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26th Jul 2018, 07:21 PM #1Most Valued Member
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Making gear blanks in the lathe - workholding suggestions
I've been making some change gear blanks in the lathe, and have made a mandrel to mount them on from some 50mm stock with a 22mm section for the bore of the gear to fit on.
The end of the 22mm section has 18mm thread and I hold the gear on with a nut and large washer.
I drill/bore the hole in the blanks and face the inner 60mm or so, then mount them on the mandrel for machining the remainder of the faces and outside diameter.
It works OK, but I've had a few occasions where I've either got a bit aggressive with the cut or perhaps the nut hasn't been quite tight enough and the blank has slipped on the mandrel.
Blank stopped, mandrel still spinning - with varying degrees of carnage depending on how quick my reactions are.
Apart from the obvious option of just taking lighter cuts (not attractive when I have approx 20mm to take off the diameter of a 180mm blank) I've been thinking along the lines of making a mandrel from larger diameter stock (so the chuck has better grip) and also so I can fit a pin in the face, and drill a matching hole in blank to transfer the cutting torque from the blank rather than relying on the just clamping friction from the nut.
Any suggestions?
Steve
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26th Jul 2018, 07:45 PM #2Senior Member
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The drive dog idea is probably going to be the easiest allowing for quick changing
Otherwise a finer pitch thread will give you more clamping load for the tightening torque if it mostly holds, if your using a Metric coarse M18 x 2.5 could try remake the mandrel with a Metric fine M18 x 1.5.
Only other thing I can think of is either a light press fit on the mandrel or some super bearing fit loctite but the loctite will take 24hrs to cure properly and will still want a locking bolt.
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26th Jul 2018, 07:48 PM #3Most Valued Member
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I have done it two ways:
loctite it onto the shaft, this can be a right bastard to get apart again, need heat or a press or both. I once had to put about 25 tonnes on a 1" shaft and bore. Trust me when i say with a clean bore you wont need another locking mechanism.
The pin trick, which works well although if stuff needs to be balanced its a problem (put two holes in i guess)
also if the gears have keys you could make a keyed arbour and broach the blanks
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26th Jul 2018, 08:20 PM #4Most Valued Member
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Another option is to place a couple of weld tacks on the gear to mandrel. Grind off when finished.
Simon
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26th Jul 2018, 08:29 PM #5Most Valued Member
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I like the idea of the finer pitch thread or buy/make a tapered lathe mandrel. Lathe Mandrels at Wholesale Tool
Phil
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26th Jul 2018, 09:18 PM #6Most Valued Member
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The easiest thing you could do is to try a 2 deg facing cut on the face that the blank is pressed against, if you leave about a 3mm shoulder on the periphery of the 50 mm you will get a lot more holding power.
cheers, shed
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26th Jul 2018, 11:48 PM #7Most Valued Member
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Are you holding more than one blank at a time?
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27th Jul 2018, 10:35 AM #8Golden Member
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Something that is extremely easy to do, so won't cost you any time if it doesn't work, is to put a piece of paper (normal office paper) between the shoulder of the mandrel and the blank, (i.e. a paper washer) then another between your clamp washer and the blank. This has worked for me in similar situations. I also use this trick in the shaper vice if a workpiece has a tendency to move with the cut.
The shoulder of your mandrel is pretty small though, having re-read your post; this technique would be more likely to be successful if it was closer to the diameter of your blank.
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27th Jul 2018, 12:08 PM #9Most Valued Member
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Thanks guys. Its just one blank at a time as they all need to be different diameters.
Some great ideas there guys - thanks.
Masterspoon - I hadn't considered my original pin idea to be a drive dog, but as soon as you used the term my thought was I could just drill and tap a hole in the blank for a pin that would rest against the chuck jaws.
The change gears aren't turning very fast so balance likely won't be an issue, but as as Cask suggested if need be I could drill another hole opposite, or I gues being threaded I could easily fill the hole with a threaded piece.
I've had a couple of months away from the workshop since the last incident (kitchen reno for the financial controller), so looking forward to getting back out there and getting these gears finished. Will report back on how its going
Cheers - Steve
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27th Jul 2018, 06:40 PM #10Senior Member
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Steve,
I had the same problems, so I made a mandrel with an internal taper and a tapered plug that tightens in the mandrel using a set screw. The mandrel had some hacksaw cuts to allow a small amount of expansion when the screw was tightened. Worked fine, held the blanks very straight and firmly, and solved the problems. Blurry photo below.
gear blank holder.jpg
Andrew.
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31st Jul 2018, 02:27 PM #11Most Valued Member
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Finally got some workshop time over the last couple of nights. The existing mandrel was still running true, but the face was a bit galled so I faced that up with an undercut so as to leave just the outside 5mm or so bearing on the work.
I decided since I'd been away from the lathe for so long to just spend some chill out time taking a light cuts rather than experiment with other drive methods and heavier cuts.
Mounted the blank up with some paper between it and the mandrel face and set to work making lots of swarf taking around ~50mm off the diameter of 3 blanks with 0.5mm DOC.
The blanks started out as the offcut plate "holes" so they've got the start of the laser?? cut in the outer edge which is hard on tooling until you get past it. Used HSS until I got past that and then carbide so I could run it a bit faster.
Got brave and increased the cut to 1mm for the last blank. No dramas so the combination of undercut on the mandrel face and the paper definitely increases the clamp force.
Can't seem to sort out the rotation on this photo, but you get the idea...
IMG_4555 (Medium).JPG
Steve
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31st Jul 2018, 11:10 PM #12Most Valued Member
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Making gear blanks in the lathe - workholding suggestions
Got a few more done tonight and gave the workholding a bit more of a push. 2mm DOC with feed of .15mm on a 220mm diam blank. With the paper and recessed face the setup was solid.
Tomorrow I'll hopefully get an arbor made for the rotary table, then I just have to cut about 500 teeth
Steve
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1st Aug 2018, 12:05 AM #13Most Valued Member
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What kind of lathe do you have oxx? Trying to guess from your photo but no dice.
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1st Aug 2018, 12:26 AM #14Most Valued Member
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It's a Takisawa TSL-1000
Basically a 14x40.
Steve
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