Improved Workholding Fixtures for Featherweight 18" and 26" Straight Edge Prisms (Useful for other similar items too)

[dgfoster]

(Please Note: While this thread will deal with fixturing my prisms specifically, I think it has application for machining other shapes that need similar flexibility in fixturing.)

In a recent related thread I reported I'd been working on the concept and execution of an improved workholding fixture to be used for machining the faces and top rail of my SE/Prism/Parallel/Level castings. These have proved very popular and about 1/3 of the people who purchase them want them machined. So, it became important to me to make machining of them faster and easier while still maintaining excellent flatness and parallelism of the faces and top rail.

Until now I had been mounting the prisms on two bolts fitted in the ends and supported by angle-iron fixtures on my mill table.

The general shape of the casting is demonstrated by the following 2 pics.

Please ignore the vertical Sharpie line drawn on the rear end of the sole below.

Here is a prism "skewered" on the bolts which are supported by the angle fixtures. The clamping straps prevent rotation of the prism on the bolts. They are placed rear (shown) and in front---not visible in this photo.

Spherical washers are placed between the ends of the prism and the angle fixture to reduce possible flexion of the prism due to bolt tension.

The red arrows indicate the location of the washers. And the front anti-rotation straps are seen in this photo as well.

This system has been quite satisfactory from a rigidity and stress-free point of view. But, its shortcoming is evident if one end of the casting needs to be raised or lowered to get the cut lined up with the casting's plane or if the casting needs to be rotated to get the second cut right at 45 or 55 degrees relative to the other face. In those cases, loosening one or both bolts puts all degrees of freedom into play---elevation of the end of the casting, fore and aft location, and rotation are all unconstrained. So, elevating one end can throw off rotation and fore and aft location etc. That was sometimes a bit frustrating as it could take a while to gradually get eveerything back where I wanted it. Admitedly, the fixture was easy to fabricate and rigid, but it ended up costing time in fiddling.

So, having plenty of time to think about solutions while autofeed moved the casting slowly under the face mill, I gradually devised a new method of work holding that allows adjustment of elevation, rotation, and fore and aft location. But, the new fixture isolates each factor so that only one degree of freedom is adjusted at any one time while leaving everything else as previously setup.

That is about enough keyboarding for one session. But I will post an image of the new system in drawing form and provide a link to anyone wishing to go into Onshape (No downloading of software needed and no spam will be received from the company) and view the drawings in full interactive 3-D imagery.

Onshape Link

This image shows the top block of the 3-block stack removed on the left so you can see the "ball joint" bolted onto the end of the prism and allowing for stress-free rotation, elevation, and fore and aft movement. All those holes will be explained later and setup of the machined pieces will be demonstrated in a brief video. By the way, the pale blue long thin bar is just a stand-in for my mill table for this illustration. It is not a part of the actual fixturing system.

The block stacks have been machined and assembled and will be put to the test tomorrow when I machine an 18 for a member here who is patiently standing by while I work this out.

Here are the machined pieces with fasteners in place. All that is missing is the straight edge casting. NO effort was made to make them look "pretty." You can see the ghosts of the dowel pins that locate the top blocks on the middle blocks.

Denis

 

[dgfoster]

I used the block setup today and I can report it works beautifully. It is rock-solid, easy to adjust and quick to set up. It took some tweaking and figuring to bring it to fruition, but I feel it is well worth the time.

Here is a 3 minute video overview of the end block. Sorry that it is rather amateurish, but time limitations sort of dictate a one and done video approach.

Video overview of setup and use.

And the following photos show how the end blocks build up. See video for color code of the fasteners and blocks.

I got the SE ,machined and checked the results on my surface plate. There is less than .0005 flatness error in the faces and the top rail is less than .001 out of parallel with the sole.

Denis

 

[JohnAU]

Thanks for posting this Denis, it’s always good to see how people use fixtures.

 

[dgfoster]

Thanks for posting this Denis, it’s always good to see how people use fixtures.

Thanks, John. I have used this system on several occasions now and it is a huge upgrade in terms of convenience and allowing systematic position adjustment.

What impresses me is how little people seem to look at such posts. So far as I know, using this 3-block approach is very uncommon (I’ve never seen it mentioned or discussed) yet, very few people have even viewed the video or looked at the thread. That is unfortunate given the potential wider application of this work-holding method for things other than these particular straight edges as it could be applied to similar 4-axis adjustable work holding. Kinda like “why bother?”…

Denis

 

[Bakafish]

Thanks, John. I have used this system on several occasions now and it is a huge upgrade in terms of convenience and allowing systematic position adjustment.

