Some questions on Momus CNC Design

[Michael In Cali]

Bob,

I purchased your plans about a week or so ago based on your videos and the good reception the plans have gotten. So far, it looks really good.

I do have a couple of questions though. I should preface these with the fact that I'm an Aerospace Engineer by training and cannot just leave well enough alone (though not well versed in CNC). I'm also cheap to a fault.

Why all the counterbores? It appears your just putting studs in most, if not all, and it appears to be redundant making the builder have to jig up for counterbores unnecessarily.

Also, why the use of all-thread in many areas where there are acceptable bolt alternatives? All thread is great for the very long rods, but many other places look like good candidates for common fasteners. You can change a stud and nut out for just a bolt.

I'm currently looking through the design looking for a bit of cost savings in the aluminum and steel parts. Whether that is buy using alternate materials or by combining parts, I don't know. But since metal is pretty much bought by the pound, combining parts, provided the overall dimiensions don't change, would likely not result in cost savings and may result in more waste material anyway. But, I'm going to try.

Just trying to understand how these approximately 200 parts go together...

Thanks,

Michael

[momus_cnc]

Hi Michael,

Thanks for the good questions. The decisions in both of these were driven by the fact that most people will be hand tapping these holes (at least I was). The counterbores are there to remove some material so you are not tapping the entire thickness of the stock. If you are not 100% straight when aligning the tap into the hole, by the time you are 3/4" deep into the material the tap is going to be hard against one side of the hole. A good recipe for breaking a tap, which is never a fun thing. By reducing the thread engagement, even with some misalignment you shouldn't have a problem. If you have equipment available to tap cleanly all the way through the full thickness of the stock, then sure, go ahead and skip the counterbores. They are an insurance measure for those of use with nothing but a tap and tap handle.

I should probably put "counterbore" in quotes, since it they don't need a square bottom shoulder. Maybe I should call them "relief holes" or some such thing. You can just run a larger size drill bit part way into the first hole to remove some material. The exact diameter isn't crucial either. As long as it is a bit bigger than the diameter of the threads. Actually, I suppose even if the counterbore were the same 1/4" diameter size as the tap, it would still remove enough material to give some relief against the tap jamming.

I spent some time carefully marking all holes on the aluminum parts first, so then I was able to drill all of these holes out in one operation. I just set the depth stop on the drill press and went through one part after the other. It just took a few minutes to mindlessly drill all of the "counterbores," and a few seconds of setup time.

I used studs since they put less stress on the threaded aluminum holes. When a stud is tightened, the primary force is one of pulling, whereas a bolt in that same hole is both pulling and turning against the thread faces. By transferring that force to a steel nut rotating against steel stud threads, you can get much higher clamping force without worrying about the aluminum threads galling or stripping out. If they weren't hand-tapped holes I'd be less worried about this, but in general I do like to put studs into aluminum. it just seems like a good practice to me. Bolts into aluminum make me very worried, unless the required clamping forces are low. Stripping a hole while tightening a bolt would possibly be even more annoying than breaking a tap in a hole, since at that point that part has already been completely finished.

Since I'm the King of Cheap I can relate to your frugality, so if you figure out any ways to shave any cost, please let us all know!

-Bob

[Michael In Cali]

Thanks for the info. In my field, we rarely have to tap anything deep. In fact, we rarely have enough metal to tap anyway and use nut plates mot always. Obviously not a cheap method.

I kind of thought that the counterbores were in the design for that reason. Perhaps in the next revision of the instructions, it could be made clear of the reason so the user knows that precision is not required.

The only possible change I've found yet is that I may delete the z-axis bar that mounts the lead screw nut and router clamp. Against earlier suggestions, I'm going to try to use my Bosch Colt router. A couple of reasons. 1) I already own it. 2) You can get precision collets for it. I don't think the precision collet is necessary, bit you can get collets that mount 1/8 bits without the extra bushing. If I come up with something, I'll share with the group. But, we haven't even ordered material yet.

A few other questions.

What is the rational for the difference in height of the left and right X-rails? I would think that mirror images (excepting the stepper motor cutout) would be simpler.

