[TrevorWhite]

Hi all. The Momus design looks interesting. Reading through the messages a few people are changing things. MDF instead of plywood. Changes to the Z axis mount. Has anyone got a stock machine built that they have been using for a while?

I am interested to know if the design is working well or is something that constantly will need improvement.

Thanks

Trev

[momus_cnc]

Trev,

As the designer of the machine, I suppose I have the longest running "stock" machine. No problems as of yet. I'm obviously a bit biased, but I thought I'd chime in with a few opinions about the points you mentioned.

Adding a thrust bearing to the Z axis: This can be a good modification to make. While many other designs take a similar approach as this machine and go without thrust bearings, steppers aren't really intended to provide thrust resistance. I had decided to not incorporate it into the machine design for a couple of reasons. The primary one is whether it would be cost effective. It would mean fabricating and purchasing more parts. The thrust forces on the stepper are fairly low, so the likelihood of wearing out those bearings without a whole lot of use is low. It would also make the Z axis taller, potentially requiring the cover to become taller as well, also requiring more material and adding weight. The other reason I decided against is that unless an external thrust bearing is adjusted properly, it is potentially a place to introduce backlash into the system (depending on type of couple between motor and leadscrew.) So it needs to be designed so that it can easily be adjusted properly. On the other hand, adding a thrust bearing could be a way of reducing backlash, if the stepper bearings have any end play. On the whole, I'd say it is a good modification to make, as long as you are aware of the minor downsides. I've contemplated adding it to a future version of the machine.

As far as MDF, I wouldn't consider it an upgrade in any way. Other than lower initial cost and dimensional consistency, it has few benefits and some really big drawbacks. It requires special care in how fasteners are placed into it, has very low stiffness, and is highly prone to creep. This last point is significant. Obviously any material that is composed of organic fibers is going to move somewhat with changes in humidity, temp, etc., but the amount that MDF can move simply due to creep is orders of magnitude larger. My opinion of MDF is that it works great for spoilboards.

-Bob

[TrevorWhite]

Hi Bob,

Thanks for the reply. I went ahead a bought the plans. They do look very professional.

I do not think I will be making any changes. I am a beginner and the learning curve will be steep enough for me right now. Thanks for clearing up the point about MDF.

I am currently in the costing phase. I really like the enclosure design. I think it makes a real difference and is a big advantage. I will stick with plywood and make the entire enclosure.

It does seem a little difficult sourcing a trim router over here in the UK. The Bosch Colt and the Ridgid? seem only available in America. I have found a couple of refurb units on ebay but they charge a lot to ship and import. I do not know how good they are either. I have read a little about the Bosch having a bit of play in the spindle. Cant really take a refurb unit back when it's been shipped from America.

Trev