Low-profile saw arbor — a use for a rotary broach!

[mcdanlj]

I made a low-profile saw arbor some years ago for 1" hole slitting saws, with a fairly small cap and only 2.5mm of radial blade engagement, with an M4 countersink screw holding the cap to the arbor to hold the blade. I originally made it because I needed to make a deep cut close to my vise with a 1/64" slitting saw, and as long as I have that, why use anything objectively worse? ☺

However, I eventually jammed the blade, which tightened the screw so tightly that I damaged a hex wrench and destroyed a hex impact bit trying to remove it. This led to two good outcomes:

1. I now know that the 2.5mm radial blade engagement is definitely enough; it stalled my 2HP PM-30MV in the low-RPM range.
2. I had an opportunity to take advantage of my newly-made rotary broach holders and broaches for something other than test cuts.

Sorry for my poor drafting skills, but this should convey the general idea:






I somewhat stupidly made the cap out of 4140, which was a real stress test for my broach, but I did eventually manage to broach a 6mm hole 9mm deep. That will give me lots of leverage to release the blade the next time I jam a blade by forgetting to lock the gibs... The cut-down threads are because I originally didn't drill and tap as deep a hole, and my M12x1.5mm tap isn't a bottoming tap, and this was the easiest way to make the cap fit. I modified the design to solve that problem if anyone else likes this and wants to follow along.








I didn't put any flats on the arbor; I haven't needed any. If I ever need to add flats to release a saw blade, I can always take care of it then.

The 1/64" blade is quite thin!



PDFs and FreeCAD files are here:

Michael K Johnson / LowProfileSawArbor · GitLab

GitLab.com

gitlab.com

I posted about the rotary broach holder I used for this, as a variant of the Hemingway kit, at:

Making a rotary broach. Twice

I have wanted a rotary broach for a few years. (A rotary broach is a tool for drilling non-round holes, such as hex or square holes.) I looked at a lot of plans online. I spent hours reading forum posts. My youtube viewing history for rotary broaches reaches back to 2018! A couple months ago, I...

forum.makerforums.info

I also have posted a simple broach design, including instructions about how to make your own with no CAD needed, at:

Experiments in making a very simple rotary broach holder

I can’t seem to stop experimenting with rotary broaches. But this one just looks like a round shaft with a slightly off-center hole in the end, so a cutaway view in FreeCAD probably shows the design better. I had the idea that you only really need a ball or two and no other bearings, just...

forum.makerforums.info

 

[woodchucker]

I hope you took a file to those threads. They have a lot of tearing, and need to be cleaned up. Glad you explained why the lead was cut the way it was. I thought there was something wrong, until I re-read.

 

[mcdanlj]

@woodchucker while there was some chatter because I didn't run the threading fast enough for the 4140, the cell phone image sharpening algorithms add perceived "sharpness" by artificially increasing local contrast, which makes everything look terrible. That image is after file and then cratex, and before cleaning the filings and cratex crumbs out. I forgot to clean before pictures, and then didn't take a new picture afterward. It would also look way better had I used the chunk of 1144 I had handy.

If I were doing it over, I would have accepted the threads on the cap as soon as they threaded nicely into the first half of the hole, and then drilled out more of the arbor and threaded it deeper when I put the arbor back on the lathe to face the outside of the cap, and drill and broach it. Cutting the threads back was a "rescue" before realizing that I didn't need to do it. But it works, so I'm not inclined to start over.

My main point in posting this was that a lot of the arbor designs I've seen have had much thicker cap/screws and a larger engagement radius, and I wanted to provide my experience that it doesn't take very much to make the blade secure. The second point was the fun/magic of using a rotary broach. That is, however, a lot easier in mild steel than 4140! But I guess if I can broach a 6mm hole 9mm deep in 4140 with a home-made broach and home-made broach holder, I've proven that it would be possible to do it easily in 1144, 12L14, or 1215.

 

[dirtmotor]

Nice tool ,been wanting to make one , after seeing yours I might drill a couple of holes in cap and make a tool kinda like the angle grinder wrench with two pins to tighten and remove .

