[peteeng]

Morning Dim - I have no commercial interests in UK or the planet for plywood. Long certified straightedges are expensive but are worth it... But making straightedges from extrusions is good enough for your endeavour. I rarely use a torque wrench. Been making things since I was a youngster so most things by feel. Bolting rails to ply I have done in several ways:
1) Using round rail with the supported extrusion is a good solution. The support extrusion is broad and takes normal bolts or timber screws well. But the preload on round rail is light so positional hysteresis can add up. Prefer medium or heavy preload square bearings
2) Square rail I use timber screws (I like Kreg screws square drives. I have banned cross head screws from my workshop, do not use them) or capscrews. Using capscrews I pilot the hole and use a taper tap to create the thread. Being tapered means when you screw the capscrew or hex bolt in, it snugs up quite tight . If the screw is to be permanent I use PVA glue in the thread as a locking adhesive. They have never backed out or come loose... They still come out with a hand tool if needed. Use as long a screw as possible and you won't have any issues. Do some tests for yourself. For general screwing I use square drive screws intended for outdoor decks. Epoxy coated they don't rust.... zinc plated screws can rust after a couple of years I've found.
3) Don't use expanding PU adhesive. Its a pain and can change the dimensions of your parts over time as it keeps expanding. I have used single pack construction adhesive and I like it. Easy to clean up squeeze out with some kero and a rag... Just as good as epoxy for the job. Being solvated means it sticks really well. In breaking tests the timber breaks not the glue. Even the cheapest glue vs premium brands
4) Yes its time to get a good general arrangement done for review
5) if your really worried about the ply foundation for the rails then use an aluminium strip thats wide under the rails. Bonded to the ply after levelling or floating up to you. A 3x50mm wide flat plate is a good width for a 20mm rail... Since a rail is near an edge usually, I have also used a light angle that goes around the edge for more support. Don't know if it helped but it can also look better...
6) If there is a cabinet maker nearby get then to cut the long cuts on their table saw. Will be straighter then you can cut with a fence and a hand saw... I usually give a cutting list to a local company of its for important parts. The edges are square and true vs my hand cut stuff. Thats one reason I'm building an 8x4 machine so I can cut stuff up properly...

Keep at it... Peter

Heres my latest stack of parts for my 8x4 machine. I'm about to start assembly of the machine base....

[dimmaz88]

Thanks Peter, I'll take all that on board.

I think as the main column is such a big unit I'll try the epoxy levelling method, that should get me somewhere close, I can then adjust by sanding/scraping.

Have you edged your new parts or painted?

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Thanks Peter, I'll take all that on board.

I think as the main column is such a big unit I'll try the epoxy levelling method, that should get me somewhere close, I can then adjust by sanding/scraping.

Have you edged your new parts or painted?

[peteeng]

Hi Dim - Don't go near epoxy levelling, Huge can of expensive worms. I painted the edges. But edge banding is faster. I have a large tin of red I'm trying to use up. What accuracy do you expect to get your lands? and what plywood spec do you have in your area? Try my brother in laws woodshop down the road Regards & Ho Ho Ho Peter

[dimmaz88]

I've seen epoxy levelling done with good results in a few places, why don't you like it?

I'm not sure on accuracy, I'm just aiming for as good as I can get without spending a fortune on specialist tools that I won't use again, or not often anyway.

"Try my brother in laws woodshop down the road"

I can get phenolic structural plywood, either poplar or eucalyptus core. I assume it will be the same sort of grade as you're used to.

[peteeng]

Hi Dim - For every Epoxy levelling done with "apparent" success there are several other failures, I have been involved with epoxy for over 40 years and I'd stay away from this apparent application of epoxy... Many people in the forum are totally in conflict with me on this, so be it...

1) Free flowing epoxy has a modulus of 3-4MPa so its rubber, do you want to mount your rails on rubber?
2) to successfully overcome surface tension, crawling and internal heating cells that create wobbly surfaces you have to cast at say 5mm thick. So do you want to mount your rails on 5mm of rubber?
3) you have to use very slow curing epoxy to prevent various thermal curing issues in this case (crawling shrinkage expansion). These types of resins require post curing, it's tough to post cure in situ on the structure you are contemplating so the epoxy modulus would be less then 3MPa &
4) these types of resins cut really poorly so if you have to correct the surface your grit paper will choke, may have well lapped/filled the plywood so much easier....