What impresses me is how little people seem to look at such posts. So far as I know, using this 3-block approach is very uncommon (I’ve never seen it mentioned or discussed) yet, very few people have even viewed the video or looked at the thread. That is unfortunate given the potential wider application of this work-holding method for things other than these particular straight edges as it could be applied to similar 4-axis adjustable work holding. Kinda like “why bother?”…

Denis

Niche subjects that apply to a handful of people take time to be discovered and appreciated, even if your target audience is fairly concentrated here. If this post saves someone time or inspires them to apply a similar solution to a different problem then you should count that as a success. To be honest, it took me a couple of reads before it was clear to me the exact problem you were trying to solve and how these blocks addressed it. I think partially because documenting technical solutions can be quite difficult, especially for the implementer, and this info could have been presented better IMHO (sometimes less is more.) The fact that you weren't dumped on by a bunch of 'you could have done that better' posts is kind of an affirmation in itself, these forums can be tough.

In the end, sharing knowledge should always be its own reward, and these details support your product story, as your castings are the real hero here.

 

[dgfoster]

The fact that you weren't dumped on by a bunch of 'you could have done that better' posts is kind of an affirmation in itself, these forums can be tough.

I'd love to see some "you could have done that better" posts. Figuring out an even better way to do this workholding would be great. Sure, I am pleased with this iteration of the solution. But, if there is a way to improve it, nobody should hold back.

and this info could have been presented better IMHO (sometimes less is more.)

I do not doubt that for a second. It is hard to know just how much introduction is helpful and how much is a putoff. Likewise, it is difficult to know how much detail to include and how to present the info clearly. I think the video probably is the best summary. But it has been viewed only by 6 or 7 members here with a few other views by friends outside PM. I felt that simply putting up the video by itself would have really left people wondering as there would be no context.

Thanks for the feedback.

Denis

 

[sfriedberg]

Quality over quantity, Denis. I haven't counted views, but I have counted likes, and it looks like nearly everyone who has viewed this has given an explicit thumbs up. You know that doesn't happen all that often, so should be proud/pleased.

 

[rmcphearson]

I've posted what I thought were, at the risk of sounding pompous, groundbreaking concepts and videos in my field (not on PM). They are/were largely ignored by the masses. I've noticed if a numbskull posts "What is your favorite toilet paper?" on the internet he will get a million views and replies.

Thank you Denis for taking the time to post your work. Since I'm not a machinist much of it is over my head, but I can tell it's valuable.

 

[stephen thomas]

Denis-
I think as others noted you are just posting in a niche section that few explore.
I only just discovered it. I seldom look or post here partly for reasons you allude to - masses commenting about things they actually did not take time to read or understand. As you know i used to write in print magazines back when that was a thing before the net. I have gotten to the point where i have little compulsion to say much about most processes & without being big-headed, have probably read or thought of most of the ones people post; with the exception that when i am deep in a project, i will google and search this site for solutions.

Maybe some of that is due to the aging effect of memory. I sometimes discover tooling in drawers that were my own solutions, and forgot i made it or how i figured it out! This is just when i actually find tooling that i was not looking for. There are still tooling sets and books i know i have that i can't find, on top of that. So now when i study solutions, it's no longer like when i was young and i retain it for later. I don't. and sometimes can't remember where it was seen to re-discover it.

Keep posting. The 'net is in some ways like when we were kids on a rainy day and just dug into the old Encyclopedia Britannica - and discovered how to make gunpowder, diffraction gratings, bricks and ceramics in the driveway from pits in the field, lead screws, casting non-ferrous out of the coal furnace in the cellar, machine tools etc, etc (Thinking of the various gen Britanicas in the house going back to 1800's). So you never know when some young kid or some old geezer will be inspired, and carry that on to inspire someone else. Even if you don't get accolades, the work spreads organically.

My SE's and prisms were longer. I did not trust end support only. The trade-off Q becomes how does deflection in the center balance with easy non-influencing fixturing. I rough planed them, had the castings heat treated at Elmira Heat treat, and shipped them. I did not believe that machining closer than a few .001's saved cost because either i had to get paid to machine them closer for the end user to save time, or he could save money and easily with a biax scrape .005 very quickly to get to the ..001 or .0005 level. Even for myself, i'd rather plane within a few .001's and find it much faster to just scrape the rest if necessary.

To directly comment on your interest in other methods, I did imagine leaving a boss on one end, and putting a center pop in the other end, to use between Hardinge fixtures (collet in dividing head, tailstock). Even acquired an adjustable height TS. Instead, i ended up usually using a swivel mag chuck with a rail for angles when that is easier than planing or milling on an angle.

This is not a criticism. Your fixture is elegant and a great solution. It makes the work more efficient & I can imagine it is a joy to use. I might need the idea for fixturing other things, too. No doubt customers prefer shiny, and very, very close. For some reason we all tend to produce and sell things that we think we'd like to buy; and generally imagine that our customers are like us in their preferences. I could never afford things growing up so i wanted the essence, the raw casting, cheap as possible, i'll figure the rest out. I got into scraping before the net and cnc made machine tools cheap to buy. I could only afford junkers or early imports; scraping made it possible to accurize them & some of the tooling i produced/sold before owning a grinder.

Anyway, I was delighted to read this.
Sorry to have missed it earlier.
You know i admire your work and thought processes!

smt