# Achieving reproducibility



## Mitch Alsup (Nov 23, 2017)

This post is a question about how to achieve repeatability--making several parts fit the same blueprint.

But first some background::

I am building several telescopes from 13" to 20" to 30" diameter apertures. I bought my milling machine and lathe in order to be able to hit better accuracies, I also bought some dial indicators, calipers, a set of micrometers, and a few other things of metrology. In many respects, I am doing "all right" and in a few I keep running into the "how did that happen" category. All of the machined material in this post is 6061T6 of extruded bar stock.

The 13" mirror cell has 6-points that through the back side of the mirror. These 6 points are on 3 balance beams. In rough numbers the beams are 4.2" long, 0.600" wide, and 0.500" tall. I can routinely rough 3/4*3/4 stock into the required dimensions within 0.0015" measured at 9 places around the parts and have 3 parts that are all within 0.0005 to 0.0015 of each other. All well and good.

At both ends of the beam I drill (0.4375) trying to get both holes exactly 3.902" apart (or as close as I can make them). After drilling and without moving the part, I switch from drilling to the boring head and bore the hole to 0.468 which is a "bearing fit" to the 12mm ball transfers that touch the back side of the mirror at very low friction levels. I start with beams that are long enough that the accuracy of the holes on (say) the left end of the beam are not critical as long as these holes are the same away from that end of the beam. When the part goes into the vise, it is supported on parallels and another parallel is clamped to the vise so that the left edge of the part can be positioned easily. The bored holes have been measured with a snap gauge and the micrometers and I am seeing a similar 0.0015"-0.002" variation in the bearing fits of these holes.

After drilling and boring all three beams, I position the table so that it has moved 3.902". I confirm this by making the first hole in a sacrificial material and measure the near sides and far sides of said hole, the average of which is the center to center distances. These are all measured with a dial caliper. I then clamp down the table, insert each part, drill and bore each part. After wards, I measure the overhang off the ends of the part and edge mill the beams to final length centered around the bored holes.

At this point I have 3 parts: one with 3.9025" hole spacing, one with 3.9015" hole spacing, and one with 3.899" hole spacing. While this is better than is probably needed for the project at hand, If anyone can help me locate the point at which I lead error creep into the machine process I would be appreciative. 

After milling the beams to length such that the material from the far end of the hole to the edge of the beam is "the same" (about 0.016" as accurately as I can measure it), I use the beam length to find the center of the beam and drill an axial hole perpendicularly through the beam. I proceed to drill a hole using a C drill (0.242") backed up with a D drill to 0.110 depth. This D drill is a nice thumb bearing fit to the miniature ball bearings (1/4×1/8×7/64) that are needed on this balance beam. The problem, here, is that the drilled hole walks about 0.002" while passing through the beam. This puts the axis off center of the beam, and makes balancing the beam in 2 dimensions extraordinarily hard. Here the question is how does one get holes on one side of a part to match up to a hole on the other side of the part to (say) 1/2 a thou? at these tiny dimensions? how does one bore a bearing fit at this kind of dimension (I don't know where to get the tooling to do bores at this size)?

The numbers above are after making this set of beams 3 times and learning the more obvious mistakes of part positioning and tolerance stackups. As I said, the current parts are usable, but future machine parts will not be so tolerant of litter errors of part position prior to machining.

Thanks in advance.


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## markba633csi (Nov 23, 2017)

Mitch I'm having trouble visualizing the parts, can you post a picture? Then we could better advise on your method
Mark S.


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## Mitch Alsup (Nov 24, 2017)

Mechanical Drawing of the parts:


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## Z2V (Nov 24, 2017)

All I can offer is “ Howdy Neighbor “


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## petertha (Nov 24, 2017)

Interesting part. I'm making a model radial engine with lots of aluminum components to +/- similar tolerances. Self taught home machinist so beware! I can offer a few observations, maybe some will help. Maybe some you are already doing.

