Had a go using motorcalc sheet but does not look right.

[irving2008]

this particular one is my design but the concept is common. If I was to do it again for a bigger diameter leadscrew (as I may be doing) I'd not use 8mm skate bearings but a 10mm x 30mm double row angular contact bearing type 3200-2RS as this has good radial and axial support and is simpler to use (one bearing instead of three), but obviously the screw root diameter needs to be >10mm.

[Bob_series1]

My quick google search for that bearing showed a cost of £15 each.

The original design the cost for the bearings was
Thrust x2 @ £1.10
Abec 7 Skate bearig x1 @ £1
£3.20

So why is this bearing worth 4-5 times the cost?

[irving2008]

Because the quality of an angular contact bearing is much higher and they are much more complex... however you can get them cheaper on eBay, though this is not a sealed bearing so that may have a bearing (excuse the pun).

If you can go to 12mm ID then here is one for £4.95

[Robin Hewitt]

Acceleration time 0.02 sec

That does sound a bit excessive :nope:

The place I came unstuck was the first time I encountered a G0 suddenly hammering the X axis in to reverse, the worst possible case when it comes to losing steps on a motor, even though you can ignore the cut pressure. My software just threw one in at the cut start, out of the blue, just for fun.

I eventually set an 'okay to hammer in to reverse' threshold, that exempted most normal cutting from accelerations, then gave it a quarter turn acceleration time on the G0's.

Setting the top speed is a bit suck it and see, but you can get an indicator by looking at the torque v speed curve for the motor to see where it starts to plummet.

I need to be a big comfort zone on the G0 speed. Having fitted new super 220V drivers to my machine it would probably be good for 25 mm/s plus, but I still have it set to 10 mm/s. I know it would work fine if I ramped it up, but I just haven't found a job yet where I don't mind risking the metal to find out :naughty:

[Bob_series1]

Should be 0.2 seconds (I think).

Yes those bearings look very good value. If you do go with them, I would be interested in a review.
For now I am going to go with your Mk 1 method, If I have permission to use your plans of course.



Robin, Whats a G0?

[Robin Hewitt]

G0 is the first G-Code, it signifies 'go to the following co-ordinates as fast as you can, doesn't have to be a straight line'.

Usually used to position the tool, cutting air.

G1 does a straight line and applies a feed rate.

G2, G3 are clockwise/counterclockwise arcs.

You can pick and choose from the rest, I ignore most of them.

[irving2008]

That does sound a bit excessive

Should be 0.2 seconds (I think).

The value of 0.02 seconds was taken from a document which I can't find now, from a major CNC manufacturer qualifying small, medium and large machines. The 0.02sec was the spec from the medium machine. Its possible that 0.02 is a bit aggressive, but 0.2 is def too slow.. The reason for using a time rather than an absolute acceleration is to make the acceleration dependent on the maximum speed, as thats where the torque is required.

I have worked out another way to estimate acceleration based on pocketing a circle, obviously to maintain feed rate as the radius reduces the rate of change of linear direction X/Y is directly related to increasing acceleration in the individual axis. This calculation shows an acceleration requirement approximately 1/2 - 1/3 (depending on the radius of the pocket) of that given by using 0.02Sec, suggesting 0.04 - 0.06Sec might be a better value. I shall be updating the sheet with the revised numbers shortly.

Yes those bearings look very good value. If you do go with them, I would be interested in a review.
For now I am going to go with your Mk 1 method, If I have permission to use your plans of course.

I am going to use the £4.95 12x32x15.9 one in my rotary table conversion; although I could get away with an open bearing there, the bigger bearing makes the machining slightly simpler and i prefer the sealed one for peace of mind. Of course you can use my plans, the technique is common.

[Robin Hewitt]

Is that .02 seconds to go from zero to top whack?

I allow about 1/8 of a second ('ish), this is what it sounds like on a G0...

[irving2008]

Clearly the value for G0 is different than that for G1.... what value do you have for G1?

Maybe I should make the spreadsheet deal with different speeds and accelerations for G0 and G1 and estimate the motor power based on which is worst case... As stated the value of 0.02 will produce an overestimate on the motor size for large routers or table-based machines but isn't too bad on a small lighter machines

[Robin Hewitt]

Clearly the value for G0 is different than that for G1.... what value do you have for G1?

Now there's a question! I'd just wired up the new drivers and ran the last job on the computer for a look see. The X axis is in reverse.

Probably 1.5 mm/s, I usually cut between 1-2 but I might drop to 0.8 for a plunge.

When the cutters get long and spindly, even slower.