1/8 Broach???

The concern here is the LENGTH of the cut, which means more metal will be in each gullet. If you cut with a broach for longer than it was designed (in this case, 2+ inches!) more metal will be in each gullet with no way to escape, which is the concern.

Looking closer at the meaning of the term, I believe I don't mean 'chip load', so much as 'load of chips', or the material in the gullet.
 
remember that your first cut is using only the broach without any shims----try it --if the broach pushes through good on the first pass which it should in a 2 1/2" thick brass pulley then you will have no problems---you can always mill your slot in your bushing a little deeper to allow for the extra length of cut needed.
 
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Dave Smith said:
remember that your first cut is using only the broach without any shims----
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What is the epiphany I should be gleaning from that? My concern isn't the 1st tooth, it is every tooth after that, which now has a gullet that has to hold 2+ inches of chip.
 
Also note the specs on the Shars Kit, which is why I bring it up: https://www.ebay.com/itm/SHARS-18PC...TING-BROACH-BUSHING-SHIM-SET-NEW/350279995974

particularly this table:
Broach​
Keyway Width​
Broach Dimensions (H x L)​
Lenth[sic] of Cut​
Min.​
Max.​
B​
1/8"​
3/16"×6-3/4"​
19/64"​
1-11/16"​
B​
3/16​
3/16×6-3/4​
19/64​
1-11/16​
C​
1/4​
3/8×11-3/4​
25/64​
2-1/2​
C​
3/8​
3/8×11-3/4​
25/64​
2-1/2​

Note that the 1/8" broach is capable of 1-11/16" length of cut. Presumably, the gullets will be filled when OP is 3/4" beyond that limit. Thus my concern.
 
A little clarification about the coupling and brass gear I broached. First the coupling was one I fabricated from 6061 aluminum. It's 2" long with an OD of 1 1/2" and an ID of 13/16". I broached a new keyway the entire length using the method described in a previous post. After the first pass I removed the broach and inspected it for chip load. None of the gullets were packed to the point the chips were putting stress on the teeth. In fact the chip load was what I would consider light. I easily brushed the chips from the broach using an acid brush.

I repeated the inspection and cleaning process after each pass. The broach on all passes was fairly easy to push through the material. At no point did it feel as though it was binding or flexing to the point it might break. My fear of doing this on a steel bushing is more the fact that the unsupported broach could flex to the point it may break once it was no longer in contact with the bushing. I don't think there will be a problem if you fabricate a bushing and shims that will span the entire length of the coupling, and if you also back the broach out and clean it at about the mid point in the cut.

As for the brass gear, it already has a full length keyway, it's just not a full 1/8" wide. The broach is only removing a few thousandths on each side. By using the previously described method the keyway can be widened without fear of breaking the broach.

One caveat: This is not my first attempt at broaching a keyway. I've done hundreds if not thousands over the last 40 years. My first attempts were supervised to be sure I used the proper methodology. Once I understood the process it was a sink or swim situation. Fortunately for me I was able to swim.
 
The thickness of the chip on a broach is determined by the height difference between the teeth. Thinner shims will reduce the number of teeth actually cutting, but will not change chip load on the teeth that are cutting. A shim of 1/2 the normal thickness will cause the first half of the broach to pass thru the bore without cutting. BTDT

Along with some of the previous suggestions, when broaching a part this long, a hole larger than the keyway can be drilled at the midpoint to allow an exit for the chips. They may not eject automatically (or even willingly), but a blast of compressed air will help.

When doing the broaching, turn the part so the side of the broach is facing you. This will allow you to observe if the broach is going off vertical and digging in. This is always a good idea, but especially in brass with its tendency to grab the tool. If it starts to dig in, you must stop, push the bushing out, remove the broach and start again from the opposite side. Never push a broach backwards out of a hole. While there is not much danger in brass or aluminum, this can roll the cutting edge. Push the bushing out, then remove the broach.

These are some hard learned lessons, I've broken a few broaches.
 
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MrWhoopee said:
The thickness of the chip on a broach is determined by the height difference between the teeth. Thinner shims will reduce the number of teeth actually cutting, but will not change chip load on the teeth that are cutting. A shim of 1/2 the normal thickness will cause the first half of the broach to pass thru the bore without cutting. BTDT

Along with some of the previous suggestions, when broaching a part this long, a hole larger than the keyway can be drilled at the midpoint to allow an exit for the chips. They may not eject automatically (or even willingly), but a blast of compressed air will help.
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Thanks for restating what I was getting at so succinctly! Thats exactly the point I was trying to make :)

In the case of Kroll (original poster), I had missed that most of the keyway was already cut, which reduced the amount of chips significantly.
 
I be,thank you MrWhoopee,I read what erichKeane saying and kinda understood but now its clear.This is fantastic information,thanks guys
 
Hi folks,

I read a few posts regarding the high pricing for broaches. All my Dumont broaches were purchased used from Ebay @ $20-$30 for As or Bs. I just looked at the pictures carefully and have been pretty lucky with used ones. They all were gently used and performed like new.

I purchased my bushings new from McMaster. They do not list manufacturer. But when I received them, they were all Dumont. They were not expensive at the time (about 2 - 3 years ago). Hope this helps.

Best Regards,
Bob
 
you are making broaching seem too difficult when it is very simple.---remember that shims just allow the broach to be moved closer for another cut. so I will renew the importance of thinner shims reducing the amount of material cut in each pass----since you made a bushing long enough to go completely
through the brass gear--then you just need to mill the broach slot in the bushing a little deeper so broach doesn't take a large cut to start with---make your shims long enough for the bushing you made. then using thinner shims for each next pass till you get the key way depth you want.--I am 76 and have broached a lot and have never broken a broach---my broach set is Hassay/Savage with a few Dumonts.---I have made all my shims and always place the thinner shim in the bushing behind the thicker shim--have fun broaching. Dave
 
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