Materials and stiffness/weight for a diy build

[Spikes]

Hi,

I've finally picked a design for my cnc and now wondering about materials and their impact on rigidity and weight. Considering that it's going to be a "cheap" build where a lot of accuracy isn't priority, joe2006 look like, and mostly routing wood and plastic, I was thinking the following:

- for the bed a torsion box in 1/2" MDF. I'm picking mdf because it's heavy and I can ensure it's straight and won't warp over time
- for the gantry another torsion box but this time with 1/2" plywood, since weight here is a concern. I'm considering using one nema23 ~175Oz/in , but it's just a thought, haven't quite got to the electronics part yet, with one acme leadscrew.

now the questions:
- should I be using 3/4 MDF for the bed instead of 1/2? 1" just seems way too much, but maybe I just don't have a clue
- should I worry about weight of the gantry/z and indeed use ply instead of MDF or is MDF ok because in the end the weight isn't going to be that different?
- I've seen people reinforcing ply/mdf with aluminum/steel, do you recommend to add 1/8" sheets of metal to reinforce part of the build? where? I've seen posts where specifically ppl use it with ply on the z axis or the gantry's sides.
- am I better off using some other material for stiffness like laminate or similar?

thanks,

Spike

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[ger21]

Build quality is far more important than materials. If you use MDF, make sure it's sealed very well. I'd recommend shellac sealer, 2 coats, then 2 coats of paint.

[Spikes]

Thank you Gerry. Any recommendation on dealing with birch playwood if I decide to go that way?

[ger21]

Do you have any woodworking experience? Tools?

[Spikes]

depends on the point of view I guess, but I'd say minimal experience, I have some tools and access to a community workshop that has more stuff if I need to (mitre saw, table saw - very old used stuff - no plainer no jointer unfortunately, currently looking for ways to do that with a jig and a table saw, but unclear on the options) . I've mostly made simple builds with 2x4s or 6s for a shed/outdoor, stained a few things, and done some nicer benches/tables with oak, danish oil finish. Not sure where that all stacks in your book.

[wizard]

You didnt mention the machines size or i missed it but that is a factor. Generally i wouldnt reccomend 1/2" wood sheet goods for any build thus would suggest 3/4" sheet goods if this is a wood build. There are lots of reasons but stiffness, eases of fastener installation, glue surfaces are some of them.

As for MDF i dont personally like working with the material so lets say im biased against the material. For me it is more of an irritant than anything else. So quality birch plywoods would be high on my list. Somebody here did a machine out of bamboo plywood which i find very interesting. For parts of a machine i can even see glue lams, chip board and othe composites being useful. If you have local access higher grade softwood plywoods like MDO (Medium Density Overlay) or marine plywood might be worth considering. You might also consider plyeood specifically designed for underlayment, the trick with construction grade plywoods is to find void free materials that are stable as many trades of construction plywood are not void free.

In a nut shell there are a lot of wood based products out there that would be very useful in a machine build. Even so it may be worth considering steel for parts of the machine. Steel for the last few years has been extremely cheap especially if buying scrap or drops. That maybe subject to change relative quickly though. Even so a gantry beam done as a steel box section beam is close to completely done.

Machine size is a factor in a build as mentioned above. If you need a very high Z you will need to choose stronger materials and construction methods for the Z and Gantry (Y). So if you choise Plywood that means box sections for everything and in places laminating sheets together to bulk up. Even then the gantry beam may need to be very large in cross section. To that end there is a sticky thread well worth reading that goes into some of the engineering and helps with understanding this part of the machine. Most of the stickies are very imformative when it comes design so do spend time with those threads.

Interestingky you didnt mention if this was a moving table design or a moving gantry. Im going to assume moving gantry. Most designs benefit from dual leadscrews on moving gantry designs so a 3 axis machine means 4 sets of leadscrews and motion electronics. In any event dont get too wrapped up in the selection of electronics until you have gotten you basic mechanical design down.