- set a hard stop on one end of the vise so the part always references that datum. This will help when you have to rotate to make the holes on adjacent faces. At least this way, theoretically, if you make one good part you are much closer to replication using same setup

- you didn't mention much about your mill & vise setup. If it has reliable DRO then fine. But if using dials you need consistent lead screw direction to take up backlash. Approach the holes from the same direction part to part. Same goes for vise. Those are close tolerances so vise jaw has to be as aligned to mill travel with DTI as you can make it.

- holes made with boring head will be the most accurate so you are doing that right (assume that's your big holes). Drills are roughing devices. 

- if the center drill is the problem hole leading you off, it might be a function of your center drill, the drill itself or chuck/arbor accuracy. I've opted for a carbide center drill similar to this one. Some people recommend specific angles to work with common drill pint angles. So far I'm getting pretty good results with a 90-deg. They are short & very rigid. Make sure your drill is of decent quality. I don't think chip evacuation is much of an issue but certainly an accurate grind is. A stubby length might help. Consider a reamer to clean the hole to specific size because a drilled hole will rarely be perfectly circular. But a reamer will basically just follow a crooked hole.

- really watch for edge burs from machining or surface unconformities that can throw off a setup. I have a plate of glass with standard 600g wet-dry paper taped on. Each part gets a light flattening with WD40. Clean your parallels/vise every time from chips & dust 

-if this is a real micro-balancing act where the center relative to outboard holes is the critical thing, maybe re-think the center hole design whereby you have a bushing with some float. Something that would allow you to fiddle it into position on a balance beam or some standard hard gage. then set with permanent locktite designed for this gap width.

Hope this helps a bit. I'd like to see more pics of your components & project


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## petertha (Nov 24, 2017)

My aluminum link rods are kind of a similar issue where I have to ensure equal hole centering & then do other operations out of the vise & onto rotary table. Here I'm referencing off a dowel pin & jig plate for replication. Your holes should lend themselves to vise setup I would think.


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## extropic (Nov 24, 2017)

Mitch,
I think there are too many issues to address in one post. What milling machine are you using and what condition is it in? Equipped with a DRO? Have you verified the table travel readings to known standards? You said you used a snap gage and micrometers to measure the bored holes. I think you mean telescoping gage, not snap gage. Correct?

You are trying to hit some pretty tight tolerances. I'm asking about the machines because it's much harder to hit those tolerances with "loose" equipment. I would never rely on my use of a telescoping gage to .001 accuracy. Some others may be able to do better. YMMV

It may be that a good portion of your perceived error is actually in your measuring equipment and technique.
Can you repeat a measurement 10 times and be satisfied with the range of observations?

Ultimately, every step needs to be reproduced precisely to achieve the tolerances you've specified.


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## Mitch Alsup (Nov 24, 2017)

Thanks everybody:

State of the machine: New as of july--new as in never used before. It is a Grizzly 8×30. No DRO, no motor drives.

I have verified that the long axis (Y) is spot on, by measuring the spindle travel with a dial indicator and comparing it to the dials on the wheels.
I have verified that the short axis (X) is off (short) by about 0.75%, by measuring the spindle travel with a dial indicator and comparing it to the dials on the wheels. This is about the error one would expect when threading the lead screw on a metric lathe with 84/93 gear ratio.
I have measured the vertical axis (Knee) is spot on, by measuring the spindle travel with a dial indicator and comparing it to the dials on the wheels.
The vise is trammed so that my dial indicator sees more roughness on the vise face surface than it sees tram-error as it moves from side to side. The vise is also trammed so that the top edge of the vise back jaw is also perfectly level with the bed. The ways the vise jaw slides on are off by 0.0015 with the left side lower and flatter and the right side starts off proper than rises by the stated amount. {It actually to me some time to get the vise to this state, it started out off by 0.004 horizontal, but a complete disassembly, cleaning, and careful assembly with dial indicators got it to where it is today.}

The ways are tight, although I have not measured them, and well lubed. The vertical axis is lubed with way oil, the horizontal axes are lube with ISO 68 for better feel at the wheels.

As parts are removed, the vise and parallels are brushed with a chip brush to remove chips and crap, then wiped down to remove oil and dust, before another part is inserted and tapped down.

