[Project_Hopeless]

For my first build I going the Baltic birch ply route, torsion box construction.

Attachment 417548

Should be plenty rigid along as I bond the pieces together properly.

Attachment 417550

I see two ways to go. Thru bolt with fender washers nylock or tee-nuts, the wood variety.

How would you attach metal parts to the the wood parts?

[ger21]

I don't like to bolt any metal parts directly to wood, as the wood will always crush some amount, depending on how tight the bolts are.
I prefer to route a pocket and epoxy phenolic or aluminum plates into the pocket, and bolt the components to the inserted plates.

[peteeng]

Hi SS - To screw to timber use T nuts or timber thread inserts. You can also thread the timber with a metal thread tap and set the screw into it with epoxy. Wax the screw before setting. The trick with timber is that you can;t torque up the screws as much as in metal so you will crush the timber. To solve that they are nipped and set with epoxy or with PVA glue. Fender washers are fine but a bit thin, you can change their shape and they will come loose. Set your threads or holes with PVA glue. RE: FEA you will overestimate the stiffness of the ply as plywood is a laminate so only the longplies do the work. For bending make sure your outer grain runs along the gantry not across to maximise stiffness. same as in other areas make sure your outer plies direction is the dirn of bending. Consider laminating thin aluminium onto the outsides of your Z axis plates it will stiffen these hugely. As drawn your tool plate is way too thin. It needs to be very stiff to not bend under cutting loads. Using drive screws like drawn will be able to create very high tool loads.

Your vertical columns may need some stiffening as well with a web. Birch is about 16GPa ? Perhaps 20GPa without considering the loss of the crossgrain stiffness .Cheers Peter S

[Project_Hopeless]

I appreciate all the comments, its a learning curve for sure.

RE: FEA you will overestimate the stiffness of the ply as plywood is a laminate so only the longplies do the work. For bending make sure your outer grain runs along the gantry not across to maximise stiffness. same as in other areas make sure your outer plies direction is the dirn of bending.

I farmed out the analysis to my brother. He actually modeled up 5 and 7 ply birch de-rating the alternate ply accordingly. Also ran the models once with parallel grain and again with 45 deg. grain. There was some differences in the bending and torsion between the two models but not much.

Consider laminating thin aluminium onto the outsides of your Z axis plates it will stiffen these hugely. As drawn your tool plate is way too thin. It needs to be very stiff to not bend under cutting loads. Using drive screws like drawn will be able to create very high tool loads.

For the Z axis I was thinking 1/2 aluminum plates. I was hoping to get additional stiffness from the liner bearings on the long plate and the router tram plate on the short plate. Even for a wood build you feel the Z axis should be stiffer?

Your vertical columns may need some stiffening as well with a web. Birch is about 16GPa ? Perhaps 20GPa without considering the loss of the crossgrain stiffness .Cheers Peter S

Not visible in the previous drawing is a large web across the back of the gantry. All in all I get a 49"x49"x 7" work envelope.
Attachment 417620

What if I backup the tee-nuts with wider hardwood blocks, increase the surface area, a dense wood less likely to crush?

[peteeng]

No need to back up T nuts, just use PVA glue to set all your fasteners and do them up tight but don't crush the timber. Looks like your doing good with the FE. Use the FE to make your decisions. Also remember that the FE is absolute and does not take into account bearing compliance, other clearances and general machine issues, friction, gaps etc so your predicted deflections are optimistic. For cutting metals you need 10N/um stiffness at the tool. For setting screws I prefer 5min epoxy. But you must wax your screws if you intend to get them out in future. If they prove hard to get out put a soldering iron on their heads and warm it up till the epoxy gives. Peter


http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf

Bolts are done up tight to generate a friction force to prevent them from loosening and to generate contact friction at the faying surface. If they are set with epoxy they are as stiff as a normally tensioned joint and become "snug". If the joint is in tension (like a cylinder head) then it needs enough preload to prevent the joint gapping. But as a general machine bolt in shear or combined small loading just epoxy is fine.

[ger21]

T-Nuts themselves are not really a problem. The issue is whatever you are bolting them will be pulled into the wood, if you tighten it enough. If you don't tighten enough, parts can get loose.

[wizard]

First thing too realize is that wood working offers literally hundreds of ways to fatten things together. You will likely need multiple methods depending upon what you are fastening to those surfaces.

Second the images you have supplied indicate to me that there are a few areas of concern with respect to dimensions of your parts. For example the uprights supporting the gantry beam look real thin in the renders.

For my first build I going the Baltic birch ply route, torsion box construction.

I don’t believe you mentioned size, though being on my cell phone at the moment I could have missed it. In any event the beam just looks thin!

Should be plenty rigid along as I bond the pieces together properly.

Well that depends upon what you want to do with the machine and your expectations. You are right that assembly is the key here. But that only Applies to a structure that meets your needs. It is far better to err on the side of a very stiff structure than one that isn’t stiff enough and vibrates all over the place.

