How To fasten rubberwood desktop to electric motorized steel base, in easy detach/ knock-down way using Fasteners? e.g. Wood inserts?

Question

How To fasten rubberwood desktop to electric motorized steel base, in easy detach/ knock-down way using Fasteners? e.g. Wood inserts?

I'm designing & putting together a workstation desk with the following traits:

• Electric motorized (lift capacity: 120 kg) & heavy Steel frame base (30kg)
  • Its a knock-down design put together using hex key-driven M6 M8 & few M10 bolts
• Rubberwood desk/ tabletop

So for consistency with the above & need for easy disassembly, I'd prefer to use similar metric bolts.

Hence, I'm considering attaching the top with metric bolts from the bottom, through the pass through layers of a steel base and a rubber cushion spacer + washer (Total: ~11mm), into matching threaded inserts.

• Upon reading online the insert types I found are these and am told that top left thread-in types for hardwood would be suitable.

• Commenters/ responders here have posted there are special types of other better Fasteners. I'd like to know, review them for additional "holding power" and related benefits they bring.

The rubberwood top is 18mm thick & pass through non-wood layers (~11mm). I do not know the final dimensions of the top, but assume a rectangle roughly 1m wide and under 2m long. [The OP is free to update these dimensions as necessary --jdv]
(OP: Size subject to change but within above & hence need insert for detachability)

My main questions are about the fasteners/ inserts.

• What is the safe maximum depth the inserts can be made into the rubberwood? Will this material hold inserts at all?
• Which insert style is recommended in this application?
  • What special type of fasteners supersede wood inserts I've mentioned so far?

Follow-on question: Does the size of the bolt (M6, M8, M10, etc.) and matching insert matter for this application?

Update Follow-up to this Question:

Noticed a lot of variety of combinations even in threaded inserts of the hardwood type: Self tapping (advise by jdv), Slotted, 3 holes/ blind holes, collar, Hex, Reservoir, etc. (Pic from some German site)

(Image is big so linked, but can be inserted if SE folks ok - Or should these slot/ holes/ etc meta parameters go to a new Q?)

What pros cons of these in my scenario and what's recommended or to avoid? Why bother asking all this?

Scouted a dozen mom & pop hardware shops I went to did not have any.(Nothing like Home Depot).

I do see some of the above variety on local business yellow pages listing site, so they do exist & have to be found, but they don't respond to me/ consumers.

I'll eventually run into and find some variants, just not sure which ones - hence, I need to know the "yes, okay, maybe, avoid, no way" of some of the common types.

High Res measurement pics:

All the layers I want to sandwich together:

The metal and rubber layers:

Width of the rubber washer:

Answer 1

I would thread holes into tabletop because I know this is strong enough because wood screws are strong enough for same purpose, and there is no insert. But if you choose inserts here is my answer.

Allowance for wood movement to be addressed. We are not yet sure if this is needed or how much.

No need for more than M6 as these are already so strong. Wood screws to attach table tops are usually thinner and the hold is weaker!

Brass or steel does not matter. Both strong enough.

Flat head or hex drive does not matter, just 2 styles. No difference in strength. Hex easier to fit clean because driver size is exact.

You want screws min 5-6mm from surface, so 12 or 13mm inserts max. But 10mm no problem and more safe. You will not use all threads anyway. Be exact when you drill holes!

For metric coarse, pitch = advancement. Pitch of M6 is 1mm so every revolution = 1mm deeper. Gives M6 x 20mm max [11mm pass through metal frame and rubber rings + 9mm engagement in insert for 1mm clearance from bottom]

Minimum 4 threads engaged is strong enough, so M6 x 16mm no problem if closest size available.

Answer 2

Edit for the new information that this is a knockdown (K/D) project.

This probably does require the use of threaded inserts
Although you can thread wood directly¹ inserts are intended to provide very durable threading for machine screws/bolts in wood and other softer materials, allowing for regular and repeated adjustments over time, especially under heavy load, or for complete removal and reinsertion of the fasteners as might be needed for knockdown furniture, especially if frequent disassembly is going to be needed.

