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Dan_the_Chemist
02-28-2019, 02:36 AM
Still designing my CNC router.

Modern airplanes are held together with a combination of mechanical fasteners and adhesives. I am looking at this component and wondering if there is any benefit to using both screws and adhesive to build this component. It is the cradle for the water cooled router. There are more features to be added, but let's just focus on the strength and rigidity of the aluminum to aluminum joint shown.

The goal is to create a part which will deform very little under the application of a transient sideways load. Would a modern adhesive actually help, or would I just be wasting time and money?

https://i.imgur.com/t4IgBzD.jpg

epicfail48
02-28-2019, 04:28 AM
Personally i cant say id see any adhesive offering anything the screws dont, but im also an amateur hackjob so make of that what you will. Rather than adhesive, have you considered adding some pins? Screws for attachment, pins for positioning and to lock down movement

bandsawguy
02-28-2019, 06:47 AM
I would say no to glue but I like the idea of say 4 dowel pins to positively lock it in place.

CCWKen
02-28-2019, 07:05 AM
My question would be; Why is that part two pieces? Trying to save a buck on thinner stock or it's necessary for the design because something is held between the two parts we can't see? :cool:

Eliminate the glue and make the cradle walls thicker. The "glue" won't help where you're going to have the sideways load. And why six fasteners? :confused:

What's the scale of this part?

RMinMN
02-28-2019, 07:15 AM
With the screws fitted in through clearance holes you are dependent on the friction between the two surfaces to resist motion. Dowel pins eliminate that as the way to resist the motion but adhesive will do the same thing and perhaps make an easier assembly but much harder to take apart again. You rely on the screws to keep the parts together but the adhesive will be in sheer mode and will take a lot to break that.

Dan_the_Chemist
02-28-2019, 09:36 AM
My question would be; Why is that part two pieces? Trying to save a buck on thinner stock or it's necessary for the design because something is held between the two parts we can't see? :cool:

Eliminate the glue and make the cradle walls thicker. The "glue" won't help where you're going to have the sideways load. And why six fasteners? :confused:

What's the scale of this part?

Scale: 1/4-20 bolts. The diameter of the cradle is 3.25 inches.
Why six: Seemed like a good number.
Why two pieces: I get my aluminum from an industrial recycler who gets drops and cutoffs and so I tend to get smaller bits. I have a supply of 2" thick by 4" wide to make the cradle, and 3/4" thick plate is relatively easy to come by. I don't have much that is thicker than 2"




Dowel pins eliminate that as the way to resist the motion but adhesive will do the same thing and perhaps make an easier assembly but much harder to take apart again.



I would say no to glue but I like the idea of say 4 dowel pins to positively lock it in place.


Dowel pins: Yes, good idea. That's going to be incorporated.

TGTool
02-28-2019, 09:37 AM
I would say no to glue but I like the idea of say 4 dowel pins to positively lock it in place.

If it's to prevent movement, two dowels is enough. If repositioning to close tolerance is important they should be as far apart as possible. Two diagonal dowels is traditional and placing them not symmetrically will insure the parts can't be reassembled in more than one position.

David Powell
02-28-2019, 09:56 AM
While a rather different application, my results are good. I built a water tank to fit under the boiler of my 2" scale model steam roller .
I used 1/4 thick plastic, forget exactly what now, it was grey.
It is screwed together with 4/40 brass screws at about 1" centres. Once I had done a trial assembly I dismantled it , ran epoxy on all jointing surfaces and reassembled. It has been in service for over 20 yrs, has been hit on gateposts, dropped on kerbs and even has a small vice bolted to it to help in " field" repairs and it still does not leak os show signs of problems. Hope this is encouraging David Powell.

cameron
02-28-2019, 10:01 AM
Two dowels will have the shear resistance of two dowels, which may or may not be enough.

The shear resistance of two dowels may be less than the shear resistance provided by the clamping pressure of six screws.

There is nothing magical about the fixity provided by dowels, any more than the fixity provided by fitted bolts or rivets.

Generally speaking, dowels are used to provide initial location, and resistance to external forces is provided by screws, bolts, clamps, adhesives or other means.

