Level Engineering, Inc. - Projects

Tolerance Stack Up Analysis of a Bicycle Frame Welding Fixture

Wed, 29 Jun 2022 20:23:18 GMT

Figure 1. Benchmark Pro and Benchmark 1.1 bike frame welding fixtures.

The Benchmark 1.1 and Benchmark Pro (Figure 1) are prosumer and professional grade bike frame building fixtures, featuring exceptional build quality with precision CNC machined parts and easy to adjust gantries. One of the most critical geometries of each fixture is the planarity of the five mounting points between the fixture and the frame tubes. The five mounting points on the bike frame are the upper and lower head tube, the top of the seat tube, the bottom bracket, and the rear axle. The bottom bracket mounting point can be adjusted laterally as to maintain planarity with the other four mounting points so it is not considered in this analysis. To better understand the possible error in planarity of each of the mounting points, we analyzed the geometric accuracy of the center plane by performing a two-dimensional tolerance stack-up analysis. The accuracy of the center plane is different from the accuracy of the linear scales. The scales on the head tube, seat tube, and rear axle gantries are pre-calibrated with little uncertainty. The exact distance from the back of the fixture to the center plane of the frame can vary as each component in the fixture has a manufacturing tolerance. This creates a tolerance chain that stacks at each part interface.

There are two possible methods for calculating the tolerance stack-up: a worst-case method, and a statistical method. The worst-case analysis adds all maximum values of the associated components in the tolerance chain. While this method provides an accurate tolerance range for the center plane, it is highly unlikely that each component will be at the maximum of its specified tolerance range. The statistical approach uses a sum-of-squares of the tolerances for each component in the tolerance chain, resulting in a narrower range while maintaining very high confidence. We measured the thickness variations of all components that are not produced in-house to create more accurate tolerance ranges than provided from the manufacturer. The tolerance bands were calculated via the student’s t-distribution two-sided 98% confidence interval, and propagated through to the sum-of-squares calculation, resulting in a final bilateral tolerance of ±0.043 in (±1.092 mm) for the Benchmark 1.1 and ±0.025 in (±0.635 mm) for the Benchmark Pro. These values represent the maximum possible variation from the center plane at any of the four fixed mounting points as can be seen in Figures 2 and 3.

Figure 2. Center plane tolerance shown at head tube on Benchmark 1.1

Figure 3. Benchmark 1.1 center plane showing possible out-of-plane shift.

The Benchmark 1.1 fixture features a large variety of components leading to more component interfaces, and therefore a longer tolerance chain when compared to the Pro. The washers used in the gantry assemblies have the greatest contribution to the stack-up analysis. The washers vary in their thickness: the tolerance range stated from the manufacturer is 0.054in – 0.074in , however, we measured a more realistic range of 0.061in – 0.065in. New washers are qualified to meet or exceed this tighter tolerance range. The variance combined with the layout of these washers can create a tilt along the gantries that shifts the position of the frame mounting points away from the center plane. The lengths of the head tube and seat tube gantries create long lever arms that cause this shift in center plane to be up to 75% of the total stack-up, depending on the mounting point. The head tube experiences the greatest potential tilt from uneven washers, while the rear axle experiences the least. The shift these washers may create was calculated with the mounting points set to the maximum travel on their gantries. For frames that are smaller than the maximum distances that the gantries support, the possible tilt from the washers will be less as the lever arms are shorter, and the possible variation (tolerance) from the center plane will be smaller than the stated value. This washer analysis along with the other measured tolerances that contribute to the stack-up explain the larger tolerance range we found for the Benchmark 1.1.