One of the reasons to like plywood over some other composites is that it holds screws really well. So much can be done with quality deck screws (and lots of glue) when it comes to assembly. It is worth considering thread inserts and machine screws anyolace where you think you may have to separate subassembleis often. The spindle assembly comes to mind here as does locations where bearings may be mounted. An option here is to use steel plates or angle iron as the mounting surfaces for bearing blicks with those pads drilled and taped. You do need to be careful with the variances in thermal expansion and woods mosture issues. The idea here is that you will have to do maintenance or replacement of your linear bearings at some point and thus will need to gain access.

Your box sections will need internal bracing to prevent parallelograming but also prevent deformation any place something may need to be bolted together. The use of bolts is again design dependent but if it makes sense in your design make sure the bolts are supported. You can laminate stock up to give you a void free section but this adds weight and cost in many cases.

Sealing was mentioned above and frankly this is very important in any desing using wood products. There are all sorts of options but a thin epoxy might be a good choice. Some materials will sosk it up faster than others though. Depending upon the epixy selected it can also be used for glue ups when final assembly comes. In any event whatever you use as a sealant, the purose is to moderate rapid mosture take up which will leadto distortion in you machine. I would seal everything including all internal surfaces in any box sections. That might be overkill in some minds but it is better to be safe than sorry.

One last thing, if you are building a large machine you will need to put a lot of effort into how the torsion box is assembed. That is you will want it to be flat and twist free. If you dont have a large work bench that might be a problem and most floors are not that flat. So put some thought into how you will initially assembe the tables torsion box. The closer you can get this to perfect the better. Consider that materials needed to fixture this in yiur costs. Obviously a smaller machine that can be built on a flat table or work bench isnt an issue.

[Spikes]

Hey Wizard,

appreciate the thorough reply, I know it takes time so thank you for the effort. Also please bear with me as I don't have enough experience to understand some of the terminology you're using so i'll be asking some clarifying questions.

For the type of machine, this is what I'm leaning toward, but I'm considering some changes like the pulleys and 2 leadscrews on the side (from the other thread), plus one big EMT pipe instead of two on the X to simplify. Gantry and table are going to be torsion boxes. And yes, it's going to be a moving gantry design.




To your question on size, I'd love to have a work area of 2'x4', however ultimately part of that will be decided by cost, and since this is also my fist machine I'm inclined to go smaller to make experimenting easier and less costly, but we'll see.

Roger for the 3/4", makes sense to me.

About wood, I don't like working with MDF either, but I have very limited budget and the price difference with baltic birch is substantial so unless I get lucky I'll have to stick with mdf for at least part of the build. The local homedepot has no MDO available and the construction plywood I looked at was pretty nasty as far as voids go hence again going back to MDF.

As to mental, I understand the preference, and maybe it's an issue of where I'm at, but I have no access to local cheap metal and with online ordering shipping costs are a killer hence the focus on a wood build (I also looked at scrapyard, but besides not finding much selling to individuals the issue is quality, ie stuff being straight to begin with).

So one thing I couldn't figure out from your post, you said: Your box sections will need internal bracing to prevent parallelograming. What do you mean with that? are we talking about the same thing when I say torsion box or internal bracing is something simpler?

Regarding a flat surface to start with my plan was to build a torsion box following this approach, which seems to give good results however I don't have a jointer and a plainer so I may have to figure something else out:
http://www.finewoodworking.com/2007/05/15/build-a-torsion-box-assembly-table

thanks again for all the input.

[genixia]

Hey Wizard,

appreciate the thorough reply, I know it takes time so thank you for the effort. Also please bear with me as I don't have enough experience to understand some of the terminology you're using so i'll be asking some clarifying questions.

For the type of machine, this is what I'm leaning toward, but I'm considering some changes like the pulleys and 2 leadscrews on the side (from the other thread), plus one big EMT pipe instead of two on the X to simplify. Gantry and table are going to be torsion boxes. And yes, it's going to be a moving gantry design.




To your question on size, I'd love to have a work area of 2'x4', however ultimately part of that will be decided by cost, and since this is also my fist machine I'm inclined to go smaller to make experimenting easier and less costly, but we'll see.

Roger for the 3/4", makes sense to me.

About wood, I don't like working with MDF either, but I have very limited budget and the price difference with baltic birch is substantial so unless I get lucky I'll have to stick with mdf for at least part of the build. The local homedepot has no MDO available and the construction plywood I looked at was pretty nasty as far as voids go hence again going back to MDF.