Two dowels is enough to provide location in one plane. Sometimes, as in fixture design for instance, two round dowels is too much, and one dowel is oval or has two sides relieved. The round dowel locates the part at one point and the other prevents rotation around that point.

Lew Hartswick
02-28-2019, 10:02 AM
Use only 4, 1/4 - 28 screws and 2 dowel pins :-) (see the coarse vs fine thread post) :-)
...lew...

BCRider
02-28-2019, 11:21 AM
It should be one piece as mentioned. There's no need that I can see to take it apart. But if you're using material you can get easily and for free/cheap and have these sizes on hand then that's more than reason enough to do it this way. While it's fine as is I can see what the others are saying about dowel pins. Especially if it'll never be taken apart. If that's the case then sure, stick some form of industrial adhesive intended to work well on aluminium between the parts. It would take the place of the dowel pins being suggested. Just pay attention to the adhesive maker's instructions on use for aluminium. It can be tricky material at times after all. And due to that you may want to consider a caustic dip to lightly etch the surface and alodine as a resistance to corrosion. But again whatever the adhesive maker recommends for surface prep.

If that's too much bother than for the size of it and given what sounds like fairly light loads I'm thinking four screws and two pins to ensure it stays. But 6 certainly won't take much longer or break the bank for cost of the project.

bborr01
02-28-2019, 11:36 AM
Use only 4, 1/4 - 28 screws and 2 dowel pins :-) (see the coarse vs fine thread post) :-)
...lew...

+1 on this design.

reggie_obe
02-28-2019, 11:58 AM
Use a single piece of steel, stiffer, cheaper.

Baz
02-28-2019, 01:22 PM
A brittle epoxy alone on bendy metal will be prone to a crack suddenly letting the whole thing go. Fixings alone in a vibrating or repeatedly varying load might progressively fret their holes and end up loose. A belt and braces approach does more than double the resilience.

greystone
02-28-2019, 01:48 PM
Depends.
In theory and practice good glue and good surfaces will be stronger than needed.
Much.

2 drilled/reamed dowel pin holes later on cannot hurt.

3 Phase Lightbulb
02-28-2019, 01:51 PM
Screw it, or screw that idea and just weld it together and machine square.

CCWKen
02-28-2019, 02:07 PM
What will that cradle assembly bolt to and how is the router to attach to the cradle? I don't believe you're thinking this through. You must have taken design classes from Brian R. :D

If you're limited to scrap, use two 2x4 pieces one above the other with the mounting towards your screws. Bore vertical holes, instead of a cradle on the other end. Use the holes to mount your router with a clamping action.

Added crude drawing:

http://bbs.homeshopmachinist.net/attachment.php?attachmentid=4740&d=1551381734

Paul Alciatore
02-28-2019, 05:32 PM
Screw and glue is recommended as one of the best, strongest joints for wood. Wood is not as rigid as metal: even hardwood is softer than aluminum. Screws in a wood joint are not typically stretched when tightened or if they are, it is only by a tiny bit. The wood fibers will compress at the threads or under the head of the screw, preventing that. If you continue tightening the screw the wood will usually fail before the screw stretches to provide much tension. And repetitive forces on the joint can gradually compress those wood fibers making the joint loose. So a wood joint that uses only screws will have more freedom to move and become loose. The glue prevents that while the screws provide good contact while the glue dries and additional resistance to failure for the life of the joint.

With metal, including aluminum, the screws can be properly torqued producing a bit of stretch in them. This pre-tension will remain for the life of the joint, holding the metal parts in good contact. This is usually enough to prevent movement in the joint.

I doubt that glue would add much to a properly designed and assembled, metal to metal joint. That being said, it may be desirable in some cases. Those cases would probably be determined by prior failures.

Yondering
02-28-2019, 06:14 PM
Modern airplanes are held together with a combination of mechanical fasteners and adhesives.

Actually, modern cars and semi-truck bodies are built that way as well (typically some form of rivet rather than bolts). Most of the reasons for doing it that way don't really apply in your case (productions speed, fastener choices, efficiency, cost, etc) but for creating a strong joint, it'll work well for you. If you prefer, you can choose to use fewer or smaller fasteners if you're also bonding the pieces together.