The Benchmark Pro uses Blanchard ground MIC6 aluminum plates throughout the fixture to reduce the variety of components and interfaces and to shorten the tolerance chain. The MIC6 plates are held to a tight tolerance range allowing for the assembly of an accurate fixture. Like the Benchmark 1.1, the components that allow for the linear motion of the gantries have the largest contribution to the tolerance stack-up. However, the gantries on the Benchmark Pro are less complicated and use high precision bearings and rails to achieve even greater accuracy of the center plane when compared to the 1.1. To account for possible manufacturing imperfections, the height of the cutouts for the rails and the depth of the pockets for the bearing standoffs are measured and shimmed to the exact dimensions. . Ring shims are added to the bearing standoff pockets to bring the assembled gantry plates into alignment and parallel with the plate behind it. Any out-of-true or out of tolerance bearings are discarded as bad bearings can cause significant tilt along a gantry reducing the accuracy of the center plane. Tightly controlling the assembly process combined with high precision manufacturing of components allows for a total planarity tolerance of ±0.025 in for the Benchmark Pro.

Smooth Haas CNC Tool Changes with an Accumulator Tank

Thu, 14 Oct 2021 22:56:31 GMT

Haas specifications for CNC machines with 40 taper spindles typically state 4.0 SCFM required at 100 psi. Additionally, 3/8” air hose and air couplers are required. Typically, machines with air-oil spindles will require a steady draw of air, while machines with sealed bearing spindles will intermittently draw air. Although the machine air requirement may average to 4.0 SCFM, the tool release piston and blow gun likely can draw significantly more air during intermittent use. Consider that a typical plastic blow gun can draw over 20 SCFM if supplied at 100 psi. We have not collected any data to contest these flowrates, but we intend to discuss how to best plumb new machines for the best performance.

We have seen many poorly plumbed machines in the field and heard many diagnoses for poorly operating tool changers that all overlook air supply problems. Tool changers make excessive noise, tools appear to stick in the spindle, the tool changer platter deflects excessively, and diagnoses focus on the spindle taper itself. We by no means intend to solve every instance of these issues with air supply considerations as there are certainly other failure modes, but we know the repeatability and reliability of Haas tool changers can be improved when plumbed properly.

A common error occurs when machine owners review their compressor specification, conclude its output exceeds the required 4.0 SCFM, and connect the machine to their air system. There is still no certainty that 4.0 SCFM is actually available at the inlet to the machine air manifold given the plumbing between the compressor and the machine. Using common ¼” air couplings at the compressor, machine, or anywhere in the system can add unnecessary restriction. Excessive hose or piping between the compressor and machine can cause unnecessary restriction leading to less than the required airflow at the machine. Even more commonly, a regulator is placed near the air supply to maintain the recommended 100 psi, but the regulator is often not capable of responding to surges in demand from tool changes and blow gun usage. Often ¼” NPT regulators are used with ¼” couplings directly into the machine, and these certainly should be considered insufficient for any Haas tool changer. The rated pressure, flowrate, or port size of the regulator is almost irrelevant since most regulators simply fail to respond in the time of the tool release piston actuation.

In this installation, the air compressor cycles between 90 psi and 120 psi, and we choose to regulate the machine air supplies to 90 psi to limit influence from the compressor tank pressure. Using a properly sized 3/8” regulator, hose, and coupling, we have found reasonable success operating umbrella style tool changers. However, significant air consumption during tool changes causes pressure surges when the regulator fails to open quick enough to meet the demand. To ensure 90 psi is available during the entire tool change, we have added an accumulator tank after the regulator. The accumulator tank is fashioned from a portable air tank and plumbed directly into the machine air inlet with 3/8” air couplings and a 3/8” full port safety shutoff valve. The safety prevents discharge if the supply line is removed from the regulator or if other elements are removed from the air system. A T-fitting also allows the 3/8” regulator to backfill the accumulator following large surges in demand. Since large surges in demand are fulfilled by the accumulator, a ¼” coupling on the regulator is found to be more than sufficient for this Haas mill with umbrella tool changer and grease packed spindle.

Although even more restrictions could be removed from this installation, the tool changer is found to have minimal impact on the pressure available in the machine air manifold. We have found this configuration to provide consistent smooth tool changing operation. Note that the accumulator tank pressure is maintained below the compressor cycle pressure, so the compressor on-time is not extended by the compressor needing to pressurize the accumulator tanks every cycle. Of course, if the air demand exceeds the compressor output, the compressor will be forced to recover the entire system including the accumulator.