I do have a stop at the left end of the vise on the non moving jaw (similar to the picture above), but it is clamped on instead of being really solid. All part positioning (above) is done to the back vise jaw and the clamped stop (which is from my box of parallels).

I did start the small hole drilling with a #2 center drill 60º--which has been used to start about 10 holes now.

And, yes, I know these are difficult accuracies to hit, I'm and just trying to figure out where my errors might be coming from. And nothing is off my more than 0.003" so I'm not doing that badly, but I need to get better for the work to come on the larger telescopes.


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## petertha (Nov 24, 2017)

Hopefully you will able to view this link on our local forum with pics (post#10). I bought a Sorensen center mike off ebay & am really impressed with how it works. I suspect the pins might be too big for your part, but maybe get you thinking about measurement in general & where the error is occurring. Sounds like you are doing lots of things right setup wise. The main thing being an independent dial showing table displacement vs relying on your dials.  https://canadianhobbymetalworkers.com/threads/sorensen-center-mike.673/

Based on what you've said I might be out of good ideas beyond integrating something into the design that allows 'tune-able' compensation beyond the limits of your machine/setup. Its too bad your center hole is in a different axis otherwise it could act as an axle pivot so the part would rotate 180-deg to make the 2 larger hole on either side & put to rest displacement measuring error creep.


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## Bamban (Nov 24, 2017)

Mitch,

I live in Anderson Mill area, you are welcome to come by and use my Bridgeport, not new as your Grizz, but at least you can compare outcome. My BP has a DRO, I have a good coaxial indicator we can use to measure centerline between holes. No power feed on the knee or the Y, I do have one for X, the L-R.

I just had a knee replacement surgery, if you decide to take me up on my offer, you will have to do all the work.


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## Jimsehr (Nov 24, 2017)

Mitch Alsup said:


> This post is a question about how to achieve repeatability--making several parts fit the same blueprint.
> 
> But first some background::
> 
> ...


I think if you are going to make a number of these parts you should look into making or have someone make you a drill fixture. With a fixture you don’t need to rely on table movement. The fixtures have hardened drill bushings that once made stay the same.  
Jimsehr


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## petertha (Nov 25, 2017)

I just want to go back to your dials & lead screw. I think you are saying you don't have a DRO but validated the X displacement dial graduations against an indicator & its sufficiently accurate. So when you are going to your hole positions you use the crank/graduations, ie. you don't have an indicator set up to independently show displacement, right? 

If so, I see one more potential error source. When you lock the tables on these typical Asian machines its incredibly easy to displace it because of the cheesy way the lock screws act against the gib strip. It imparts a small tourque & can easily move the table behind the scenes & your dial is not the wiser. I'm not saying your mill is exactly like mine but maybe test this. I had the exact same issue on my lathe. It was actually the DRO installation that flagged this. I would set the position to 1.000", lock the table and it easily move to 0.998" to 1.002" as evidenced by the DRO.

This is an old pic before the mod, but I replaced these plastic knobbed screws with a modified bolt that has a bearing ball affixed to the end. It imparts force to a little brass shoe that is like a cylinder with its end chamfered to match the dovetail. So it slow friction & nil torque imparted to the shoe which clamps linearly against the gib strip. Now I get virtually zero table movement when I lock it down. (ps - a DRO is a great investment!)


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## petertha (Nov 25, 2017)

This is what I made for my lathe (same tightening displacement problem & similar principle). I just inserted it into the set screw hole. The screw itself was countersunk to kind of cup the ball. Note, you have to have a way to remove this in the case of a blind hole, either by moving the table to expose it or chewing gum on a stick etc


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## extropic (Nov 25, 2017)

Mitch,
I've never seen an indicator with graduations that include "spot on".
I don't know what "spot on" means relative to deviation over distance nor even the resolution of the indicator.
When we're trying to converge on tolerances in the "tenths", via the internet, sharing a common understanding of the details is critical, IMO. When I read others using adjectives rather than units of measure to describe something (precision?) I can only infer that it's 'good enough' for the author.

