Decided to finish my CNC machine this summer.

Size will be 8x4. Servos and control box I bought from Anoel. I hope to be making chips by the end of July.

This first pic is a cross section of the possible y-axis. The roller bearings are in purple.
Main frame is blue
Guide frame is in red
and supports in black

I didn't draw the face plate which will hold the router mount this is attached between the two black angle pieces.

X-Axis is the same style only the bottom red and black pieces won't be there. Should have a more complete drawing this weekend.

I would like to take this time to say thank you to everyone. I've been reading posts for the last 2 years on this site and the community is one of the better ones on the net.

-Matt

I dont get it, I cant not see how anything rolls, could you please clarify?
colin

It looks to me like the pink parts are skate bearings which are secured by the green bolts, they roll on the red angle plate.

The purple rectangles are skate bearings. And the green are bolts, like Splint mentioned. I'll be working on it this weekend so hopefully I will have some real pictures to post.

I'm going to pick up new x axis framing today.

Finally an update--

I scrapped the original design and went with this one using roller bearings. I have a question if I eliminated the lower guide bearings (red) for the yaxis. For those of you with similar designs are you having any problems with the router wanting to lift off the rails?

Let me also add that the wood base is just a support for right now, once the machine is about half finished the base will be built with aluminum.


Thanks, should have another update next week.

I decided to work on the drawings during the winter here's a preview of them so far:

Ok, making an update. As you can see I went back to my original design with the angle aluminum for the gantry ends.

The problem with this design is that the gantry won't slide smoothly the bearings don't properly align with the rails and when pushed down the rail cause the gantry to either grind down into the rail or raise up off it. I fixed this with a second set of inside bearings and a new problem arose.

The new problem is when the gantry is pushed from one side there is "backlash" in the opposite end and it lags slightly.

Another issue is the angle steel that the bearings run on tends to flex a little when I do this as well. It is entirely possible that the steel angle is flexing because it is only bolted on the two ends for now. But I tested a piece by hand and even over a one foot range it still flexes. Currently it's 2" x 2" x .125" I might switch that out for thicker 1/4" angle.

After looking at some machines on here I realized I made a HUGE mistake in design. The large machines tend to use manufactured linear rails of some type while the smaller machines use a mixture of DIY/Manufactured bearings. I am thinking that the bigger things go the tighter tolerances you have to get.

My next step from here is I am going to machine some V bearings and try those on the two inch angle. If that works I'll buy some stainless and machine V bearings for all my bearings and do something similar to shopbot design. My concern though is after all that work, the backlash problem from above will surface again. But this will eliminate the question I had earlier about gantry lift.

Shopbot seems to over come this problem of backlash from a wide gantry design, so I may end up making my gantry supports 6" wider (the current alu angle you see in the pictures is about a foot wide, you can also see in the last autocad pic the entire design.)

More pics coming this weekend.

-Matt

Shopbot seems to over come this problem of backlash from a wide gantry design, so I may end up making my gantry supports 6" wider

The ShopBot is driven from both sides, so you can't get the skewing of the gantry you're seeing. My machine is the same way, and a lot of others are noticing it too. Bottom line seems to be that you must drive the gantry from both sides to prevent this. I'm pretty sure even a very solid and heavy steel gantry would do the same thing to some extent, unless the bearings are spaced very far apart, maybe 2 feet or so, with good linear bearings.

Gerry,

Does shopbot have a motor on each side? I thought issues with phase difference were problems with that?

Do you think the same problem would happen with linear pillow block style bearings? (problem is trying to find some 10 feet long)

-Matt

Yes, they have a motor on each side, and yes, I think it will happen with "real" linear bearings. Although with linear bearings, if it does it it will possibly bind up.

Hello,

what was wrong with the design from Post #5? Looks quite good to me...

k

Keyne,

The problem with that design was that I couldn't ever get the bearings to set straight on the rail. The lock washers cause the bearings to get cocked when the nuts were tightened down. I also designed the gear rack to be on the outside so there won't be any way to stop the gantry from lifting off.

Gerry,

Your gantry skews as well?

Adding another motor to the opposite side is out of the question. I don't want the added headache/cost/etc. (or at least for the moment let me dream that is won't come to that)

I have some pillow blocks with roughly 4 feet of linear rail that I'm going to try out tomorrow. I would think if the gantry does this on only a 1 foot section, vising the rail down and the blocks to the gantry will be able to tell me.