Why 3/8 ply? Not real common in my experience. 1/4" is common and it appears that 1/4" will suffice in this application. That would let the user just buy two full sheets of ply for the whole project. Never a bad thing to have some extra ply around the shop anyway.

Again, thanks for your thoughtful plans (even though I seem to question your every design decision ).

- Michael

[momus_cnc]

Against earlier suggestions, I'm going to try to use my Bosch Colt router. A couple of reasons. 1) I already own it. 2) You can get precision collets for it. I don't think the precision collet is necessary, bit you can get collets that mount 1/8 bits without the extra bushing. If I come up with something, I'll share with the group. But, we haven't even ordered material yet.

Sounds like good arguments in favor of the Bosch to me!

What is the rational for the difference in height of the left and right X-rails? I would think that mirror images (excepting the stepper motor cutout) would be simpler.

Yeah, it probably would be simpler. I designed it with the left rail a little bit smaller in size because the bearing forces that are acting on it are less. And also because I really am supremely cheap. By reducing it in size it saved a few cents on metal cost, and a little bit of shipping weight (I was ordering from online metals). So the smaller rail then resulted in the slightly taller dimension of the base to compensate.

Why 3/8 ply? Not real common in my experience.

This has come up a little bit in the first Momus Design thread that was started here on cnczone (the one in the main DIY woodworking forum). Around where I live it is a very common plywood size. It is in every Home Depot, Lowes, etc. So I used what was the ideal thickness plywood for each area of the machine. It wasn't until I started getting feedback from builders that I realized that plywood availability seems to differ considerably by geographical region. I've had a few guys who couldn't find any 3/8" ply of any sort in their areas, which totally took me aback. Luckily, the base construction isn't that complicated, so with a little careful studying of the exploded view drawing it should be obvious where to change a few dimensions if you need to use a different thickness. I think one guy ended up just building the whole base out of 3/4" ply. I think if it were me, instead of dropping down to 1/4" as you suggest, I'd go up to 1/2" or even 3/4", at least for the top skin (part F in the exploded view) as it is the bed area, and the sides of the machine also attach to it. The other parts should be fine if they are 1/4"

-Bob

[raynorj]

hi all, I have now spent many hours looking at the plans and on occasion totally confusing myself. Still I think now I have a handle on the beast.
I have made both rails the same and in my case down here in NZ I have had to go metris so we have the 1.25" angle which remains same but rails (304 stainless) are 40 x 6mm. Unfortunately such rails ase guillotined from plate by the European supplier and seemingly run through a routing/milling operation to meet straightness standards. this may not be "flat" enough and I may have to surface grind both edges of each bar. at this point have cut drilled and tapped and after assembly will set up on surface plate to check out how good it is before making choice. A friend also making a self designed machine for metal has just completed setting up his z axis and has it to 5 thou top to bottom of travel which is fairly impressive.
Anyway a question I had was about the fact that there is no thrust bearing assembly for the "Z' drive with the coupling and the stepper taking the load.
Is there any issue with this???
My Y gantry is RHS steel 75x40x3mm which I hope to lighten out by cutting out some triangular holes top and bottom. At moment it is already made and drilled.
Rayna

[momus_cnc]

Anyway a question I had was about the fact that there is no thrust bearing assembly for the "Z' drive with the coupling and the stepper taking the load.
Is there any issue with this???

Yes, it isn't the ideal situation to have the stepper taking the thrust load. I decided to give this setup a try and see how it worked. So far I haven't had any problems. We'll see if it hurts the longevity of the motor bearings. A thrust bearing on the Z leadscrew would be nice, but it would also take up some more room. If I could come up with an inexpensive and elegantly compact solution I might add it in. In the meanwhile, I haven't had any problems with the current arrangement. I think it might depend also on what kind of work you are doing with the machine. If you are doing primarily full 3 axis work, like cutting highly contoured surfaces with lots of up and down Z movement, then the Z might want that bearing plus some other mods as well. Like a multi-start acme screw. Any good thoughts on how a thrust bearing could be incorporated?

-Bob

[bjesson]

Against earlier suggestions, I'm going to try to use my Bosch Colt router. A couple of reasons. 1) I already own it. 2) You can get precision collets for it. I don't think the precision collet is necessary, bit you can get collets that mount 1/8 bits without the extra bushing. If I come up with something, I'll share with the group. But, we haven't even ordered material yet.

