Ball Screw History
by David M. de Caussin
Fadal Ball Screw History:
In 1979 we started in with Twentieth Century ball screws and as production grew in the mid 80’s, NSK was added as an alternative source. We avoided having a single source for all products used in building the Fadal Machines.
It was after Fadal was sold to Giddings and Lewis in 1995 that as Twenth Century was no longer a supplier, we started using Steinheim ball screws during production of the Fadal machines.
The INCH Pitch Ball Screw:
Initially all machines were using the “Inch” pitch ball screw. All machines used a .200” pitch per turn ball screw helix. That is a fairly fine pitch screw, actually it was the same pitch as a Bridgeport Manual Mill. The advantage with the fine pitch is that a smaller motor can do a lot of work. The downside is that the rapid speed is limited to 400 IPM (2000 RPM x .200”).
The motor we used was the Glentek 4050 motor (MTR-0002). That motor provides 37 in/lbs of torque, combined with the .200 pitch we were able to get almost 1200 lbs of continuous trust for milling and drilling at a cost that was very efficient.
With the introduction of the 6030 and the 8030 machines, because of the length of the screws and the mass, we increased the diameter of the ball screw and balls. When you increase the diameter, you can increase the ball size and still maintain the same thrust ratio. A lager ball size is desirable for longer duty cycle.
We used a .250” pitch ball screw for the XY, but wanted to increase the Z axis thrust so we kept .200” pitch for the Z axis. Because of the increase mass, he had Glentek develop Glentek the GM6000 motor. This motor went from a 4” diameter (MTR-0002) to a 6” diameter (MTR-0010) with almost 2x the magnet length (stack). The larger motor had 2x the torque and doubled the machining thrust to over 2400+ lbs of thrust.
We also started using the new MTR-0010 with the 4020 machines for customers that had demanding applications that required more thrust and higher duty cycles.
The METRIC Pitch Ball Screw:
Starting in 1991, we had to increase the rapid speeds in order to be competitive. To increase the rapid speed, you just can’t spin it faster. All ball screw manufactures have a rpm limitation. The typical limit ranges between 2000 to 2500 RPM. The RPM limitation is based on how the balls recirculates as they return to the beginning of the circuit. Recently, there are internal designs for recirculating the balls instead of using external tubes which can rotate up to 3000 RPM. Even at 3000 RPM we could only increase the rapid traverse from 400 to 600 IPM.
The solution was to increase the pitch of the ball screw.
Going from an Inch to a Metric pitch might seem like a strange change? Why go from a .200” pitch to a .3937008” pitch instead of a .400” or .500” pitch? We went from inch to metric because at the time, the USA was in the process of converting from Imperial to Metric. So, going to the Metric pitch at the time made the most sense going forward. Going from .2 pitch to .3937 allow the increase from 400 to 787 IPM rapid travel.
It was decided to favor thrust over speed so instead of going to 10 mm with the Z axis, we used an 8 mm pitch screw.
Higher Pitch required a larger motor:
In order to maintain the same thrust performance, going to high pitch screw for faster rapids requires more torque. It was at this time we made the MTR-0010 motor standard for all machines with metric ball screws.
The Reality Of Higher Rapids:
It’s difficult for a machine with 1200 IPM rapid travel to have the same thrust as a machine with 400 IPM rapid travel. To go faster you either increase ball screw rpm and/or increase the pitch of the ball screw. Since ball screws have a RPM limit and you can’t increase it more than 50%, the only solution is to increase the pitch. Increasing the pitch allows for faster rapids but proportionally reduces the thrust for drilling and milling. Not only does it reduce the thrust but it decreases the response time that the motor can control a change velocity as with contouring. Going from 400 to 1200 IMP is a 3 to 1 torque increase requirement. Imagine having a mountain bicycle that can only start moving in 3rd gear rather than 1st gear! While it might be ok for straight paths but with stop and goes it greatly reduces the performance.
In order to maintain similar performance, one can increase the thrust of the motor but that requires going from a 75 in/lb to a 225 in/lb motor which greatly increases the costs for power consumption, plus the cost for much larger motors and amplifiers; easily a $15,000. cost increase to do it correctly.
Recently, a major manufacture came out with a “High Speed” machine option. They increased the pitch of the ball screw for faster rapids but didn’t increase the motors torque profile proportionally. The result was the “High Speed” machine that did slightly reduce the tool change time, but the machine did not perform as well as the customers “low speed” machine when machining their everyday parts! Needless to say, the “High Speed” machine wasn’t very practical unless your just drilling small holes and doing light cuts!
