ok, first attempt using sketchup 8

[jcb121]

My advice on sketchup, I see you are scaling the supported rail and the mounting blocks as one,

If you double click the entity, then you will be able to select them separately and then select the rail and press "S" then drag it to the length needed.

[deannos]

My advice on sketchup, I see you are scaling the supported rail and the mounting blocks as one,

If you double click the entity, then you will be able to select them separately and then select the rail and press "S" then drag it to the length needed.

Thanks for that tip, i thought it didn't look right but couldn't put my finger on it

[routercnc]

Hi Jazzcnc,

I totally agree with you that I beam would be sufficient for cutting wood. I just wanted to show Deanos the improved performance of RHS to explain Jonathan's point about how it could be better if you wanted to make it do more in the future.

I also can't fault your logic and argument on the cost side because that is very important for a home build.

Just for completeness, and not to start a war (!) I've done the calcs for the sizes you mention:

___________________I beam____RHS
Width _______________100______80
Depth _______________180_____160
Flange/wall thickness ___8_______6
Mass (kg) ____________34______32

Tool deflection (mm)______0.009___0.00067

[all assuming 150mm between gantry centre and tool cutting tip - the moment arm trying to twist the gantry]

So the I beam still looks reasonable.

[JAZZCNC]

Hi Jazzcnc,

Just for completeness, and not to start a war (!) I've done the calcs for the sizes you mention:

" For to win one hundred victories in one hundred battles is not the acme of skill. To subdue the enemy without fighting is the acme of skill." (Sun Tzu)

It's Aussie day so no worries Mate.

[deannos]

" For to win one hundred victories in one hundred battles is not the acme of skill. To subdue the enemy without fighting is the acme of skill." (Sun Tzu)

It's Aussie day so no worries Mate.

I count myself part Aussie, having done nearly all my schooling there, :tup:

[Swarfing]

Looks like bloody good odds to me especially from such a simple and quick build

[deannos]

Looks like bloody good odds to me especially from such a simple and quick build

I agree, that's why i've decided to go the RSJ route, especially as i don't have any fancy tools. But i can weld and this will be far the easiest route for me, and there won't be a lot of drilling and tapping either. The only thing that worries me slightly is the weight of the gantry, i know weight is a good thing, but would this be to much to get decent speeds. Now i'm not looking to break any speed records, just to get a reasonable feed etc. The motors i have at the moment are nema23-3NM and MSD542 drivers with 50v power supply.

Dean

[Jonathan]

I also can't fault your logic and argument on the cost side because that is very important for a home build.

Same here. Thought that it was obvious that you just make it as strong as you can afford. I made mine as strong as I could afford at the time, but not as well as I could and I've been waiting for the funds to improve it for some time.

One last thought:



You could weld a steel sheet (red) on to the back of the RSJ (blue). Then you've got the torsional stiffness of box section and the convenience of RSJ. It needn't be very thick, I think 3-4mm sheet would be fine. Adding several of the green triangular pieces might help, but it's a lot of effort to cut them out without a guillotine. Only problem with all this is the welding might distort the top surfaces, so it's best to do it from the start.
Other option is use box section but add plates on the top and bottom so that you can put the linear bearings off center to get the ballscrew in a good position. Not sure which would cost more...

The only thing that worries me slightly is the weight of the gantry, i know weight is a good thing, but would this be to much to get decent speeds. Now i'm not looking to break any speed records, just to get a reasonable feed etc. The motors i have at the moment are nema23-3NM and MSD542 drivers with 50v power supply.

Dean

[deannos]

Adding several of the green triangular pieces might help, but it's a lot of effort to cut them out without a guillotine.

I have a very nice angle grinder that would do the job.

Other option is use box section but add plates on the top and bottom so that you can put the linear bearings off center to get the ballscrew in a good position.

You mentioned earlier about the ballscrew being in a good position, i'm not to sure in what you mean. If you mean to bring it out closer to the Z plate, could i not use spacers on the bearings

[Jonathan]

You mentioned earlier about the ballscrew being in a good position, i'm not to sure in what you mean. If you mean to bring it out closer to the Z plate, could i not use spacers on the bearings

The best position for the Y-axis ballscrew is between the Y-axis rails so that there can be no movement of the linear bearings without the ballnut moving. More precisely the ballscrew should lie on the plane occupied by both the Y-axis rails, parallel to the rails. With the RSJ design it's close enough, with the previous design using two 80x40 box sections and 10mm plate it would be perfect.

(meant to post this in the previous post)

The only thing that worries me slightly is the weight of the gantry, i know weight is a good thing, but would this be to much to get decent speeds. Now i'm not looking to break any speed records, just to get a reasonable feed etc. The motors i have at the moment are nema23-3NM and MSD542 drivers with 50v power supply.

As long as you use RM1610 ballscrews to move the gantry it will still be fast due to the higher pitch and more importantly get better acceleration than RM1605. You'll want a 1:1 ratio with 10mm pitch, but I used pulleys anyway to reduce resonance. You can always swap the pulleys round to get a 1:2 ratio for better resolution. If you get the end machining for the ballscrew where the pulley mounts made 8mm, same as the motors, instead of 10mm then swapping the pulleys is simple. For instance use two 15T on Y and 30T on X you can swap them all round to get 15:30 if required.
Also I got the end machining for the pulley made 25mm long instead of 15mm as it makes it much easier to align and mount the pulleys.

RM1605 may still be fast enough, but when you're getting them from linearmotionbearings2008 on eBay I don't think there's much if any difference in price between the two so no point. Especially since you can use pulleys to get effectively the same pitch (if required, for the vast majority of things it wont be) as RM1605 with better v&a as the screw is only spinning half as fast.