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Thread: Paging the brains trust...
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10th Jun 2019, 10:21 AM #1Philomath in training
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Paging the brains trust...
Since coming back from Tas, I've had a backlog of projects that I'm slowly working through. Next on the list is one that I can't work out the 'best' way to do it, so interested in other ideas or thoughts on the two methods I propose below
The part
The part is cylindrical, the bore is 1.75", OD is around 2" and about 4.5" long. On one end is a helical gear (larger than the OD). The ID runs on a shaft so has to be to size. The OD has another gear mount on it so also has to be to size. To cap it off, the bore, gear and OD all need to be concentric. The blank supplied is only a few mm smaller in OD, around 20mm longer and there is enough meat on the OD for the gear, so the sized part is no problem.
On the lathe
I had thought about mounting the blank in a 4 jaw at one end and the other in a fixed steady. Boring the ID to size and then putting a plug in the end (nominally the gear end) that without disturbing the set up, I then drill a centre hole in. I can then remove the steady, centre the ID on the 4 jaw at one end and use the centre at the other. Machine the OD and gear blank before then moving to the mill to cut the gear.
On the mill
Another option is to attach a plate to the gear end of the plate and then secure that to the mill table. Use a boring head on the mill to first bore the ID and then change the set up to bore the OD while not moving in X or Y, guaranteeing that the OD(s) and ID are concentric. A bit awkward to do though (long stick out on the boring bars)
On both methods the end result would still have to go back on the lathe for a final polish/ hone.
Any thoughts?
Michael
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10th Jun 2019, 10:27 AM #2Most Valued Member
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10th Jun 2019, 11:11 AM #3Most Valued Member
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Hard to say for sure without a picture or drawing but I'd be inclined to put it on the Kearns HBM to do the bore. A couple of V blocks if required - it's why I have a ground to size matched pair. Turn the OD to size, make sure there's a section you can indicate on, rough-bore the ID on the lathe then finish to size on the HBM. Advantage is, with a decent setup, you can take the workpiece out of the V blocks for a trial fit and it'll go back in the exact same place (provided you've clamped the V blocks to the table of course).
Now if you meant how to do it with the equipment you have - probably I'd use the lathe. Carefully. Sounds like a job for a one-off arbor - bore the ID to finished size, make an arbor with centres to hold the ID, turn the OD.
PDW
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10th Jun 2019, 11:20 AM #4Intermediate Member
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G'day Michael
If i had to do it, I would bore it to size using the lathe as a horizontal borer, i.e. mounting the tube on a support on the cross-slide (obviously more work involved). If it has been clamped down without inducing distortion the boring operation using a boring bar driven by the chuck and supported in a rotating tail stock steady should produce a bore that is both on size and without too much induced distortion.
I would then mount the cylinder on a mandrel (custom made if required, or using one of the expanding types) and thereafter do all machining operations with it on the mandrel between centres.
I am not certain if it would be as accurate as you desire, but with care it could be pretty good.
If I have missed something in your explanation and what I have proposed is not feasible, please ignore.
Regards
Quentin
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10th Jun 2019, 12:14 PM #5Philomath in training
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An internal mandrel would be a good idea. I did not consider one initially as I haven't got anything like that but to keep the part on centre as well as stop squashing the end, it would probably be worth while making one. If I screw & dowel in from a flange on the mandrel to the end with excess length, that would provide power transfer as well (no dogs harmed in this machining operation...).
More to think about.
Michael
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10th Jun 2019, 01:52 PM #6Most Valued Member
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What are the tolerances?
Why not make a threaded mandrel, if you already have a mandrel of suitable length but not the OD just make 2 bushes to insert into the bore.
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10th Jun 2019, 02:44 PM #7Golden Member
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I’d probably go down the threaded mandrel route. Always more than one one to do it, but to ensure concentric it’s you could turn the bore and rough out the exterior, then mount on the mandrel and finish turning the exterior which will ensure concentricity, the to the mill for gear cutting
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10th Jun 2019, 02:45 PM #8Philomath in training
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Both the ID and OD are going to have to be within a couple of thou I think to avoid rattle (another thing to check). The ID in particular is effectively a bronze on steel plain bearing. The plan for that was to bore to be close and then get a cylinder hone to take it to size/ improve the finish.
