Making of a Smart Trainer: Machining the Hammer’s Casting

Making of a Smart Trainer: Machining the Hammer’s Casting

Before the sleek, black direct drive trainer enters your training station; it was first a silver casting, waiting its turn in our computer numerical control (CNC) machine to become the Hammer.

This casting is the shell of the smart trainer, housing all the electronics and components that make the Hammer work. Before each casting is sent off for paint, it undergoes a 13- to 15-minute machining process in our HAAS v4 here in Madison.

For an in-depth look at this process from inside our CNC machine, we turned to Eric Albers, CycleOps Product Manager, and Nate Dowse, CycleOps Manufacturing Engineer.

In the video above, Eric and Nate walk us through the casting machining process from start to finish.

Hammer after being finished
Before the Hammer looks like this...

A Brief Background on Machining

CNC machining is one of the many ways we manufacture our products here in Madison, Wisconsin. It was important to us to keep the casting machining in-house, and so we purchased a HAAS v4 to be used exclusively for machining the Hammer.

When we first began this process, each casting would take 17- to 18-minutes to complete. We’re fortunate enough to have a top-notch team of machinists here at Saris, and so we gave them the green light to explore ways to save time without sacrificing quality.

Jesse, the head Hammer machinist, and the team were able to bring this time down to a 13- to 15-minute window. They did this by testing the path of the tool during the process, along with determining which tool is used when, and for how long. Every efficiently helps us continue to keep up with the demand.

However, even with the improved efficiency, in the manufacturing world spending nearly a quarter-of-an-hour on one product is not ideal. Here at Saris, we believe in doing business the right way – even if it takes longer.

We’ve committed to a longer process because doing so allows us incredible attention to detail. We are not only able to do minor corrections in real time, we’re also able to nip any machining issues in the bud immediately.

Our obsession with a flawless casting doesn’t end there. Once the casting leaves the CNC, each unit is hand-filed for another 10- to 20-minutes to ensure it meets our rigorous credentials before going out for its coat of colors.

Hammer before being finished ...it looks like this.

Before Machining Begins

Once a batch of castings arrives from our supplier, a member of our manufacturing team inspects each shell. If the casting passes the acceptance credentials, it makes its way to our shot-blast machine.

Conveniently located next to the HAAS, this machine shoots teeny, tiny BB shot at the casting. This process removes 99% of the flash, while also adding texture to the metal to better help the paint adherence.

Once the casting has been through the shot-blaster, it is lined up for the CNC machine.

How We Machine the Hammer Casting

drive pulley preparation

1. Drive Pulley Preparation (2:00)

Here we see the CNC probing the main bearing surface for the drive pulley. An integral part of what makes the Hammer tick, this helps keep the belt in-line between the drive pulley and the resistance unit. Having this perfectly lined up not only reduces vibration, which is incredibly important when you have a 20-pound flywheel moving quickly, it also reduces wear in the belt.

There aren’t many wear items in the Hammer, but the belt is one of them. And while we designed the Hammer to have a field-replaceable belt, it’s important to us that each belt lasts as long as possible. Machining the casting to keep these parts true is a critical part of that. Since we started manufacturing Hammers over a year ago, we have yet to replace a belt in the field. The drive pulley preparation has played a very large role in that.

plunge cut for axle bore

2. Plunge Cut for the Axle Bore (3:16)

Next, the HAAS v4 will make a couple of passes on the casing with a plunge cutter. The more passes you take, the more accurate the cut for the bore. It’s all about how the machine loads while cutting a circular diameter. As we cut more and more material, the tool will load so much that it’ll barely kiss the sides of the casing to ensure a very accurate bore.

the machine spraying

3. What’s all that spray? (4:18)

If you’ve watched the video, or are following along, you’ve likely noticed all the liquid spray. What you’re seeing isn’t purely water – it’s a non-corrosive water-coolant mixture.

By now we’re about four minutes into the process and we’ve not switched tools. This mixture is key to keeping this important tool cool. This tool is highly versatile and allows us to cut on X and Y, but also Z, in addition to plunging. After all that work, it needs a cool down to keep it clean and working well.

undercut process

4. Undercut (5:26)

With a quick tool change, we’re now seeing the undercut tool in action on the backside of the Hammer where the non-drive endcap sits. Using the undercut bit gives us accurate spacing for dropouts, which is essential for bike fits.

This is especially true for the Hammer, a smart trainer that boasts future-proofed quick release and thru axle compatibility – no adapters needed. For cyclocross and mountain bike riders like Nate and Eric, they have a personal appreciation for this feature when they’re riding at home.

axle press process

5. Axle Press (6:28)

Remember that axle bore we plunged out? Now it’s time to press in the axle. Doing this within the CNC machine ensures that nothing has changed. The casting hasn’t touched, removed or shifted since we machined everything out. This allows us to press the axle perfectly; right down the center of the previously drilled bore.

Machining faced pieces

6. Machining Faced Pieces (8:27)

The environment of the CNC allows us the best control of manufacturing conditions, which is important when we’re facing pieces together. With this step, every screw boss coplanar will become perfectly aligned with the other. Here we mount the side cover to the screw bosses, and affix the circuit board and resistance unit.

It is essential that we keep the correct distance between the circuit board and the resistance unit due to the temperature compensation sensor. Mounted to the circuit board, the temperature sensor must be close enough to make the adjustments necessary to keep your ride accurate as the trainer heats. At the same time, the sensor can’t be too close in case it is nicked.

tap and drill

7. Tap and Drill (10:07)

Almost done! Ten minutes in and we’re tapping screw holes to make sure everything is secure. Drilling follows this process, which can be visually noted in the video by the pecking motion of the tool. This motion removes metal chips building up in holes, which will allow the tap to accurately come through.

clean up process

8. Clean Up (12:55)

Jesse, the Hammer machinist, enters the picture now that the machine has done its job. Here he’s blowing off the casting with air, and preparing it for another visual inspection by the team. If the casting checks all the boxes, it’ll get packed with its brethren to be sent off for paint.


Once painted, these smart trainers will make their way back to our Madison facility to be calibrated, boxed and sent off to their new homes.