[Roadsidejobshop]

Hello people,

I've been trying to decide on which type of mill to build and considering my availability to machines and tools, I have ruled out a RF45 clone, a fixed gantry mill (unfortunately, i really like them) and have decided on an inline CNC mill for small aluminium parts production. The bed is 200x350mm and has a Y-axis travel of about 250mm, gantry height is about 200mm(between table and gantry bottom) and X-axis gantry has a travel of about 350mm. Z-axis should have a travel of 150mm.

All rails are HIWIN HGR15, frame is made out of steel tubing, current weight (in the model) is around 101.3 kilograms (i'm guessing around 160-170 with all the parts).

I also did some preliminary FEA testing on the individual parts (after having calculated the cutting force for the largest cut I could imagine taking, 6mm endmill, 2mm cut depth), which came out to about 800 newtons. Using this I found that all parts have very small deflection (Gantry was highest, around 0.012mm when 800 newton force applied to upward, leftward and inward to the front face(2400N total).

Anyways, thanks for the feedback, motors are planned to be salvaged NEMA 23, which will be later swapped out for either closed loop steppers or maybe even servo motors (if i ever get that much money).

Thanks!

[Hezz]

Hello people,

I've been trying to decide on which type of mill to build and considering my availability to machines and tools, I have ruled out a RF45 clone, a fixed gantry mill (unfortunately, i really like them) and have decided on an inline CNC mill for small aluminium parts production. The bed is 200x350mm and has a Y-axis travel of about 250mm, gantry height is about 200mm(between table and gantry bottom) and X-axis gantry has a travel of about 350mm. Z-axis should have a travel of 150mm.

All rails are HIWIN HGR15, frame is made out of steel tubing, current weight (in the model) is around 101.3 kilograms (i'm guessing around 160-170 with all the parts).

I also did some preliminary FEA testing on the individual parts (after having calculated the cutting force for the largest cut I could imagine taking, 6mm endmill, 2mm cut depth), which came out to about 800 newtons. Using this I found that all parts have very small deflection (Gantry was highest, around 0.012mm when 800 newton force applied to upward, leftward and inward to the front face(2400N total).

Anyways, thanks for the feedback, motors are planned to be salvaged NEMA 23, which will be later swapped out for either closed loop steppers or maybe even servo motors (if i ever get that much money).

Thanks!

Looks to me like a good little design. As always with weldments you can never get them exactly perfectly aligned so there needs to be some way to get the final alignment of all the axis dialed in. I think you could put a double steel plate on the lower table and shim it to align all of the axis and compensate for any error in the frame. I would also put .250 inch thick steel plate webbing sheets between all of the uprights on the gantry supports to make it really resistant to twist. But it would probably cut aluminum just fine the way it is. But why not max out the potential of this design since it is small enough to do cheaply. And I think it has good potential. You could then cut some steel if you needed to. You could also put steel sheet webbing on the z axis support beam to stiffen it up a bit without too much added weight. I think a tweaked version of this would be a good design. The low height wide gantry looks to be especially good and should be more resistant to vibration than a tall design.

[Eldon_Joh]

so your Z travel is similar to a more standard milling machine of similar Y travel, meaning: you seem to be swapping Y with Z.

any reason for this?

the 15mm rails may be the limiting function, they have very little dampening coefficient.


a machine as small as you have designed, for the loads you mentioned (6mm diameter end mill, 2mm depth of cut) you can fill and or surround your machine with either.. concrete.. or dry sand. either will be good enough. you will easily meet the limitations imposed by 15mm rails.


anyhow, i'm not sure which is X and which is Y, but your Z axis has to stick out at least as far as your X or Y axis plus clearance. this is not really optimal.

optimally, the spacing between your two X and two Y rails has to be at least as wide as your X or Y travel.

again, not really optimal.

however, the 6mm diameter end mill, 2mm depth of cut can be handled by a 1/8th hp or perhaps 100 watt spindle, so, you should be able to throw this together and it will "just work"

[Roadsidejobshop]

I will certainly build in some way of adjusting the axes, so it produces square cuts. I am planning on skinning/webbing the entire lower frame (except right next to the beds - so I have a flood coolant runoff area). I did do some calculations for cutting steel and it should handle 1mm deep cuts of steel pretty easily (with 10mm endmill). With the extra stiffening pieces/webbing it should now handle a 6mm endmill doing a 6mm deep cut with a deflection of 0.014mm (but real life is often not as optimistic). I will modify and improve the a bit more and see what the end result will be. And as always - I will try to fill the 80x40 tubes with epoxy granite to give it a bit more dampening cabability and weight.

