Working On A New "Timber Frame" Design and Build.

[LorenBMS]

Pete, Thanks for the article. I agree the AL/PLY/AL would be a great construction material. I have been considering doing a composite beam for my gantry. I have looked at pre-made carbon fiber sheet stock, but it breaks the budget. Considering the aluminum option too. At this point, I will wait until I have the gantry beam fabricated and can do some deflection tests on it. If it looks like it needs to be beefed up at that time I will bond the aluminum sheet to the exterior faces.
Here is the beam cross-section. 6"wide x 10" tall.

To overcome the lower compressive strength of the plywood for fastening hardware to it all of my hard attach points like the linear rails and racks are mounted to wider 3/8" aluminum bar stock which will have tapped holes for the hardware attachment. Then the aluminum will be bonded and screwed to the plywood. For the connections between the gantry side plates and columns and columns to the gantry beam, I have internal bolt plates to fasten too, so the compressive loads of bolting are spread over a larger area.

About how large is this next machine you are working on?

[peteeng]

Hi Loren,
The next machine has a foot print of 1200x1200mm. Its that size as that's the std sheet width. Look up the Frankenrouter thread.
https://www.cnczone.com/forums/austr...hlight=peteeng

I highly recommend using furniture thread inserts into timber. The cast ones are good, the machined brass ones are excellent. If you epoxy them in they are really good. Saves mucking around with extra metal. On some machines using hardwood ply I have screwed straight into the timber. I use a tapered tap so as the screw goes in it tightens. If you PVA glue the thread it stays tight. If you epoxy it its there for good, wax the screw/bolt. Have to use a soldering iron to warm the bolt to get it out.... I've also started using the new truss type head timber crews, they are good as well.... Peter

[peteeng]

Hi Loren - Some ideas - did a quick calculation on your gantry. 1) I recommend making it solid plywood no internal bits this would weigh 25kg/m 2) if you make the same size al gantry it would be 12mm thick to be same lineal weight. So a 10"x6"x1/2" AL RHS would still weigh 25kg/m. Sorry about mixed units!! But the Al section would be 2.39x stiffer due to Al modulus 70GPa vs F17 ply at 14GPa and birch at E=10GPa. So depending on your gantry rigidity target depends on what you need to do.

My 3mm thick steel gantry weighs 14kg/m at 1.5m long. So maybe some holes in the middle plies to make it a little lighter but gantry weight doesn't matter much if its stiff.

To make it simpler plate the front and back of the gantry with 6mm al and you would have a very stiff structure plus some metal for threads. The vertical being 10" is stiff within itself. Since most cutting loads are in the horizontal direction its good to make the XY direction structure very stiff. the timber core will add dampness and have very good shear transfer. Gantries are not "long" beams and shear plays a big part in short beams ie shear deflection can be the issue vs beam bending... near solid sections solve a lot of those issues...Peter

[LorenBMS]

I am using thread inserts in a couple of places in the construction where a bolt plate was not going to fit. I have already made the bolt plates so going in that direction this time.

Thanks for looking at the beam design. I looked at making the gantry beam solid wood, and it will add about 90lbs to the assembly and I just do not have the equipment to true a 6"x10" solid beam. My plan was to fill the beam with rigid urethane foam which has a 103psi compressive strength. Doing small pours so I don't bulge the outer faces.

Blocking in the beam, spaced out along the length is probably my best way to help resolve shear deflection in the cross-section of the beam instead of the PU foam.

All a compromise for shop capabilities and budget. :-(

[peteeng]

Hi Loren - yep everything is a compromise I suppose that's why we don't stop at one machine... Peter

[LorenBMS]

I have been making progress on fabricating the base. Beams are all laminated, ripped to width, cut to length and finished up cutting mortises in all of them. Ready to assemble the base frame work next.

[LorenBMS]

I had a productive day assembly the base. All the beam joints are mortise and floating tenon, glued and screwed to hold the clamping so I could keep moving.

I still have some bracing to add from the long beams to the legs.

[peteeng]

Hi Loren - I've found pocket hole screwing allows alignment of parts vs end screwing which makes that impossible. Plus they pull up very tight. Its now officially looking like a machine! Peter

[LorenBMS]

Thanks it is starting to look like some sort of machine.

Good to know on the pocket screws. Actually all those joints have domino tenons in them and the tenons located the parts. They also make a joint stronger than screws.

[peteeng]

Hi Loren - You sure your not building a pool table? Their easier then CNCs. Peter

[LorenBMS]

Haha...It sure looks like it could turn into a pool table.

[LorenBMS]

The build has been progressing along. I painted the base over the weekend. I have been waiting on some aluminum angles to arrive to make some tie brackets for connecting leg braces between the base beams and the base legs. The angle arrived yesterday and I made all the tie brackets and started installing the braces on the base. Hopefully will finish that up this evening. Next up will be an epoxy leveling pour on top of the base frame beams. Which should give me a new zero plane for building the machine table.

Maybe a point of interest, the base weighs just over 450 lbs.

[peteeng]

Hi Loren - WOW so many bolts!! I do like torq head bolts, Hate cross head screws they should be banned. Why not use timber battens in the corners vs metal? either way is OK but I li9ke to stay in the same medium if possible...Peter

[LorenBMS]

Yes, a lot of fasteners. They are "cheap" and spread the load out. Battens?? Are you referring to the alum. angle?

[peteeng]

Hi Loren - A batten would be a square piece of timber cross screwed in the corner like the metal brackets. The triangular pieces of timber used as braces would be called knees by a boat builder. Peter

[LorenBMS]

I can't really say I have a reason for the metal angles vs wood blocking other than it was easy. I'm by no means a wood working purist.

I installed the rest of the leg bracing and started making and installing the epoxy dams this evening.

[peteeng]

Hi Loren - The pour is going to be interesting - we wait apprehensively. Peter

[LorenBMS]

Dams installed and sealed. If things go as planned I will do the pour tomorrow.

[peteeng]

Hi Loren - I recommend you brush the cavity with epoxy before you pour. This will allow the epoxy to "wet" the surface before you pour. You can mix a small batch and brush it on and let it get quite tacky before you mix and pour the main pour. This way the tacky epoxy will fill and seal any holes before the main pour. Its surprising how much epoxy can leak through a pinhole.... Peter

[LorenBMS]

I know a pour can leak like a sieve. A sticky situation...lol