My CNC Router Build Adventure

[peteeng]

Hi Jayne - use deep engaged threads a full tap depth. That will be strong enough. Plus use long bolts so its stiff in local bending. helicoils are great for repair but they will drive you mad doing that many. Consider through bolting with brackets, will be easier and stiffer and the edge quality won't be critical. Peter

Hi Mactec - then what are all those bolt I can see in the image?

[mactec54]

Hi Mactec - then what are all those bolt I can see in the image?

You are a designer you should know what the bolts are doing, most have no function at all other than holding covers Etc. on, some Robots like these do have different heads that are bolted on, most all the major parts are cast Aluminum and steel machined parts.

Here is a snip of a base for a medium size Robot

[peteeng]

Hi Mactec - The circled bolts are structural they hold the main elements together, hardly "no function". At some level every machine is held together with bolts. If holding rails on with bolts is valid then holding the machine structural parts together is valid with bolts....whether cast parts, machined parts or plate parts doesn't matter...just have to understand how bolts work....Peter

[StrawberryBoi]

Yes, bolting is the method of choice for connecting robot sections across almost all designs. They require regular-ish maintenance and are also frequently upgraded. Bolting provides reliable pre-load against mating features as well as nearly infinite disassembly.

I will caveat that knowledge with that my experience is with medium to small, medical, and specialty robots. I also currently work at a robotics company that is also funded by two large robotics companies. All products across these companies are fully bolt assembled, mostly at joints.

As I've mentioned before though, when precision is required (and these robots are generally only precise in the sense of motion, not absolute mechanical geometry), the industries will build a full rigid assembly with bolts and other methods and then final machine the end to end interfaces in a single op to achieve the tolerance. Specific part stackup is thus not as critical. Even in those cases a bolted assembly can be then unbolted and rebuilt, so long as all fits are true locating OR you re-machine the interfaces post re-assembly.

[mactec54]

Hi Mactec - The circled bolts are structural they hold the main elements together, hardly "no function". At some level every machine is held together with bolts. If holding rails on with bolts is valid then holding the machine structural parts together is valid with bolts....whether cast parts, machined parts or plate parts doesn't matter...just have to understand how bolts work....Peter

I said that the Heads are Bolted on The Head is a miner part of the Robot construction

[mactec54]

Yes, bolting is the method of choice for connecting robot sections across almost all designs. They require regular-ish maintenance and are also frequently upgraded. Bolting provides reliable pre-load against mating features as well as nearly infinite disassembly.

I will caveat that knowledge with that my experience is with medium to small, medical, and specialty robots. I also currently work at a robotics company that is also funded by two large robotics companies. All products across these companies are fully bolt assembled, mostly at joints.

As I've mentioned before though, when precision is required (and these robots are generally only precise in the sense of motion, not absolute mechanical geometry), the industries will build a full rigid assembly with bolts and other methods and then final machine the end to end interfaces in a single op to achieve the tolerance. Specific part stackup is thus not as critical. Even in those cases a bolted assembly can be then unbolted and rebuilt, so long as all fits are true locating OR you re-machine the interfaces post re-assembly.

You only have to look at the auto industry bolts are used everywhere, The main chassis though are welded, the use rivets for same parts also

Yes and the Bolts at the joints do what ???

Most are not a structural element, only where the interchangeable Heads and the Base joints are bolted that are structural

[StrawberryBoi]

EDIT: Deleted to avoid another nonsensical argument cycle unrelated to the OP.

[peteeng]

Hi All - I built a model of Jaynes wall. One is welded the other is bolted. Its steel but if aluminium the answer is the same just more deflection. I used M10 C8.8 bolts to hold it together preloaded to 22kN the nominal preload for a C8.8 bolt. I used a friction coefficient of 0.35 at the connections and ran a full contact non linear analysis. The wall is 350mm high. I applied 300kgf in the middle of the top. Unlikely that Jaynes machine will see this sort of load. Non of the connections slip. I also left the face of the wall off to remove the flexural stiffness from the model. So the wall is a shear beam which loads the connections higher. The bolted wall has a 2mm gap between the webs and the top/bottom plates so the load travels entirely through the bolts and its faying surfaces ie no mechanical affect do to edge contact. I've used nuts but the answer would be the same using thread holes.