You're getting lots of good advice here and I don't think I can help anymore.

To satisfy my curiosity please reply: You said you used a snap gage and micrometers to measure the bored holes. I think you mean telescoping gage, not snap gage. Correct? Post a picture?


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## Mitch Alsup (Nov 25, 2017)

petertha said:


> I just want to go back to your dials & lead screw. I think you are saying you don't have a DRO but validated the X displacement dial graduations against an indicator & its sufficiently accurate. So when you are going to your hole positions you use the crank/graduations, ie. you don't have an indicator set up to independently show displacement, right?
> 
> If so, I see one more potential error source. When you lock the tables on these typical Asian machines its incredibly easy to displace it because of the cheesy way the lock screws act against the gib strip. It imparts a small tourque & can easily move the table behind the scenes & your dial is not the wiser. I'm not saying your mill is exactly like mine but maybe test this. I had the exact same issue on my lathe. It was actually the DRO installation that flagged this. I would set the position to 1.000", lock the table and it easily move to 0.998" to 1.002" as evidenced by the DRO.
> 
> This is an old pic before the mod, but I replaced these plastic knobbed screws with a modified bolt that has a bearing ball affixed to the end. It imparts force to a little brass shoe that is like a cylinder with its end chamfered to match the dovetail. So it slow friction & nil torque imparted to the shoe which clamps linearly against the gib strip. Now I get virtually zero table movement when I lock it down. (ps - a DRO is a great investment!)



I validated the long and short horizontal axes with a DI on a magnetic base measuring the position of the quill (several times.)
The long axis has its dial exactly match the DI (1.000 on the dial == 1.000 on the DI) after 1 inch of travel with DI at 1.000. move DI and reset it to 0.000 then move axis again.
The short axis has its dial read 1.000 when the DI reads 0.9997 EDIT:: make that 0.997 not 0.9997, and this repeats as the DI is repositioned.

I will have to watch the table as the locks are applied. However in the case of these 3 balance beams, the table remained locked for the machining of all three beams.

I may have to do a ball bearing trick too.

Thanks.



extropic said:


> Mitch,
> I've never seen an indicator with graduations that include "spot on".
> I don't know what "spot on" means relative to deviation over distance nor even the resolution of the indicator.
> <snip>
> ...



Spot on means the reading on the dials are exactly the same reading as on the dial indicator.
I did mean telescoping gauge (I had heard they were also/sometimes called snap gauges) In any event, the box they came in says telescoping gauges.


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## TakeDeadAim (Nov 25, 2017)

I think you may have a number of factors at play here.  When holding this type of tolerance lots of little things can have an effect.  Making sure any backlash in the screws is taken out before moving the table, runout in a drill chuck, having one lip a bit long or the angle a bit off on the drill, just to name a few.  As far as boring small holes goes you just need a tiny boring head and a tool ground, (or purchased) that is small enough to do the size your trying to do.  Micro 100 makes a whole series of small boring bars for just this purpose.  

I could tell you how I would do this, but it would be pages of written procedure.  If you lived near me Id be glad to have you over and help you work this out.  Want a trip to the cold, snow and ice?

Doing this level of work requires adherence to procedure, a good set up, sharp tools.  Repeatability requires doing the correct procedure the same way multiple times.  This is what separates the men from the boys as the saying goes.  Im not sure if you watch any of the YouTube machinist creators but, looking at the process and time spent doing set up by guys like Tom Lipton (OxTools),Robin Rinzetti and Stefan Gotteswinter you will get what Im talking about.  Having the tools is a great first step, having the experience.... well it takes experience.

This is not easy, look at the results you give us above, your doing well!  Keep at it and you will get it.


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## Mitch Alsup (Nov 25, 2017)

TakeDeadAim said:


> I could tell you how I would do this, but it would be pages of written procedure.  If you lived near me Id be glad to have you over and help you work this out.  Want a trip to the cold, snow and ice?



maybe in the summertime......



