Broaching Keyways with the Lathe

[Paul Jones]

I have watched several videos on YouTube for broaching internal keyway slots using the lathe carriage while holding the part in the lathe chuck with the lathe totally powered off. In this example I cut a 5mm wide by 2.51mm deep slot in a 40 tooth gear bored to 18mm ID. The gears are for a replacement change gears to a 12” swing geared-head lathe (see Homemade Change Gears).

Using the 90 degree end of a 0.500” boring bar with a 3/16” square HSS tool blank ground with 10 degree relief angles, I cut the 5mm wide keyway slot. Grinding the cutting faces on the HSS is done with the longer tool blank. The shorter HSS cutting tool is cut from the larger piece of HSS stock using a Dremel tool with a reinforced abrasive wheel and before completely cutting all the way through, the shorter section of HSS is snapped-off in a vise.

The boring bar set-up uses another longer piece of 3/16” square HSS tool blank to align the boring bar horizontally using a test dial indicator. Also, the QCTP is adjusted to a height to center the cutting tool with the keyway slot (and knowing the distance from the cross slide top to the lathe center). I used a digital height gage to set the HSS tool height.

Broaching began after replacing the longer HSS blank with the newly ground broaching tool. Make sure the lathe chuck is locked in the lowest spindle gear to prevent it from moving. I also used a soft jaw ratchet bar clamp to ensure the chuck could not move. Each broaching pass was feed in increments of 0.001” to 0.002” with the lathe cross slide in order to prevent the cutting tool from hogging into the ID wall. Use plenty of cutting oil. Originally I had planned to use a 0.625” diameter boring bar that holds ¼” square HSS cutters so I could grind the larger HSS tool down to 5mm (0.197”) wide. However, in this case, the 18mm (0.709”) ID hole provided too little clearance for both the boring bar and cutting tool. I used the 3/16” (0.1875”) square HSS set to two different vertical heights to cut the keyway slot to the final 5mm width and checking the slot width using a gage pin before removing from the chuck.

[kbalch]

Thanks Paul! I've added your Keyway Broaching Technique to our Machining category, as well as to your builder page: Paul Jones' Homemade Tools. Your receipt:

Keyway Broaching Technique by Paul Jones

tags: lathe, keyway, broach

[DIYer]

Very informative post, Paul Jones. It's always nice to learn something new.

[olderdan]

Hi Paul
Nice work and presentation as ever.
I have in the past used the topslide with the feed screw removed with a hand linkage.
At the moment I am trying to design a motorised version with a 1 inch stroke working on the saddle to machine some small gears integral with a cam.
I look forward to posting it if it works.

[Paul Jones]

olderdan,

I think you will have a winner with the motorized version. I found that grinding the correct angles on the cutting tool for the material being broached was extremely important and more so when motorized. The boring bar and cutter can hog into the inner wall with too much feed and potentially ruin the part. It is not a big deal when driving the cutting manually but will be a problem with motorized. You might want to consider adding some type of mechanical clutch on the driving mechanism to stop the stroke.

Thank you for the comments and looking forward to seeing your broaching system.

Paul

[olderdan]

Thanks for your observations, you are right about needing some sort of friction clutch in the drive and it is something of a stumbling block at the moment but I am still chewing on it.
Any thoughts.

[Paul Jones]

olderdan,

How about a simple miniaturized clutch assembly with two small rotating plates and an adjustment nut with a compression spring that put pressure from behind on one plate and the friction between the two causes the clutch to slip when too much resistance occurs on the output shaft due to the tool jam.



I don't have access to my CAD so I quickly (and very crudely) drew the clutch cross section in Excel to help with my explanation. Sorry about the drawing.

Paul

[olderdan]

Your clutch design looks sound and thanks for that, the problem I have is lack of space to fit an intermediate clutch to the gearbox I have, this an old but very powerfull mobility scooter 24v drive but it will have to have an overhung eccentric so no room for an outrigger bearing.
Your drawing started me thinking that I could maybe make an adjustable spring loaded conrod.
Anyway thanks for your help, I think I need to think about something else for a while and have a fresh start.
Regards

[ncollar]

Paul
I am working on a stroker right now. I have done it using a boring bar and manually back and forth, not fun at all and time consuming. I am using a gear head motor with a pivot point like an old locomotive wheel. The mount bolts onto the compound on my South Bend and can use the compound or the saddle screw to move sideways. I am concerned about the size of my gear head motor but that will dictate the amount of cut. It will be better than using the carriage and wear on the bed. I'll be posting soon.
Nelson

[tonyfoale]

olderdan,

I think you will have a winner with the motorized version. I found that grinding the correct angles on the cutting tool for the material being broached was extremely important and more so when motorized. The boring bar and cutter can hog into the inner wall with too much feed and potentially ruin the part. It is not a big deal when driving the cutting manually but will be a problem with motorized. You might want to consider adding some type of mechanical clutch on the driving mechanism to stop the stroke.

Thank you for the comments and looking forward to seeing your broaching system.

Paul

I use a pneumatic cylinder to drive a slotter. There is no need for a clutch because the force can be limited by the set pressure. Just like electrics, pneumatics offer a lot of options for control. You could simply have a manual valve with powered or spring return. If you wanted to be more sophisticated you could set it up to automatically oscillate.
I have such a device for my mill for making clutch baskets and hubs.