In practice I'd do this a little neater, I'd make the brackets a single piece vs two and shape things a bit rounder. But the model is solid and accurate, apples to apples.

The coefficient of friction for steel/steel is 0.35 and Al/Al is near 1.0. The software max friction allowed is 0.8 for some reason...

The first model the welded one did better. This is because of the web warping at the ends as it is supported assymetrically. So I put on a small plate at the end in Model 2. Welded deflection Model 1 0.212mm bolted 0.297mm Model 2- 0.212mm to 0.203mm so its a tad stiffer. So from this model I see no reason against bolting.

bolted model 2 - deflection of wall including end support
distorted - showing excessive warping of end webs
slippage - showing zero slippage in the connections
unsupported end - showing deflection of welded 0.212mm and bolted 0.297mm which can be fixed by edge stiffening or using symmetric supports...

Using a face bolted structure allows adjustment of alignment at the build and any time fwd of the build. It lowers the edge accuracy requirement to being able to be done hand routed, it allows mixing of materials eg steel and aluminium to take advantage of their properties (eg stiffer steel brackets vs al), its a damper construction vs welded. I'll leave it at that....

I've always considered bolted construction to be 50-75% efficient but maybe I have to revise this upward a bit.

Everyone chooses different paths for the build and that's their prerogative. Just would like to say there's more than one way to do something successfully.

Peter

[JayneV]

Last night I added some brackets to the wall uprights to visualise how to bolt the parts together. I have also added a single piece end plate to the front and rear of the machine for additional stiffness between the left and right wall sub-assemblies. I don’t know how much benefit the end plates will provide.

I’m still not happy with gantry beam. I’d like to experiment with a built up box section design made from flat bar. See attached for a cross section view of the beam. That is a very rough concept sketch made in a drawing app on my phone, not drawn to any scale. It needs a lot of refinement. You will notice in the sketch the X axis linear rails are placed above and below the beam. The reason I did that was to reduce the amount of overhang of the Z axis. This might cause problems with getting the rails aligned in space if the top and bottom surfaces of the beam are not exactly parallel along the beam length. Any thoughts?

[peteeng]

Hi Jayne - lovely brackets. The bolts need to be as close the ends of the webs as possible and as close to the corners as possible. Ensure it can be assembled, crossed bolts can be tricky. Maybe have inboard holes offset from outboard so they can be closer to the edges.

Placing the rails on top and bottom implies that you will have the vertical web edges machined and the top and bottom plate is machined at the lands (or faced flat) as well. Its unlikely you can hand route the edge to the tolerance required to not have the cars jam. But I can see a theme appearing and thats good...

The top and bottom approach is valid but you loss out with the saddle bearing access, usually means going above and below the gantry bearings, thats your next step, sort the saddle... Peter

[JayneV]

Hi Peter,
Thank you for your continued support and advice. I’m not following what you mean with a couple things in your last post, could you clarify?

(1) Is this what you meant about offsetting inboard and outboard holes in the brackets? I’ve previously had trouble assembling brackets with a cross bolted arrangement. My work around was going to be careful selection of fasteners, such as using bolts and nuts with low profile heads and ensuring bolt lengths are no longer than necessary so that they can be located as close to an ideal position as practical.

(2) I didn’t understand what you meant in the last paragraph about saddle bearing access.

To help me out with correct terminology, is the saddle the fixed part of the Z axis as shown in the attached image?

