Composite Mini Benchtop Mill / Router

[ficklecycler]

Hello All,

Long time listener, first time caller.


I've been enjoying playing around with a 6040 router I got around a year ago and while it's been a blast for a lot of projects so far, I found its limitations pretty quickly. I wanted to build a CNC before I got the 6040 but was sure I'd make lots of mistakes and decided some training wheels would be nice to have first. I am intrigued by the idea of using it to assist in making an epoxy granite machine and want to post it here to hopefully get a little feedback on my design direction. Nothing is set in stone (hur hur) and you'll probably notice lots of little parts missing here and there.


Goals:
-Small Foot Print - I only have so much benchtop, and I don't have an engine lift to move some behemoth around.
-Stout enough to mill aluminum well. I don't expect crazy speed or depth, but I'm hoping for good surface quality.
-Accuracy of .01mm - hopefully I can do better, but I don't need more than this.
-Reasonable cost - I haven't tallied it up yet, but I'm definitely not trying to spend 3k on this thing. Hopefully half that? I'm going to be using ebay more than misumi lol!
-Easy to mold forms - molds are going to be a lot of work, and I don't want to make mistakes. KISS mentality here.


Features / Thoughts :
- Fixed gantry style
-24"x24" foot print with a 14"x12" working area with ~120mm height. As is, I can reach further than this by a smidge, so it could surface itself.
- Form is designed as 4 castings, we have the main pad, that houses the Y axis, then 2 risers that support a beam that carries the X and Z axis. They are all held together with 6 lengths of 3/4" all thread and nuts on each side. I've considered casting the all thread into the 2 riser pieces and having the 2 other sections be removable. This way I can disassemble the X axis if I need to move this around. It also breaks this into what look like really easy to form molds vs making a large u shaped mold for the X axis with risers integrated.
-I am hoping to cast all the inserts in the mold and bolt the linear rails directly to the epoxy. I don't have access to a mill so I might be leveling with a skim of epoxy and a reference surface.
-I haven't gotten as far as picking out a mix of stones.
-Loose dimensions here. Main base is 4" thick 24x24. Risers are 4.5" thick. X axis is 8"thick and 7" deep.
-Z axis is all .5" aluminum plate possibly mic 6. Has ~135mm of travel.
-Nema 23 Steppers, ball screws, 20mm linear rails. All going with cheap stuff. Send recommended sellers my way.

Thanks for any input or advice!








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[ficklecycler]

Trying to attach pictures again. They showed up in my previous post during editing then disappeared?

[RCaffin]

Make allowance for aligning the Z axis rails to be at right angle to the X & Y axes.
Make allowance for aligning the spindle axis with the Z axis rails - on both axes (X & Y).
Minimise the distance between the collet face and the lower Y axis rail.
Let the Y axis go a bit further to one side: that is a good place to fit an ATC should you ever wish.
I would really try to have a bit more clearance under the Z axis: sometimes you want to mill (not route) a high object.
If you make the table a bit longer on the X axis you will have room to mount an A axis rotary table.
Those blocks of epoxy granite are going to be extremely heavy if solid. Of course, weight is good, but don't collapse the bench.

If you want better than 0.01 mm you will need coolant flow through the blocks and air conditioning. It may not be justified.

Rather a solid MILL, that design.

Cheers
Roger

[ficklecycler]

Make allowance for aligning the Z axis rails to be at right angle to the X & Y axes.
Make allowance for aligning the spindle axis with the Z axis rails - on both axes (X & Y).

Couldn't I accomplish this with thin shims or setscrews on this current design? Or are you thinking of a different method?

I would really try to have a bit more clearance under the Z axis: sometimes you want to mill (not route) a high object.

Minimise the distance between the collet face and the lower Y axis rail.

Totally agree that more Z could potentially be nice especially if I try to put a vice in there. I have seen some builds try to build in MORE of everything and then end up with a weaker structure because of the distances they have to span so I'm trying to avoid going overboard. I don't understand though how I could add more clearance under the Z axis and simultaneously minimize the collet face to y axis rail distance. If I raise Z aren't we just going to have to reach further to the work area or am I missing something?

Let the Y axis go a bit further to one side: that is a good place to fit an ATC should you ever wish.

So that would be an auto tool changer? Not familiar with any tight budget ways to accomplish that. A quick search for spindles with that ability looks like it'd blow my whole budget. Do you know of any methods that are friendly to the cash poor hobbyist?

If you make the table a bit longer on the X axis you will have room to mount an A axis rotary table.

Interesting idea, it might be weird having the machine hang off the bench a few inches.

Those blocks of epoxy granite are going to be extremely heavy if solid. Of course, weight is good, but don't collapse the bench.

I'm certainly hoping they are, maybe I can reinforce my bench! I should do a calculation to estimate the weight. I wish I had enough room to put this on a rolling stand or even permanently mounted to the floor.

If you want better than 0.01 mm you will need coolant flow through the blocks and air conditioning. It may not be justified.