I could go on - just figure a better way... get your GA done.... The GA is the first real milestone of a project. Until you have that you don't have a possibility of a machine. Also remember that you can throw a lot of timber at a part and it won't be much weight. Structural hardwood ply 700kg/m3 steel 7800kg./m3 so 7800/700= 11x to get to same weight so 3mm steel = 33mm of ply.... the ply is therefore much stiffer at same weight

Peter

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Hi Dim - For every Epoxy levelling done with "apparent" success there are several other failures, I have been involved with epoxy for over 40 years and I'd stay away from this apparent application of epoxy... Many people in the forum are totally in conflict with me on this, so be it...

1) Free flowing epoxy has a modulus of 3-4MPa so its rubber, do you want to mount your rails on rubber?
2) to successfully overcome surface tension, crawling and internal heating cells that create wobbly surfaces you have to cast at say 5mm thick. So do you want to mount your rails on 5mm of rubber?
3) you have to use very slow curing epoxy to prevent various thermal curing issues in this case (crawling shrinkage expansion). These types of resins require post curing, it's tough to post cure in situ on the structure you are contemplating so the epoxy modulus would be less then 3MPa &
4) these types of resins cut really poorly so if you have to correct the surface your grit paper will choke, may have well lapped/filled the plywood so much easier....

I could go on - just figure a better way... get your GA done.... The GA is the first real milestone of a project. Until you have that you don't have a possibility of a machine. Also remember that you can throw a lot of timber at a part and it won't be much weight. Structural hardwood ply 700kg/m3 steel 7800kg./m3 so 7800/700= 11x to get to same weight so 3mm steel = 33mm of ply.... the ply is therefore much stiffer at same weight

Peter

[pippin88]

I have practical experience of what Peter is saying about epoxy levelling.

Even with super slow hardener, epoxy did not self level unless 5-6mm deep.

Epoxy is soft. Over time it has crept - there are outlines of rails / bearing blocks in the epoxy.

Would not do again.

Steel filled epoxy using a straight edge to replicate the straight edge surface is quite different.

[dimmaz88]

I hear what you're saying, I had wondered about the density of the epoxy, figured it wouldn't differ much from ply.

Replication is a method I'd like to use, it's just having a good enough straight edge to use as reference. I don't fancy spending £5k on one though!

[peteeng]

Hi Pippin thks for chipping in and Merry Xmas to you and your family!! Replication or casting a surface is fine in a hard filled epoxy. But if you have a straightedge to use for casting then you may as well lap it.
Merry Xmas Dim - If you have a 2m extrusion thats stiff and you have 3 of them you can lap them "flat" using whitworths method. I've just done that with some concrete pads to create surface plates. Then you have a good reference surface.

Hope Santa treated everyone well....

[routalot]

I'd be interested in seeing a model of the proposed machine,with notes to explain how it all works.

[dimmaz88]

Peter, I've been toying with the idea of making plywood/MDF straightedges. I'm thinking of making them 2400 x 500 x 36mm with 5mm aluminium epoxied on the edge, this will make the reference edge. Do you think they'd be stable/stiff enough if they're coated with epoxy/paint?

Routalot - Here's a video created with Fusion's machine builder, I thought I'd shared it already but I don't think I have.

https://drive.google.com/file/d/113K...usp=drive_link

[peteeng]

Hi Dimm - May as well buy a 2.5m long level that has a milled face. If you did go the extrusion/plywood path you would have to make it symmetric ie Al/ply/AL so it shrank/expanded evenly. The ply expands much more then the AL so it would bow. If you made this how would you get the edge straight? There are various rectangular tubes available - 100x50 150x50 and 200x50 that are straight when I put my 1m edges along them. 500mm seems to be very deep? Extrusions are hot stretched in their final process so are quite straight. You could lap them against each other? How accurate do you need them? Peter

[dimmaz88]

Thanks Peter, I'm probably overthinking it again. I did think to buy 3 lengths of alu extrusion so I could check and scrape them, I imagine a milled level should suffice for the accuracy I need.

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Thanks Peter, I'm probably overthinking it again. I did think to buy 3 lengths of alu extrusion so I could check and scrape them, I imagine a milled level should suffice for the accuracy I need.