I see two ways to go. Thru bolt with fender washers nylock or tee-nuts, the wood variety.

How would you attach metal parts to the the wood parts?

There are literally dozens of ways to approach this and as mentioned above you may need to choose different methods for different parts of the machine. As a few people have already mentioned one of the big issues with wood is that it crushes easily and that depends a great deal on the type of wood and where it is cut from the tree.

For things like linear bearing saddle block you will likely want to run machine screws into the supplied holes on the bearing block. The problem here is that machine screws and washers easily crush most Wood products so you need a hefty backing plate ideally of steel. Some 3/16” thick hot rolled steel cut to therequired length can do the trick here. Steel is cheap for this use but there are dozens of other materials that can be used. The goal is to spread out the clamping force to prevent crushing of the wood at installation time but also to prevent loosening up under operation.

Most fender washers are too thin to be useful in this application. You end up with a saucer shaped thing when tightened under even light load. Given that there is a class of washers designed for carriage bolts that where used by the ARMY in shipping boxes of various types. These are fairly heavy steel that would not reform easily at all. Sadly I never tried to buy such washers so I’m not even sure where you would get them. Here is an example of a light duty variant: https://www.mcmaster.com/carriage-bolt-washers Though the washers I’m think about where at least twice s as think and had nail holes instead of drive ears.

Since you are building up torsion boxes you have a lot of options if you think ahead. Here are some ideas:

1. For something like a linear rails create a backing bar out of steel with properly drilled and tapped holes. With a little fore thought in gantry design and assembly these rails could be embedded prior to gantry construction. With the a suitable width this would practically eliminate any pull through even if there are soft spots or voids.

2. All thread can be embedded in your gantry build to achieve steel studies for the mounting of hardware. These can be embedded in epoxy between two piece of plywood and ideally would have a short leg bent in the all thread to create an “L”. If you don’t want to bend all thread J or L boots for concrete work might work.

3. There is a massive range of thread inserts made for wood, with the so called T-nuts being of only one type. Some may be better than others depending upon the situation. You should familiarize your self with all of the possibilities because the right choice can make life much easier.

4. Sometimes space can become a problem when trying to size a backing to bolt metal pieces to. What you can do in these situations is build in metal inserts (pipe with the ends squared off) and clamp between two smaller backing plates. It helps a great deal here to have much thicker material to mount the pipe into. So let’s say on the surface of your gantry beam you don’t have a lot of room for a backing plate but have dead space inside your gantry beam. Make the beam extra thick in these area to then sleeve with tubing. You can do the same thing with threaded inserts made of bar stock. The important things here are square faces and suitable thickness of material to prevent cam out. Note the goal here is a bit different than normal thread inserts in that you want the much longer insert to enable resisting rocking loads in a tight location. One example might be a bearing block for a leadscrew. In a nut shell the opposite of the all thread suggestion above for studs.

5. Don’t forget screws designed for wood. There are a variety of modern deck and construction screws out there that are perfectly suitable for mounting things like drag chains, sensor brackets, E-sTop boxes and the like. You do run into the problem of torsion boxes being to thin to really hold screws of any type. It is for these randomly mounted devices that thicker materials are often a better choice in torsion beam constructions. Even so I would add additional material in places where you know you will be adding things like drag chain brackets An 1-1/2 to 2 inches can make a huge difference in those screw holding over time. Since you likely have a good idea as to where these items will be added it is pretty simple to add build up material at torsion box assembly time. So don’t Dinis modern wood screws especially the construction focused ones. The only real issue to their use is suitable material thickness. Also pay attention to the heads, you want to avoid the so called bugle head dry wall screws. Instead look to cabinet screws with flat beads , construction screws and joist hanger screws.

6. While on the subject of joist hanger screws, the part of the hardware store that sells joist hangers will have all sorts of prefabbed plates and brackets for joining wood product together. I’ve used such hardware to slap together tool stands and even a router table (non CNC). This isn’t fine wood working but it does get things done. With a good screw gun the assembly is fast. In any event I digress, the point here is that you can find rated wood screws easily these days. The supplier of those screws also mass produce steel products that might be of interest.

[Project_Hopeless]

For things like linear bearing saddle block you will likely want to run machine screws into the supplied holes on the bearing block. The problem here is that machine screws and washers easily crush most Wood products so you need a hefty backing plate ideally of steel. Some 3/16” thick hot rolled steel cut to therequired length can do the trick here. Steel is cheap for this use but there are dozens of other materials that can be used. The goal is to spread out the clamping force to prevent crushing of the wood at installation time but also to prevent loosening up under operation.

My vision or maybe its delusion, is evolving. Like you suggest I'm now thinking 1/4" thick cold rolled bar stock threaded for backing plates.