 

[mcdanlj]

Yeah, I brought up that approach on Mastodon:

Michael K Johnson (@mcdanlj@social.makerforums.info)

@MisterHW@fosstodon.org The *whole point* of this arbor is its low profile, both side profile (minimum diameter) and bottom clearance (thin cap/screw). I have made cuts with it where I had less than 0.5mm clearance between the part and the arbor. In any case, I have proven that [a rotary broach...

social.makerforums.info

If I didn't have a rotary broach and didn't want to make one, I'd probably drill two holes in the cap and use a pin wrench, as used for angle grinders. My angle grinder pin wrench is adjustable, so the pin pitch wouldn't even matter much.

But why not just embrace the magic of rotary broaching?

I'd think you would want the holes to be circumscribed by a circle smaller than the arbor hole, so for these blades fitting entirely within a 1" circle.

 

[mksj]

The one concern is the self-tightening under load often makes it difficult to release the blade. Another option is to use an Allen head screw and a wave washer. It retains the same low profile as the Allen head is recessed in the end cap. I also added some flats to hold the arbor if needed. The run-out when finished was under 0.001" and takes a wide range of saw blades.

 

[mcdanlj]

As always, your workmanship is superb and puts mine to shame... ☺

Right, the self-tightening under load was what finally doomed my M4 CSC version of this I made years ago.

A 6mm hex hole 9mm deep (what I broached) has a lot more engagement than a cap screw. The M12x1.5mm thread is a steeper taper than a smaller machine screw and so should release with lower force, too. The 5/16" screw you use is of course pretty similar to M8, which would also use a 6mm wrench, but have only 4mm of engagement, and the thread form presents a much lower effective angle. The shallower pitch would reduce the locking forces from self-tightening. In retrospect, I think that using a coarse pitch would be better, so that it would self-release more easily; using a fine pitch was almost certainly a mistake on my part.

So far, I have just used the spindle flats, for which I have a wrench handy anyway, since I want it mounted when I'm removing the blade. I'd rather not slip while holding the arbor and cut myself! ☺ Most every time I use it, I ponder cutting wrench flats on it, and I keep not bothering.

Your design with the blade indexed to the cap rather than the arbor is less fiddly than mine, and for thick blades is clearly far preferable. Honestly, if I had been starting over from scratch, I almost certainly would have done the same; the original for this arbor was a very early project. Though I think I would still have done my integrated screw with the much larger broach instead of using a cap screw, and just have enough female thread inside the arbor to cover a wide range of saw thicknesses.

I think if I do this again, I'll use 1144 as you spec.

Maybe I'll update my design for larger, steeper thread pitch and locating the blade on the cap as you do, but keep the broached hole. Because rotary broaching is fun magic!

I merely broke the edge on my cap, but you beveled yours. What purpose does the bevel serve?

 

[mksj]

There are many ways of doing these things and at the end of the day all that matters is that it works for you application. The bevel on the edge gives a bit more clearance just how I did it. The 1144 works well for many application, I also use a lot of O1 and 4140 for these types of projects, more of what I have in the size needed and I am not a big fan of turning 1018. I like your broaching, but the wave washer provides an interface that make the release much easier, and the other aspect of the design was a wider thickness range of cutters potentially could be used, but for smaller Bridgeport knee mills I would not want to use anything bigger than maybe 3/8" cutter for slotting, and in most cases I use it for thin blades. Reading the reviews of those that you can buy, often they were pretty poor even for the expensive ones. A good reason to make your own if you have a lathe, and also a nice project to do.

 

[mcdanlj]

Yeah, I bought one of the commercial arbors when I first bought my mill and before I knew better, and what utter garbage it was. The only reason I still have it is as stock from which to make something better, maybe.

One more benefit of the separate screw in your design is that if you want, you can slot and broach it for a key, for keyed blades. I had thought there was no point in the key because the cap held tight enough to stall without one, but now I finally realize that the key would actually stop the self-tightening. I had assumed the only reason for the key would be to be a driving element.

 

[mcdanlj]

Thanks to @mksj for the simplification.




I haven't made this new design, but it's how I would make it today. I still like my huge hex hole, if only for the joy of using the rotary broach. ☺