[louieatienza]

Thanks for the reply..

I also had this though initially, but after further analyzing the situation I have to disagree.

Assuming that the right angle join between the gantry side plate and gantry base plate is rigid, the linear slide bearing block will experience mostly only vertical and horizontal loads. Only if that right angle join is allowed to flex will the bearing block see a moment force, this is why I have gone to the trouble of reinforcing that join with another aluminum block which is fastened to both the side and base plates.

Am I missing something?

My only thought is it is a bit superfluous... Provided the bolt holea of the LM blocks allow it, it would be easier to bolt the uprights directly to the center of the mounting plate, countersunk bolts centered between the LM bolts. Plus there are less joints which is always better.

[Pysiek]

I have a question. Why a lot of guys who build their own CNC router machine offset the gantry to back? I'm talking about the gantry side plates? Why they are not straight up?

[wendtmk]

But as the OP said, if the joint doesn't flex, then the load is vertical, not twisting. (although technically, everything flexes a little.)
Even though the weight is outside of the blocks, the blocks still see a mostly vertical load on them.

Everything flexes. Steel, aluminum, etc. The moment arm is still outside of the bearings rather than centered over the top of them. The blocks and rails will see both a vertical load and a rotational load.

[LeeWay]

I have a question. Why a lot of guys who build their own CNC router machine offset the gantry to back? I'm talking about the gantry side plates? Why they are not straight up?

This puts the spindle more on top of the bearings for support. It moves the spindle back some too, which may help with travel size.

[JermNZ]

This puts the spindle more on top of the bearings for support. It moves the spindle back some too, which may help with travel size.

Yep, I think it's ideal to have the COG of the gantry centered between the bearing block pairs.

[jfong]

The OP is using THK SSR20's linear tails and from his cad drawing its looks to be XW type carriage blocks.

These can handle considerable loads. They can handle 19.6kN in the vertical load direction with a maximum static moment of 180N-m. That is per carriage block.

With a aluminum extrusion gantry and cutting forces from a 1.5kw spindle, it seems unlikely he will exceed those limits. But I'm not a mechanical engineer so I'll leave that to someone who can calculate actual cutting forces.

FYI I'm using some ebay THK SSR20's in my gantry build and I just took a look at the data sheet to see how much load they can handle. I'm not to worried about exceeding those limits for my CNC which will only be cutting plywood.

[JermNZ]

The OP is using THK SSR20's linear tails and from his cad drawing its looks to be XW type carriage blocks.

These can handle considerable loads. They can handle 19.6kN in the vertical load direction with a maximum static moment of 180N-m. That is per carriage block.

With a aluminum extrusion gantry and cutting forces from a 1.5kw spindle, it seems unlikely he will exceed those limits. But I'm not a mechanical engineer so I'll leave that to someone who can calculate actual cutting forces.

FYI I'm using some ebay THK SSR20's in my gantry build and I just took a look at the data sheet to see how much load they can handle. I'm not to worried about exceeding those limits for my CNC which will only be cutting plywood.

Hi jfong,

Yes I also read these values on the spec sheet, they are indeed XW carriages.

I almost couldn't believe the 19.6kN rated loading, If i'm not mistaken that's 1960 kg per block??!

Regardless the important part we are considering here is the rated moment load which is indeed 180 Nm, which should be sufficient in this situation.

I do agree that in an ideal situation the load would be centered on the bearing block but I have to take other issues into consideration for this design. I have also seen other machines designed in this way which seem to work well.

[Pysiek]

This puts the spindle more on top of the bearings for support. It moves the spindle back some too, which may help with travel size.

I agree on centering the spindle between the bearings but on travel size it still stay the same because you move spindle back which takes the travel distance from front. So no matter what you do , keep gantry side plates straight or make them so spindle is centered ,both options give you same travel distance

[JermNZ]

I agree on centering the spindle between the bearings but on travel size it still stay the same because you move spindle back which takes the travel distance from front. So no matter what you do , keep gantry side plates straight or make them so spindle is centered ,both options give you same travel distance

It can indeed help with travel distance, in the case where the spindle axis is in front of the forward most bearing block.

In this case you will need to have a bed that is slightly longer than your bearing travel distance to gain use of the full travel. If you step the gantry side plates back so your spindle is within the bearing block footprint, the bed length can be reduced, resulting in max cutting length with minimum bed length.

I hope you can understand my explanation.

[Pysiek]

It can indeed help with travel distance, in the case where the spindle axis is in front of the forward most bearing block.

In this case you will need to have a bed that is slightly longer than your bearing travel distance to gain use of the full travel. If you step the gantry side plates back so your spindle is within the bearing block footprint, the bed length can be reduced, resulting in max cutting length with minimum bed length.

I hope you can understand my explanation.

Yes definitely I do. That's how my table is set up. Bed itself is longer the the bearing rails so I can use that front space. So basically it all depends on design. Still no matter what method you use you always achieve same travel distance. You can make the plates straight up and down but then you will have to have that extra long table. Or you can make the plates offset so spindle is located in center between the bearings and have the table little bit shorter.
For me it was easier to make the plates straight because then I can just layout holes , drill and mount it. If I want the gantry to be offset then I will either have to spend numerous hours of fabricate them by hand or go to local CNC shop and pay them some pretty good money to make me two.
But now When I have my CNC running I can make them by myself and maybe upgrade the machine :-)

[wendtmk]

The OP is using THK SSR20's linear tails and from his cad drawing its looks to be XW type carriage blocks.

