The design is finished

[plcamp]

Well it's time to get off of my duff after spending many hours of time lurking in the weeds and reading the accounts of many DIY'ers who have preceded me into the realm of the 'home made' CNC router.

I first explored many of the DIY plans available and began thinking this would be a great way to approach the project, however, as I looked closer I noticed aspects that if re-designed would suit my particular needs better. Kudos to the individuals who spent countless hours designing and prototyping their machines, but they just didn't seem quite right for me.

So the first decision to be made was of the fabrication method to used. I explored the T-Slot extrusion bolt together option and concluded that the cost was beyond the scope of this project, after all it's a hobby and not a business venture. So my choice of construction materials will be 3/4" Birch cabinet grade plywood.

The second consideration was just how big the router should be. I wanted it to be larger than many of the turn-key tabletop models available from numerous sources on today's market, but smaller than what I would consider a full out 4x8 production machine. With available floor space and economics in mind I decided that the machine would focus around a 4x4 foot print.

Now the challenge:
1. 4x4 wooden support frame
2. 3/4" plywood 'Y' axis gantry and support
3. 3/4" plywood 'Z' axis
4. NEMA 23 drives
5. 1/2"-10 5 start Acme lead screws
6. 2HP router motor

My design as it stands yields: X-Axis travel of 34"
Y-Axis travel of 44"
Z-Axis travel of 6" This height was chosen to accommodate a future 4th axis rotary.

Simple modifications for both X and Y axis could yield a full 48"x48" work area, however I'm leery of possible rotational 'whip' that could develop with the longer lead screws. A rack & pinion drive would certainly eliminate that potential but at a higher build cost.

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[ger21]

Leadscrew whip is the least of your worries.

When the spindle is moving along the gantry, and stops, then reverses direction, your entire gantry is going to oscillate back and forth like a pendulum, probably moving about 1/4", if not more.

Unfortunately, you've based your design on the least rigid machine out there in this size range.
Your gantry sides, motor mount plates and screw support plates need to be made much stronger, or you won't be able to take advantage of the speed that the 5 start screws provide.

The gantry sides should be a minimum or 2 layers of plywood laminated together. Even that will flex. A torsion box like side would be much stiffer, but will be considerable larger.
Also, a triangular shape would be much stronger than the "L" shape, as there would be a lot more material to reduce flexing.

[plcamp]

Gerry, thank you for your feedback on my design. This is exactly what I'm looking for as this is virgin territory for me and I look forward to any and all comments for improving this project.

Please note that the gantry itself is of a torsion box design, and I had hoped that the gantries front and rear vertical extensions would provide some additional side to side stability. The 'L' shaped gantry supports were simply chosen for weight consideration. Perhaps moving the Acme Nut support rearward and extending it upward would provide some additional support also. Doubling the gantry vertical supports is also a valid idea, thank you for mentioning that.

I can send another pic of the support scheme mentioned above for your review if needed.

Paul

[ger21]

and I had hoped that the gantries front and rear vertical extensions would provide some additional side to side stability.

They need to go all the way down to the bottom of the sides.
Don't worry about weight. Weight is your friend.

Make changes, and keep posting pics. Designing a CNC is a process that often requires many changes along the way.

[plcamp]

They need to go all the way down to the bottom of the sides.
Don't worry about weight. Weight is your friend.

Make changes, and keep posting pics. Designing a CNC is a process that often requires many changes along the way.

Gerry, following your lead I have made the following changes to my design;

doubled the gantry riser thickness to 1-1/2" as well as increasing it's surface area
extended the vertical gantry extensions to the bottom edge of the riser to serve as stiffeners and added a box section
doubled the mounting thickness for the backlash nut

Motor mounts are still WIP

Paul

[bhurts]

You should also consider not offsetting the z screw. It will work much better in the center of the axis. It may be a little more work but having it to the side like that will most likely cause it to rack one way on the way down and the opposite on the way up.

Ben

[plcamp]

Thanks Ben, I had first thought of placing the 'Z' screw on center, and may still do so. It is only slightly more complex than offsetting the screw. The pair of guide wheels on the backside of the gantry were of dual purpose controlling torsional twist on the gantry and to provide additional stability to prevent racking ( I hope), at least that was the plan. I can make the necessary modifications and submit a picture for further review.