Similar performance may be achieved in a facility with large diameter air supply lines regulated to 100 psi. In this case, the large supply lines would act as the accumulator assuming the air consumption was relatively small compared to the capacity available in the lines. Full size 3/8” supply hose and couplings would still be required to avoid unnecessary restrictions. However, significantly large supply lines would be required to offer similar surge capacity to a 5 or 11 gallon accumulator tank. Consider that typical ¾” pipe has ~0.0229 gallons per foot. Neglecting restrictions, 218 feet of ¾” pipe would be required to match the capacity of a 5 gallon accumulator tank, so significantly larger pipe should be considered

Measuring Chamfers

Tue, 07 Sep 2021 17:20:17 GMT

Here at Level Engineering we take measuring chamfers seriously. We have designed and manufactured a device to accurately measure the width of a 45 degree chamfer on a straight edge of a machined part.

How It Works

The device works through the combination of an anvil and a micrometer thimble. We machined an anvil that has features to locate itself on a straight 45 degree chamfer and has a bore to let the spindle of a micrometer thimble pass through. This anvil is secured to the micrometer thimble so that the spindle can pass freely through the anvil bore and touch off against the flat surface of the chamfer it is measuring (Figure 1). Figure 2 shows a diagram of the measurement that is taken.

Figure 2: Diagram Representing The Measurement Taken By The Device

How to Derive the Chamfer Width

The measurement from the device gives the user the depth of the chamfer. To derive the width of the chamfer, we use some basic trigonometry. For a 45 degree chamfer, the width of the chamfer can be calculated from the following formula.

Width = Depth / 0.707

The table below shows some common width values for a 45 degree chamfer.

MicaSense RedEdge Upper Enclosure: From Billet to Final Product

Tue, 11 May 2021 23:05:36 GMT

Manufacturing the upper and lower enclosures for MicaSense’s line of RedEdge cameras is a great example of the state-of-the art in Level Engineering’s manufacturing capabilities.

Part 1. Machining

The upper enclosure begins as a 3 ½” x 2 ½” x 1 ¾” billet of Aluminum. This billet starts its journey in the 4th axis of our Haas CNC Mill. The fourth axis enables us to perform three operations with a single setup. In the same program, the final operation is done on the part using a soft jaw fixture. This means every time the operator opens the doors on the mill, a finished part comes out and a blank goes in, resulting in increased machine throughput.

To overcome tool mark issues on cosmetic surfaces of the finished part, we adopted a brush grinding operation in the mill. Our experience has taught us that brushing within the machining cycle is far more economical than adding additional mechanical finishing processes such as vibratory tumbling. The end result of dialing in the brushing feed rate and bristle construction is a clean, tool mark free surface ready for anodization.

Part 2. Finishing

The fine crinkle finished aluminum on many popular electronics including Apple MacBooks is highly desired, but quite difficult to achieve in low-medium volume production environments. Although typically specified as a light bead blast with Type II clear anodize, Level Engineering has migrated to chemical processing after extreme attention is paid to reduce tool marks during milling. The enclosures are finished with proprietary etching and Type II clear anodize at an outside vendor. We are able to adjust our machining process and work closely with the vendor to achieve our desired finish.

Part 3. Laser Engraving

After anodization, the enclosures are laser engraved with the customer logo and product information. We use custom 3D printed and machined fixtures, made in house, for our Epilog Mini Helix 18 laser engraver to engrave very small text in places with minimal margin for error.

Part 4. Post Machining

The last step of the manufacturing process is to machine some of the anodize surface away to expose a conductive surface at the mating features.

Post Machined Part

Part 5. Packaging and Shipping

Finally, each part is heat sealed in a plastic bag and shipped to the customer.

