Thread: A few questions

a "ballscrew nut" is the peace that bolts to your carrage , and travels along the ballscrew when it spins

pitch is how far the "nut" will travel with one turn of the thread , measure from one side of the screw thread to the next thread , same spot on the "tooth " , some acme/ballscrews , have double or tripple/ quad , turns , so the next thread might not be " one rotations " thread ,

gecko's are the drivers , a box that controls the stepper , an amplifier of the Break out boards signal with direction control and speed control , the difference between a gecko and a china stepper driver is "chalk and cheese" , also some geckos are so idiot proof , there near on impossible to blow up , ( higher priced ones ) dead short them , connect wires up wrong ,over voltage , none will hurt a ( select model ) gecko

there are heaps of different steppers , 4 wire , 6 wire , 8 wire , your stepper controller ( gecko ) has to match the wire out of the motor
they also come in different sizes (power) , measured in oz/in or Nm , the number stated on the motor is its "hold weight " how strong the motor is holding its position , the size of the motor ( foot print ) is in a "nema size" 17 , 23 , 34 and gear boxes use the same size units, they also have different shaft sizes , and number of shafts , dual shaft is good so you can but a hand adjust wheel on the back

so your motors might be a " 4 wire , 50oz/in , nema 17 " stepper

so 【Germany ship】NEW 3PCS CNC NEMA34 Stepper Motor 1232OZ-IN,5.6A,118mm,4leads CNC | eBay would be a 4 wire , 1232oz/in , size "nema34" stepper with single shaft

Re ballscrews and balscrew nuts vs ACME thread vs Conventional screwthread.

You are familiar with a conventional screwthread as used on nuts bolts and screws, a triangular shaped thread with the thread sides angled from 47.5 degrees (BA threads) to 55 degrees (BS threads) or 60 degrees (metric or UN threads). These threads have reasonable load carrying ability and strength, but can be stripped reasonably easily if overloaded and are not intended for repeated loading and releasing. Male and female portions tend to bind as loaded to minimise self loosening tendencies and remain tight in service. Modest backlash until locked together


ACME threads are a near square threadform, they have a side angle of around 80 degrees, and substantial flat portions at the top and base of the threadform. They have a substantially greater advance (or pitch) than an equivalent screwthread, and the larger thread profile and reinforcement provided by the flat tops provides a lot of strength. Generally used for vise screws and lead screws in machines, they tend to be non binding and well suited to repeated drive and reverse applications. Much less friction in use than an equivalent screw thread and typically similar backlash unless special compensating nuts are used. May be multiple start, with a threads starting typically 180 degrees apart for 2 start thread, 120 degrees apart for 3 start thread, or 90 degrees apart for 4 start threads. Multiple start threads have longer pitches and hence require fewer turns to travel the same distance without affeting load carrying ability.

Ballscrews and their nuts are a later evolution, with the thread form having a semicircular profile. The nut houses a number of ball bearings and passages that return them from the exit point of the nut to the entry point so that they recirculate constantly. Typically a low friction, low backlash, high speed alternative to an ACME thread. Can be multistart if required for the application.


Re Pitch, this is the distance any screw will advance with one rotation. Normally can be measured between corresponding points on the threadform, but it is safer to rotate the screw one turn and measure the distance that the nut moves, as it will increase in the case of multistart screws. For imperial threads, pitch is normally expressed as threads per inch (TPI) while for metric threads it is normally expressed in mm. For example a 5/8inch arbour thread for a saw would be 5/8 x 18TPI while the nearest metric equivalent would be 16mm x 1.5mm. There is a difference of about 0.1mm in both diameter and pitch between the two sizes, but the male and female threads will not fully engage (fasten) if mixed.


Sorry for getting technical, but hope this helps you understand the differences.

Thank you Mal
As clear as mud
Actually, I did understand it

I am re-making my CNC Shark HD and in the process found the nuts for the ACME? nuts was made from HDPE
The Z-Axis nut looked a bit worn so I wanted to replace it.
Made an inquiry to New Wave Automation about replacement nuts. OMG $60 plus shipping for 3 plastic nuts. They tell would not tell me the pitch of the screws.
As I am replacing all HDPE parts with aluminium, I asked them about whether the motors were strong enough for this or whether I should buy stronger ones. There is nothing on the motors to identify them and all I was told was that the control box is set to suit the motors and that, if I replace them they will be out of sync. Not happy Jan.
So I wanted to find out whether I could buy taps and make the nuts myself in brass or such.
Unfortunately my local bearing shop could not help me apart from telling me the diameter and TPI of the screws but not the angle/pitch. Looks like I need 3 different taps for 3 different screws

Saisay.