- Michael

I only suggested not to use the Bosch because of the substantial change in the design to integrate it. But by all means go for it!! Since I am done messing around with creating new clamps for the Rigid 2401 router, and I was initially going to use the Bosch Colt palm router, I will make up some clamps for it and give it a try. i prefer the Bosch over the Rigid any day. If I will try to get some clamps cut and mount the Bosch between tonight and tomorrow. I will also send my mechanical drawings to Bob so he can incorporate them into future revs of the plans which will give this design more potential and maybe help people save a few bucks if they already own a specific router model.

Billj

[groswald]

Bob, et-al;

I was thinking about a thrust-washer solution for the Z myself. What if one were to raise the motor just enough on stand-offs to allow room for a shaft collar, thrust washer, and 1/8" support-plate on top of the current Z axis mounts? Put a thrust washer and second collar below the plate and voila - no axial load on the motor. (I just happen to have all of the above lying about from the design I no longer plan to build)

I figure stand-offs no more than 3/4" should be plenty, and 1/2" might be enough. The only issue I can see is a possible clearance problem with the lid?

This would have the added benefit of eliminating the need for relief arcs in the Z axis motor mounts.

I think I have some 1/8" plate and assorted nylon stand-offs in the shop. I will try and mock it up when I get the Z motor mounts done and post some pictures for your thoughts. The nylon mounts would be for testing only, replaced by aluminum if it actually works.

Regards,

Randy

[bjesson]

Bob,

I should preface these with the fact that I'm an Aerospace Engineer by training and cannot just leave well enough alone (though not well versed in CNC). I'm also cheap to a fault.

Michael

You kind of answered your own question initially...
I am not speaking for the designer just out of experience of building this design and also by some of the notes he had in the plans.

I initially thought the same as you for counterbores and though maybe a lot of cap head screws would be used, but not the case for the cap head screws although I did change the studs in the four main carriage bolts to 1/4-20 stainless cap head screws and am in the process of changing them to 5/16-18 capheads to make it sturdier.

First, the plans are designed for a beginner with not many tools, with that said...the counterbores have two functions. I believe it is stated in the plans that they were used to lessen the depth to tap and once you start building they act as a depth guide for the studs. Granted they are not at all neccessary but do not hurt the design at all either.

Use of all-thread...

Again, this is a beginner to intermediate project and also attempts to use readily available, easy to find hardware. all-thread can be located at just about any hardware store in existence. Using alternate hardware like cap head screws for instance can be difficult to find locally especially in some of the sizes required for the build. Yeah I know there's the internet and everything is available but that adds shipping, etc. and sometimes it is just easier to go to the store to get what you need These plans give a good starting point to create a good machine, as many of you will do and I did, once it was built I changed things around to be easily servicable and to get rid of some of the studs.

Cost savings and alternate use of materials...

Sure you can change materials for some parts, sure you can shave a few bucks here and there. And if you have the right equipment you dont have to get the exact pieces of metal from the BOM. I had a few pieces like this and having a mill i was able to use some of the scrap pieces in other areas. But, this design is trying to eliminate having to use and specialty equipment. When it comes down to it we will all use our own experienceto get the job done accurately and quickly, not a single one of these machines will be built the same way but in the end they will all look similar.

FYI...I spent less then $200 on metal and that was with some extra "oops" pieces.

Hope this helps a bit!!

Billj

[bjesson]

sorry, had the older posts sorted first and just realized after all that typing that the questions were already answered

[Michael In Cali]

sorry, had the older posts sorted first and just realized after all that typing that the questions were already answered

No problems. Still some good info to be gleened.

[momus_cnc]

I was thinking about a thrust-washer solution for the Z myself. What if one were to raise the motor just enough on stand-offs to allow room for a shaft collar, thrust washer, and 1/8" support-plate on top of the current Z axis mounts? Put a thrust washer and second collar below the plate and voila - no axial load on the motor.

Randy- if you get a mock-up of this done at some point, please post a photo. The cover clearance issue could be dealt with easily enough with some minor mods.

-Bob