1

Suddenly the two hardware stores near me have all sorts of varieties of spade bit sets from multiple manufacturers displayed in prominent aisles, which makes me wonder whether that's the reason I am tempted to use a spade bit for this job, or else spade bits have always been there and it's only now I'm noticing.

Anyhow, I'd like to confirm.

A quick summary of the project:

I am hanging a chin-up bar on drywall, as shown in the picture below. From two previous questions on this site I know that:

  1. Several pieces of two-by lumber, along with 3/8″ - 5″ lag screws, are needed to take the load from the metal frame to the studs, and
  2. A lag bolt, a fender washer, a regular washer, and a lock washer are needed to attach the metal frame to the two sheets of 3/4″ plywood.

burying a bolt in plywood

What is the right tool for burying a 3/8″ bolt, along with washer and fender washer, in plywood? The lag bolt appears in the bottom half of the image. Also, my hunch is that I do not need to glue the two sheets of 3/4″ plywood together; would you care to comment?

Regardless of whether I would use a spade bit or something else (a sawtooth bit? a hand chisel?), it's difficult to measure this sort of thing at the outset (at least using an amateur's handheld drill). Is my guess correct that I need to do this by trial and error: shave a bit, put the bolt and two washers; shave some more; until the head of the bolt no longer protrudes?

If you are following the previous two questions and the present one very closely, you will notice that I am no longer discussing the need for three kinds of attachments (A, B, and C in the previous two questions), but only two, as the picture above shows. I asked based on the posted dimensions from customers' reviews and expected that the manufacturer had cleverly separated the holes by exactly 16″, the common dimension separating studs. Since then the chin-up bar arrived in the mail, and the separation is actually 52 cm (~20.5″). As a product meant for worldwide consumption, this makes sense, and, ironically, it simplifies the job on my side.

Comment

I didn't know this when I asked the question, but it turns out that in this project I don't have the luxury of choosing whether to bury the bolt and washers in the plywood. That decision is forced by the available bolt lengths. The 3/8″ bolts at the largest chain of stores are available in only 2″, 2 1/4″, 2 1/2″, and 3″ length. Since I'm going with acorn nuts to avoid getting cut from the exposed bolt, I need to use 2 1/2″ bolt and to bury them. At the unavailable 2 3/4″ size (as luck would have it) I could have drilled a hole in the drywall for the bolt head and kept the plywood intact.

5
  • 3
    Off hand I would rather pop a hole in the drywall, than weakening the plywood. For making holes in drywall, a hammer feels nice.
    – crip659
    Aug 6 at 18:18
  • For that job, almost any bit that will make the hole will do. Spade, forstner, hole saw.
    – crip659
    Aug 6 at 19:23
  • Forstner bit would be my choice. Though I'm not sure I would buy one for a one-off job.
    – SteveSh
    Aug 6 at 19:45
  • Don't really need such long screws into stud. Shorter ones going 3/4 way into stud should give enough strength.
    – crip659
    Aug 6 at 22:48
  • @crip659 the stud is probably wider than the 2" shown in the drawing
    – Jasen
    Aug 7 at 2:07
3

Spade bits don't make for the neatest cuts but as it's hidden it won't matter so much

I'd drill the hole with the spade bit first as if you drill the main bolt hole you might end up with the spade chattering around to start with (if the bore of the hole is bigger than the widest part of the spike on the spade)

In terms of measuring the depth, get your bolt plus washer etc and line the face of the washer that touches the wood plus the bolt head up against the cutting part of the spade, then use some electrical tape wrapped around the bit to mark where the face of the bolt head is. Now just drill til the tape is level with the surface of the work. The ply will compress slightly when you bolt it all up too..


By the way, if the diagram is a vertical section, I'd hesitate to mount it on the wall in the way you depict in the diagram - if the yellow square is a horizontal wood between vertical studs, it's asking a lot for drywall to resist being broken by hanging a significant weight (a person?) on anything attached to the lower bolt. The downwards force of the weight will server to try and rotate the whole diagram in an anti-clockwise direction, smashing the drywall. I'd mount your proposed bracket in a vertical stud so that the stud takes the load and resists the twisting, not the drywall