A threaded mandrel is a possibility, although I would need to see how much it would deflect. I do have some M16 nuts & all-thread somewhere I think. Regardless, I may have to make put a conical seat on one or both ends of the part to centre it, or come up with an expanding mechanism.
Michael
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10th Jun 2019, 03:30 PM #9Most Valued Member
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What would you do with the threaded rod?
Are you not able to just bore within tolerance?
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10th Jun 2019, 04:05 PM #10Philomath in training
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I had thought you meant use some threaded rod with 2 bushes (step profile) in the ends of the bore.
I should be able to bore to tolerance but I am concerned that I need a good finish on that bore, hence the thought to hone it rather than rely on a turned finish. I have not got a reamer that large either.
Michael
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10th Jun 2019, 04:15 PM #11Most Valued Member
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Recently did that with the pulley/gear assy off of my metal planer which is why I used the Kearns HBM. It ended up a nice running fit and I've probably got 4+ hours of continuous use on it since. Planing isn't a fast activity....
If I was doing it on the lathe I'd definitely do it on a mandrel between centres. I definitely wouldn't be using 16mm allthread, I'd be making a custom mandrel no less than 40mm diameter for stiffness given you've got 1.75" bore to play with.
It's only 4.5" long, you're probably overthinking it. You say the helical gear end is bigger than the rest of the OD so I assume reasonably more than 2" (hard to say without a photo or dimensioned dwg). Assuming reasonable wall thickness that end I'd probably just dial it in using a good 4 jaw and bore it out without further ado. I can hold 0.0003" taper on my Chipmaster over that distance, your CVA should be able to equal or better that.
PDW
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10th Jun 2019, 04:18 PM #12Most Valued Member
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Sorry Michael,didn't mean to mislead, unless I was doing a job that only had to be about a size would I even consider threaded rod for an arbor, mine are either directly mounted into the spindle or set up in the chuck.
With regards finish once again I would of thought a suitable finish would of been achievable by boring only.
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11th Jun 2019, 08:20 AM #13Most Valued Member
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Hi Michael
I agree with PDW.
Make a custom mandrel for turning between centres after boring the ID.
Machine the mandrel (between centres), so as to create a shoulder for the gear to rest against then machine a decent washer for the other end.
Screwcut a thread for the retaining nut and then you can clamp the component onto the mandrel without any distortion.
Phil
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11th Jun 2019, 08:43 AM #14Philomath in training
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Consensus seems to be that an internal mandrel is the way to go. I have some steel around that size so I can do that. One thing that still worries me if using an internal mandrel, how to get a good fit between it and the part. I would expect to only have a thou or two clearance between the two parts but that is still clearance. I could make an expanding section to lock things up a bit although for a one off I don't want to spend significantly more time on a mandrel than on the part if I can help it.
I did think about putting a couple of O-ring grooves in the mandrel and effectively use an O-ring as a stiff spring to centre the part. The other way could be conical seats at each end. In this I'm not so concerned about the bore, but making sure that a) the ID and OD are concentric, and b) I don't get chatter on the OD. Previously I have found when turning a heavy mass supported on relatively flexible ends at otherwise 'normal' speeds that the mass can bounce up and down. While the pattern is really pretty, it is not a bearing surface worth a damn.
Yes, I know I may be over thinking this, but would prefer to spend some time now thinking rather than doing the job twice because I didn't think. Boring - no problem; turning the OD - no problem; cutting the gear - no problem; getting everything concentric to within say a thou -
Michael
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11th Jun 2019, 08:49 AM #15Most Valued Member
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Is there going to be any heat treatment involved? If so it's nearly impossible to rely on any dimension ore heat treat to not move.
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