therefore, in order to bring the center down I suggested you nail the clips on the inside of the frame.  Making the ledge/rabbet deeper makes the job much easier but then you will need a seperate ledge that is higher for the upholstery fabric.

I have read that the spring should be 3 inches shorter that the depth of the frame and stretched to fit the clip.  Is this the dimension that you recommend?  

    Yes,  that is correct.  

Also, what length helical springs to you recommend for the no sag springs installed every 4 inches?  Four inches seems reasonable but they would not be preloaded and they, like the no sag springs, might be better if they were shorter??  

    A no-sag spring every 4 inches is fine,  the outer edge spring should be about two inches away from the side rail,  if the last spring is too close to the frame edge it won't do anything.      

Finally, how many helical springs should be used for a chair seat?

    One helical spring every fourth loop should be enough.  The helical springs should be slightly stretched when they are in place.


 Thanks again!    My pleasure,  your questions are all good questions.