CNC milling machine plan

[Vard]

Hi everybody. I want to DIY a CNC machine. The board panel is 40" length and 23" width. It should be a wood engraving/carving machine for miniature text making. Now I am seeking recommended CNC stepper motors and acme lead screws for it. What would you recommend me? Please specify motors oz size and lead screws size (diameter, TPI and starts). I will appreciate any help.

[louieatienza]

Originally Posted by Vard Hi everybody. I want to DIY a CNC machine. The board panel is 40" length and 23" width. It should be a wood engraving/carving machine for miniature text making. Now I am seeking recommended CNC stepper motors and acme lead screws for it. What would you recommend me? Please specify motors oz size and lead screws size (diameter, TPI and starts). I will appreciate any help.

There are a few here who would recommend first figuring out how much your axes will weigh and how much force you'll need to push each axis plus cut through your work. AFAIK grossly oversizing your steppers may not be a good idea.

That said, if you're doing mostly engraving there shouldn't be too much cutting forces involved. The mechanical resolution will be dictated by the leadscrew pitch, but there's a "give and take" with steppers between resolution and speed. The finer the screw pitch, the slower the machine but more mechanical resolution, and vise versa. If you can make the g-code for your engraving, for example, that doesn't have the machine jump from place to place, then rapids speed may not be as important because the mahine won't travel far enough to reach max speed between cuts.

On the other hand, it would be pointless to have that mechanical resolution, if the backlash in your screws or bearings, or flex in your system, exceeds that resolution. So deciding what resolution you'll need will be important as well.

[Vard]

Originally Posted by louieatienza There are a few here who would recommend first figuring out how much your axes will weigh and how much force you'll need to push each axis plus cut through your work. AFAIK grossly oversizing your steppers may not be a good idea. That said, if you're doing mostly engraving there shouldn't be too much cutting forces involved. The mechanical resolution will be dictated by the leadscrew pitch, but there's a "give and take" with steppers between resolution and speed. The finer the screw pitch, the slower the machine but more mechanical resolution, and vise versa. If you can make the g-code for your engraving, for example, that doesn't have the machine jump from place to place, then rapids speed may not be as important because the mahine won't travel far enough to reach max speed between cuts. On the other hand, it would be pointless to have that mechanical resolution, if the backlash in your screws or bearings, or flex in your system, exceeds that resolution. So deciding what resolution you'll need will be important as well.

Ok, louieatienza. Your advice is really important for me. I have some more questions please. What size of screw pitch or TPI would you recommend for wood engraving/carving? And what power size should be used (etc. Nema 23, 270 oz/inch)? Please help me to realize the precise screw TPI and stepper motors power? The X axis moving gantry totally weighs about 10 lbs (spindle included).
However, I would appreciate any advice from you.

[handlewanker]

Hi, backlash, inherent in Acme screws, is a problem that so far has not been overcome, but it can be reduced with compensated Acme nut design.

For a size of 40" X 20" approx, you would better be counting on spending some money on ball screws, on the X and Y axis at least and Acme on the Z axis, but as the Z axis is only a short drive a ball screw won't cost much more, and is less complicated to apply.

You want to do engraving of text on wood, so rapid reversals of the cutter will be happening more frequently than if'n you were milling objects in alluminium or wood etc, so ball screws will not display jagged or "bumpy" edge cuts when the X and Y axis reversals happen.

Doing engraving, as opposed to milling or routing, means you could use a moving gantry design as cutting forces for engraving are much less than milling or routing.

Engraving cutters are more "pointy" with less contact area than side force end milling or routing bits, and so use less force to drive them, but the speed wants to be on the higher side, and a motorised router type head will probable be more usefull and cost effective than a seperate spindle/motor belt set-up.
Ian.

[Vard]

Thanks, Handlewanker. Please see the following links and tell me if those stepper motors and their driver are suitable for wood miniature engraving. Besides, i have order XYZ ballsrews, they are more precise than ACME.

eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices

eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices

Thanks!

Originally Posted by handlewanker Hi, backlash, inherent in Acme screws, is a problem that so far has not been overcome, but it can be reduced with compensated Acme nut design. For a size of 40" X 20" approx, you would better be counting on spending some money on ball screws, on the X and Y axis at least and Acme on the Z axis, but as the Z axis is only a short drive a ball screw won't cost much more, and is less complicated to apply. You want to do engraving of text on wood, so rapid reversals of the cutter will be happening more frequently than if'n you were milling objects in alluminium or wood etc, so ball screws will not display jagged or "bumpy" edge cuts when the X and Y axis reversals happen. Doing engraving, as opposed to milling or routing, means you could use a moving gantry design as cutting forces for engraving are much less than milling or routing. Engraving cutters are more "pointy" with less contact area than side force end milling or routing bits, and so use less force to drive them, but the speed wants to be on the higher side, and a motorised router type head will probable be more usefull and cost effective than a seperate spindle/motor belt set-up. Ian.

[handlewanker]

Hi, ballscrews are your only option for the accuracy you require.....less headaches with backlash developing later as in Acme screw/nut systems.

