Plywood and V Bearings

[spalm]

Well I received some V bearings yesterday, installed them on a test board last night, and am very pleased with the results. They track extremely well. I am using the eccentric mounts that allow the bearing sets to be pinched together. I plan to rebuild my machine to use them instead of pipe. We will see what problems I run into along the way.

I will be able to use assemblies from machine #2 to make the build go a bit faster, but I want to spend some time making it cleaner also. Parts to keep are the leadscrews and their mounts, and also my completed Z unit to start with.

The plan is to make a JGRO sized machine with a wrap around gantry. The bed will be torsion box with 1.25” angle iron bolted to the bottom edges and use two V bearings and two skate bearings as shown in the photos.

The bottom of the gantry will be a torsion box. I hope to be able to center drive the gantry as the V bearings should not allow any skew or racking.

The top of the gantry (Y) will be a wood box. I can not decide on whether to use angle iron on the top and bottom, steel strips on the front, or an 8”x22” steel plate for rails. The Y bearings will be a plywood plate with 4 V bearings, two at the top, and two at the bottom to ride on this steel. I am thinking about placing the Y leadscrew at the top of this box.

The gantry sides will probably be 3/4” ply with a stiffener glued or bolted to the outside.

Not sure if I can pull off running the bearing axels through just the plywood, or if I will need some kind of attached plate to strengthen this.

Steve

[ger21]

Steve, try brushing a coat of epoxy on the plywood (after the holes are drilled, try to get it in the holes too.) I just did this to my gantry sides. They are lightweight, or ultralight MDF, which is very soft and porous. After coating with epoxy and sanding, they are glass smooth and very hard. Now that I think about it, try making them from epoxy coated MDF. The epoxy will soak deep into the edges making them rock hard. The only issue is the cost of epoxy. A quart of West System w/ hardener is about $35-$40. But if you're going to spend that much, for those few small pieces, you might want to just buy aluminum.

[Rance]

Steve,

You're killing me. You mean you are going to scrap your machine #2? You don't know how much of an inspiration that was to get me going (designing) again. I'm so close to actually cutting some wood its not funny. However, my 1st machine (my own testing grounds) will be a very small variation of yours.

So tell me, why are you scavenging from machine #2? What are its weak points?

As to your rail idea above, its good to see new ground being broken. Are your GrooVy Bearings sufficient for thrust loads in the application you are about to put them in?

Rance (still asking noob questions)

[spalm]

Hi Rance.
How ya doing? Long time no talk.

Well, I never did really finish that last machine. The gantry sides are still MDF with ugly pine stiffeners and extra holes. I never did put the bottom skin on the torsion box bed and the constant pressure caused it to bow up a little bit. The X leadscrew is still mounted on the top left hand side which is in the way and weird looking. The half circle cut outs on the bed to hold the rails is not as precise as I would like. Most of these are cosmetic, but instead of rebuilding it as is, I decided to try some new things.

Problems with #2:
The gantry can rack about 1/4”. The left side is held in place with the leadscrew. The right side can be pushed back and forth. Not sure why. Think it is inherent in this bearing arraignment. Dual leadscrews would probably solve it for all practical purposes. I have never been too fond of my lower gantry torsion box. I believe I got lazy and only put three ribs in there. If this was stronger, I think it also would help.

The Z structure has some flex. If I do repeatable rapids of the Z into the table with the motors off you can see the router lift up in the air about a 1/8” inch (I know, I know, but I need to stress test it). I think this from a combination of the bearings rolling back on the Y, the rack of the X, and flimsy non-supported bearing holder inside the Z (study Gerry’s or Lionclaw’s Z for a better design)

Now, that said. I have high hope for these V bearings. They are the same type as shopbot . They are double angular contact and are made for doing just this. They are supposed to ride on precision rails but Ynneb claims they will seat themselves on angle iron.

Steve

[ger21]

Originally Posted by Rance Are your GrooVy Bearings sufficient for thrust loads in the application you are about to put them in? Rance (still asking noob questions)

Rance, check out this. http://www.bwc.com/cadFiles/dualvee.pdf

[ynneb]

I hope to be able to center drive the gantry as the V bearings should not allow any skew or racking.

Remember the title of my post " Built on a foundation of past mistakes"
One of my previous mistakes was not driving my gantry from both sides. On my current machine is driven on both sides via a drive shaft that connects to the one servo.

I did get racking on my first machine, mind you the gantry had a 1.5m span.( (Current machine 2 m span) I guess if your gantry has a short reach and is driven in the center you might be ok.

Yes the bearings do seat the steel well. There is an initial seating that happens quite quickly and as the corners of the angle iron flatten out its stabalizes due to the surface area increasing. This may sound crude to most machinists, however it has worked very well for me.

Good luck on your machine Steve. You can always Skype me on "ynnebbenny" if you wanted to talk more about all this.

