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wierdscience
04-29-2017, 02:34 PM
Got a project I would like to do where I need a small ball leadscrew and was wondering about the smaller 1204 metric units coming in from China.Anybody tried these?Opinions?Backlash?

https://www.banggood.com/250mm-SFU1204-Ball-Screw-With-BK10-BF10-End-Supports-For-CNC-Parts-p-1101188.html?rmmds=search

Last ones I used were from Taiwan,but much more expensive.

J Tiers
04-29-2017, 03:27 PM
Initially thought of something along the lines of the "Malay boot".

Maybe that IS pretty close, if the accuracy difference tracks the price difference, and you use it in a CNC application.........

enginuity
04-29-2017, 05:31 PM
Tormach uses Chinese ballscrews and I think they are pretty good.

Chinese ballscrews are like Chinese bearings - usually pretty good initially - just don't last as long so your maintenance costs are higher.

olf20
04-29-2017, 06:42 PM
A friend of mine used some of these (not exact ones) and had trouble
with them whipping. I guess it depends on the overall length.
olf20 / Bob

elf
04-29-2017, 07:23 PM
You may want to do some more shopping: https://www.aliexpress.com/w/wholesale-sfu1204-ball-screw-250mm.html?spm=2114.01010208.0.0.dcFF0j&initiative_id=SB_20170429152209&site=glo&groupsort=1&SortType=price_asc&g=y&SearchText=sfu1204+ball+screw+250mm

Jaakko Fagerlund
04-30-2017, 06:21 AM
A friend of mine used some of these (not exact ones) and had trouble
with them whipping. I guess it depends on the overall length.
olf20 / Bob
Whipping comes from having not bearings supports on both ends and/or the length to diameter ratio being too small and/or rotating them too fast. If the thing needs to turn at fast speed, is has to have supports on both ends and the diameter thick enough for it to not bend from the rotation.

olf20
04-30-2017, 08:24 AM
His was on a small cnc router. According to their specs they
should have work. The ball screw were not perfectly round.
Worked good at slower speeds.
olf20 / Bob

J Tiers
04-30-2017, 11:42 AM
Whipping comes from having not bearings supports on both ends and/or the length to diameter ratio being too small and/or rotating them too fast. If the thing needs to turn at fast speed, is has to have supports on both ends and the diameter thick enough for it to not bend from the rotation.

Depends on what you mean by "whipping".

They would have to spin very fast indeed tor eally "whip" from rotation. But on a router, if the axis motor is strong, and the cutting force is high, a thin screw might deflect sideways due to the forces. Especially if it is not made very well, and the balls do not all contact the screw at once. If one "side" of the screw contacts more than the other, that could put a bending force on it that can start the problem.

Bearings at both ends help a good deal, but may not be enough to fix it when you have a long, thin, imperfect screw, and fairly high cutting forces while the nut is at the far end, so a lot of the screw is unsupported. It's just like a long thin column, and can be unstable in the same way.

Basically a bad design, or a good design pushed past its limits.

Jaakko Fagerlund
05-01-2017, 03:31 AM
Depends on what you mean by "whipping".

They would have to spin very fast indeed tor eally "whip" from rotation. But on a router, if the axis motor is strong, and the cutting force is high, a thin screw might deflect sideways due to the forces. Especially if it is not made very well, and the balls do not all contact the screw at once. If one "side" of the screw contacts more than the other, that could put a bending force on it that can start the problem.

Bearings at both ends help a good deal, but may not be enough to fix it when you have a long, thin, imperfect screw, and fairly high cutting forces while the nut is at the far end, so a lot of the screw is unsupported. It's just like a long thin column, and can be unstable in the same way.

Basically a bad design, or a good design pushed past its limits.
I meant just that, the screw starts whipping around from the rotational forces. Long unsupported section bends from its own weight already and is thus out of center of axis of rotation and whips once turned.

The problem source was not descriped any further by olf20, so hard to say what was wrong as there was just a mention of "being in spec", whatever that means? Made of steel?

olf20
05-01-2017, 07:15 AM
I don't remember how long they were, maybe 36 overall.
They were supported on both ends. He had purchased them
so he could get more speed on his router. They are ok, but
for the price they have limits.
olf20 / Bob

J Tiers
05-01-2017, 11:27 AM
I meant just that, the screw starts whipping around from the rotational forces. Long unsupported section bends from its own weight already and is thus out of center of axis of rotation and whips once turned.

The problem source was not descriped any further by olf20, so hard to say what was wrong as there was just a mention of "being in spec", whatever that means? Made of steel?

It would seem that at the sort of speeds normally used, the screw would need to be already bent before it would really whip enough to make a difference in a wood router. What sort of RPM are we discussing? Normal cutting travel, or some sort of fast "rapid" as it returns to 0-0 or the like?

Spec = "made of steel".... LOL Could be about it....