Cool Screws vs Glass Scales:
Glass Scales - With the introduction of the Metric ball screws we also began working closely with Heidenhain to provide glass scales as an option. While direct feedback greatly reduces mechanical backlash and the positioning error that comes with the thermal changes as the screw heats up and expands then contracts but it does bring in other characteristic such as overshoot and response bandwidth reduction. One of the most difficult issue is contamination, which comes from coolant from being located in the machining envelope. One of the standard requirements by the manufacture was to an air-purge system to each scale, which we did. Unfortunately, the air-purge system requires the air supply system to be very dry else the air-purge system introduces moisture in the glass scale. One simply thinks adding a “Air Dryer” to the air supply will take care of the problem. In theory that is true but the reality is that is a high maintenance item and without constant maintenance the dryer fails and the glass scales become contaminated (lousing feedback counts). This requires removing the scales, sending them in for cleaning and then reinstalling them. Today it is not a surprise when talking to a customer that there glass scales are disconnected because the cost to have them cleaned is expensive.
Cooled Screws – After working with glass scales we began researching alternatives to the thermal expansion that occurs mostly during rapid moves. For machines of this size, a rapid move the length of the screw can cause a growth of .0001” per stroke. This growth is predictable, and some manufactures have attempted to calculate it and have the CNC compensate for it. Unfortunately no one has been very successful because without direct measurement of the screw temperature, its not more than an educated guess.
Another method is to actually stretch the ball screw the amount you want compensated; in other words if you have .002” growth, you need to stretch the screw by .002”. Physically stretching a steel ballscrew .002” is no simply task. We review the requirements and concluded we could not make this a standard feature because of the costs required for casing changes to strengthen them plus the ball screw mounts and bearing would too expensive for our customer base to absorb.
With that we determined with the larger metric screws we could core drill the screw and easily inject coolant into the screw that would virtually eliminate the heat problem.
in doing this we used the spindle cooling system and developed a design that as the spindle heated the Z-axis head and expanded in the Y direction, the Y-axis screw would also grow and follow the head as it grew or shrank. So here we had a system that was better than the Glass Scale solution because both remove the heat problem for the screws and corrected for the thermal growth from the spindle system.
