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View Full Version : Two to three point contact tip from Steve Barton / Viperspit



pinstripe
08-15-2017, 06:40 PM
I was watching Steve's latest video and he had an interesting suggestion to increase rigidity that I haven't seen mentioned elsewhere. He had a typical bore / shaft / set screw arrangement where the set screw pinches the shaft in the bore.

To turn it from a two to three point contact, he backed off the boring bar slightly and moved it away from the set screw side. This creates some clearance so the shaft makes contact at three points. He said it can make a noticeable difference in rigidity.

Thought some of you might find it interesting.
https://www.youtube.com/watch?v=RxL8FWMMync&t=18m04s

toolznthings
08-15-2017, 08:52 PM
Steve has a great channel and I hope more people subscribe. Very informative and down to earth.

danlb
08-15-2017, 09:13 PM
I don't see how this could possibly be better than a good press fit. If it were, wouldn't we use a triangular broach for all shafts?

My understanding is that set screws are to prevent pull out.

Dan

Illinoyance
08-15-2017, 10:40 PM
That offset bore has been around many decades. It was used on very large diameter shaft fits. It allows the gear, flywheel, or whatever to slide easily onto the shaft. Set screw pressure and/or tapered key forced the shaft against the true part of the bore.

J Tiers
08-15-2017, 10:49 PM
A lot of press-on knobs DO use a three flat system, and it works quite well.

The issue with a round bore and a set screw is that the bore has to pass the round shaft. Therefore it must be larger. The shaft is held by a line and point contact, and, probably held off center. (can be on-center in at least one axis by offsetting the hole TOWARD the set screw.)

With two flats, the shaft is held against the two flats, it can be on-center in two axes, and the set screw holds it in TWO line contacts plus a point. It would be more rigid, of course.

Without seeing the video, I have no idea how he is making a two-flat hole and getting three point (line?) contact. I don't feel like watching a half hour video to find out.

Press fits (and shrink fits) are a different animal, they have contact all around, and the matter does not come up.

LKeithR
08-16-2017, 12:39 AM
...I don't see how this could possibly be better than a good press fit...

I don't think it is. More of a gimmick than anything else. In the world where I work my customers want round
bores and round shafts. If I started giving them holes that are elongated from the start I wouldn't get much
return business...

J Tiers
08-16-2017, 12:51 AM
I don't think it is. More of a gimmick than anything else. In the world where I work my customers want round
bores and round shafts. If I started giving them holes that are elongated from the start I wouldn't get much
return business...

So that is his deal? A sort of ovaled hole, wide enough to pass the shaft in the middle, but smaller at the end so it hits the shaft in two places?

Seems hard to do accurately, and hardly worth the trouble.

A regular triangular hole would be better, easier, and give good "two line and one point" contact.

pinstripe
08-16-2017, 02:00 AM
So that is his deal? A sort of ovaled hole, wide enough to pass the shaft in the middle, but smaller at the end so it hits the shaft in two places?

Not really. It's two holes, one smaller than the other and offset to one side slightly to create clearance opposite the set screw. It should create three firm contact points (including the set screw). If you click the link I posted above, it takes you to the section where he discusses it and shows the CAD. The rest of the video is about a different topic.

I've never tried it, just posting what he said. He is a career machinist and said that it has worked for him to solve chatter problems in the past.

Steve copped a bit of flak here for posting his videos. I know his style doesn't suit those that want condensed information, but he has some good tips. I'm not suggesting that it's his original idea, only that I haven't seen it mentioned elsewhere.

DR
08-16-2017, 08:42 AM
The three point contact in boring bar holders has been around for years. The holders use a horizontal teardrop shape. The set screw wedges the boring bar into the taper establishing the three point contact. One nice thing about these type holders is they hold an assortment of bar diameters all at the same center height.

I couldn't find a picture of the bar holder I'm thinking of. It's kind of like the three point contact of a round bar in a tap holder shown in the link.

https://www.rdgtools.co.uk/acatalog/3TO10TAP.JPG

A.K. Boomer
08-16-2017, 09:14 AM
I explained for years on here as to why I hate the two point contact systems esp. for holding endmills (is it weldon?)

if you can slip an endmill in and out of a holder then guess what? it means there's clearance - and if your jamming the cutter over to one side with one set screw your not only running off center your only holding it by two points and one of the points is extremely localized --- not good and in fact stone age in comparison to a typical collet system...


they are so piss poor they should be abolished but you can go out and buy them still today...