I would suggest some sort of toughened epoxy, which will have a little bit of give to it to account for flex, while still retaining high strength. Surface prep is important of course.

Seems to me that you've got nothing to lose by bonding those two parts together. I would do it, if it were my choice.

gellfex
02-28-2019, 06:23 PM
I'm in the dowel pin camp, but if you really want to waste some time you can mill X shaped slots on one side and a corresponding raised shape on the other. I've done it in certain circumstances, usually a rod that I didn't want to rotate into a plate held by one screw.

darryl
02-28-2019, 06:54 PM
If that isn't supposed to come apart, it's a no-brainer for me. More than one aspect of it will be enhanced by using a bonding agent. It appears to me that the bolts are meant to be installed and left, otherwise no need for the recesses. As such, each bolt will become a dowel pin by virtue of being surrounded by the bonding agent which prevents any sideways movement. I would caution to not over-tighten the bolts. JB weld that baby.

From the length of the bolts, and the fact that the heads are recessed, it's obvious that something else will be in that sandwich. I'm guessing a copper coil flat pack through which a coolant can be pumped. A bonding agent will largely improve the heat transfer ability.

cameron
02-28-2019, 07:08 PM
IF you don't need dowels for accuracy of initial assembly and subsequent re-assembly, don't use them. That would be a waste of time. Just use an adequate number and size of fasteners to take the loads.

Mindless addition of belts and braces is not an indication of good design, it is an indication that the designer doesn't quite know what he's doing.

PStechPaul
02-28-2019, 11:51 PM
You might try some high bond double sided adhesive tape:

https://www.mcmaster.com/fastening-tape

I had a product where a shunt with a bakelite base was mounted on a powder coated electrical box, with two #10-32 screws and nuts, and the torque on the current carrying bolts would cause the shunt to twist. I used some ordinary double sided foam mounting tape and it held very well.

johansen
03-01-2019, 12:12 AM
forget the dowels. you may want the slop in the bolt holes for tramming the spindle.

J Tiers
03-01-2019, 12:28 AM
I'm kinda liking the suggestion CCWKen made. I see a lot of similar machinery made that way, and it is a good, positive holder. You can put the pieces on a backplate and then bore the holes, if alignment is key, although for the router application, really close lineup may be overkill.

PStechPaul
03-01-2019, 01:25 AM
Taper pins might be better than dowel pins. No need to bore or ream the holes so accurately.

https://www.mcmaster.com/=1gzpe0n

https://www.mcmaster.com/mvB/Contents/gfx/ImageCache/983/98390A163p1-b02-digitall@1x_636616341650329395.png?ver=ImageNotFou nd

elf
03-01-2019, 04:38 AM
CCWKen in post #17 has the best solution. It also happens to be the way I made mine :cool: It's a lot easier to bore a round hole than to mill a round bottom slot.

ulav8r
03-02-2019, 01:35 AM
Two screws to hold in alignment, then tig weld it.:cool:

Punkinhead
03-02-2019, 08:50 AM
Dowels are for locational accuracy during assembly/re-assembly. Dowels are hardened and brittle and can break when shear loaded. In the machines I design I don't put dowels anywhere that they might see shear loads. For example, just yesterday I was re-using another engineer's design for hanging a SCARA robot from the roof of a machine. He used dowels to locate. Not only are the dowels likely to shear when the robot is working, but the guy assembling the machine is likely to break one off just lifting the robot into place and getting the screws started. I removed the dowels from the design and will use precision machined edges to align the robot to the mounting surface.

3 Phase Lightbulb
03-02-2019, 09:44 AM
Screw and glue is recommended as one of the best, strongest joints for wood.

Screws are for terrible hacks :)

The wood itself should provide the joint strength aided by some glue to help keep the joint from releasing.


https://www.youtube.com/watch?v=UDQ_aS8qvaU

DICKEYBIRD
03-02-2019, 11:05 AM
I'm not going to add yet another "...all those other ideas suck & my way is the only way" post because you'll do it with what you have out in the shop & it'll work great! Just wanted to say "Happy Saturday" guys & have fun makin' cool stuff this weekend!:D