Sparky_NY
05-01-2017, 02:35 PM
It would seem that at the sort of speeds normally used, the screw would need to be already bent before it would really whip enough to make a difference in a wood router. What sort of RPM are we discussing? Normal cutting travel, or some sort of fast "rapid" as it returns to 0-0 or the like?

Spec = "made of steel".... LOL Could be about it....

Actually, ballscrews 'whipping" is a well known and documented behavior. The term is "critical speed" It is not uncommon at all to run up against the critical speed limits in DIY designs at speeds not all that fast. The main factors determining critical speed are the screw diameter, length, and how it is supported by its bearings.

Here is one description: http://www.beaver-online.com/uploaded/files/Critical%20Speed%20Formula.pdf Most ballscrew makers have info on calculating critical speed in their technical docs. As a example, bridgeport boss cnc's use type A mounting for the Y axis because the screw is short, and type C mounting for the X axis which is much longer, the screw diameters are the same.

Keeping the screw rpm down is one of the main reasons cnc routers commonly use pretty steep pitch ballscrews. They want fast speeds but also keep screw diameter to a reasonable size, all to avoid hitting the critical speed.

J Tiers
05-01-2017, 03:08 PM
Actually, ballscrews 'whipping" is a well known and documented behavior. The term is "critical speed" .....
Keeping the screw rpm down is one of the main reasons cnc routers commonly use pretty steep pitch ballscrews. They want fast speeds but also keep screw diameter to a reasonable size, all to avoid hitting the critical speed.

AAAAAAAAAAANNNNNND that would be why I asked about the actual rpm.

Sparky_NY
05-01-2017, 07:26 PM
AAAAAAAAAAANNNNNND that would be why I asked about the actual rpm.

It often happens at the speeds "normally used" in wood routers, IF approached without knowledge or checking the critical speed. The rpm can be surprisingly low with the diameter and length of screws very commonly used.
The RPM alone won't tell anything without the mounting method and screw diameter.

Lets say the RPM was 866, what would that tell you? Is that below the critical rpm? You can guess, or calculate it and not have any surprises, its pretty simple.

J Tiers
05-01-2017, 07:37 PM
It often happens at the speeds "normally used" in wood routers, IF approached without knowledge or checking the critical speed. The rpm can be surprisingly low with the diameter and length of screws very commonly used.
The RPM alone won't tell anything without the mounting method and screw diameter.

Lets say the RPM was 866, what would that tell you? Is that below the critical rpm? You can guess, or calculate it and not have any surprises, its pretty simple.

It would tell me "fast enough to be considered". If you said 60 RPM, I''d be thinking it was not a problem unless crazy skinny. or bent. Just wanted a range. I have absolutely no idea how fast they TYPICALLY DO turn in actual use in actual units, which is why I asked.

Sparky_NY
05-02-2017, 07:57 AM
It would tell me "fast enough to be considered". If you said 60 RPM, I''d be thinking it was not a problem unless crazy skinny. or bent. Just wanted a range. I have absolutely no idea how fast they TYPICALLY DO turn in actual use in actual units, which is why I asked.

DIY routers (and hobby level commercial units) typically use stepper motors. Your average stepper motors top "useable" rpm is 1700 tops due to torque rolling off vs rpm. That motor either direct drives the ballscrew or through a 2 or 3:1 belt reduction. Sooooooooo absolute max rpm of the ballscrew? 1700rpm, more likey about half of that. 1000rpm ballscew with a .2 pitch comes out to 200IPM which is considered slow for a router which is why coarser pitch screws are the norm, it allows faster speeds within the motors real world rpm capability.

Now...... a ballscrew at 1000 rpm is easily OVER the critical rpm unless attention is paid to the mounting method and diameter. Lets use a bridgeport boss again as a example, its ballscrews are approx 1-1/8 dia. Half that dia can easily handle any loads encountered, the big dia is because of they wanted 200 ipm max speed and the critical speed. Modern VMC mills with servos directly coupled to the screw often run 800ipm, the screw diameters are BIG !!

Cnc, especially routers, speed is highly prized, sometimes overrated, especially for hobby users.

Sparky_NY
05-02-2017, 08:18 AM
Regarding ballscrew accuracy, there are grades for ballscrews. The cheap import screws are normally C7 grade which is a rolled vs ground screw. Still, they are usually accurate enough for hobby users.

Bridgeport boss CNC's originally used C3 grade screws but later models went to C5 grade.

The import screws are really not bad at all for hobby use. Cnczone is loaded with build threads, with excellent results using those import screws. They have become somewhat a standard for hobby builds.

For reference, here is a doc that shows the specs for the different accuracy grades for ballscrews (Page 8), its a industry wide JIC standard. http://www.cbmind.com/linear/thk/pdf/Ballscrew%20Product%20Specifications.pdf

J Tiers
05-02-2017, 10:58 AM
According to that , the C7 is a couple thou per foot, or maybe 0.005" per metre, which is not too bad.