As to mental, I understand the preference, and maybe it's an issue of where I'm at, but I have no access to local cheap metal and with online ordering shipping costs are a killer hence the focus on a wood build (I also looked at scrapyard, but besides not finding much selling to individuals the issue is quality, ie stuff being straight to begin with).

So one thing I couldn't figure out from your post, you said: Your box sections will need internal bracing to prevent parallelograming. What do you mean with that? are we talking about the same thing when I say torsion box or internal bracing is something simpler?

Regarding a flat surface to start with my plan was to build a torsion box following this approach, which seems to give good results however I don't have a jointer and a plainer so I may have to figure something else out:
http://www.finewoodworking.com/2007/05/15/build-a-torsion-box-assembly-table

thanks again for all the input.

If you don't have a good flat surface to assemble your torsion boxes then the first consideration should be to get a good flat surface big enough to assemble your boxes on. Your wood will comply to every bend, wave and undulation in the assembly surface during assembly, and then your torsion box will be a mirrored replication of those imperfections.

A good source for a relatively good and cheap surface is Craigslist - look for granite countertops being discarded during a kitchen remodel. Most kitchen countertops are 27-29" wide, ideal for your needs.

MDF is your best option. It is dimensionally stable in all directions, and easy to work with, although a dust mask is an absolute must. I echo the sentiment about sealing it well - moisture absorption is woods biggest weakness. Most wood (including plywood) absorbs fairly slowly but can result in a 6% change in dimension across the grain. MDF resists absorption through the material but sitting liquid will cause it to delaminate and bubble at the point of application.

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[wizard]

If you don't have a good flat surface to assemble your torsion boxes then the first consideration should be to get a good flat surface big enough to assemble your boxes on. Your wood will comply to every bend, wave and undulation in the assembly surface during assembly, and then your torsion box will be a mirrored replication of those imperfections.

Good points here. It is possible to set up a jig to allow for the assembly of a very flat torsion box but it isn't easy nor is it cheap requiring material a person might not have. The idea of a used granite counter top is a good one and I suspect that any flat surface would help.

One idea for a flat surface, especially if one is to go with a larger machine is to pour an epoxy surface plate on a garage or cellar floor. It is an interesting solution to building a relatively large flat structure. Of course one could flatten a table that is less than flat this way also

As for those tiny undulations I ran into this problem on a traditional router table I built years ago. The table ultimately has a curve (only at one end) in it, that while not much is enough to frustrate me at times. The design has worked well for me other wise as I put a lot of mass into it to keep it stable.

A good source for a relatively good and cheap surface is Craigslist - look for granite countertops being discarded during a kitchen remodel. Most kitchen countertops are 27-29" wide, ideal for your needs.

I might add that counter tops sometimes show up at thrift stores along with large tables and other "flat" solutions. I like this thinking though

MDF is your best option. It is dimensionally stable in all directions, and easy to work with, although a dust mask is an absolute must. I echo the sentiment about sealing it well - moisture absorption is woods biggest weakness. Most wood (including plywood) absorbs fairly slowly but can result in a 6% change in dimension across the grain. MDF resists absorption through the material but sitting liquid will cause it to delaminate and bubble at the point of application.

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It will take a lot to convince me that MDF is my friend. It certainly has some good properties for CNC building but I will/would look for alternatives.

While I agree with the idea that common construction plywood is out of the question there are still home center solutions to consider. There is a grade of plywood designed for sub floor usage that is also void free thus eliminating the need for underlayment in construction. The material is a few steps above your run of the mill construction plywood. If you find a lumber yard willing to stock or order it you can also get what amounts to construction grade birch plywood. This is basically birch plywood without the nicely finished surface plys.

[Biggs427]

+1 for sealing. I sealed my birch machine with 2 coat of epoxy and after one year it still looks ok.

As wizard said, MDF doesn't hold scews that much, but threaded inserts can be epoxied and, given that it is done correctly, it provides far better holding capacity than screws and is relatively cheap.

This is an example of how I did it on my machine: https://www.cnczone.com/forums/cnc-w...ml#post2004418