The only down side with the cooled screws was that in using the Dow-Frost solution it was great for thermal transfer but if didn’t provide good lubrication to the seals at screw entry/exit of the coolant and over the years could leak because of wear. One solution that is available is to replace the Dow-Frost with a very light oil. Oil does have the thermal transfer efficiency as the Dow-Frost, but it is still good and commonly used by Machine Tool Builders for cooling the spindle plus it provides lubrication for the Cooled Screw seals.
Ball Screw Parts:
Below is a list of many of the BLS parts that have occurred over time. You’ll see that list contains BALLSCREW ASSEMBLYs. These are all the components that are used beside the Ball Screw any most of the time you’ll only need to replace the ball screw and possibly the bearings for the Bearing Mount which support the ball screw.
This included many parts besides the Ball Screw.
BLS-0000 = BALLSCREW ASSEMBLY, Y; 15, 2016L, 3016L IN
BLS-0001 = BALLSCREW ASSEMBLY, Z-28; 15-4020 IN
BLS-0002 = SCREW SUPPORT, Z-28; WITH BRG IN
BLS-0003 = (USE BLS-0027)
BLS-0004 = BALLSCREW ASSEMBLY, X; 15, 2016L, 2216, 40 IN
BLS-0005 = BEARING MOUNT, X; 15-3016 IN/L'S
BLS-0006 = BALLSCREW ASSEMBLY, Y; 20/40 IN
BLS-0007 = BEARING MOUNT, Y; 15-3016 IN/L'S
BLS-0008 = BALLSCREW ASSEMBLY, Z-20; 15-4020 IN
BLS-0009 = BEARING MOUNT, Z-20/28; 15-4020 IN
BLS-0010 = (USE BLS-0032)
BLS-0011 = BEARING MOUNT, X; 4020 IN
BLS-0012 = BLSCRW SUPPRT, X, WITH BRG; 15XT/L'S IN
BLS-0014 = BEARING MOUNT, Y; 4020 IN
BLS-0016 = BALLSCREW ASSEMBLY, X; 6030
BLS-0017 = BEARING MOUNT, X; 6030/8030
BLS-0018 = BLSCRW SUPPORT, X, WITH BRG; 4020IN-8030
BLS-0019 = BALLSCREW ASSEMBLY, Y; 6030/8030
BLS-0020 = BEARING MOUNT, Y; 6030/8030
BLS-0021 = BALLSCREW ASSEMBLY, Z; 6030/8030
BLS-0022 = BEARING MOUNT, Z; 6030/8030
BLS-0023 = SCREW SUPPORT, Y/Z; 6030
BLS-0024 = BALLSCREW ASSEMBLY, 28Z; 2216-5020A MM
BLS-0025 = SCREW SUPPORT, MM 28-Z WITH BRG
BLS-0026 = BALLSCREW ASSEMBLY, X; 2216 MM
BLS-0027 = BRG MNT, X-AXIS; 2216, 3016, 4020A, 5020A MM
BLS-0028 = BLSCRW SUPPORT, X, WITH BRG; 2216/3016 MM
BLS-0029 = BALLSCREW ASSEMBLY, Y; 2216/3016 MM
BLS-0030 = (USE BLS-0035)
BLS-0031 = BALLSCREW ASSEMBLY, 20Z; 2216-5020 MM
BLS-0032 = BEARING MOUNT, Z; 2216-5020A MM
BLS-0033 = BALLSCREW ASSEMBLY, X; 3016 MM [40.5]
BLS-0034 = BALLSCREW ASSEMBLY, X; 4020 MM
BLS-0035 = BRG MNT, Y-AXIS 2216-5020A; X-AXIS 4020 MM
BLS-0036 = BLSCREW SUPPORT, X, WITH BRG; 4020MM
BLS-0037 = BALLSCREW ASSEMBLY, Y; 4020-5020A MM
BLS-0038 = (USE BLS-0035)
BLS-0039 = THRUST BEARING RETAINER
BLS-0040 = BEARING RETAINER WITH C'BORE
BLS-0041 = Z AXIS SCREW SUPPORT BLOCK
BLS-0042 = METRIC Z AXIS SCREW SUPPORT
BLS-0043 = GUIDE, SPIRAL COVER, IN, Y-FRONT; 20-5020A
BLS-0044 = GUIDE, SPIRAL COVER, IN AND MM, Y-REAR; 20-5020
BLS-0045 = GUIDE, SPIRAL COVER, MM, Y-FRONT; 20-5020A
BLS-0046 = NUT RETAINER, BLS TAIL; 6030-8030 INCH
BLS-0047 = NUT RETAINER, BLS TAIL; 2016-5020A INCH
BLS-0048 = BRG RETAINER/CNT PORT, 6030 BRG MOUNT
BLS-0049 = NUT RETAINER, BLS TAIL; MM SCREWS
BLS-0050 = LIMIT STOP, X AXIS; 4020
BLS-0051 = PAD, X-ADAPTER LIMIT 4020
BLS-0052 = Y; Z AXIS SCREW SUPPORT MOUNT
BLS-0053 = Y; Z AXIS SCREW SUPPORT HOLDER
BLS-0054 = ADAPTER X-BALL NUT(EARLY 6030)
BLS-0055 = LIMIT STOP, X-AXIS (6030)
BLS-0056 = LIMIT STOP, Z-AXIS DOWN (6030)
