# Keyway Broach



## YYCHM (May 18, 2020)

I'm attempting to make a 3/16" keyway broach for use on my lathe.














I'm using a remnant of 4340 I got from @John Conroy (Thanks John!).

I'm to the point of creating the cutting edge and hardening it.  At what angle should I grind the edge to?  For hardening I was planning on heating the edge to cherry red and quenching in water.  Any feel for how long I would need to maintain the critical temperature?  I have read 1 hour per inch of thickness.  The blade is 3/16" X 1/2".  This tool shouldn't see any shock loading.  Is annealing necessary?

I guess the other question is, do I harden and grind the edge in or grind the edge and harden it?

Any input is appreciated.  I'm exploring new territory here.


----------



## RobinHood (May 18, 2020)

I would machine / grind all the angles you need with some allowance. Harden it. Finish grind it and hone the cutting edge.

2*-3* clearance angle is what I would go for. Maybe 5* angle for the cutting edge - more will have a tendency to dig in. Once you have your tool made, try it on some scrap material (the same type that you are going to broach) to see if the tool performs to your satisfaction. Modify if required. It will also show how much deflection you get and how large the cutting forces are.


----------



## YYCHM (May 18, 2020)

RobinHood said:


> I would machine / grind all the angles you need with some allowance. Harden it. Finish grind it and hone the cutting edge.
> 
> 2*-3* clearance angle is what I would go for. Maybe 5* angle for the cutting edge - more will have a tendency to dig in. Once you have your tool made, try it on some scrap material (the same type that you are going to broach) to see if the tool performs to your satisfaction. Modify if required. It will also show how much deflection you get and how large the cutting forces are.



2*-3* ?  20-30?  I held a protractor up to the image @Brent H posted and got 20.


----------



## YYCHM (May 18, 2020)

A little more progress.  It now has a cutting edge and it just made center height without resorting to milling a step in it.  The way this material behaved on the grinder has me wondering if I need to harden it at all?


----------



## Brent H (May 18, 2020)

Hey Craig - I had a bigger relief angle as I was plunging into a blind hole. - left lots of clearance so material wouldn’t build up behind the cut on the way down.  

you will need to adjust the angle depending on application and material being cut.


----------



## YYCHM (May 18, 2020)

@Brent H Is your broach rectangular in cross section or does it have some tapered relief on the sides like some cut off blades have?

Craig


----------



## RobinHood (May 18, 2020)

Hi Craig,

thanks for posting pictures. I just marked up the angles I was referring to.


----------



## Brent H (May 18, 2020)

It is straight, that being said a 1 or 2° angle would be fine - its all about taking shallow cuts.


----------



## YYCHM (May 21, 2020)

So far this isn't going according to plan.  The tool is spinning the tool post away from the cut and I'm not sure I want to crank the tool post down any harder than it already is.  The tool is just rubbing a keyway into that aluminum.

Not sure what to try next?

Is there something thin I could put between the tool post and tool post mount point to provide more friction between the two?

A shorter tool would help I imagine.


----------



## johnnielsen (May 21, 2020)

Do you have t slots on your cross slide? If so, you can make a solid type of toolpost like a Gibralter type. It is just a big chunk of material with a hole for a boring bar. Put a keyway cutter (piece of square high speed steel ground to suit) in the boring bar. That is how we used to do odd internal keyways. This is hard on the arms but it works.

If you don't have t slots, you can attach braces that use the front and back surfaces of the cross slide to keep the tool post from turning.

Another alternative is to make a lever type pusher, kind of like a horizontal drill press quill  but I would mount it to a heavy non rotating tool post like the gibralter. This is much easier on the arms but requires time to build the mechanism. See attached url.


----------



## RobinHood (May 21, 2020)

YYCHobbyMachinist said:


> Is there something thin I could put between the tool post and tool post mount point to provide more friction between the two?



you could try a single sheet of blotting paper. It would take away the metal to metal contact. And potentially any unevenness between the mating surfaces.

how sharp is your cutting edge? it needs to be razor sharp. As you state, reduce the amount of stick-out so that you have just enough length to go through your part, no more.


----------



## YYCHM (May 21, 2020)

RobinHood said:


> you could try a single sheet of blotting paper. It would take away the metal to metal contact. And potentially any unevenness between the mating surfaces.
> 
> how sharp is your cutting edge? it needs to be razor sharp. As you state, reduce the amount of stick-out so that you have just enough length to go through your part, no more.



I put some 0.002" thick brass shim stock under the tool post and then the tool started moving in tool post.  So I choked up on the tool to get three screws on her.  At that point I was actually able to get a sliver of swarf to form on the tool edge and push it out.  Yea progress  After two passes like that I discovered the tool had lost it's edge

So looks like I'm going to have to harden the tool after all.  I wouldn't say it was razor sharp to begin with, but I'll strive for that after hardening it.

Interesting learning experience that's for sure.