If that doesn't work next thing will be to fire off some emails to people with large machines and see how they eliminate it. Or I wonder if it's possible to "dial" it out on software side.

-Matt

Rather than drive it from both sides, a cheaper idea might be to use cable and pullies to stop the racking. You would proberbly have to make at least one pully adjustable in its centre point so you could keep the cable lengths exactly the same.
Two cables.
Ends are attached to gantry.(red)
Cable starts on one end of gantry and terminates on the gantry diagonally opposite.
The blue circles are the pullies fixed to the corners of the table.
When setup correctly with quite a bit of tension the gantry can only rack by streaching the cables. You would use a couple of turnbuckles to tension the cable.
The cable lengths must match or it will rack the gantry automatically, so you would need to tune the setup.

I've seen this idea used to control the tracking of objects in the past.
It looks like you would be easily able to mount something like this under your table.
What do you think? Would it work for something of this size?
Paul

Gerry,

Your gantry skews as well?



Yes, it does, and it doesn't take much force at all. I have always planned on 2 motors, though, so it won't be an issue for me. But I was quite surprised how flexible the gantry ended up being.

Paul,

Could you go into a little more detail, I'm trying to understand your pic, but I'm just not getting it ;) Where do each of the pulleys attach?

Gerry,

What were you going to use for the controller and stuff? Who makes a 4 motor controller board? Or can you just jumper two motors in parallel into the geckos?

Maybe I should make a smaller machine, then as money permits add to the larger one.

Paul,

Could you go into a little more detail, I'm trying to understand your pic, but I'm just not getting it ;) Where do each of the pulleys attach?



Edited the post with more detail.
Paul

I've got Xylotex drives, a 3-axis + a single axis.

Here is a picture of what Paul was talking about maybe this will illistrate it a little better.. In this situation id does not matter where the leadscrew is placed.

I've seen this idea used to control the tracking of objects in the past.
It looks like you would be easily able to mount something like this under your table.
What do you think? Would it work for something of this size?
Paul

This looks great! Now I've decided to go with a single center screw for sure, and if the other things I've come up with to limit the gantry twisting aren't adequate, I'll add this. This, I am sure, is only as good as the base that it is tied into is solid, and mine is built like a tank.

Thanks for the response Matt. I will also go with Vee-rollers, but making a good rail is not that trivial either. Early shopbots used the cable drive system.. i figure there must be a reason why they went to dual r&p. i will go with dual belt drive.
K

Here is a picture of what Paul was talking about maybe this will illistrate it a little better.. In this situation id does not matter where the leadscrew is placed.
Thanks Joe.
Nice drawing. Much better than my quick paintbrush effort.
Paul

My old drafting table used a cable to keep the Tsquare square. Same concept. It did not use the crossover in the center. I don’t see the advantage of crossing the cables. Guess I am missing something. Anyone tried this?

Steve

My old drafting table used a cable to keep the Tsquare square. Same concept. It did not use the crossover in the center. I don’t see the advantage of crossing the cables. Guess I am missing something. Anyone tried this?

Steve

My guess is that the drafting table needed a "cable free" drawing surface. If I remember those dino's right they routed the cables around the perimeter of the drawing surface. It seems to me that the advantage of crossing the cable (I would just use one) would be that there would be a shorter cable length and therefore less potential cable stretch.

Also, if you use a drafting table model you will have to use double pulley blocks on 2 of the corners.

Huh ?
One single cable around the perimeter, single pulley block in each corner, no crossover = shorter cable, less stretch potential.

Must be some other reason.

(Dinosaurs do have their instructional purposes.)
Steve

Spalm,

The reason they are crossed is so that if the gantry is pushed on one side the other side is pulled via the cable.

I've gotta make a few phone calls today and see if I can't come up with another motor this seems to be the easiest way of dealing with the whole situation.

Huh ?
One single cable around the perimeter, single pulley block in each corner, no crossover = shorter cable, less stretch potential.

Must be some other reason.

(Dinosaurs do have their instructional purposes.)
Steve

I don't see any way to do it with "One single cable around the perimeter, single pulley block in each corner, no crossover". :confused:

My bad, Guess I was up too late last night and my mind was stuck on that drafting table in which the Tsquare slid on the cable. Yes you need the cross over.

Thank you for not laughing at me,
Steve

Pics of the controller and one of the servos