I have (0) zero experience with a CNC router and I would like this to be a positive one from the first power on test.

Paul

[wizard]

Thanks Ben, I had first thought of placing the 'Z' screw on center, and may still do so. It is only slightly more complex than offsetting the screw. The pair of guide wheels on the backside of the gantry were of dual purpose controlling torsional twist on the gantry and to provide additional stability to prevent racking ( I hope), at least that was the plan.

My first impression was where is your gantry beam? It is skinny to the point of invisibility in your pics. I'd seriously consider a much larger cross section and similarly built up supports. Baltic Birch can make a perfectly good gantry, but you need to give it a chance and you do that by paying close attention to the cross sections and the way the pieces are assembled.

I can make the necessary modifications and submit a picture for further review.

I have (0) zero experience with a CNC router and I would like this to be a positive one from the first power on test.

Paul

Someplace in this forum, about a month or so back, there was an extensive series on gantry design. Someone even submitted a nice spreadsheet to calculate some parameters. I found the article enlightening as it reinforced practical experience with design effort.

[hanermo]

Agree with others .. way too small, and very low support on torsion.
Strength is not the problem, rigidity is. Its plenty strong enough, no-where near rigid enough.

The verticals should be triangles.
The beam dimensions should be about 1/2 height of z axis, or about 2.5 times larger.

The goal is to minimise deflection.

Picture standing on the tooltip, with machine tilted on its side.
The beam (structure, spindle mount) shoud not flex more than 0.01 mm.

This gives you a good mental idea of what, ideally, you are looking for.
You wont achieve this, but it describes the forces involved very well.

1 hp of cutting force (spindle cutting power) is about the same as standing on the tooltip (about 80 kg).

[plcamp]

Wow, you guys are giving some really good advice and I'm trying to incorporate them into my design. Really wish I could get a FEA analysis of the beam so I can see just how much deflection is taking place, but I don't think I will be able to accomplish that.

The gantry box as it stands at the moment is 4in x 5in with a Z travel of 6in, overall Z height is 16.75in.

Trying to get pics of a larger piece of equipment can sometimes be difficult as some loss of component detail gets lost. Hopefully this pic will clear up some of the detail.

[ger21]

Really wish I could get a FEA analysis of the beam so I can see just how much deflection is taking place, but I don't think I will be able to accomplish that.

I think your Z axis will flex more than the gantry beam, although I'd try to increase the height of the beam another inch or two. If you make the gantry beam a torsion box, with about a 2"x2" grid framework, you can use 3/8" or 1/2" skins and it'll be pretty stiff, relative to the rest of the machine.

I think that you're sacrificing quite a bit of rigidity to accommodate your future 4th axis. You might want to consider making the base of the machine a foot longer, and mounting the 4th axis on the end, with the center of rotation even with the table. This will let you lower the gantry 2-3", which would make the Z axis quite a bit stiffer for non 4th axis cutting, which will probably be most of your cutting?? That's how I plan on doing my 4th axis.

[plcamp]

I think your Z axis will flex more than the gantry beam, although I'd try to increase the height of the beam another inch or two. If you make the gantry beam a torsion box, with about a 2"x2" grid framework, you can use 3/8" or 1/2" skins and it'll be pretty stiff, relative to the rest of the machine.

I think that you're sacrificing quite a bit of rigidity to accommodate your future 4th axis. You might want to consider making the base of the machine a foot longer, and mounting the 4th axis on the end, with the center of rotation even with the table. This will let you lower the gantry 2-3", which would make the Z axis quite a bit stiffer for non 4th axis cutting, which will probably be most of your cutting?? That's how I plan on doing my 4th axis.

I had considered a similar option for the 4th axis by extending the gantry beyond the X axis guide rails (ie; CAMASTER) allowing the 4th axis to mount parallel to the X. A good idea for a metal machine, but I am really nervous about trying this with a wood machine because of counterbalance and vibration issues that would surely be troublesome.

