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7X12 Taper Attachment

YYCHM

YYCHM

(Craig)
Premium Member
The MT2 Taper Attachment I got on Amazon (delivered from India) is limited to 7mm offset. I've had numerous occasions where I needed offsets of 10mm or more. Is there a valid reason it should be limited to 7mm? Something to do with dead/live center point angles?

I could expand it's limits on my mill by at least another 5mm or so, if there is a reason not to?
 
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Maybe show us a pic or link but I have an idea what you have. There is probably some safe limit to how much stagger between centers, but of course its a function of stock length. If you have a 300mm bar between centers with 7mm tailstock offset, that will yield a different (shallower) angle then a short 75mm bar with the same 7mm offset. Its hard to say if they are observing some rule of thumb or just made it that way based on size of machine or whatever.

Technically, when using the typical 60-deg male between centers and tailstock offset method, you should be center drilling with a <forgot the exact name, type R = radial?> center drill. These have a slight curved profile & will always have a tangent line of support on the 60-deg center cone even when offset. A 60-deg center in a 60 deg cone doesn't really match in a consistent spot... even though lots of guys do it that way.

I'm about to go down this path with a converted boring head. Probably not answering your question directly but hopefully we can find a rule-o-thumb. Anyway take light cuts to begin with. A lot of the support stability is lost with offset centers.
 
This is the taper attachment.

1.JPG


I could expand it's range by lengthening the slots on the back of it.


2.JPG


I want to put a 16 deg taper on this 2.5" long piece.

3.JPG


I'm thinking attempting to do the taper with compound rest and the piece unsupported isn't a good idea.

Thoughts?
 
Just eyeballing looks like you are not far off the 2-3X stick-out ratio. At least I think that's the rule-o-thumb for steel & if so should be ok if the setup is rigid & light cuts finishing. I'd be inclined to cut it with the compound set. But if you don't have full travel to traverse the 2.5" cut in one pass, then you have to take a different approach: cut one end, move the carriage then 'join' the other side so the surface becomes coincident.

The offset tailstock geometry looks pretty steep in this case. Not sure if this is your big end diameter but just shows an example.. I assumed 1.5" and you can see the offset dimensions required & not much meat at the end for the center drill.
 

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You could do it with the compound but it will take time. I never used a taper attachment on the 7x12, but if i had one i would have used it rather than try the compound way. Although i would say your concern of an unsupported end is warranted given the strength of the chuck and comparable size of the work piece. In a larger chuck i would say unsupported is fine but not here, its not going to kill you by any means if it goes caddywhompas if you should try it, but you might damage the tool, bed and chuck if it gives out.
 
You could make a chart that gives you X and Z coordinates (measurements) and plunge cut using a parting tool with the end supported with the tail stock. Leave about 10-20 thou for a clean-up pass with the compund set at your desired angle. Might help with the lack of rigidity issue.

That Lazy Machinist (Marc Lecuyer) shows how to in his video where he makes a domed pulley. See here:
 
PeterT said:
Just eyeballing looks like you are not far off the 2-3X stick-out ratio. At least I think that's the rule-o-thumb for steel & if so should be ok if the setup is rigid & light cuts finishing. I'd be inclined to cut it with the compound set. But if you don't have full travel to traverse the 2.5" cut in one pass, then you have to take a different approach: cut one end, move the carriage then 'join' the other side so the surface becomes coincident.

The offset tailstock geometry looks pretty steep in this case. Not sure if this is your big end diameter but just shows an example.. I assumed 1.5" and you can see the offset dimensions required & not much meat at the end for the center drill.
Click to expand...

What I want to achieve is more aggressive than I described. The included taper angle is more like 32 deg. I want to taper 1.25" dia down to 0.5" dia over a 2.5" span. Hmmm..... I'm actually at 2xDia stick-out. Maybe I'm ok, trying the compound?

Reducing it in steps is always another option.
 
I think your only viable option is the compound. That is too far a span for offset taper method. Even a conventional lathe taper attachment (the typical rear mounted bar that guides the cross slide) will not typically accommodate an angle like that. There is nothing wrong with using your compound in this manner, that's what its intended to do. Robinhood's parting tool suggestion is to get the bulk of material removed leaving you a series of steps. But you still require the compound travel to make the finish cut at its angle.

How much travel do you have on your compound?
 
I'm not sure we are saying the same thing on stick out (material outboard of chuck jaws). The main reason its keep under say 3X diameter is material deflection. If you take a perfectly parallel cut on a long stickout bar with carriage locked, it will measure larger diameter on the outboard end than near the chuck end. Why? Because the cutting tool pressure is deflecting the bar so its not taking full DOC. Near the chuck, deflection is less so it will be closer to DOC & therefore target diameter. Think a cantilever beam with a weight on the end. The longer the beam or larger the weight, the more deflection. That's why we do spring passes & lighten the DOC during finishing. Same deal for boring, this time the boring bar is the deflecting beam. The softer the material, the worse the deflection will be. That's why I think the 2-3X is a rule of thumb based on steel. If you put the same size piece of plastic, it will deflect much more. So less stiff materials need to be choked up more or better yet between centers.

Now what is INSIDE the chuck jaws is important too. Maybe that's called STICK IN Haha. If we don't have sufficient jaw gripping contact area on the bar, it may loosen & bad things happen. Why? Because the same cutting pressure on the outboard end of the bar acts like a lever & wants to pivot the stock about the jaws. I've never seen a guidance on chuck side like like grip length = 1X diameter or whatever, because I think it depends on other variables - how much grip pressure the jaws can apply, DOC & conventional stickout. I know its real though, sometimes you have to go to a 4J to get another jaws worth of contact area. If the stock spins in the jaws that's bad. If it spins & completely looses its grip, that's much worse.
 
YYCHobbyMachinist said:
What I want to achieve is more aggressive than I described. The included taper angle is more like 32 deg. I want to taper 1.25" dia down to 0.5" dia over a 2.5" span.
Click to expand...

Something not adding up. I get these angles.
 

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This is what I mean by overlap because your 2" compound travel cant span the full 2.5" taper in one pass. After roughing, you would finish cut one end with compound (red arrow). For example maybe the 0.5" end is the target diameter. Then you would re position the carriage over, keep compound angle locked of course, wind back the compound leadscrew and make a new series of tapered cuts until it 'meets' this prior surface (green arrow meets red arrow). Its a bit fiddly, you have to ensure you don't under or over cut or else the taper wont be a continuous taper.
 

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PeterT said:
Something not adding up. I get these angles.
Click to expand...

Me bad, you're right.

Makes me wonder if my between centers offset calculation is correct now.

O = (L (D-d)) / (2 x L1)

O = offset (.47)
L = Len between centers (3.125)
L1 = Len of taper (2.5)
D = Large end dia (1.25)
d = Small end dia (0.5)

BTB - What program are you using to generate those images? You seem to whip them up pretty fast.
 
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Modifying the taper attachment won't be as easy as I thought.

4.JPG


Nice tool marks in the slot eh.
 
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