Jayne

[peteeng]

Hi Jayne - to have the bolts in the same plane is ideal as then the loadpath is straighter. But this means the holes have to be further out from the corner so the bracket has to be thicker to accommodate that stiffness loss. I'd have the low holes out and the inner holes in rather then staggered. I think a thick bracket with the holes as far apart as possible allowing assembly is the best solution... or use bought brackets with webs they are quite stiff due to the web.

Yes your correct with the saddle and yours even looks like a saddle. the carriage or saddle is usually the bit that takes bearings in two directions.

Its ideal on the saddle to have the opposing cars opposite each other but then how do you get access to the bolts? You can do this by using flanged bearings and use the small side for one set and the large side for the other set of bolts but this is complex and means you cannot get access to some sets of bolts unless you pull down the machine to get at them (how do I know that?) . The usual remedy is to have the saddle longer then the inside bearings so the outside bearings have bolt access clear of the gantry. You will figure this out once you detail out the saddle and consider how to deal with the bolts. You can also go sideways but then the saddle gets wide and the you lose travel.. All a juggle for you to wade through...

https://www.journeymans-workshop.uk/lathemill.php

Peter

The saddle is an interesting (meaning painful) part. I have tried to remove it from various designs unsuccessfully. A lifting gantry config will remove the saddle. Structurally it plays a small role yet is important as it holds the bearings. Its sort of a middleman that takes a border tax but sits back most of the time not doing much.

[JayneV]

Hi Peter,

Thank you for explaining what a saddle is and it's purpose. I like your tax collector analogy, there is always some kind of tax to be paid in life, even within the design of a machine.

I had the same thoughts about using bought brackets for their built in webs. Is there any advantage or disadvantage to using one single, wider bracket compared to two narrow brackets? The brackets I drew up are made from 25x25x6mm Al angle cut to desired size.

The configuration you described for the saddle sounds like both the X and Z axis carriages are mounted on the saddle. I have seen machines set up with the Z carriages mounted to the saddle and others with the rails mounted to the saddle. Is it important to have easy access to the carriage bolts after it has all been assembled?

Jayne

[peteeng]

Hi Jayne - I'd have to model the two, to figure which is better. But my guess is webs win. But choose which you like for now, you have bigger fish to catch. Yes good observation on the cars. I prefer to have the rails on the Z axis side vs on the saddle this puts the 4 cars on the saddle. If the z axis is high I find it better to have the rails on the Z. I think the standard arrangement is to have the front of the saddle with the rails. But you will figure this out as you go. A machine settles in and you will need to pull up the bolts after it runs for a while. Sort of a first service. Ideally all bolts are accessible so they can be checked as needed. They can loosen under service loads and vibrations until it really settles in. Peter

edit - in my first machine there were some blind holes in the saddle. These were deep in the machine which means to get to them required the z axis to be removed and broken down. The machine developed a wobble so I pulled it down and checked the torque of all the bolts and rebuilt. No change, so pulled it down again and removed all screws and retorqued. Turned out the bolts had bottomed in the hole so although they torqued correctly it did not quite pull the cars up. So try to use clear holes wherever possible so bolt length does not matter and try to have access to nuts or screws so they can be checked as easily as possible...

[StrawberryBoi]

It's a good practice, to avoid what happened to pete in the blind holes, to allow an excessive amount of space beyond the end of the bolt in your threaded holes and then to also measure the bolts you purchase to ensure you were supplied the correct lengths. That or of course, allow for through holes and bolt length to vary.

I've had holes that were a full diameter of excess thread depth and then someone assembled it and it turned out the bolts McMaster sent us were mislabeled and were too long. Even an institution that almost never makes errors like McMaster can goof on the bolt lengths. Fastenal made a similar error for us before, where we ordered a bulk quantity of 1/4-20 bolts and there were a few dozen short and long ones in the bulk package of thousands. It only happened the one time, but it's shockingly easy to miss in your excitement to assemble something and you end up with engineers wasting company time trying to figure it out...