You are correct, I would estimate that the router would be used 99% of the time for 'flat' work, but a rotary option would enhance the versatility of the machine.

What do you think about instead of extending the machine table, fashioning a removable section that would expose a sub-frame for mounting the 4th axis at the C/L of the table? Do you think this would compromise the integrity of the base?

[ger21]

Yes, your machine would need to be considerably more massive to go the Camaster route.

I've seen some dsigns with removable beds. I'm not a fan of that though, as to me it would be too much trouble to change over. If you do go that route, you just need to design the frame to be strong enough to support it. It probably needs to be much more substantil than it is now.

[plcamp]

Yes, your machine would need to be considerably more massive to go the Camaster route.

I've seen some dsigns with removable beds. I'm not a fan of that though, as to me it would be too much trouble to change over. If you do go that route, you just need to design the frame to be strong enough to support it. It probably needs to be much more substantil than it is now.

Design update;
1. The gantry support riser will be laminated doubling it's thickness to 1-1/2"
2. The gantry torsion box front and rear component now extend downward for additional riser support as well as upward for motor bracket support
3. The X motor mount brackets will be laminated doubling their thickness to 1-1/2"
4. The gantry torsion box front and rear components were increased by 1"
5. The gantry support risers were shortened 1" thus lowering the gantry to spoil board distance
6. Added spoil board and T-Track

I may also laminate the Z axis components as it is now a 'U' configuration rather than a box because the motor being placed on center rather than offset

Please note that the pics are illustrated with a 1/2" ball cutter chucked into the router as this will most likely be the largest cutter used for any rotary operations.

[plcamp]

Well the comments have stopped and I would like to thank all who have responded. Many of the suggestions have been incorporated into the design and now its time to get serious about the project. I have the power supply, controller and the 4 stepper motors ordered and tomorrow I will make a trip to the lumber yard to purchase the material needed for the base table. So I say let the fun begin and the sawdust fly. Pics to follow.....

[plcamp]

Ok, materials have started to arrive and construction of the base will begin within this coming week . Seems like forever since my last post and I'm really anxious to get the project started.

The current design is focused around the Porter Cable #690LRVS motor, yes I know it is only rated at 1-3/4hp maximum but I do not intend on taking 1" single pass finish cuts with it. So the question is about how to control the spindle speed from the software, is this a viable option?

What about the PID controller?

[plcamp]

The base frame is complete with the exception of the spoil board, 'T'-tracks and finish. I'm undecided on color at the moments but there are still plenty of parts that need to be fabricated so plenty of time remains for any color options. Paint or some form of sealer is a must as the entire machine is fabricated from wood.

Now for some pictorial representation of the project and the progress to date;

Note the middle right picture which shows '0' light under the straight edge indicating all mating components are at the same level. The frame is square across the 66-1/2" diagonals to less that 1/16" and is rock solid. Now if the gantry turns out this solid I will be extremely pleased.

Ignore the messy shop area......

[PaulRowntree]

Cool! If it is not too late to make suggestions, try to add a few diagonal braces on the sides. Or perhaps skin in a few sides with sturdy plywood. It will keep the table from racking when the machine gets to whacking ...

[plcamp]

Thanks for the kudos, and from one Paul to another Paul I can say that it's never to late to make changes, heck the BIG 3 do it on a daily basis. Diagonal braces on 3 sides are easy, that way they stay open to collect tons of dust and debris, which I'm going to call free ballast. However, I do want to keep the front open for placement of my computer and such. I'm going to try and get a weight for the base frame tomorrow just to satisfy my curiosity as it has now become a 2 person task to pick up and move.

Paul

OK, I just weighed the base and it tips the scale at 192# and from this point it can only get heavier. Now I have to devize dolly to move the thing into it's final resting place when completed.

[plcamp]

Fabrication on the router base continues with the addition of some truss bracing. Prior to this addition I could not get the frame to 'RACK' or shift at all, and with these braces it definitely is not going to move (I know famous last words).

Again please ignore the cluttered shop, I'm still trying to find a place for all of my stuff and this 4x4 frame is only adding to the confusion. This is something like organized chaos.