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A Shop-Made Compact Rotary Broach
- Thread starter randyc
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Cobra's drawing is dimensioned fully, unlike the drawing from post #1. Cobra has .050 of freedom per side at the ball pivot. The shank and tool have a 1:1 length to pivot ratio, so that the .050 of movement at the eccentric shank is converted to a proportional displacement as rocking motion at the tool. The pivot has enough range of motion to not bind. The tool tip tracks on center and the broaching motion cuts.
I don't think your tool is rocking enough, which is why it isn't cutting. Possible causes are not enough shank eccentricity, or tool motion binds somewhere and the loss is preventing rocking due to clearance or due to changing the proportionality of the tool lengths on either side of the pivot.
Clearances of mating parts not shown above.
Cobra's plans show all dimensions.
I don't think your tool is rocking enough, which is why it isn't cutting. Possible causes are not enough shank eccentricity, or tool motion binds somewhere and the loss is preventing rocking due to clearance or due to changing the proportionality of the tool lengths on either side of the pivot.
Clearances of mating parts not shown above.
Cobra's plans show all dimensions.
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- Mar 25, 2013
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- 5,231
I may be off base here, but when I look at this pic:
There appears to be no clearance between the broach holder and the shank. The broach holder seems to be a tight fit in the shank which would not allow "wobble" at this joint? Am I seeing this correctly?
Here is a pic of mine which shows a generous gap to allow for wobble.
There appears to be no clearance between the broach holder and the shank. The broach holder seems to be a tight fit in the shank which would not allow "wobble" at this joint? Am I seeing this correctly?
Here is a pic of mine which shows a generous gap to allow for wobble.
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- Nov 25, 2015
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The broach holder and tool shank don't need to wobble. The holder wobbles in the larger housing.
That housing should be the one causing the driving and retracting as it turns, or in the lathes case, as the tool turns in the holder.
That housing should be the one causing the driving and retracting as it turns, or in the lathes case, as the tool turns in the holder.
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- Mar 25, 2013
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Maybe I am using the wrong terms here. To clarify, I am calling the thing that goes in the mill spindle (and is eccentric) the Shank. That is how it is labeled in the drawing above. My device does not have a broach holder, so the broach just wobbles in the shank. If you have a broach holder you must have diameter clearance between the shank and the broach holder to allow wobble along the Z axis.
Last edited:
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- Nov 25, 2015
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so I have been doing a bunch of research, I haven't found a square broach yet. All the hex broaches do not have a large dish in the center. Most are flat. I am wondering if that might be causing you an issue. Rather than folding the chip over, the prongs are just cutting in, and unable to move the larger mass of material since it didn't remove it to begin with..
Your dish is quite pronounced.
Can you try making a new tool.
I would even venture to make a hex (you can always use a hex for a hex key later).. so try making it without a dish.
see if it works. you don't even need to harden it if you use the plastic to test.
If it does work, you should be able to harden it and try it on metal.
If that works, move on to the square, and leave it flat.
you can always come back and add a dish later.
Your dish is quite pronounced.
Can you try making a new tool.
I would even venture to make a hex (you can always use a hex for a hex key later).. so try making it without a dish.
see if it works. you don't even need to harden it if you use the plastic to test.
If it does work, you should be able to harden it and try it on metal.
If that works, move on to the square, and leave it flat.
you can always come back and add a dish later.
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- Joined
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I forgot to mention, but it was flat enicialy and did not work, so then I dished it.....still no cigar.
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Problem no.1.I thought it was 0.050 total clearance, now I see you stated it is 0.050 per side. Problem no.2. I made my broach 5mm too short,it should be 50.8 (2") and it is 1.771". Someone or EVERYONE mentioned that the length is important and it make sense.
As soon as I get a chance,I will make the changes. This is South Africa and we have Load shedding now,meaning our power company shuts down our power for certain amounts of time so I can't do my work. I will keep ypu updated when I can.
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- Mar 25, 2013
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I see you have clearance and it seems adequate. I believe my shank is offset by 0.050 to give a TIR of 0.100. My clearance was generous for a safe margin. I am not sure the broach length is critical, although a longer broach will rock less at the work surface.
I am not sure what is wrong with your device. You might try center drilling the back of the broach and running it without the broach holder just as a test. That would eliminate some variables and shorten the device for more rocking action.
I have not tried mine on steel, but it cuts easily through brass and aluminum.
I am not sure what is wrong with your device. You might try center drilling the back of the broach and running it without the broach holder just as a test. That would eliminate some variables and shorten the device for more rocking action.
I have not tried mine on steel, but it cuts easily through brass and aluminum.