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You haven't asked about this but as mentioned already in the Comments it is an important factor.

Allowance for wood movement
The key thing with any tabletop made from solid wood is to allow for seasonal movement.

All solid wood experiences seasonal expansion and contraction perpendicular to the grain direction, see previous Answer to which direction does wood expand?

For any type of glued-up panel — regardless if made from many smaller pieces² or two or more wider boards — or single very wide boards, where the width is approximately 460mm (~18") you need to start considering movement. At widths in excess of 600mm (24") some allowance usually becomes vital. The wider the tabletop the more allowance is needed.

From what we have been told up to to this point there may or may not be enough allowance naturally available in this desk due to the size of the holes in the metal frame. Note that the smaller the machine screws used here the more allowance is built it, because it leaves more space around each fastener — approximately 12-13mm for each pair of screws if M6 are used, based on the stated size of the holes.

See previous Answer for a little more on how you would normally attach a solid-wood tabletop.

If you don't build in some allowance for movement the table could experience serious problems. Normally the issues are breakage or tearing out of the screws, or warping or cracking of the tabletop. Here there's also a chance the steel framework could be distorted because the pressure exerted by expanding or shrinking wood can be considerable.

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¹ Threads in wood are of course more durable in some harder hardwoods, e.g. maple, but they can work even in softwoods and weak hardwoods like poplar. In softer or more crumbly woods (which includes some good hardwoods) the threading can be substantially reinforced by dribbling in some superglue into the hole.

² These smaller pieces may be called staves, hence "stave construction" although regrettably this is now most commonly, and incorrectly, sold as "butcher block".

Answer 3

The question at hand is "how deep should I make a hole in relatively soft wood for a blind insert for an M-sized bolt?"

A good rule of thumb is don't remove more than 2/3rds the material in any given dimension when doing joinery. It isn't a hard and fast rule, but a nice start. In this case it'll work just fine because nothing here is critical.

The follow-on question, which size of M bolt to use is up to you. You want to choose the size appropriate for the entire dimension of the panel, with larger being better for shear and tensile strength. But your main challenge will be getting good thread engagement at such shallow depths, since it'll be the threads giving you that tensile strength. As long as you can safely move the item by gripping the panel this will probably work. But depending on the entire dimensions you may find that only 6 points of contact mean threads rip out easily no matter what.

For this wood use the screw-in self-tapping inserts, with or without glue.

Choosing more expensive fasteners made out of tougher material might be required.

Allowing for panel movement will be necessary in most cases. For this design that'll mean slotted holes in the metal along the axis where the panel is across the grain.

However, much of this is application driven. How strong does this have to be? If this is just a big table then typical installation and typical fasteners will be fine, even with only 1/3 fastener thread engagement. But maybe you need to be sure? In which case you can epoxy the inserts, use larger inserts designed to engage more fully with the material, and use fasteners with finer threads and higher strength (and, if possible, maximize your thread engagement).

A word on failure modes

There are two failure modes here:

1. The inserts tear out of their self-tapped threaded holes. The weakest part of any metal-to-wood contact (or, indeed, any good glue joint) are the wood fibres around that contact. This is the most likely failure if the top is lifted too many times or if the frame is very heavy; the inserts will simply rip out. You could also tear them out simply by torquing the bolts down enough, as the leverage of threads is almost certainly stronger than the insert-to-wood joint. This can be somewhat mitigated by also using some sort of glue when installing the inserts.
2. The metal threads in either the insert or the bolt fail, either because the material weakens, or is fatigued because of a thread-mismatch or cross-threading, or simple mechanical fatigue. Larger fasteners don't necessarily have larger threads, and most of the stress is going to be on the threads fully engaged in the insert. It is possible that these threads will let go from stress, or strip out over time if they are removed and replaced often. This is unlikely in normal use, but it is possible. There are a lot of cheaply made bolts out there that often have poor threads or use metal with poor strength. If this table was subject to a fair amount of vibration you can apply thread-locker to the bolt threads when installing them.