First Proper CNC Router

[Boyan Silyavski]

Whats so great about A882 drivers, Iv seen a number of people recommend them.

Leadshine are a brand name already that is proven these last years for top quality and reliability. Or you could buy a copy of a copy and cross your fingers it will work properly.

[diycnc]

Thanks for the advice. I will defiantly use fixed supports on both ends of the ball screw. it will add no extra cost, and as JAZZCNC suggests will improve rigidity.

I have been working through some of the final details of the design. major changes since last time are i have added the motors belts and pulleys.

Y axis motor is attached to the gantry end plate (which is now 15mm thick, 10mm just look to flimsy!) with 4 50mm long spacers. 1:1 drive to the screw is achieved with two 15 tooth htd 5 pulleys and 15mm wide belt. Can anyone suggest if this is enough teeth on the pulley, or point me towards the datasheets and formulas to work it out. i assume there is a max load per tooth to keep below.


X axis motors is attached the base frame with a long 5mm thick steel plate (I think i need to added a stiffener to this, looks very long and floppy at the moment!). The motor hangs down quite far to avoid the piece of extrusion connecting the bottom of the gantry end plates. I tried removing this extrusion, and replaced it with two small plates to connect the ball nut to the end plates, but a little FEA showed this was a very bad idea! The long end plates twist allot!!
1:1 drive to screw again with 15 tooth pulleys. I can increase to bigger pulleys here if needed.
Larger taper lock pulleys and 15mm wide belt are used to connect the two screws. I will size the pulleys later to suit the available belt length and tensioner adjustment. Belt tensioner will just be couple of bearing on bolt in a slot, no need for anything complicated here.


Next thing to do is add all nuts and bolts to the model to make sure nothing clashes!

I also managed to pick up a used kress 1050 for little over £100. Would rather have one of the Chinese water cooled ones that everyone else uses, but at 300+ they are pricey. If i dont like the Kress i can always sell it on and probably get back all my money anyways.

[Davek0974]

Looks good but i have a general dislike of motors mounted on long tubes - not too stable and prone to loosening.

[Clive S]

Y axis motor is attached to the gantry end plate (which is now 15mm thick, 10mm just look to flimsy!) with 4 50mm long spacers.

Have you thought about how you are going to tension the belt?

[diycnc]

Have you thought about how you are going to tension the belt?

I was planning on slotting the holes in the gantry end plates so I could just slide the whole motor up and down.

[routercnc]

My first ever machine had 4 posts like that and it took a while to get them all the same length. I didn't have a lathe back then. If they are out a bit the motor will be skewed and the belt won't run straight on the pulleys.

The other more popular option is to put the motor behind what is in your drawing the small orange box section, and run the belt horizontally out the back of the gantry, between the upper and lower beams. You just need a big L bracket to mount to and it is a bit more tucked out of the way.

For your other question - there are lots of pulley engagement formulae around but you can tell by looking that you are OK there with ~180 degree contact.

Make sure you have pulley / belt clearance should you ever want to do 1:2 instead of 1:1 to get more speed, and have adjustment in the motor plate to accommodate.


On the overall design it will work fine, but do think through if the underslung ballscrews will be easy to set up and align. Imagine that the base frame cross members (which the ballscrew blocks mount to) is not perfect, and end up on a different plane to the rails. It will tend to bind the ballscrew at some point and you will have to shim something in the system.
If you go the more traditional route with the ballscrews mounted down the side, driving the side members of the gantry directly (no underslung part), I think all the connections can be made with the appropriate 'slip plane' to get it aligned. This will quicker, cheaper and simpler to build and would be my preference.

Up to you but just make sure you can visualise the adjustment process for any scenario if it is not perfectly built.

If you are going underslung just to stiffen the gantry I'd put the efforts in on the gantry itself, e.g. plating across the back etc. (Not saying it needs further stiffening mind).

[diycnc]

I have a Clark mini lathe so getting them the same length should be easy.
I did consider putting the motor behind the gantry... can't remember why I chose not to. I'm sure there was a reason. I'm gona look at this again cos it would be a nicer design.
Thanks for the pulley info, I will take a look at the design formulas for reference anyway. They might come in useful one day.

[JAZZCNC]

The motor location is a no-brainer to me. Machine the gantry side to accept motor with a slot for adjustment and lengthen ball screw so goes thru side then put the belt on outside of gantry out the way from chips etc and put a cover over. Simple and neat.

Pulley size is little small but nothing to do with teeth but because you won't have enough Boss left after machining for grub screws. This will then mean putting grub screws thru the teeth which isn't ideal.
I find 20 teeth gives good balance leaving enough boss for M5 grub screws. Certainly no less than 18T.

[diycnc]

On the overall design it will work fine, but do think through if the underslung ballscrews will be easy to set up and align. Imagine that the base frame cross members (which the ballscrew blocks mount to) is not perfect, and end up on a different plane to the rails. It will tend to bind the ballscrew at some point and you will have to shim something in the system.
If you go the more traditional route with the ballscrews mounted down the side, driving the side members of the gantry directly (no underslung part), I think all the connections can be made with the appropriate 'slip plane' to get it aligned. This will quicker, cheaper and simpler to build and would be my preference.

Up to you but just make sure you can visualise the adjustment process for any scenario if it is not perfectly built.

If you are going underslung just to stiffen the gantry I'd put the efforts in on the gantry itself, e.g. plating across the back etc. (Not saying it needs further stiffening mind).

Thanks for the advice. The way i imagine it is the screws first need to be on the same plane as the gantry travel. With the current design this can be achieved just with shims. Then the screws need aligning to the axis. Hopefully there will be enough play in the holes in the BK supports to allow enough adjustment. If not i can simply drill over sized holes in the underside of the frame, and use either nuts inside the tube, or if i cant get a spanner in there make a small plate with 4 threaded hole. This will give me plenty of adjustment to compensate for the inevitable poorly assembled frame!