Should You Add Linear Scales to Your Machine?

Reader Question: Our shop is ramping up to buy new equipment before end of the year and we’ve narrowed our choice down to a single machine. However, we are stuck on if we should spend the extra money for scales on all axes or if the quoted accuracy of the machine is sufficient for our needs.

Miller’s Answer:

Scales are a great option for shops who demand a lot from their machines in terms of accuracy. They can deliver more of the accuracy you expect, help stabilize a machine in a challenging shop environment and limit some of the chase associated with challenging offsets and blends. While your team may understand that scales make a machine more accurate, knowing how the scales will influence machine accuracy is important for this decision.

On a typical machine, your controller commands an axis to move a certain distance. Based on the pitch of the ball screws, and number of pulsecounts of the servo (finite “increments” per one rotation of the servo) the machine knows that a certain number of rotations of the ball screw are needed to travel said distance. It completes this number of rotations, then the control goes to its next move. However, what if the ball screw is a little longer than before due to thermal growth? Or, a more common error, what if there is backlash in the system via a worn screw or slop in the coupling? While modern controllers have all sorts of solutions for these errors like thermal algorithms and backlash compensation, these are all passive controls which rely on what the control thinks is true about the system. Scales are active feedback to the controller of what it actually is doing.

Image a rigger on the ground spotting a crane operator overhead. The crane operator can surely make the big moves to get near the target, but the rigger on the ground is on the radio relaying the fine adjustments to put the cargo in place. In the same spirit, scales do not drive an axis — this is still the job of the servo and ball screw — but a scale acts as a spotter for each axis and tells the controller to a finer scale how close it is to the desired position. The servo can then react quickly to settle more precisely on its destination for a given move.

You’ll generally see scales rated in resolution, which is how precise they can measure. For this to have a positive impact on machine accuracy, that resolution will be much smaller than what the machine is already capable of achieving. When choosing whether or not to have scales for a machine, this is more of a binary decision of yes or no, based on what the builder has predetermined is a good fit for that machine platform. You likely won’t be responsible to pick type (glass or magnetic) or resolution. 

Bolstering accuracy of the machine can mean many things to many shops. For example, this could apply to a micromachining application or a critical dowel hole distance in high-volume automotive. Despite the drastically different use cases, the biggest reason both shops might add scales to machines is to help solve thermal issues in their shop. For micromachining, this could mean mitigating the slightest outside variances, while the automotive shop could be a thermal nightmare with large bay doors flowing in summer heat or skylights beating sunlight directly on the column of a machine. Scales tend to wash out the thermal swings in the shop as they react to the overall environment and do not generate their own heat, like the ball screws creating a good reference between a known distance and the ball screw’s possible current length.

One very important thing to recognize about scales on a machine is that they only influence positional accuracy. I’ve written before about the ways a machine can be accurate, including positional, geometric, volumetric and dynamic. While scales can lead to obvious improvements in positioning and some tangential gains for overall volumetric and dynamic accuracy by nature of tighter positioning control, it has zero influence over the geometry of the machine. Scales cannot straighten individual axes or make two axes more perpendicular; this is up to the care taken during the machine’s construction. With this recognized, I would encourage you to look at what is important for the parts you produce and if the added expense for scales is solving the issue you hope it will solve. If your main concerns are tolerances of form versus tolerance on position and size, then maybe scales are not needed or won’t have the affect you desire.

When deciding to put scales on a machine, I recommend you have the mentality that the scales will not drastically improve the machine accuracy but will only bolster it. Despite the scale’s resolution, this is not magically your machine’s new quoted accuracy. As part of a control system, you must still accept some reasonable level of tolerance on the feedback of servo and scale, otherwise machines would be tuned so tightly they would be slow and jerky.

My point is this: Scales add positive feedback to what the machine can already do, thereby delivering the accuracy you expect, even in changing thermal environments. If you have certain types of work in mind, buy the machine that can handle the work through its own inherent accuracy, then have confidence the scales will be there to pick up the micron-level loose ends and deliver that accuracy consistently over time.

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Do you have a machining question? Ask the expert. John Miller leans on more than a decade of industry experience to answer machining questions from MMS readers. Submit your question online at mmsonline.com/MillersEdge.