I’ve tried three different knobs (part #s 11K5029-KCNB, COM-10597 and CL170846CR) to fit my encoder (part #PEC12R-4220F-S0024) but no luck. The 1/4” diameter one is too big, and the 6mm diameter ones are too small. The only knob shown on the product page for the encoder is too big/wide - I’ve got the encoder on a PCB so I need a fairly low profile one.
Your encoder appears to have a 6mm flatted shaft type, while the 6mm knobs you mention appear to be designed for a splined shaft, which is likely why they appear too small. Seeking out a knob designed for a flatted shaft, such as the CL170850BR would likely yield better results.
The encoder data sheet shows the shaft is smooth 6mm with a flat for a set screw.
The Grayhill 1/4" hole knob is way too big for a 6mm shaft. The one linked on the product page is the same.
The SparkFun knob is 6mm but it’s for toothed shafts, so it won’t fit.
I can’t find a datasheet for the CLIFF knob so don’t know why it doesn’t fit.
This is the least expensive in stock knob I could find that’s 6mm, no teeth, with a set screw. (ouch it’s 7x the price of the encoder)
This is a search for in stock 6mm shaft and with set screw knobs.
I’d be searching other vendors for a less expensive option if nobody here can find a more cost effective option.
I found the Cliff Electronics product sheet that specifies the attachment system for each of their knob series.
This Bourns encoder needs a knob with an “SF” in the part description. Most of the Digi-Key parts are “D” and “SPL”, here’s a search showing the 4 “SF” variations and they are under $3.00US, but they are huge.
According to the data sheet I found many of their smaller nicer looking and cheaper knobs are available with set screw attachment. I just can’t find them at Digi-Key.
Newark has a bunch of choices that they can get from the UK warehouse with many under $5.00 a few under $2.00. Make sure you check the data sheet to verify they use set screws, the few I checked were all OK.
Unfortunately the URL from my search that was specific enough to get only set screw knobs does not work when used as a link so I made a less specific search with a link that you can modify to duplicate my search.
Start at the following link and narrow the search to these three suppliers, Mentor, Multicomp, and Multicomp-pro by control clicking their names.