> Doing this level of work requires adherence to procedure, a good set up, sharp tools.  Repeatability requires doing the correct procedure the same way multiple times.



I have literally watched hundreds of hours of Lipton, Abom79, Fenner, Doubleboost, .....
I think the problem described above is one that an experienced machinist could see in a few minutes of watching my work habits, procedures, setups, and measurements.



> This is what separates the men from the boys as the saying goes.  Im not sure if you watch any of the YouTube machinist creators but, looking at the process and time spent doing set up by guys like Tom Lipton (OxTools),Robin Rinzetti and Stefan Gotteswinter you will get what Im talking about.  Having the tools is a great first step, having the experience.... well it takes experience.
> 
> This is not easy, look at the results you give us above, your doing well!  Keep at it and you will get it.



I have no choice but to continue, the mirrors for the scopes are already in hand.


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## Silverbullet (Nov 25, 2017)

Sounds like some good advise, I might add some may be to backlash , the take up backlash may in fact only be when it's locked down.


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## Mitch Alsup (Nov 25, 2017)

Silverbullet said:


> I might add some may be to backlash , the take up backlash may in fact only be when it's locked down.



I "take out the backlash" by using several means::
a) move the table in only 1 direction after finding an edge
b) setup a DI when the distance table moves is small (taking backlash out of the equation)
c) move the table 40/thou the other direction before moving it forward.

As to a): When I use an edge finder, I move the part up towards the spinning edge finder, watch the finder become cylindrical, then watch for the kick. With my edge finder, I can set the dial to 0.000, back the part away 40/thou, come back, see the kick, and be within 1.5/thou on the dial. I'm not really happy with this lack of precision/accuracy, but this is what I can do right now.

As to c): the dial movements of backlash are on the order of 10-12/thou, moving the table 40/thou means the grease will have moved in the position bearings and hopefully eliminate squishy "readings".

I think I have a number of small issues; all adding up to give me the errors reported above.

Thanks again.


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## BtoVin83 (Nov 25, 2017)

I’m going to simplify your process to keep from being wordy. If I understand correctly you have some type of fixture and prep the pieces and bore one end of the beams. Then you step off the center distance and verify, lock everything down and proceed to bore the opposite end. In essence all three beams have been removed and reinserted into the fixture. If this is the case a .003 variation is pretty good, in the shops I worked in would not use that procedure unless we had at least a +/- .01” tolerance. The center drill and drilling process should have nothing to do with the error as the boring should take care of any discrepancies.
If this is not the case disregard my ignorance.
If it is the case and I needed tighter tolerances I would proceed like you did in the first phase. When boring the opposite end I would use your system that verified the center distance on each beam eliminating most errors except that of measuring.
You can have fast or accurate but rarely can you have both at the same time.


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## mksj (Nov 25, 2017)

Mitch Alsup said:


> I think I have a number of small issues; all adding up to give me the errors reported above.



I agree. You also have a lot of cumulative errors and also at the limit of of your caliper/equipment resolution. You can often measure a part with a caliper 3X and each measurement will be different. Just switch from a drill to a boring head and changing the height of the head or knee cen change position, just lots of sources of errors.

You are really at the limit of what you can get out of the mill as setup, and are doing pretty decent at that. You can get a DRO for as little as $250 or less with 5 micron scales (which have their own deviations), this takes some guesswork out of the dials and table movement locking, which can easily account for the error you are seeing. It would also give you more precise center to center hole dimensions.  As others mentioned, just locking down the gibs can introduce significant error, although there are a few tricks to minimize it. Mechanical edge finders and also the basic electronic touch ones, have not been very accurate in my experience, certainly not at the 0.001" level. I use a mechanical Haimer which is good for both edge and center to ~0.0004" and also have an electronic probe center finder which is about the same accuracy or a bit worse. You also have eccentricity of the collet, angular changes in the head, etc. Part of the process is reproducibility of procedure, but you still have movement of the machine, and also the process of drilling or boring holes. Aluminum can deform or twist on clamping. Drills tend to walk very easily, the longer drills can deflect quite a bit. When I need hole location precision I center drill the hole with a Keo countersink first and than follow with a stub drill. On precise boring dimensions I use a reamer. My goal is to hit 0.001" or better tolerance when it is required. Without a DRO, decent edge finder, etc. I was lucky if I was at 0.003" on a good day.