----------



## Brent H (May 22, 2020)

Hey Craig,

busy as heck here but thought I could send you some additional pictures of the tool as well as a boring bar adapted for the same purpose.  The tool worked best in the mill as it is round.  


















note the tool taper on the top


----------



## historicalarms (May 22, 2020)

Craig, I have used "home built" keyway cutters a bit but never used the lathe as a "pusher" but most of the principles are the same I would think. When I say this thread yesterday I was going to comment but then my computer shut down so sorry im  bit late to the party.
    anytime I used a home built broach , I also used a homebuilt "backing shaft, same as a store-bought one, this eliminates the "backlash/ flex" that you mention today. it just doesn't allow the cutter to deflect away from the work piece.
   Another attack you might like to try is to use a mill cutter in your milling machine and pre-cut most of the slot material out first before broaching, you anly need to "broach" the back corners out then, not so much back force on your cutter.
   I might also suggest trying to use your mill as the broaching machine. Just leave your work piece mounted on the table "raised up a bit for cutter clearance underneath". now mount your cutter into the quill chuck and, using the mill down-stroke gearing, cut straight down on your aluminum. there will be less "side flex" on one straight down push than a sideways slide using a bunch of lathe apparatus.
   for what its worth I used square HSS cutters ground to size with an ordinary bench grinder for my homebuilt broaches and then cut individual teeth with a Dremmel tool ...sure not fancy but they worked the couple of times I used them.
   I should clarify that both times I used them ,I didn't use a lath or my mill, I used an ordinary hyd bottle jack the first time but had a Hyd press for the second attempt.

    Edit again; Just visited the vintage Safe thread and Robinhood used his mill quill just as I had envisioned it to work.


----------



## David_R8 (May 22, 2020)

For what it's worth I wonder if a HSS cutoff tool held on its face might work better for you.
When I cut the key way in my pulley bushing I used a 1/8" HSS cutoff blade. It worked well enough in aluminum. The only challenge I had was that the keyway was 1.25 long and the tool flexed and didn't track in a straight line so the sides of the keyway were not parallel. I think it would have been prefect if the keyway were not a long.


----------



## Tom Kitta (May 22, 2020)

I add soon a video of cutting on a milling machine. Everyone is into a lathe - time to use something new. A mill, or even a drill press.


----------



## YYCHM (May 22, 2020)

Tom Kitta said:


> I add soon a video of cutting on a milling machine. Everyone is into a lathe - time to use something new. A mill, or even a drill press.



I'd be using the mill if I had something bigger than a mini-mill.  Can't picture using a drill press?  How would you advance the tool to achieve the desired keyway depth?


----------



## Tom Kitta (May 22, 2020)

You can easily use a drill press or a mini mill (you can move the whole head up and down). Behold the video. Its easy. Its a good workout for your arm.


----------



## YYCHM (May 22, 2020)

David_R8 said:


> For what it's worth I wonder if a HSS cutoff tool held on its face might work better for you.
> When I cut the key way in my pulley bushing I used a 1/8" HSS cutoff blade. It worked well enough in aluminum. The only challenge I had was that the keyway was 1.25 long and the tool flexed and didn't track in a straight line so the sides of the keyway were not parallel. I think it would have been prefect if the keyway were not a long.



Were you trying to cut to depth and widen the keyway in multiple passes?  I think the only way this works is to cut the width to depth in multiple passes to reach depth.


----------



## David_R8 (May 22, 2020)

YYCHobbyMachinist said:


> Were you trying to cut to depth and widen the keyway in multiple passes? I think the only way this works is to cut the width to depth in multiple passes to reach depth.



I thought that I could cut to depth then drop down the width of the blade and again cut to depth. 
I think my mistake was that I didn’t drop down the full thickness of the blade. So it tracked correctly for the first 1/4” or so. 
If I were to do it again I’d grind a HSS bit to the width of the key way. 


Sent from my iPhone using Tapatalk


----------



## Tom Kitta (May 22, 2020)

If you want to cut sideways your HSS has to be very rigid. For small stuff like 3/16 I did in the video you cut full width. Cut, move 1/1000 or so and cut again. It may flex at 1/1000 so you move another 1/1000 and again and 3rd time it cuts. So on and so forth. Plenty of lube.

This thing has to be RIGID setup. The more rigid it is, the better. My boring bar in the video is too long - hence issues with cutting just 1/1000 - it flexed away. If it was shorter it would probably be quite OK with that cut.


----------



## historicalarms (May 23, 2020)

YYCHobbyMachinist said:


> I'd be using the mill if I had something bigger than a mini-mill.  Can't picture using a drill press?  How would you advance the tool to achieve the desired keyway depth?



    If you have a "boring tool head", they have a fine "sideways adjustment" that would make using a drill press a charm, otherwise you would be down to using a dial gauge & re-clamping after every cut.


----------



## Tom Kitta (May 23, 2020)

There are two way vices available or a milling table for a drill press. Or boring head. Given it takes a lot of strokes I don't think re-clamping is an option unless you really have lots of time or figure out how to do it in just few seconds. Maybe also there could be a jig made for a drill press.


----------



## YYCHM (May 26, 2020)

I think it's time to give up trying to use the lathe to broach and go a different route.











Modified the blade profile to match @RobinHood suggested profile.






Hardened the blade and ground a keen edge on her.






Rigged up this RIDGID tool post/holder.

Things were going better until I realized the saddle was now lifting and twisting on the ways


EDIT - It just occurred to me, flipping the blade over and cut the keyway towards the out board side of the lathe should prevent the saddle from lifting.  I'll try that.


----------



## RobinHood (May 26, 2020)

Good choice on making a solid tool post.

I would really shorten up your tool. Make it just long enough so that it passes through your part plus 100thou.

You could also try to use the tail stock to push the tool post from behind. It would stop the carriage from lifting. The push force would be in line with the tool, thus no lifting moment on the carriage. It is almost like using a mill quill...


----------



## Tom Kitta (May 26, 2020)

Yeah there was flex in my boring bar - you need really rigid setup.


----------

