Today, I milled two slots!
4mm wide by 5mm deep slots in the steel strips shown in the previous post. The slots will have simple covers placed into them.
Next I've devised a bit of a mad scheme to drill the array of M6 holes in the bed:
Then checked that it was sufficiently parallel - couple of whacks with the nearest hammer and it's now within 0.05mm over 400mm.
Now to continue the drawing...
Ok, broken tap situation resolved...
I intend to buy the fasteners I need for this tomorrow, so time for some rough calculations to check what grade I should use. From the THK documentation, I gather that the tightening torques for the M8 and M6 fasteners in the rails should be 30.4Nm and 13.7Nm respectively. They don't recommend a grade of fastener (as far as I can see), so we are left to work out the tensile strength required ourselves.
First step, use the torque value to calculate the tensile force on the fastener. This is the same formula as working out the axial force you get for a given torque on a ballscrew, so I don't even need to look it up (although can be more accurate by including more in this):
This gives the force, for a given torque (tau), coefficent of friction (mu, ~0.2 for steel) and pitch. Machine screws are graded based on the tensile stress, so we need to divide this value of force by the cross sectional area of the screw to arrive at the stress, in N/mm^2. The stress area is roughly the minor diameter of the fastener, so roughly the diameter of drill you would use. The area is then clearly just the area of a circle.
So, e.g, the M6 fasteners are 1mm pitch, 13.7Nm and steel, so the force is 13.7*2*pi*0.2/0.001=17kN. Divide that by the stress area, 17000/(pi*2.5^2)=865Mpa. Bolts are graded such that the first number is 1/100th of their tensile strength rating, so e.g. a 10.9 grade fastener is rated for 10*100=1000Mpa. A rule of thumb suggests to only tighten a bolt to about 80% of its tensile strength rating, so in my case I will be OK with 10.9 grade fasteners for the M6 screws (especially as the actual stress for the given torque will be a bit less due to things I missed out for brevity).
Now I just need to remember to bring my torque wrench home...
Also noticed that RS have the caps to help prevent debris getting into the linear rails, so I'll get some of those.
Last edited by Jonathan; 04-01-2016 at 01:24 AM.
good to see your doing something at last!
if you need a lift with ought let me know. Although I about give myself a hernia the other week lifting the motor ontop of my mill.
Today I almost finished tapping the bed and made some bits for other projects / people. That's about it ... will buy some steel flat bar I need tomorrow to finish off the bed.
Last edited by Jonathan; 04-01-2016 at 11:36 PM.
I made some progress - over the weekend I machined the end pieces for the bed:
I have now left the bed with "Leytoner De Montford LTD" in Leicester to get both sides surface ground. First time I have needed someone else to machine something for me...
Next step is to acquire a straight edge asap to align the rails and decide on what bearings to use for the ballscrews (not sure on angular contact vs taper roller) so I can machine the housings.
Last edited by Jonathan; 11-01-2016 at 01:56 PM.
this thread makes me wish I had space to set up my RF45 clone and finish the conversion... I too did a huge part of the machining on a round coloumn mill. Will post photos when I find them
Just wanted to chime in on the bearings front. I would suggest you go with angular contact bearings and ones specific for ballscrews or ones specifically made with medium preload as standard.
Angular contact bearings have lower thrust rating than taper rollers but you will not need to add any spacers to get the preload to them right and also have lower friction. Additionally they take up less axial space. There is also a lower limit to the ID of taper rollers which from memory is 14.989mm (imperial).
Did you mention what size/grade ballscrews you are using?www.emvioeng.com
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11-01-2016 #29Did you mention what size/grade ballscrews you are using?
This document compares the virtues of triplex angular contact bearings vs a pair of tapered roller bearings and seems to conclude that the main limitation is the additional friction. In terms of stiffness, they are pretty much the same. I don't really consider the additional torque to be an issue, as that is a secondary consideration compared to the bearing axial stiffness - in essence I can just use slightly higher rated motors, which wont be a concern if I use my own. I will use at least 15mm bore bearings, so the available sizes are not an issue.
Some time ago, I did buy some angular contact bearings from aliexpress with 12mm bore that were supposedly matched pairs. Although they have the markings of matched pairs, they don't actually seem to be so, hence I am a little wary of trying that again with more expensive bearings, e.g. BS1547TN1 or BS1747. Interestingly the load rating of ballscrew specific bearings is similar to that of similarly sized taper roller bearings - probably the stiffness requirement dictates a certain load rating.
From what I've read/calculated the stiffness of generic single row <60° angular contact bearings in matched pairs is low compared to the screw/not or simply unclear/unspecified. There's a formula floating around for calculating the bearing axial stiffness for given dimensions, which I can do from the data on the SKF site, but the resulting values don't give much confidence. I fear that if I use these the stiffness of the bearings would be noticeably less than the ballscrews, so when combined in parallel with the nut & screw stiffness, it would not be a well balanced system, hence I want to at least get 60° contact angle bearings, if not roller bearings.
Last edited by Jonathan; 11-01-2016 at 02:54 PM.
The stiffness you get from the bearings is also dependent on the preload class you use. Just like in the ballscrews, you can have zero backlash (P0) but you can also have P3 and P5. And of course there is the precision class and clearance to them too. The more accurate, the stiffer the bearings can be claimed to be.
You are right about the price though. On My KRV I went with heavy preload and spent around £240 for 2 sets.
You should also think about the stiffness of rotation perpendicular to the axis of the ballscrew. this will govern which way round you put the bearings. I forget which orientation is stiffer (face to face or back to back)
In the end I bought IBC bearings with 60deg angle medium preload. You can find all you need here: http://www.ibc-waelzlager.eu/dmdocum...t_Bearings.pdf
The ones I would have bought had I not bought the ones I did are: http://www.qualitybearingsonline.com...ct-20x47x15mm/
Another slightly overlook aspect is the locking nut you use should be as good as you can get it. The squareness of the thread to the face is not great in some of the chinese nuts I have come across. In my case I also wanted to lock on the front face as opposed to on the thread. Too a while to find anyone in the UK what had them.www.emvioeng.com
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