MattiJ
08-16-2017, 09:17 AM
Multifix holders work bit similar fashion but all of my boring bars have flats so..
http://www.rotagriponline.com/components/com_virtuemart/shop_image/product/00b__Boring_Bar__4c2cd54047d92.gif

J Tiers
08-16-2017, 09:46 AM
I explained for years on here as to why I hate the two point contact systems esp. for holding endmills (is it weldon?)

if you can slip an endmill in and out of a holder then guess what? it means there's clearance - and if your jamming the cutter over to one side with one set screw your not only running off center your only holding it by two points and one of the points is extremely localized --- not good and in fact stone age in comparison to a typical collet system...


they are so piss poor they should be abolished but you can go out and buy them still today...

Don't hold back, tell us what you really think!

BUT.....

1) All the holders I have are so snug that the EM has to be held in for a little bit until the trapped air leaks out.... and they remove with a pop as the air rushes back into the partial vacuum..... That "clearance" is minimal.

2) If the hole is offset toward the setscrew by half the total clearance, the off-center is compensated. The makers know that.

3) The setscrew hits a flat (assuming you use the thing correctly) so it is considerably better than a tiny point contact. The EM will turn to orient the flat to the bottom of the set screw. You get an area contact and a line contact, not two points.

Say whatever you want.... I like the holders, and have never had any problem with them. Collets, now, yes, I have had problems with those. And they have no positive way of holding the EM, just pure friction, although they are a valid and useful way of holding end mills

wombat2go
08-16-2017, 10:03 AM
The vee drive inboard boat I had, used a 1.125 inch stainless shaft.
the coupling at the gearbox was keyed and split.
The 'horizontal' Unbrako screws ( about 3) clamped the coupling around the shaft.
When the boat is at speed and the throttle is backed off, there is a force acting on the propeller shaft tending to pull the shaft out of the coupling.

A.K. Boomer
08-16-2017, 10:10 AM
Iv never had a problem with a collet that did not turn out being my fault,

take a look at your well used EM's on the non-set screw side, esp. the high end and the lowest contact point where it's just coming out of the collet, they will look different than the set screw sides due to all the fretting that's going on when the damn things wobbling around in it's holder,,, can find you some on-line examples of it if you want? there's plenty of them...

CCWKen
08-16-2017, 10:17 AM
I was pretty bored with the video and the camera work and upset that the title had nothing to do with the body of the video. The grinder didn't appear until the end of the video and sounded like a commercial for the grinding motor. Nothing about how the surface grinder was made into an ID grinder. Piss-poor camera work made it worse. Glad I was able to skip over large portions of it. :mad:

J Tiers
08-16-2017, 11:59 AM
Iv never had a problem with a collet that did not turn out being my fault,

take a look at your well used EM's on the non-set screw side, esp. the high end and the lowest contact point where it's just coming out of the collet, they will look different than the set screw sides due to all the fretting that's going on when the damn things wobbling around in it's holder,,, can find you some on-line examples of it if you want? there's plenty of them...

I have not found that to be the case, and certainly no problem. And I am more concerned with the sharp end. I never see a problem with the sharp end that is in any way caused by what microscopic "looseness" might be found in certain cases with a holder, especially if not tightened properly.

Bottom line is that the holders, if they are decent ones, just work perfectly fine, never causing a problem. If yours fit like a pencil in a pickle barrel, I suggest you replace them (or the cheap end mills), with something that is made right.

Arcane
08-16-2017, 12:13 PM
These which work on the same principle.

http://www.homews.co.uk/LrgDivDog2.jpg

A.K. Boomer
08-16-2017, 12:14 PM
to each their own - i prefer having a solid wall of holding material clamping down evenly in all directions instead of literally having the tool oscillate around the entire circumference when side loaded, but that's just me...

pinstripe
08-16-2017, 12:30 PM
These which work on the same principle.

Yes, but just to be clear, this isn't the shape he is cutting. The dog is designed to hold many different diameters, whereas his is for a much smaller range.

Arcane
08-16-2017, 01:47 PM
Yes, but just to be clear, this isn't the shape he is cutting. The dog is designed to hold many different diameters, whereas his is for a much smaller range.

That's exactly why I said "same principle".

J Tiers
08-16-2017, 03:35 PM
to each their own - i prefer having a solid wall of holding material clamping down evenly in all directions instead of literally having the tool oscillate around the entire circumference when side loaded, but that's just me...

You and the guy in that video.... distorting the description to suit the argument better.

He keeps saying "two point clamping".... when it is obvious that there is at worst a LINE and a point, changed by his little mod, to "two lines and a point".

You likewise claim the thing is going around like a rowboat oar, but have no proof, just your assumption.

And you are saying "two point" just like the video guy, when it is obvious that at worst it is a point and a line. And, in actuality, the setscrew has a wide end, and the cutter has a flat to match, so there is something of a flat surface contact there, to provide a better restraint.

