[Catmandu13]

Short Time Lurker, first time builder.

Looking to build a hobby cnc for doing home projects, but very likely for future small scale production (<50pc a month) For small signs, engravings, robotic platforms,and enclosures. I'd like it to be rigid enough for aluminium, to have the option, but I don't currently have a need for it.

The planned specs so far are:
3/4" MDF
1/2-10 5-Start Screws and antibacklash nuts.
300/925/1025mm SBR20 Rails and Bearings (purchased)
3 Axis G540 Kit from CNCRouterParts(debating on the 4 Axis for future expansion/addiction)
PorterCable 892 Router

Attached is what I've gotten designed so far(Criticism welcome!), the Z Axis however is not finalized. It's currently drawn at a 5" width, but I think it may need to go wider.

It'll be a long build, as i only have about 10 or so hours a week to work on it, and limited workspace with below freezing temps outside, but should be fun!

[OCNC]

What are the footprint dimensions? I assume that you're going to get the vertical rigidity for the X-rails from the table (not shown) that the unit sits on. That will be a key part of build. Looks good.

Chris

[Catmandu13]

The base the X rails are attached to is 48"x 39"x 3/4". This will then be resting on one side of a spare dining room table in my basement (dimensions approximatly 48"x72" with 3.5" posts and 3" supports for the table bed, 1" thick).

Hopefully that'll be rigid enough, but it is in the back of my mind that I may have to build a proper table for this in the future.

[louieatienza]

Looks cool, though, judging from other MDF builds here and elsewhere, you might benefit by doubling the MDF on the uprights, or some other form of bracing, as the legs will tend to flex....

[Catmandu13]

Y'know I saw that, and i had it in my notes, but totally forgot to design it in. Thanks for the reminder!:cheers:

[Catmandu13]

I do occasional dovetail work with a jig, but after reading through the dovetail threads, I'm considering leaving the x axis bed length short where the stepper motor is so the full length of the front can be used to for the other board orientation.

But to do that I'll have to put the stepper in the back...is there a setting in mach3 to account for that?

I'll also have to remove the support mounts on the front, so ill probably double up on the mdf there.

[Trotline]

No real reason you couldn't leave the space for the overhang to one side or the other. Plenty of room on either side of the X-axis stepper for all but the very widest dovetailing operations..

Luke

[ger21]

'm considering leaving the x axis bed length short where the stepper motor is so the full length of the front can be used to for the other board orientation.

But to do that I'll have to put the stepper in the back...is there a setting in mach3 to account for that?

Mach3 doesn't know or care where the motor is. You can put it anywhere you want. I'd put the motor on the back to allow the entire end to be used for clamping.

[steve123]

I like your design. It's interesting how the X rails are attached to the base instead of the bed. That will cut the bed sag down a lot. On the other hand, bed sag is now out of sync with the gantry. If you put something heavy on the bed it will dip in the middle, and the z height will end up higher there. I don't know how heavy it would have to be to matter. Maybe the torsion box won't dip much.

I would add a vertical stiffener to the outside of the gantry verticals. It would be sticking out 4 inches or so, and run from the top to bottom. I would also extend the bottom piece under the gantry (that connects to the lead screw) wider, so that you can bolt up into the new stiffener. This will cut down on sideways gantry flex. I think something like this is always needed on MDF "High gantry" designs like yours.

[Catmandu13]

Spent the afternoon moving some stuff around a double checked my table downstairs. Good thing too, it's a couple inches shorter than i thought. So with that in mind, i've shortened the Y and the the bed accordingly, reinforced the front end of the bed to account for the dovetail option, and doubled up the mdf on the gantry sides.

steve, I agree on the vertical supports, but space wont allow for it without shortening the bed width even further. The table is pushed against a wall to allow for a small walkway. I could see about some L side bracketing but not sure what else to do.

[CaptainVee]

Looks pretty good.

I'd say, leave room in the set up/design to easily add a torsion box below the x rails later on. I think you may need the stiffness there if the rest performs well.

That area will flex at higher loads.

[Catmandu13]

I'm about one weekend away from starting to cut the gantry parts. Before I do though, is 4 inches high enough clearance from the bed, or maybe even too much? Im sure it all depends on the purpose of the router, but as its primarily for hobby, im not entirely sure what all i might end up doing. What is the average height of most?

[rbraeking]

Doubling up on your y axis vertical supports will not add nearly the stiffness as using perpendicular vertical stiffeners. Boxing it like a torsion box would make it even stiffer. My MDF machine uses a box for the gantry which makes it very stiff. I mounted the electronics within this gantry but am considering moving them to a console as the vibration has a tendency to loosen connections.
Bob

[Catmandu13]

I could probably torsion box the sides at a width of an inch or so. Think that would be enough?

Starting cutting gantry parts today. I forgot how much I hate my portable table saw. I can eyeball accuracy better than the guide can hold. A new one stand type is defiantly on my wish list.

When attaching linear rails to MDF, machine bolt/nut, or drywall screws?

[rbraeking]

An inch and a half of boxing is worth 2 layers of material screwed and glued.

When using MDF, drywall screws don't hold well unless they are just there to clamp a glue joint. Thru bolt if you can. Use cross dowels on perpendicular joints.

[harryn]

Consider to build up fixed sides the length of the X axis, eliminating the need for the Y uprights. Just run the Y gantry across the top of these fixed sides. This can be 3- 4 sheets thick, just bolted to the outside of the base box.

MDF is acceptable for compression, but it is medocre for gantry uprights.

In general though, looks like you are on your way. Good luck.

[Catmandu13]

Good thing i haven't cut too much!

Now I've talked myself into a 4 axis system. I was thinking about just continuing on with the system as designed and just moving the two x-screws closer to the uprights, but today it occurred to me that if i go down that path, i can redesign the gantry. This way i can get rid of the undercarriage, which will allow for a fully supported bed and just move the Y over it. OR, maybe leave some of the hieght to allow for more of a vertical support on the backside.

...back to sketchup! (and another couple weeks of pondering)

[Catmandu13]

Small update, some cuts made last weekend. This weekend, a plate for the z axis to mount to. I wasn't planning on using aluminum in the build, but a few small plate became available.

[Catmandu13]

Should have a gantry assembled by the weekend. I'll be making another trip to the store to get a decent respirator before making any more MDF cuts though.

In the mean time. Here's the revised design. So far, I've got the "C"-beam and the outer two uprights cut and drilled. I attached them and was quite surprised how sturdy it was! I imagine with the inner uprights it should be decent enough for trials.

Another learning experience: I had planned on using T-Nuts with Cross Dowels. I got about 8 of them mounted on the c channel before i realized the compression between the joints wasn't there because of the threading of the T-Nut. Replaced with washers and all was fine.

[rbraeking]

It appears that your design is progressing nicely. I'm sure you have designed thrust bearings for your lead/ball screws. The bearings in the steppers are only designed for rotational loads.

Bob