These can handle considerable loads. They can handle 19.6kN in the vertical load direction with a maximum static moment of 180N-m. That is per carriage block.

With a aluminum extrusion gantry and cutting forces from a 1.5kw spindle, it seems unlikely he will exceed those limits. But I'm not a mechanical engineer so I'll leave that to someone who can calculate actual cutting forces.

FYI I'm using some ebay THK SSR20's in my gantry build and I just took a look at the data sheet to see how much load they can handle. I'm not to worried about exceeding those limits for my CNC which will only be cutting plywood.

Yes, but that 19.6 kN is in the purely vertical. Says nothing about horizontal or rotational loads. Ideally, the load would be directly over the bearing itself, not outboard of it.

[LeeWay]

It really isn't an outboard load. As a rigid structure, the gantry could be virtually anywhere and fastened to the 4 blocks and still be considered a vertical load. Gravity will still push straight down on the bearings. Not sideways.

[wendtmk]

It really isn't an outboard load. As a rigid structure, the gantry could be virtually anywhere and fastened to the 4 blocks and still be considered a vertical load. Gravity will still push straight down on the bearings. Not sideways.

That would be true only if everything were "perfectly" rigid. But it's not. It will flex, and cause a rotational load, if the mass is outside the bearings. There's a moment arm there, however short.

[LeeWay]

You will have that moment no matter where the load is mounted. It is spread over the 4 trucks, so the ones on the other side help to cancel it out. It does need to be as rigid as possible though. You are correct.

[wendtmk]

You will have that moment no matter where the load is mounted. It is spread over the 4 trucks, so the ones on the other side help to cancel it out. It does need to be as rigid as possible though. You are correct.

Not if the load is mounted directly over the bearing trucks. Then the force is purely vertical on the bearings/rails. With the mass mounted outside of, or inside of, the bearing trucks, you will have a rotational force on the bearings/rails.

[LeeWay]

Ah. Don't forget about the Z axis change of direction.

You are thinking the plate will bend. The gantry will weigh the same no matter where it hits a scale at. If the plates are solid and do not give, the trucks don't care where the actual load sits.
On a milling machine, the table spans much farther than the trucks. They do not see the kind of moment you are thinking about. If they do, then it is well within the bearing specs.

[wendtmk]

Ah. Don't forget about the Z axis change of direction.

You are thinking the plate will bend. The gantry will weigh the same no matter where it hits a scale at. If the plates are solid and do not give, the trucks don't care where the actual load sits.
On a milling machine, the table spans much farther than the trucks. They do not see the kind of moment you are thinking about. If they do, then it is well within the bearing specs.

Look again at the renderings in post #10. The plate will flex, and the sides of the gantry will flex. The only concern for the gantry itself is the total amount of mass. How that mass is transmitted to the bearings/trucks is what needs to be taken into account. A typical milling machine uses cast iron, which is much, much more rigid in practice than plate metal, and you are talking about the table, not the head/gantry. Typically, on a milling machine, the table moves, and the head remains stationary, and the mass of the table exerts a vertical and possibly a horizontal force on the bearings. The bearings/trucks, in the OP's design, will see rotational force.

Now, as to whether the bearings/rails can stand that constant rotational force depends on the specs for the bearings and rails. Haven't yet seen what the specs for rotational forces are for the specific bearings and rails the OP is using.

[jfong]

SSR rails can take loads in all 4 directions although mostly in the vertical direction. THK also makes the HSR versions which are 4 way equal load bearings. Size 20 rails are also on the smaller end of what THK makes. Big VMC mills use much larger versions.

Put it this way, a single SSR bearing block can handle atleast a couple hundred pounds in any direction with a 180Nm moment. Cutting forces from his spindle would probably not exceed that.

You do realize 19.6 kN is almost 4500lbs force. Max loads in other directions are less but they are still very significant.

[wendtmk]

SSR rails can take loads in all 4 directions although mostly in the vertical direction. THK also makes the HSR versions which are 4 way equal load bearings. Size 20 rails are also on the smaller end of what THK makes. Big VMC mills use much larger versions.

Put it this way, a single SSR bearing block can handle atleast a couple hundred pounds in any direction with a 180Nm moment. Cutting forces from his spindle would probably not exceed that.

You do realize 19.6 kN is almost 4500lbs force. Max loads in other directions are less but they are still very significant.

Look at the static permissible rotational moment for the HSR bearings/rails. It shows considerably less, at .0844 kN-m, than the 13.5 KN for vertical and horizontal forces.

[JermNZ]

Have made some good progress this week.

Drilled and tapped over 60 holes.

Attachment 273370

Test fit up looks good.

Attachment 273372

Managed to weld the base frame up and it came out pretty well. Holes seem to be within a mm over the whole length so with the play in the linear slide mounts I should be able to get them nice and parallel. This is a bit of a relief as constructing the base was one of the projects biggest hurdles, having had limited experience with this type of relatively high tolerance fabrication.

Attachment 273374
Attachment 273376

Now to find someone to powder coat the frame and it should be ready for assembly.