BLS-0057 = LIMIT STOP, Z-AXIS UP (6030)
BLS-0058 = LIMIT STOP, Y-AXIS VMC 6030
BLS-0059 = LIMIT STOP, Z-AXIS 6030 UP 6.0
BLS-0060 = BALLNUT SUPPORT, X-AXIS
BLS-0061 = SCREW, X-AXIS BALLNUT SUPPORT
BLS-0062 = COVER, X-AXIS BALL SCREW CPLNG
BLS-0063 = GUIDE, Y-SCREW COVER 6030 FRONT
BLS-0064 = GUIDE, Y-SCREW COVER 6030 REAR
BLS-0065 = LIMIT STOP, X-AXIS VMC 15XT
BLS-0070 = LIMIT STOP, VMC40 X-AXIS
BLS-0073 = BALLSCREW ASSEMBLY, X; 8030
BLS-0074 = (USE BLS-0024 OR BLS-0031)
BLS-0081 = BALLSCREW ASSEMBLY, X; 5020A MM
BLS-0082 = LIMIT STOP, ROD/SPRNGS ASSEMBLY; Y-AX
BLS-0083 = LIMIT STOP ASSEMBLY, Y AXIS; 6030
BLS-0084 = LIMIT STOP HOUSING; Z-AXIS
BLS-0085 = LIMIT STOP ROD, Z-AXIS
BLS-0090 = LIMIT STOP PAD, Z+; 15/15XT
BLS-0105 = LIMIT STOP PAD, Z+; 2216-5020A
BLS-0113 = LIMIT STOP PAD, Y+; 6030/8030
BLS-0114 = LIMIT STOP PAD, Y-; 6030/8030
BLS-0118 = BALLSCREW ASSEMBLY, X; 15XT/3016L INCH [36.5]
BLS-0125 = BALLSCREW ASSEMBLY, X-AXIS; 4020A MM
BLS-0128 = PLT, SCRW SUP, X, IN/MM; 4020-8030
BLS-0132 = PLT, SPR CVR, IN AND MM, Y-R; 20-5020A
BLS-0137 = BALLSCREW ASSEMBLY, X-INCH; 4020
BLS-0138 = BALLSCREW ASSEMBLY, Y-INCH; 4020
BLS-0139 = SCREW SUPPORT, X-AXIS; 15XT/3016L
BLS-0140 = SCREW SUPPORT, X; 3016/40 MM
BLS-0141 = METRIC LIMIT STP SLEEVE 2.50
BLS-0142 = X AXIS LIMIT STOP SLEEVE 1.25"
BLS-0143 = Z Y AXIS SCREW SUPPORT SHIM
BLS-0146 = BEARING MOUNT, X; 4020 IN PLT
BLS-0147 = BRG MNT, X-AXIS; 4020 PALLET/SLANT MM
BLS-0148 = BALLSCREW ASSEMBLY, X; 4020 IN PLT
BLS-0149 = BALLSCREW, X; 4020 PLT AND SLANT M
BLS-0179 = (USE BLS-0016)
BLS-0186 = BALLSCREW ASSEMBLY, X; 3016 INCH [36.5]
BLS-0195 = SLEEVE, LIMIT STOP ROD; 7.25; 6030/8030
BLS-0196 = ROD, LIMIT STOP; Z-DOWN 6030/8030 39.62
BLS-0223 = AXIS LIMIT STOP SLEEVE 1.0"
BLS-0233 = ROD, BALLSCREW FLUID; 4020 X-AXIS MM 49.5
BLS-0236 = ROD, BALLSCREW FLUID; 4020 Y-AXIS MM 31.7
BLS-0237 = COOL ROD, 27.45 M; 2216-4020 Z
BLS-0245 = ADAPTER PLATE, AXIS MOTOR; 3020/4525
BLS-0249 = BALLSCREW ASSEMBLY, X-AXIS; 3020
BLS-0250 = BALLSCREW ASSEMBLY, Y-AXIS; 3020
BLS-0251 = BALLSCREW ASSEMBLY, 24Z-AXIS; 3020/4525
BLS-0256 = BEARING MNT SET, XYZ; 3020/4525
BLS-0267 = BALLSCRW ASSEMBLY, Y-AXIS; 4525
BLS-0268 = BALLSCREW ASSEMBLY, X-AXIS; 4525
BLS-0279 = BALLSCREW/MOUNT ASSEMBLY, Y-AXIS; 6535
BLS-0280 = BEARING MNT, BEARING, SET, XYZ; 6535
BLS-0281 = BALLSCREW/MOUNT ASSEMBLY, Z-AXIS; 6535
BLS-0284 = NUT, RETAINING-JAM; 6535 BALLSCREW
BLS-0288 = (USE BLS-0327 AND BLS-0328)
BLS-0295 = BALLSCREW/MOUNT ASSEMBLY, X-AXIS; 6535
BLS-0303 = BRNG MNT ASSEMBLY, XYZ; 3020/4525
BLS-0304 = BLSCREW SPRT, X, WITH BRG; 40/3016IN
BLS-0305 = BLSCREW SUPPORT, X, WITH BRG; A MDLS
BLS-0343 = ADAPTER PLATE, Z-BRAKE MOTOR; 3020/4525 SMNS
BLS-0370 = BEARING MOUNT, X-AXIS; TRM
BLS-0385 = BEARING MOUNT SET, XZ AXIS; HMC 400
BLS-0392 = BALLSCREW ASSEMBLY, 32Z-AXIS; 3020/4525
BLS-0396 = BALLSCREW ASSEMBLY, X; 3016L MM [36.5]
BLS-0438 = BALLSCREW ASSEMBLY, Y-AXIS; 4020-5020 MM SEALED
BLS-0439 = BALLSCREW ASSEMBLY, X-AXIS; 4020-5020 MM SEALED
BLS-0440 = BALLSCREW ASSEMBLY, 28Z; 2216-5020 MM SEALED
BLS-0441 = BALLSCREW ASSEMBLY, 20Z; 2216-5020 MM SEALED