Broach--max. Length Of Cut, Length Of Bushing

[Reeltor]

Another new to broaching with questions.

Are Collard Bushings measured from the top of the collar to the bottom of the bushing -OR- from the under- side of the collar to the bottom of the bushing?

Here is some background: I picked up a Nuline #10 set from Enco on-sale and with a 30% off code at a really good price. Quality seems pretty good, but NO instructions came with the set.
Specifically I want to broach a 1/4 keyway in a gear that I am making. The face of the gear is 2" plus a 1/2" collar. While waiting for delivery, I on-line chatted with Enco Support and asked if the "C" Bushing is at least 2-1/2" long and can I broach a keyway with a length of 2-1/2". Enco responded "yes" to both questions.

When the set arrived, I found that the measurement of 2-1/2" is the total length of the bushing, including the collar and undercut space under the collar. So I made a new bushing 3" long intending to use it to broach the gear blank.

After reading the recent post on Broach Info for the beginner, I went to the Dumont site and found that their "C" bushings are 2-1/2" long and state the maximum length of cut is 2-1/2 inches. Both the Dumont's and Nuline's broach have 22 teeth.

Did I waste my time and materials making the extra long bushing? How do they measure a collared bushing?

 

[Wreck™Wreck]

Not a waste, you will need to make a piece of steel that will push the broach through the bushing until the last tooth clears the bottom of the part and falls out, you shouldn't have a problem with an extra 1/2". If the cut is to long the chips will fill the gullet and bind the tool, if so mill the slot in the bushing deeper and use more yet smaller shims as the limiting factor is the gullet volume.

One could push such a broach through 6" of bore with a bushing that is deeper and using many small shims and many passes, very time consuming however.
Be aware that in soft easily cut materials such as aluminum the broach may pull itself away from the bottom of the slot and make a much deeper seat then intended, push it 1/2 way through and then take it out of the press and look at the exit to determine if this is happening. You wouldn't think that this could happen but it often does.
Good Luck

 

[tertiaryjim]

Wreck is rite! I've broached just over 3" through parts.
Machinist handbook has all the common OD.and ID. keyway information in handy charts.
If yours isn't common there's enough information to figure sumthin out.
The tooth extension/taper on the broach will catch up to the cut and still cut its full designed depth so loading and binding has to be watched for.
When I did deep broaches I cut the slot so the broach slid in the full depth of the bushing so it could be started straight.
Checked that it remained straight and used made shims thinner than factory, then next pass use the factory,then add a thin shim.......
Just add an extra shim equal to the added slot depth on the last pass.
Also had to use a punch for extra reach to push the broach out. The broach will still be cutting so a stout punch is a must.
Use lots of good cutting oil.

Jim

 

[Reeltor]

Wreck and Jim,

Thank you for your response. I need to review my Machinery Handbook to digest what you posted and ask some more (hopefully intelligent) questions.
Are Collared broach bushings measured from the outside top to outside bottom?

 

[Wreck™Wreck]

Wreck and Jim,

Thank you for your response. I need to review my Machinery Handbook to digest what you posted and ask some more (hopefully intelligent) questions.
Are Collared broach bushings measured from the outside top to outside bottom?

As far as I know there are no US published standards regarding push broach bushing lengths, one would suspect that the length is measured from below the collar as this would be the "usable length". I do a good deal of broaching monthly and have never measured this feature and when the commonly available bushing is not long enough I just make one.
Also you do not need charts to tell you the depth of square internal keyseats. If the Customer requires extra clearance just add it to the depth.
The bore + 1/2 the key, an example would be a 1" bore with a 1/4" key would be measured from the bottom of the seat to the far side of the bore at 1.125.

The MH has tables for subtracting the arc which will need to be removed when making external seats, this does not apply to internal seats.

 

[Wheels17]

I've never used a broach before, and I'm kind of in the same situation as Reeltor, but with an additional problem.

I'm setting up to broach a 1/8" keyway in a gear that is 1.67" thick. I bought a B broach and a 3/4" B bushing from Shars and I may be paying the price for saving money. First, the bushing is only about 1.44" under the collar, so I have a fair amount of unsupported broach at the exit. Not a problem, I have a lathe and mill and can make a bushing that is longer, 1.75" under the collar. I used the Shars bushing dimensions as a model and tuned a few of them up. The Shars had a diameter of .7485 on a 3/4" bushing. Mine is .7495. The Shars bushing is sloppy on the broach, so I took .002 off the slot when I milled it. Nice sliding fit on mine.

The stacked bushing and broach lead measure .7565" on the Shars and .757" on mine....... Which brings me to my question. This can't be right in either case. Shouldn't the lead and the bushing be .7500" to fit in the bore at the start of broaching? Or is there something I'm missing? Fortunately, I didn't break the part out of the setup so making the slot .007 deeper is trivial, but I wanted to be sure there wasn't something I didn't understand.

 

[Wheels17]

I deepened the broach bushing slot .0075 and it worked fine. The broaching length was longer than recommended, so I backed the broach out and cleaned the teeth when it seemed to be getting tight. #3 arbor press worked well.

 

[macardoso]

Be aware that in soft easily cut materials such as aluminum the broach may pull itself away from the bottom of the slot and make a much deeper seat then intended, push it 1/2 way through and then take it out of the press and look at the exit to determine if this is happening. You wouldn't think that this could happen but it often does.

I've never seen anyone offer this advice but it is absolutely true and WILL ruin parts (personal bad experiences!). This has happened to me almost exclusively when running the broach through the part a second time to clean any burr and open the fit up ever slightly. I could see the broaching with a thin shim to have the same issues.

 

[SamI]

I suspect that it may be a problem on thin parts where the broach has insufficient support. Or the broach bushing does not extend all the way through the part.

If I was broaching something like a gear and there would be less than 3 teeth engaged in the cut at the same time I'd knock up a sacrificial collar to broach through with the gear offering support to the broach. It could also be an issue with a hydraulic press, or possibly my home made one having a bit of slop in it. An arbor press helps keep things much squarer. I never thought I would enjoy broaching but with an arbor press over a hydraulic it's a much more enjoyable experience.

 

[SamI]

I deepened the broach bushing slot .0075 and it worked fine. The broaching length was longer than recommended, so I backed the broach out and cleaned the teeth when it seemed to be getting tight. #3 arbor press worked well.

Also worth pointing out that it is very east to chip the teeth when backing out of a cut. I have tried this in the past when trying to broach extremely long keyways but now I would go straight through in one pass unless I absolutely have to back it out. I guess if it was getting tight that may be a sign of the flutes packing up so it may have been unavoidable in your situation.