If you are making multiple parts, then I agree with others in setting up a jig with alignment pins that reference off of known holes.


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## kd4gij (Nov 25, 2017)

When you can, a DRO would be a big help.


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## Mitch Alsup (Nov 26, 2017)

BtoVin83 said:


> I’m going to simplify your process to keep from being wordy. If I understand correctly you have some type of fixture and prep the pieces and bore one end of the beams. Then you step off the center distance and verify, lock everything down and proceed to bore the opposite end. In essence all three beams have been removed and reinserted into the fixture. If this is the case a .003 variation is pretty good, in the shops I worked in would not use that procedure unless we had at least a +/- .01” tolerance. The center drill and drilling process should have nothing to do with the error as the boring should take care of any discrepancies.



You have my procedure correct.


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## Wreck™Wreck (Nov 26, 2017)

If positional accuracy of holes is your ultimate goal there are a lot of jigbore machines that no one uses today floating around used, tooling for them may be a bit more then you are willing to spend however, an old Moore would be a good choice.
Good Luck


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## Mitch Alsup (Dec 5, 2017)

I thought I could amuse the group here with how the original topic turned out:




Here we see one of the beams after it has had its back shaved and one end lightened so that the CoG is coincident with the center of the axle. It is also sitting on its holding bracket. It is balanced to within about 0.1 grains and has very low friction, at the ends of the beam are ball transfers. 3 beams and 6 ball transfers support the back side of a 13" telescope mirror 1.1" thick and 10.3 pounds in weight (or just under 2 pounds per ball).

Originally the idea was to have the bearings pressed onto an axle and into their races. But after screwing around with that for a few hours, I could never get the bearings to rotate smoothly and at low friction. So, I used the micrometer and measured the ground shaft at 0.125,1". I sanded them down on the lathe with 1000 grit and WD-40 to 0.124,9 and they would fit into the 1/4-1/8-3/32 bearings. It was this set of shafts that would not roll smoothly. So I took the shafts down to 0.124,6 and the beam would spin 20+ times when kicked with the finger (snap). However, this (also) meant the beam could slide on the axle and was not laterally located.




a couple of brass spacers locate the axles well enough (about 0.003). Originally the 4-40 screws were to allow lateral placement of th beam, but now this will be done with the brass spacers.

So now we have 3 sets of beams pivoting in bearings, and we need a means to hold the beams in exactly the right spots and build the rest of the framing around them.




I carefully drilled 6 holes in a flat piece of round (scrap) plywood (baltic birch) to locate the ball transfers. I also machined some bushings that just fit the ball transfer bores at the ends of the beams, and slotted the back sides of the brackets to fit the y-frame holding these bracketed beams. Two of the brackets are simple slots at 30º, the third bracket has a linear slot and two 60º slots.

The original idea was to have the slots grab onto the y-frame. Due to drill drift in the beam bearings and in the brackets, I was forced to relieve the slots so the y-frame can giggle--thus inducing no stress on the axle--thus placing the ball transfers at exactly the right position. Later, after the upper frame has been attached to the lower frame and the things sanded and painted, these brackets will be epoxyed onto the y-frame while in the support plate.




You might notice that the beams and brackets have lettering to denote which bracket goes with which beam and in which orientation. The balance is close enough that the ball transfers are also indexed to a particular bore in order to remain balanced.

As it sits, the y-frame can jiggle in the brackets and pop in and out effortlessly--inducing no stress on the axles. I did assemble this flipped it over, put in the ball transfers and used another flat wooden plate to check friction--it is very low (lazy susan low).

I also checked deflection from the axle-shaft-4-40 screw and found around 0.0060" with 8 pounds of force on a single beam which is "acceptable". Should this prove problematic, I can support the axle with little brass triangles between the floor of the bracket and soldered to the spacing tube.


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