So I was mitering some chain stays this afternoon and something wasn’t right. I have my drawing printed out 1:1 for reference. I marked my miters and set up my Miter’s touch. My drawing is 425mm and I set it up accordingly. I have a Bicycle Academy LCFF and it is also set up using the horizontal dimension which comes out to a little over 420mm. All checks out thus far. I miter the dropout end of the left stay, set it all up in the Miter’s touch, cut the BB miter, clean up the cut and when I place it in the LCFF it’s nearly 10mm short!? I double check and nothing has moved and the stay is the correct length on the drawing.
So I placed the stay in the LCFF and moved the BB Tower in to hold it and measured the horizontal distance and it’s 415mm and the center to center is 425mm. I checked the assembly and set up instructions and all looks good.
My question is: has anyone else had an issue like this? Did I set up the Miter’s Touch with the wrong dimension? It doesn’t seem so since the stay matches the drawing.
I’ve built quite a few frames on this jig and don’t remember this specific issue.
The stay can’t be two lengths so if the stay is the correct length, then the scale on the frame fixture is out. Are you sure you didn’t use the horizontal measurement on the chainstay fixture? The chainstay fixture doesn’t know about BB drop, so you use the actual length measurement for that. Not the horizontal. So it’s either that, or you have the BB drop wrong, the wrong size of BB shell, or the scale on the frame fixture is off. Shouldn’t be difficult to check any of those things. My bet is you’ve cut the stay using the horizontal measurement.
After checking everything again I’m still not sure where the discrepancy lies but it is most likely the chain stay fixture setup. I’ll have to review and make sure I have those instructions correct.
The jig has a laser adjustment for straightness but I don’t remember setting up vertical alignment, meaning that the BB drop you measure on the dropout beam could be off. Also the length of the main beam changes with temperature, that may be off as well.
I have found similar discrepancies when measuring things off the jig and on the jig (I own the same BA one). That said, 10mm seems like a lot, I usually notice a 2-5mm gap, which I adjust on the jig (like I move the jig to fix it) as I trust more my tools than the jig itself.
@Matt thermal expansion should be the same for all the steel parts. The Farr jig is aluminum, so if there is a 10 degreeC swing the results are 425.09435 for aluminum and 425.05525 for steel. Either way it’s really unlikely that the problem is temperature control in the shop.
Maybe the drawing/print is a top view rather than normal square to the stays?
I think you are right, I don’t think I correctly grabbed the square view of the stays when projecting the view in fusion. After all this the stays are 423mm actual so nothing to be concerned about. I still can’t figure out why the projected reading on my jig is off though. For these 425mm stays the horizontal length is 420.8. When I set that on the jig (with the correct bb drop) the center to center was over 430mm. At this point, I will just make note and set the jig accordingly from now on.
@Magnoliacycles I like to make construction planes in fusion for these kinds of views. Then you can rename them something obvious to you! It’s also helpful to use these planes to set up custom views (which also get names).
