I recently brought the above and would like to convert this machine to CNC. I have read a couple of excellent build logs that Tweaky and Robin Hewitt have written. My question is regarding the diameter of the ball screws to use. The machine currently has a 24 mm dia for the x and y lead screws. As Robin found out there is a tight fit between the underside of the table and the base on the cross slide for the ball nut housing. I would like to use a 25 mm dia screw as this is close to the current lead screw. The two sizes of ball nut, 20 mm and 25 mm, have a dimension of 44 mm across the double 'D' and 48 mm respectively. I have measured my machine and the distance between the base on the cross slide and the majority of the table is 44 mm. The two ends of the table are closer and would need to be machined away to give the whole table the same clearance. If I were to machine a pocket where the base is on the cross slide to a depth of 5 mm from the original top of the base I should have room for either size ball screw. I am just not sure if a 20 mm ball screw would be OK. The table on this machine is 800 mm long. I would be interested in your thoughts.
Has anyone had any experience with ball screws from Marchant Dice Ltd?
Thank you in advance
Hi and welcome to the forum.
The sizing of the screw depends on three things.. Firstly the speed you plan to rotate it and how its physically mounted - too thin a screw will 'whip' at speed. on an 800mm length of 20mm fixed in bearings both ends the critical speed is 2700rpm, thats 13.5m/min traverse on a 5mm pitch - I dont think that an issue!
Secondly, what is known as Euler buckling... the tendency of the screw to bend under the load (think of it as a column with a weight on the top). Again for a 800mm long, 20mm screw in a fixed bearing at each end held in tension the load is around 7000N (7 tons), again not an issue for a mill of this size (max cutting force probably a tenth of that)
The thrid factor is simply the load rating of the ballscrew and nut... can it transmit enough force through the thread of the screw.. looking at a typical manufacturers catalog (Nook), a 20mm x 5mm nut can handle 10800N dynamic load for 1million rotations or 21000N static thrust. The screw will buckle long before the nut fails!
I'd go with the 20mm...
IIRC the ribbing under the bed gives more clearance than the last 3/4" at the ends, so you can fit the larger X nut if you don't mind removing the tapered Gibb completely everytime you want to get at it. The table would need to tilt.
The Y screw nut fixing runs in a horrid-zontal, U shaped slot. Obviously the nut extends beyond the nut fixing and has to fit under the ironwork. Lengthening the slot would weaken it substantially. This required a bit of angle grinding from below for even the 16mm nut. You may need to deepen the Y screw entry hole on the front of the mill so you can fit the screw a bit lower down.
I found the screws securing the Y nut collided with the column, thus limiting the rearwards Y travel a tadge. Mount the nut further forwards and you may limit it in the other direction. Compromise and lots of measuring required. I got an extra couple of mm by using button heads. Durned difficult to get the angle grinder in there to relieve the iron.
I preloaded all the screw mounts and double nuts to around 1/4 ton, well within the capability of a 16mm screw, but that does set my top load limit for zero backlash.
The accuracy limitation seems to be the head rotating around the centre axis of the round column. My Y axis cuts to better than my digi-calipers can measure, 5um. X leaves a 25-50um excess depending on the final cut. You can't remove 25-50 microns if the head can shy away and rub, you could go round until Dooms Day and it would still be there :naughty:
Thank you for your reply. I have attached three photos. One picture is of the mill that I have. Note there is no round column. One picture is of the current nut which is bolted to a raised base on the cross slide casting and the last picture is of your cross slide casting which looks the same as mine. It is that raised base that I thought of machining to give more clearance for the X nut housing. The top of the current X nut does collide with the last 100 mm at each end of the channel under the table but has 45 mm approx clearance for the rest of the travel. The FSU nuts for 20 mm are close to colliding so this is why I thought of machining the base down by 5 mm. This would give me about 1/2 to 3/4 mm deep pocket to locate the X ball screw housing.
Thank you for the heads up on the Y axis, did not think about that.
Marchant Dice Ltd can do some ball nut / screw assemblies with 0.00 to 0.15 mm back lash. That would do me fine. Based on irving2008 comments and yours I will go for 20 mm screws, 25 mm will be pushing my luck.
Warco only support the X axis screw at one end, thrustwise. If you stick with that arrangement you may need a fat screw. If you hold X in tension you can save yourself a lot of work trying to fit fat ballnuts.
If you are planning on using the Warco thrust races, you are probably also planning to skip putting a screw on the Z. This is a recipe for disappointment.
My experience with Marchant Dice has not been wonderful. I replaced his "zero backlash" screws with Gary's screws, then added springs. Kevin once kept me waiting weeks for a piece of 10mm rod.
Thank you once again Robin
I want to control x , y & z via CNC. In these few short posts and your machine build log I have gained enough knowledge to go forward. I need to do a lot of measurements / drawings before firming things up. Once I have sorted out what I'm going to do I will post a machine build log with pictures.
I will take on board your comments about Marchant Dice and look else were.
Thank you once again for your help
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