Finished Parts Ready For Shipping

Vacuum Pump System for Vacuum Testing

Wed, 15 May 2019 17:44:07 GMT

A vacuum pump was sized and assembled in a portable sound deadening enclosure. The complete system included the vacuum pump, vacuum reservoir, cooling fan, and fused line input module all conveniently mounted in a typical wheeled beverage cooler. These coolers make excellent affordable sound deadening enclosures! Typical small vacuum pumps are unable to startup under normal system loads. However, this system has been configured for automatic in-situ startup based on the customer's specified time to start. Contact us if you are interested in a similar system.

Current Switch PCBA

Mon, 29 Apr 2019 21:53:40 GMT

A customer required a custom PCBA allowing for measuring and interrupting current for the development and prototyping of an industrial product. This PCBA would allow the customer to perform basic tests of their concept product with minimal investment and using the MCU platform they are familiar with. The customer’s specifications were:

3x Optoisolated Digital Inputs
1x Hall Effect Current Sense Input
3x Relay Outputs
USB Powered
Interface to Mbed Nucleo development board

The PCB was designed in Altium Designer and prototypes were assembled by Level Engineering

3 Axis Motion Control Using Trio MC403-Z Motion Coordinator

Mon, 02 Jul 2018 16:01:30 GMT

From http://www.triomotion.uk/public/products/p821-p825.php

Trio Motion’s low cost MC403-Z motion coordinator may be integrated with three servo/stepper motor drivers of the user’s choice to provide control of a motorized XYZ stage. This system abstracts low level behavior to allow a user to easily:

Perform multi axis coordinated movements such as linear, circular, and helical moves
Home to limit switches to guarantee repeatable movements
Set soft and hard position limits to protect actuators from overtravel damage
Interpret G-Code to allow for integration with existing CAM software

Trio Motion’s MC403-Z is a low cost ARM11 processor based motion coordinator. The “P822“ variant allows for control of three axes of motion via step/direction outputs. It is controlled and programmed via its Ethernet interface using Trio Motion’s Motion Perfect software in the TrioBASIC language or industry standard IEC61131-3 languages. It has the ability to read/write digital I/O to allow for datum/homing operations, overtravel protection with limit switches, and communication with user interface components.

This MC403-Z variant has the ability to abstract control of three stepper/servo motor drivers using step/direction signals from its Flexible Axis Ports. With each axis configured to step/direction mode (ATYPE = 43), the pinout of these connectors is as follows:

From Trio Motion MC403-Z Hardware Overview Document

Each of these flexible axis ports should be connected to the appropriate inputs of the stepper/servo motor drives. For example, the step/direction and enable inputs for an Applied Motion SV2D10-P-NE servo motor drive as as follows:

From Applied Motion SV2D10 Hardware Manual

Once the drives are connected to the MC403-Z, each axis may be configured in MotionPerfect software using TrioBASIC with commands such as ATYPE (to select the type of axis being controlled) and UNITS (to convert from “steps” to physical units). Movement parameters may be modified by using commands such as ACCEL, DECEL, FASTDEC, JOGSPEED, SPEED, and CREEP.

After pressing the “Drive enable” button in MotionPerfect, moves may be made using commands such as MOVEABS (absolute linear move), MOVE (relative linear move), and MOVECIRC (relative circular move).

In order to establish the origin of the machine or prevent mechanical overtravel, switches may be added to any digital input of the MC403-Z. To use these switches as limit switches on an axis, the FWD_IN and REV_IN TrioBASIC commands are used. Software travel limits may also be implemented using the FS_LIMIT and RS_LIMIT commands. To use these switches for homing, the DATUM_IN command is used to define the corresponding homing input and the DATUM command is used to start the homing sequence.

Below is an example of how the Trio MC403-Z may be connected to Applied Motion SV2D10 servo drives and J-series servo motors along with Omron proximity sensors (PNP NC) to create a 3 axis motion control system with datum or limit switches:

Below is example code showing some basic setup and movement with Axis 0:

Reference Documents :

Trio Motion MC403-Z Hardware Overview
Applied Motion SV2D10 Hardware Manual