Plus, even with your "solid support" (so called), the end of the cutter is STILL "orbiting" the centerline with respect to the collet, because the cutter bends with cutting forces as it spins. Nothing you can do about that, even with a shrink-fit holder.

It's just a hurricane in a thimble..........

A.K. Boomer
08-16-2017, 04:22 PM
A line and a point is the correct way of looking at it and I will give you that, but 90 degree's of loading from both said line and said point make the tool holder incapable of doing what a tool holder should be doing, holding the tool instead of counting on a push-in loose fit... and hate to tell you this but at both 90 and 270 degree's that's what your counting on when using an endmills sides - which incidentally is what endmills are "mostly" designed to be used for,

arguing that the endmill itself is going through it's own deflection patterns is senseless, it's a given and fairly uniform in comparison to "erratic oscillations" of the tool holding capabilities itself and in a geometry area that amplifies the deflections.

their junk holders - they always have been and always will be - it takes real measurable clearance to get the tool in and out, this clearance increases as time going by - not so with the brilliantly designed self adjusting taper system...

J Tiers
08-16-2017, 07:29 PM
..... it's a given and fairly uniform in comparison to "erratic oscillations" of the tool holding capabilities itself and in a geometry area that amplifies the deflections.
.....

Well, you have to prove that there are "erratic oscillations".... AND that they are large enough to make any difference in comparison to deflection of the tool.

I'll give you that an end mill 1/4" long and 1/4" diameter does not deflect of itself. {meaning shanks are same, but cutting end is length given} You need to accept that one 2" long and 1/4" diameter DOES deflect all over the place, far more than any of the "erratic oscillations" as you call them.

So essentially, what you are arguing about is WHERE in between those two limit points there may be a deflection that starts to exceed the inherent deflections of the tool.

Have fun with that, because I am not going to argue for any specific length.... you will have to do both siides of the arguing.

While you are at it, instead of tossing unsupported statements around, you should start to put numbers on these "erratic oscillations" you are so proud of. Do not forget to account for the elastic deflection of the shank as it is deflected sideways and has to bend up in order to move to the side because it goes up a slope.... and the length of shank in the bore, etc, etc. And also don't forget that a short cutter (less deflection of the tool) also has considerably LESS movement if there is any of your "oscillations", since the exposed length is much shorter than the length of shank that is in the hole.

While you are doing that, the rest of us will be doing work using end mills in Weldon type holders, with no troubles whatsoever from those violent "erratic oscillations".

A.K. Boomer
08-16-2017, 08:31 PM
Lol --- place would not be the same without you JT, keep em coming:)

wombat2go
08-16-2017, 08:51 PM
The lathe dog is free to displace radially

If anybody can give me the holder OD, bore, the end mill shank OD, and the approximate thread and torque of a set screw,
i can
run a 2D FEA stress, strain to get the deflection radially.


that can be compared with the radial clearance of the bearing race ( if known)
Regards

CalM
08-16-2017, 09:02 PM
Set screws should be installed in pairs, positioned at a right angle to one another.

Norman Bain
08-17-2017, 05:17 PM
Lol --- place would not be the same without you JT, keep em coming:)

I think Jerry is steadily working up a case for use of end mills in likes of jacobs chuck.

J Tiers
08-18-2017, 12:20 AM
I think Jerry is steadily working up a case for use of end mills in likes of jacobs chuck.

AKB has already got that sewed up. :D

With 3 line contact, instead of the "point contact" that he complained about, I'm sure he REALLY LIKES the chuck idea. He's just too shy to post pictures of himself using drill chucks for end mills and crowing about how good it is compared to the dreaded "Weldon holder" that I often use.

I've only used a chuck a couple times, I'm sure AK has MUCH MORE experience using them. :D

Paul Alciatore
08-18-2017, 10:07 PM
I made my own boring bar holder. I bored the 1/2" hole to receive the bar. Then I slit it on one side. Then added a SHCS across that slit to tighten it down. That SHCS bends the steel holder around the boring bar and gives me full contact all the way around it. No set screws. No two point contact. No three point contact. Just a complete 360 degree wrap that brings the boring bar and the holder into very intimate contact, like one, single, solid piece of steel. I guess you could call it an infinite point contact. It's a little larger than the design in the video, but it works great. I have not had any problems with the size or the SHCSs. I can't imagine how it could be made any more rigid.

softtail
08-18-2017, 10:39 PM
Paul, I would be interested in what the results would be if you put some blue on one part or the other to see what the contact actually is. While well within the realm of 'plenty good enough', I bet it's not as uniform or total as one may think. I like to use barrel nuts when possible to facilitate (more) even clamping.. the bolt going through the slit and bringing the sides together causes binding and uneven clamping.