Thanks!

Looks to me like a good little design. As always with weldments you can never get them exactly perfectly aligned so there needs to be some way to get the final alignment of all the axis dialed in. I think you could put a double steel plate on the lower table and shim it to align all of the axis and compensate for any error in the frame. I would also put .250 inch thick steel plate webbing sheets between all of the uprights on the gantry supports to make it really resistant to twist. But it would probably cut aluminum just fine the way it is. But why not max out the potential of this design since it is small enough to do cheaply. And I think it has good potential. You could then cut some steel if you needed to. You could also put steel sheet webbing on the z axis support beam to stiffen it up a bit without too much added weight. I think a tweaked version of this would be a good design. The low height wide gantry looks to be especially good and should be more resistant to vibration than a tall design.

[Roadsidejobshop]

Very observant, but not entirely true (on the Z and Y axis swapping). It is related to it, but the main reason for this design is that the parts I want to start manufacturing at max 10mm thick and I don't plan on making any large parts on it (most of the stuff I want to make fits into the palm of your hand). Also, the accuracy does not have to be within 0.005mm - even if the part is off by 0.1mm I'm well within tolerance (as they are bolt-on accessories, no pressfits or anything).

I will ask why do you think the 15mm rails will be the biggest issue? The router at my work (granted, its a router) has 20mm rails and runs through aluminium really well(and even with rack and pinion, the accuracy is acceptable at around +-0.1mm). I constantly see people building routers with 20 or even 25mm hiwin rails and then run their machines at 20-30 inches a minute and seem so happy with the results. With 15mm rails I should be able to run my machine comfortably at around 25-30 inches. The spindle I am planning is going to be a 1 kw router (18 000 rpm max) that feeds into a straight shank ER16 collet spindle. I would have two speeds: 1:3 and 1:6. That should give more than enough variation for both steel (although I am 90% sure I'll never try to run steel parts on it) and aluminium (3000 and 6000 rpm respectively).

Thanks!

so your Z travel is similar to a more standard milling machine of similar Y travel, meaning: you seem to be swapping Y with Z.

any reason for this?

the 15mm rails may be the limiting function, they have very little dampening coefficient.


a machine as small as you have designed, for the loads you mentioned (6mm diameter end mill, 2mm depth of cut) you can fill and or surround your machine with either.. concrete.. or dry sand. either will be good enough. you will easily meet the limitations imposed by 15mm rails.


anyhow, i'm not sure which is X and which is Y, but your Z axis has to stick out at least as far as your X or Y axis plus clearance. this is not really optimal.

optimally, the spacing between your two X and two Y rails has to be at least as wide as your X or Y travel.

again, not really optimal.

however, the 6mm diameter end mill, 2mm depth of cut can be handled by a 1/8th hp or perhaps 100 watt spindle, so, you should be able to throw this together and it will "just work"

[Eldon_Joh]

if you get new, preloaded rails you'll be fine.


alternatively redesign your entire system to a dual bridge, or bridge style gantry machine where you should be able to save 100 kilograms of mass and get the same rigidity.


as it is now you sort of have a machine that could be made 100 times longer in the X axis with almost no additional loss of rigidity.

[Hezz]

if you get new, preloaded rails you'll be fine.


alternatively redesign your entire system to a dual bridge, or bridge style gantry machine where you should be able to save 100 kilograms of mass and get the same rigidity.


as it is now you sort of have a machine that could be made 100 times longer in the X axis with almost no additional loss of rigidity.

Not exactly sure but I think this might be something like Eldon_John is talking about. It's very similar to your design but much more rigid and has better travel in the bottom axis.

Sorry for the blocky model. I was just trying to show a quick concept. IF you were to go with this you would certainly have a nice little benchtop machine that was very rigid.