A C8 or C10 would be double or quadruple that error, and would rapidly become intolerable for metalwork. Having had a goodly number of large items waterjet cut, that C8 or C10 level of error has caused issues in certain cases, but for most purposes, if there are not too many other errors, the C7 would be fine.

I notice that there is NO LIMIT for the fluctuation (irregularity) within either one revolution or within 300 mm for the C7 and worse screws. Irregularity may be more of a problem than a simple accumulating error.

C5 and better are all more than twice as accurate, and all have limits on the irregularity. I suspect the price is significantly higher.

And, I wonder if the bargain site screws even meet C10. You may be getting whatever was in the reject bin and was sold out the side door of the factory. How many people can measure the error per metre of what they got?

Sparky_NY
05-02-2017, 11:24 AM
C8 and C10 are real rude and crude, the type used for elevating hospital beds and such. I have never seen screws of that low a grade for sale, the lowest I see usually is C7. A interesting point, C7 grade is normally a rolled screw, better grades are ground. I am a believer is using good parts, I have never used a rolled screw but rather ebay sources ground screws. I also pay close attention to the support bearings which are often overlooked. As a result, I have had excellent results in retrofits I have done. I consider the screw, its support bearings and the condition of the machines ways the key to a well performing machine.

A side note..... many/most of the cnc softwares have screw mapping as a feature. You can enter correction factors in a table to compensate for any screw errors. ALSO... ballscrew makers usually offer a scew map data sheet as a option, it gives the exact deviation as measured for the screw purchased, that data can be entered directly into the cnc software screw mapping. Short story, a lower grade screw, together with screw mapping can give results very close to a higher grade screw. You can map the screw yourself with a indicator and jo blocks of course. There are a few ways to approach things in cnc, much like a manual machines many optional methods.

Now... for a wood router, C7 grade is probably plenty good enough. Metal working machines are another story, especially the cross slide of a lathe where any error is doubled on the part diameter.

Again, a ton of retrofits are detailed on cnczone using import ballscrews and the boys seem quite happy with them in general. One fellow in China especially is highly regarded. (linearmotionbearings is his ebay ID)

You are absolutely correct in assuming C5 costs far more than C7. What is a $75 screw in C7 might be $500 in C5 and kill $1000 in C1 . I once got a brand new name brand C1 off ebay for a lathe I did for under $100, so deals can be had. I also picked up a few C5 ones, used, about 4ft long, just under a inch diameter for $50 each that are going into a RF45 mill I have. One last point, C5 and better are truely zero backlash, C7 can have 1-3 thou backlash as bought (double nuts and such can improve this) In DIY builds, the support bearings often contribute more backlash than the screw itself.

J Tiers
05-02-2017, 11:39 AM
.....
A side note..... many/most of the cnc softwares have screw mapping as a feature. You can enter correction factors in a table to compensate for any screw errors. ALSO... ballscrew makers usually offer a scew map data sheet as a option, it gives the exact deviation as measured for the screw purchased, that data can be entered directly into the cnc software screw mapping. Short story, a lower grade screw, together with screw mapping can give results very close to a higher grade screw. You can map the screw yourself with a indicator and jo blocks of course. There are a few ways to approach things in cnc, much like a manual machines many optional methods.
.....

Mapping of ballscrews and mapping of granite flat errors, may all be done on the same photocopier in china........ Some may be legitimate, of course, especially when getting a name brand NIB ballscrew surplus.

I expect very few have the ability to map the screw. The CNC software may be able to accept the values, but YOU have to do the measuring. Unless the software reads values from a DRO, there is no way for it to KNOW what the error is.

So, what does the user do? Measure movement with a 12" "guessing stick" (caliper)?

I'd suggest that a real C7, straight up, no compensation, is as good as "reading the dials" on any used industrial machine such as most of us have, or any new chinese machine in the hobby grades.

Sparky_NY
05-02-2017, 11:51 AM
Mapping of ballscrews and mapping of granite flat errors, may all be done on the same photocopier in china........
I'd suggest that a real C7, straight up, no compensation, is as good as "reading the dials" on any used industrial machine such as most of us have, or any new chinese machine in the hobby grades.

I see you also appreciate the chicom QC data sheets LOL

I'd say your assumption on the C7 being about equal to reading the dials on a manual machine as a very safe bet. If a hobby machine can hold a thou around the area of a Kurt vise that is probably good enough for 99% of the hobby use encountered. Dimensional errors in your finished part are far more likely to come from errors in the endmill diameter, touching off and even your measurement technique/equipment. My machines are far more accurate than the finished parts I often end up with for those very reasons.

skunkworks
05-02-2017, 02:36 PM
If you are worried about wip - spin the ball nut instead...

sam