A few observations from a nut who has never made an ACME screw or nut but has done both for screwthreads with tap and die or screwcutting lathe.

They would use HPDE or a Delrin like plastic for the nuts because they are strong, wear resistant, low friction, and a machinable plastic. If you could replace the nuts with ali ones, they most likely would have worse wear over time (more backlash) and would probably have rougher contact surfaces and higher friction. Depending on material grade and surface finish the could gall and bind on the screws.

As I said I have never cut ACME threads, but I doubt that you would cut an accurate internal thread for them with a tap. I don't know if ACME taps are available as I have never sought one, but suspect that they would be rare and expensive beasts if they exist. For CNC lead screws and nuts you would want something fairly accurately machined.I think that individual OEM replacement HDPE would be the way to go if needed.

There was discussion last year in the metalwork section about a Preston (Melb) factory closing down. They had 40 + years experience and a lot of excess stock of ACME threaded screws and nuts, it was their main product line. Owners were retiring and clearing business, but I don't know if they have gone or not.

I know that this is contradicting your wants/beliefs, but helps to give you a better understanding of the issues involved.

Thanks Mal
I will continue with the old ones and decide what to do in the meantime.
I still believe they are fleecing customers charging $60 for them but that is the evil of monopoly

You say that the Z axis nut is showing signs of wear. Is this obvious in backlash at the spindle, or just visible as discoloration on the unit?
If there is excessive Z backlash you may be able adjust it by slitting part way through the block at rightangles to the leadscrew with a hacksaw and pinching the gap with a couple of screws and nuts, provided that there is enough material for this to work. This will reduse the area in contact with the screw and increase wear in the contact area, but if the backlash is making the machine useless, it would at least get it going. It is obviously a butchers solution, but a little better than no solution, and it is adjustable to take up backlash as it develops over time. Have sketched out the idea assuming a simple block of material for the nut.

It is possible to make your Acetal or Delrin nuts. Here's a link to one method.
Making Acetal leadscrew nuts the easy way
You can also use a pourable plastic. Just make a dam around the screw. The above used round stock but I see no reason why square or rectangular wouldn't work too.

If it was my machine and was after better accuracy and longevity I would go ballscrew on the Z axis. You then can use the old nut to double up on another axis giving less backlash and then would only need one more nut to double up the third axis. Then you'd end up with a durable and accurate Z axis which carries the most load and less backlash on the rest. Ideally go full ballscrew but that's costly. Even if you use ebay ballscrews you'll be better off than the acme/delrin setup. Also ballscrews will use less torque than acme so you'll get a slight performance increase too due to less friction.

I've seen brass acme nuts. My mill had these until I CNC'd it and went dual nut ballscrew.

Steve

saisay
You may like to think in terms of a proper ballnut and screw.
You would also need the mounting blocks and bearings, and the ends would need to be turned to the correct diameter as well.
What size do you have on there at present?

The bit about the motors being out of sync is true but garbage. So long as the motors are of a fairly similar voltage and size I could not forsee too much problem with swapping the motors for some others so long as the physical size would fit too.

Originally Posted by Bob Willson

saisay
You may like to think in terms of a proper ballnut and screw.
I bought this one, hope it fits.
You would also need the mounting blocks and bearings, and the ends would need to be turned to the correct diameter as well.
What size do you have on there at present?
1/2" round x 8 tpi, trapezoidal for the Z axis (I think).
1/2" round x 8 TPI for the east/west (forgot which axis)
M10 x 8TPI for the north/south, according to the bearings man.

The bit about the motors being out of sync is true but garbage. So long as the motors are of a fairly similar voltage and size I could not forsee too much problem with swapping the motors for some others so long as the physical size would fit too.

Thank you Bob
I have already bought the mounting blocks and adjustable bearings for the Z axis, should be here next week.
I have finished setting up the table, ready for the gantry tomorrow.

Thank you maggs, that sounds very interesting.

You guys are great