[spalm]

Gerry, the only place I am concerned about reinforcing is where the axles go through the plywood. Maybe I could just epoxy that area. Do you think it would soak into Baltic Birch as well as it does in MDF?

Benny, I hope I can drive from the center as it will only be 24” wide. Otherwise, I will need your help on purchasing belts and pulleys. I will wait until the end to see if I think one screw will work.

I am thinking about taking the easy way out and making the Y beam out of glue up of two 3/4” pieces of ply sandwiched together. Routing vertical dados on the back of the front piece for threaded rod to pass through to hold down the angle iron, and routing horizontal dados on the front of the back piece for threaded rod to attach to the gantry before glue up. Or should I build a thicker torsion box?

Steve

[ynneb]

The further the bearings are away from each other, the better the stability of the whole thing, both racking wise and gantry remaining flat.
I really like the Shopbot as a model/guide since it takes all this into consideration.
If you havnt already made the base of you machine, it may pay to make it lightly longer than your plans, that way you can widen the gantry and spread the bearing mounting wider. Know what I mean?

Drill the mounting holes in the angle iron a bit bigger than the bolts. Pre tap the bolt holes on the frame. This way you can lay a smear of body bog between the frame and the angle iron. You then tighen and level just one angle iron, using the bolts and a straight edge as you go. This will make sure you get a nice even and solid monting.

Do this to one side only, then place your gantry on the fixed track and do the same with the othet angle iron. Always checking the gantry movement and also the flatness etc. This is a bit of mucking around but well worth getting right in the first place.

Dont worry so much about if the rails are level with each other in terms of height. When you mill your sacrificial board on the table this will true the whole lot up in terms of the machine movement. I hope I am explaining this in an understandable way.

[ger21]

Originally Posted by spalm Gerry, the only place I am concerned about reinforcing is where the axles go through the plywood. Maybe I could just epoxy that area. Do you think it would soak into Baltic Birch as well as it does in MDF?

I'll be doing a few pieces tonight, so I'll let you know tomorrow. From now on, I'll be using the epoxy as a sealer for any wooden parts I use on any future CNC machines. It's a bit expensive, but worth it to me. Also gives you a nice smooth, sealed surface for paint.

[Rance]

Looks like those bearings will be just fine.

Originally Posted by spalm I am thinking about...making the Y beam out of glue up of two 3/4” pieces of ply sandwiched together. ...Or should I build a thicker torsion box?

Steve,

If you don't have the inner 'ribs', then its not a torsion box and is it nearly as strong, I doubt it. Why are you thinking of leaving them out, is it because of the difficulty of getting the ribs tight? As an alternate method, maybe use dowels for the ribs and lay them in simple bull-nose slots cut out from your table saw. To cut the slots, you effectively angle your fence ever-so-slightly (maybe 5 degrees) and run the panels through. The slots don't have to be all that exact either. Just err on the narrow side so the dowels sit on the edges of the slots rather in the valley. Is this clear or do I need to draw a picture?

Rance

[spalm]

Rance, I think I talked myself into building torsion boxes all around. I was just trying to cut corners. I shouldn’t do that. I have time to build this one a little nicer. Thanks for keeping me honest.

Gerry, I think you are on to something. When I built my canoe , it was out of thin ply that I painted with epoxy. It is very strong. It can take paint or be left as is and looks pretty nice.

Benny, I hear ya. I really haven’t built anything yet. I just picked 8” as the gantry width because I had to pick something. Also the 2 foot X 4 foot footprint fits well into my shop (and I already have leadscrews for this size). I don’t weld, so this will be a bolt together assembly. I think I will build the gantry first.

Not sure what this is good for, but I snipped an interesting mounting use for these bearings out of the BW pdf.

Steve

[ger21]

Originally Posted by spalm Gerry, the only place I am concerned about reinforcing is where the axles go through the plywood. Maybe I could just epoxy that area. Do you think it would soak into Baltic Birch as well as it does in MDF?

Steve, here's whay I see. On the faces of the MDF, the epoxy penetrates about 1/64" (I routed into it to see this). If it was warmer, maybe more, but a light, brushed on coat entirely soaked in. As for the edges of MDF, It will continue to soak in as long as you keep applying it, until it starts to cure.

As for the Baltic Birch, it only really soaks in to the end grain on every other ply. West System says heating the epoxy will result in better penetration, but I didn't try it.

Here's what I think is the best way to do it. Drill the mounting holes oversize with a forstner bit as close to going through as possible, but don't go all the way through. Fill the holes with a thickened epoxy, and when cured, drill for the bolts. This will give you a very solid mount that won't compress.

This is similar to what's recommended by West System, without actually bonding in the bolts. http://www.westsystem.com/frames/tier1/usingepoxy.htm

When I get around to it, my bigger, faster router will use a LOT of epoxy composite construction, from laminated beams (like LVL's) to possible carbon fiber. I've been doing a lot of reading on the subject over the last year. If you can build big, fast boats with it, why not a small CNC router?