I think you're right, I really don't need it.

Rather a solid MILL, that design.

Glad to hear it and thank you for the suggestions!

[RCaffin]

Couldn't I accomplish this with thin shims or setscrews on this current design?
Yes ... but don't rely on the stiffness of the rails themselves. They ARE stiff, but your design justifies greater reinforcement.

Totally agree that more Z could potentially be nice especially if I try to put a vice in there.
Just don't go overboard.

I don't understand though how I could add more clearance under the Z axis and simultaneously minimize the collet face to y axis rail distance.
True. You need to increase slightly the Z travel. Again, only by a bit. Imho, what you have is a bit small.

[I]So that would be an auto tool changer? Not familiar with any tight budget ways to accomplish that. [/I
Chuckle. yes, some $$. But there is nothing worse than completing a build only to realise that you have failed to leave the opportunity for a bit of expansion. But if you can't see an ATC in your future - forget it. I use ER collets, so an ATC is unlikely for me.

it might be weird having the machine hang off the bench a few inches.
No, just make the mill table a bit longer so you can put an RT at one end and still have enough travel.

Rule of thumb for accuracy: to halve the error, you increase the cost by 10x. Sigh.
I was making a number of units and had made up a nylon machining jig for 10 at a time. It worked well. Then we went from mid-summer to mid-winter, and the between-unit gap on the jig went from 20.00 mm to 19.97 mm. Thermal shrinkage from 30 C to 3 C. I was able to deal with that OK, but temperature matters!

Cheers
Roger

[skrubol]

I'd be a little concerned about mixing granite and aluminum unless this will be in a climate controlled area, as aluminum has about 3x the thermal expansion of granite (23 vs around 8 x 10^-6 /K.) Not sure if that has any real impact on something this size, the only place they meet is the linears on the gantry. If you leave one of the rails 'floating' (torqued down, but without any guides to keep it from shifting,) that may not be an issue.
To get more Z reach you might need to put the Z rails on the spindle carriage and put the blocks on the other side. Each design has it's issues.

[ficklecycler]

No, just make the mill table a bit longer so you can put an RT at one end and still have enough travel.

Understood. Can't say where I'll land yet but this is exactly the kind of design feedback I was hoping to get. Thanks!

Rule of thumb for accuracy: to halve the error, you increase the cost by 10x. Sigh.

Yow! Yeah, that's not going to work with my budget haha!

[RCaffin]

Another rule of thumb for you:

Servo motor with 512 line encoder or stepper with similar spec
GT2 belt reduction of 3:1 (NOT trapezoidal profile!)
Reasonable quality 5 mm pitch ball screw with double nut
Double ball race at drive end (preferable AC, but DG works)
Linear rails in solid base

You should get close to 10 micron repeatability. Try to match that on a manual mill!

Cheers
Roger

[ficklecycler]

I'd be a little concerned about mixing granite and aluminum unless this will be in a climate controlled area, as aluminum has about 3x the thermal expansion of granite (23 vs around 8 x 10^-6 /K.) Not sure if that has any real impact on something this size, the only place they meet is the linears on the gantry. If you leave one of the rails 'floating' (torqued down, but without any guides to keep it from shifting,) that may not be an issue.
To get more Z reach you might need to put the Z rails on the spindle carriage and put the blocks on the other side. Each design has it's issues.

Thanks for the info. I haven't thought too much yet about material expansion. I wonder how difficult it would be to make the entire mill frame out of epoxy granite. Have you seen any examples of a compact Z axis' built completely out of EG? If I did that then I'd still have the Y axis plate to deal with and that's a really impractical part to make out of EG when I can just get some MIC 6 cut to the right size instead. I may just end up living with some material differences for practicalities sake.

One thing working in my favor is that I only have a 5 degree temperature fluctuation in my garage most days.

[RCaffin]

I only have a 5 degree temperature fluctuation in my garage most days.
Blimey!
I have been out there when it was 0 C, and when it was 40 C. Mind you, I don't think I enjoyed either time.

I think your biggest concern with differential expansion should be with the long X axis.

Cheers

[skrubol]

Thanks for the info. I haven't thought too much yet about material expansion. I wonder how difficult it would be to make the entire mill frame out of epoxy granite. Have you seen any examples of a compact Z axis' built completely out of EG? If I did that then I'd still have the Y axis plate to deal with and that's a really impractical part to make out of EG when I can just get some MIC 6 cut to the right size instead. I may just end up living with some material differences for practicalities sake.

One thing working in my favor is that I only have a 5 degree temperature fluctuation in my garage most days.

Steel has a coefficient of expansion of around 10 x 10-6, so plenty close to granite. Not sure how much the epoxy affects the thermal expansion either, one random site I found said 45-65 for epoxy, so mixed with the aggregate, who knows where it ends up.

With only a 5 degree fluctuation, it's probably not something to worry about. Only be looking at about a .005% relative change over that range.