3. There is a massive range of thread inserts made for wood, with the so called T-nuts being of only one type. Some may be better than others depending upon the situation. You should familiarize your self with all of the possibilities because the right choice can make life much easier.

I think tee-nuts are out, 1/4" cold rolled, drilled and tapped. From some of my research on the boat building forms I see they counter bore all the bolt holes and pour an "epoxy annulus" to help with side loads for rigging and winches. I'm thinking about doing this also.

What epoxy type brands would you recommend for embedding inserts or building "epoxy annulus" in wood?

[ger21]

I use US Composites, with appropriate fillers. It's very inexpensive compared to most.

[peteeng]

Much easier to thread the wood and set with epoxy. The epoxy annulus would be a soft zone compared to the wood (EP 3000MPa vs Birch 10000MPa plus). You want to couple the wood and the fastener in as stiff a manner as possible. On boats epoxy annuluses are used within sandwich construction to replace foam or balsa to couple the fastener to the skins. No need to do this in Birch. If the fastener is intended to be taken in and out a lot then use a metal thread insert. If the joint is permanent then the screws are just there until the adhesive goes off. I use the attached quite a bit and they work very well for things that have to be taken in and out a lot. I use them for hold downs on my router bed. they are placed in a 200mm grid on a 30mm thick plywood base. I have also used them edge grain.

The only thing I think I'd back up is the rails. Id use hard washers vs a strip unless you want to use a strip. If I'd use a streip I'd use it on both sides of the ply to create a sandwich about twice thew width of the rail. this all depends on how much leverage you are going to apply to the rail? Cheers Peter S

[routalot]

How easy is it for any of us to find proper woodscrews these days?It seems to be a lost commodity,along with the knowledge of how to insert them properly.Almost every vendor seems only to stock screws intended for rapid driving with no pilot hole and a thread running all the way to the head.Clearly the market values speed of driving more than anything else.A hundred years ago,in the age of wooden framed aircraft and America's Cup yachts with hollow wooden masts it was possible to find both good screws and the people who took the time to use them to their greatest advantage.

At least we can find a good selection of bolts and ensure that the plain shanked variety are used where shear loads occur.With the various threaded inserts now available it can be quite challenging to position them accurately enough for the bolts to engage with the thread cleanly.I suppose living near a man with a mill having a DRO is a good start.

[ger21]

How easy is it for any of us to find proper woodscrews these days

I get mine from here: https://www.mcfeelys.com/

Modern screws are far superior to old style wood screws.

But when building with wood and epoxy, I use machine screws threaded into the wood.

[Project_Hopeless]

I use US Composites, with appropriate fillers. It's very inexpensive compared to most.

I assume you use the thin resin/hardner to wet out the wood beforehand? To set the insert do you add filler to the thin epoxy or use a thicker different epoxy?

Much easier to thread the wood and set with epoxy. The epoxy annulus would be a soft zone compared to the wood (EP 3000MPa vs Birch 10000MPa plus). You want to couple the wood and the fastener in as stiff a manner as possible. On boats epoxy annuluses are used within sandwich construction to replace foam or balsa to couple the fastener to the skins. No need to do this in Birch. If the fastener is intended to be taken in and out a lot then use a metal thread insert. If the joint is permanent then the screws are just there until the adhesive goes off. I use the attached quite a bit and they work very well for things that have to be taken in and out a lot. I use them for hold downs on my router bed. they are placed in a 200mm grid on a 30mm thick plywood base. I have also used them edge grain.

When using inserts and epoxy is it better to drill the hole slightly larger than the ID (root diameter) so the treads bite the wood or drill a hole equal or larger than the OD and fill the void with epoxy? I see both methods in my searches. For the inserts, flange or no flange? Does the tread type on the insert matter now that it is set in epoxy?

[ger21]

Yes, I use the thin epoxy first, then add fillers to it.

[Project_Hopeless]

Yes, I use the thin epoxy first, then add fillers to it.

What filler would you recommend? Is strength compromised much with the filler?

[ger21]

I use West System 404 for bonding fasteners

[Project_Hopeless]

With epoxy'd inserts in birch plywood are hardwood or softwood inserts preferred? Does it matter?

[Biggs427]

On my machine I had better retention with softwood insert. I drilled the hole slightly bigger (+1mm or 1.5mm bigger diam) than I'd drill for softwood.

I used 403 filler because but 404 like Gerry said would have been better.

[ger21]

I personally don't like threaded inserts. They can be difficult to install, and may cause the ply's to separate. I've bonded in nylon locking hex nuts, but a solid steel insert is the best method, as it can distribute the load better.

If you do use them, I don't think the epoxy would add much if any strength.

[Project_Hopeless]

On my machine I had better retention with softwood insert. I drilled the hole slightly bigger (+1mm or 1.5mm bigger diam) than I'd drill for softwood.

Bigger than the recommended drill size or the OD of the insert?