I couldn't comment on the stepper motor type as it would depend on the slideway friction and forces in the drive train before the cutter gets to do it's work, but as it's only engraving, I should compare it to other engraving designs so far shown on this forum without considering any milling designs.

The Ebay offer for the stepper motors and driver are a package deal, and if'n it turns out you want more power you'll have to go and buy the whole lot again....better to cut and try using a single stepper bought second hand and try it in the X axis to see if'n it's capable......you can have more power, but you can't have less to do the job, so the second Ebay option might be the way to go...buy with capacity in mind.....the difference in price is only $62 more for the bigger ones.

Milling, (which probably won't interest you), is a whole different approach (a very much more rigid machine requirement), as you usually want to do long X axis runs and fast traverse returns with end mills and facing cutters, which absorb quite a bit more power than a single pointed engraving bit.
Ian.

[Vard]

Ok, buddy, thanks again. And can you give me some advice about stepper motor torque power? What do you think, 270oz/in stepper is enough for XYZ movements load? The load weighs about 5-6 kgs.

Originally Posted by handlewanker Hi, ballscrews are your only option for the accuracy you require.....less headaches with backlash developing later as in Acme screw/nut systems. I couldn't comment on the stepper motor type as it would depend on the slideway friction and forces in the drive train before the cutter gets to do it's work, but as it's only engraving, I should compare it to other engraving designs so far shown on this forum without considering any milling designs. The Ebay offer for the stepper motors and driver are a package deal, and if'n it turns out you want more power you'll have to go and buy the whole lot again....better to cut and try using a single stepper bought second hand and try it in the X axis to see if'n it's capable......you can have more power, but you can't have less to do the job, so the second Ebay option might be the way to go...buy with capacity in mind.....the difference in price is only $62 more for the bigger ones. Milling, (which probably won't interest you), is a whole different approach (a very much more rigid machine requirement), as you usually want to do long X axis runs and fast traverse returns with end mills and facing cutters, which absorb quite a bit more power than a single pointed engraving bit. Ian.

[handlewanker]

You'll only have to worry about inertia loads when you want to fast traverse....for a load of 6 kG abouts, and engraving with a small pointy cutter, it will depend on the pitch of the screw....the coarser the pitch the more torque required, whereas with a finer pitch it's like driving in low gear, you won't get very far fast but you'll carry a big load.

If'n you're worried about the possibility of under powering the cutter drive force, it's minimal compared to milling, so without actually having the machine to play with I'd go for the bigger steppers which will only set you back $60 more.

It's very hard to hypothesise on power requirements, most of the time it's a rule of thumb, but better to be big than small.....most people who skimp on the stepper power eventually have to invest in a total new package and drive set-up....better to be big than small......if'n you go small you just overload the motor and you only find out when it smokes...LOL.

In electronics you calculate the power requirements and then double the component rating value to allow it to run within the load design without overheating.....you NEVER go exactly to the component maximum rating EVER.....and steppers will need to be running in a stalled condition to hold the position, and that means the power is constantly on for whatever length of time required.

One of our forum posters NeilW could probably help you with more info.
Ian.

[louieatienza]

Originally Posted by Vard Ok, louieatienza. Your advice is really important for me. I have some more questions please. What size of screw pitch or TPI would you recommend for wood engraving/carving? And what power size should be used (etc. Nema 23, 270 oz/inch)? Please help me to realize the precise screw TPI and stepper motors power? The X axis moving gantry totally weighs about 10 lbs (spindle included). However, I would appreciate any advice from you.

Sorry I missed this... You're really not pushing a lot of weight. Even so, consider Gerry our moderator has a larger machine, and uses 250 in-oz steppers and antibacklash nuts with 2-start ACME screws. If you use ballscrews, they're way more efficient than ACME screws, which would reduce your motor size even more.

My guess would be to use 1/2"-10, 2-start ACME screw and anti-backlash nuts. This will give you a mechanical resolution of .001". Your axes are small enough where any lead error can be virtually erased by tweaking the motor configuration. You probably could use 180 in-oz steppers; even then I would guess that you could still push a 1hp trim router to its limit.

Ballscrews are a different beast. The cheaper rolled screws you see on eBay and such can have a lead error almost comparable to ACME screws. And not all ballscrews are anti-backlash; removing play has to be done wither with a double-nut, or replacing the bals with larger ones. They can be noisy as well. Precision gound ballscrews would be best, but the cost can be prohibitive. Now that everyone is bidding for them it's getting harder to find good deals. You need proper end bearing blocks to take advavntage of a ground ballscrew's accuracy, otherwise you're throwing money away. And if you need the ends machined it is way more costlier. And none of this would matter if you used something like a Dremel as a spindle, whose runout is probably more than the backlash or lead error of a ballscrew. So you'll need a better quality router or spindle as well.

Another thing to consider is what kind of linear rail system you plan to use. It might not be a wise idea to pair ballscrews with a skate bearing system, unless you use something like CNC Router Parts carriages. Just the same, you could use ACME screws with either a home made linear rail, or round rail or even profile rail.