[mactec54]

Last night I added some brackets to the wall uprights to visualise how to bolt the parts together. I have also added a single piece end plate to the front and rear of the machine for additional stiffness between the left and right wall sub-assemblies. I don’t know how much benefit the end plates will provide.

I’m still not happy with gantry beam. I’d like to experiment with a built up box section design made from flat bar. See attached for a cross section view of the beam. That is a very rough concept sketch made in a drawing app on my phone, not drawn to any scale. It needs a lot of refinement. You will notice in the sketch the X axis linear rails are placed above and below the beam. The reason I did that was to reduce the amount of overhang of the Z axis. This might cause problems with getting the rails aligned in space if the top and bottom surfaces of the beam are not exactly parallel along the beam length. Any thoughts?

A build like this Bolted together, you have to many joints, the angle iron Bracket / Cleat would want to be welded at least for one of the connections, so the angle iron could be welded to the uprights, this would cause almost zero distortion, and eliminate ( 1 ) bolted joint, unless this is a semi box structure it will be a fail, look at my build from 10 years ago to see how each part is joined to the next to form a bolted structure, computer simulation is good but is not the end result, which can be a fail as well.

https://www.cnczone.com/forums/cnc-w...-friend-3.html

[mactec54]

It's a good practice, to avoid what happened to pete in the blind holes, to allow an excessive amount of space beyond the end of the bolt in your threaded holes and then to also measure the bolts you purchase to ensure you were supplied the correct lengths. That or of course, allow for through holes and bolt length to vary.

I've had holes that were a full diameter of excess thread depth and then someone assembled it and it turned out the bolts McMaster sent us were mislabeled and were too long. Even an institution that almost never makes errors like McMaster can goof on the bolt lengths. Fastenal made a similar error for us before, where we ordered a bulk quantity of 1/4-20 bolts and there were a few dozen short and long ones in the bulk package of thousands. It only happened the one time, but it's shockingly easy to miss in your excitement to assemble something and you end up with engineers wasting company time trying to figure it out...

Threaded holes should only be the depth for max strength needed for the bolted joint, or fastener being used, any deeper would be a fail.

Why blame McMaster, the inventory check on arrival would of found this, or if the person who was assembling the parts should of also checked that he / she had the right bolts for the job.

[StrawberryBoi]

Your close mindedness is always fascinating.

[mactec54]

Your close mindedness is always fascinating.

Your lack of basic engineering principals is even more fascinating, you should stop pretending to be what you are not.

[JayneV]

I appreciate all comments. I am here to learn and share my journey with other like minded people. Please keep any type of bickering out of this thread. There are always multiple solutions to any one problem and hearing different opinions is very interesting and informative. Healthy debate is good and I welcome it, all I ask is that you are all respectful.

StrawberryBoi, thanks for the tip about double checking bolt lengths before installation. It would be difficult to track down a poor connection due to a bottomed out bolt in a blind hole. I can see how it would be easy to pickup an incorrect length bolt out of a box of hundreds and not notice it’s the wrong size.

Mactec, I may have not mentioned it before, the current design with all those brackets is aluminium. I could build the same design out of steel with welded connections, eliminating most of the brackets. There are valid reasons for choosing either material. I actually have two copies of the model in CAD, one for an aluminium base and one for a steel base. For the steel base, I am planning to take Peter’s suggestion and use tabs and slots to locate and weld each part. I need to update the steel version of the model to include the tabs and slots then I’ll generate 2D drawings of the parts to send to laser cutters for quotes.

I don’t have a problem with using bolted connections. I work on large aircraft and the entire wing is held on by a handful of bolts and these bolts remain untouched after installation for the life of the aircraft unless there is an unusual circumstance that warrants their removal. Why do you say that threaded holes deeper than what is need for max strength if the joint is a fail? Surely the hole would need to be a little deeper than the max depth of the bolt to eliminate any possibility of the bottoming out. Could you clarify what you meant?


Jayne