5
  • If spade bits don't make the neatest cuts, might I get a clean(er) cut using a sawtooth bit? Is a sawtooth bit suitable at all for this job?
    – Sam
    Aug 6 at 18:33
  • The edges of the hole will probably be neater with a sawtooth bit; you can also run them backwards to start if they are a very coarse tooth and the cut will be gentler, but getting the part drilled plug out might not be so neat. A new spade bit will be neat enough on the hidden side that it's what I'd go with. If you were looking for absolute precision/neatness, a bit that is used for drilling hinge holes in kitchen cabinet doors would be the tool.
    – Caius Jard
    Aug 6 at 18:38
  • It will be hidden, so I don't care much about neatness, but I'm worried about the first layer(s) of the plywood chipping off. Your idea to run the bit (whether spade, sawtooth, or Forstner) first in reverse is excellent!
    – Sam
    Aug 6 at 18:59
  • i.stack.imgur.com/rWqlD.jpg - this is what a slightly used spade bit drilling into hardwood beech looks like - not the ugliest cut, softwood might have been a bit more chewed up.. higher grade spade bits have points on the outer edges of the spade that can help cut the grain rather than ripping a chunk out as the spade aligns with the grain direction
    – Caius Jard
    Aug 6 at 19:16
  • Purely hypothetically (I've never done tasks out of order), if you have a big pilot hole that a holesaw or spade bit or forstner bit won't center on, you can make a hole in another chunk of wood and clamp that onto the final piece to give you a guide for starting the hole. (Totally diverging, you can do the inverse with a holesaw and its cutout if you need to actually screw the guide in place.) Aug 7 at 13:27
4

I like using Forstner bits for jobs like this vs. spade bits. They cost a bit more but they make a much cleaner hole and don't have a tendency to tear or split the wood like spade bits do.

Most hardware and home center stores have them these days.

2
  • A Forstner bit is a sawtooth bit but with the sawtooth part replaced with a smooth (yet still cutting) circle; is that right?
    – Sam
    Aug 6 at 18:41
  • I've not heard of a "sawtooth bit" before but a quick search indicates that these are a type of Forstner bit with saw teeth vs. a knife edge around the circumference of the bit. I'd say either would work for your application.
    – jwh20
    Aug 6 at 18:51
1

I would use a structural screw rather than a lag bolt. Lag bolts are yesteryear's technology. Aside from being stronger and not requiring pilot holes, structural screws have heads that will bury themselves in the wood without the need for washer or drilling a recess with a spade or forstner bit.

As for the metal frame connection: again, I would use structural screws. Simply install the screw through the front of the metal frame, through the wood blocking, through the drywall, and into the wall stud. This would be much simpler and much stronger than mounting the metal frame to the plywood.

If I understand your OP correctly though, the metal frame you currently ahve does not allow for mounting on 16" centers. I consider this a defective design for the American market and would not use it. I'd simply get a pullup bar designed for mounting on 16" studs. Then put wood blocking on top of the drywall (to prevent crushing the drywall) and then mount the metal pullup bar frame through the blocking and into the wall studs at 16" on center.

I've attached a photo showing a Bora lumber rack designed for carrying very heavy loads of lumber on a wall. As you can see, the application is very similar to installing a metal frame pullup bar. I have installed this lumber rack in my workshop with structural screws into the wall framing. I've attached a photo stolen from Amazon though. The photo shows a structural screw holding the metal frame to the wall stud.

enter image description here

7
  • Washers spread the load, which I would still do given large tear-out forces here. The recess is for a bolt, not a screw and I don't think there are "structural bolts".
    – jay613
    Aug 6 at 20:18
  • 1
    Structural screws have a washer head. The washer head it designed to prevent hurricane-level tear out forces. That's part of what makes them structural screws. If the washer heads are good enough for securing a roof, logic would dictate that they are good for holding pull-up bar. Moreover, using structural screws would definitely make the entire assembly stronger by avoiding boring out 1/2 of the thickness of one of the plywood boards.
    – stmp945
    Aug 6 at 21:38
  • BTW, here is a link to an example of the structural screws I am referring to: lowes.com/pd/…
    – stmp945
    Aug 6 at 21:39
  • That sounds like a good idea, but just to clarify: you are suggesting using a structural screw instead of a log screw (top of the image). Attaching the metal frame to the pair of plywood boards still needs a bolt and nut (bottom of the image). There is no way to attach a structural bolt/screw to a nut; is there?
    – Sam
    Aug 7 at 3:54
  • I've edited my answer to answer your question.
    – stmp945
    Aug 7 at 15:51
0

I'd use a spade bit because a forstner/sawtooth bit is a lot more expensive, and just produces a cleaner cutout that you don't need. A spade bit is a perfect way to produce a flat rough cutout.

Yes, go a tiny bit at a time til you can fit the two washers in the hole without their protruding. You don't have to totally bury the back of the bolt in the plywood. Just the washers. The end of the bolt will easily get driven into the drywall when you drive the boards in with the big lag screws. Or, as suggested in a comment, just cut a hole in the drywall and don't countersink the bolts in the wood at all.

You should glue the boards together and trim them flush and square if necessary after gluing. It will look nicer. The bolts should have a tiny amount of play in the holes you drill through the plywood, so the two boards might become slightly misaligned when you tighten things down if they aren't glued. It'll just look bad.

1
  • Yes, if you don't countersink the wood at all you should pre-cut into the drywall to avoid the washers acting as spacers and warping the wood. You don't need any particular kind of bit, as you'll be going all the way through the drywall. Whatever you have that is as big as the fender washer. And if you don't have one big enough, use what you have and enlargen the hole with anything. A saw, a file, your finger will do the trick.
    – jay613
    Aug 6 at 20:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.