[mike sr1]

I am kind of a rookie when it comes to CNC thats for sure!

I never really paid much attention to backlash as its so low that its kind of irrelevant to the parts I make until last week! I had a part that is two separate halves/pieces but the profiles need to match when they are bolted together, they machined fine, drawing is correct etc but after machining, the profiles didnt match exactly?? I think I narrowed it down to the fact that I cut the profile on the parts in two different directions, I never gave this a thought but it stands to reason when there is some machine backlash involved they need to be cut in the same direction to match up perfectly. I learned something new to me in the process,they arent off by much but are not perfect, sure gave me something to stew over this week ha!


I really havent read anything that pretains to this situation, and am wondering if some of you guys have had a similar situation or did I just miss the boat on something?

[RussMachine]

Could be tool pressure. Did you do a 'skim' cut for the final pass along the mating surfaces?
Your endmill might have a dull spot, leaving a slight ridge.
Also a dull endmill could leave a slight taper.
A climb cut and conventional cut will leave differences in the finish as well.
There are lots of things that could affect mating surfaces.

[kstrauss]

How bad is the mismatch? Are we talking about 1/2-thou or 5 thou? Are you cutting the two parts from a single lump of stock? Posting a sketch of your work might help.

[mike sr1]

Russ,

The mating surfaces are fine, its the profile that is off a bit. the cutter is sharp and relatively new. Finish cut is .010 just to make sure they clean up nice.

Kstrauss,

About .003 mismatch and I am guessing at that, not too visible but I can feel it. The parts are cut out of separate pieces of stock, I cut 5 parts at a time of each one. The profile cut is in opposite directions on the different groups of parts.

This is just a problem that is new to me and one I hadnt considered untill now, but then all my parts are singles and dont have to match up to another part with the exact same profile. I can fix them I believe by putting them back on the fixture with the two halves bolted together and re cutting the profile or maybe a small chamfer.

I can lay out the fixture with a different hole pattern and cut the profiles of both parts in the same direction and see if that fixes it.

I will post a pic or two.

Thanks for the replies.

[mike sr1]

Attachment 232478Attachment 232480Attachment 232482

These are pics of the parts in question,

I make 5 at a time on this fixture of two different groups of parts, I mounted two different singles ones to show the direction of profile for each part, which when mated they are not profiled in the same direction and I am wondering if that could be the reason they are off a bit?

the last pic, where the left bolt is, is where its off a bit, pics tend to make things look better than they actually are, maybe I am just too picky?

[TurboStep]

My first thought was that you may be using the protrusion on the inside to locate the two halves. If the protrusion and matching hole were not machined in the same ops as the respective outside profiles then a mismatch is easily possible. On the other hand, if I stretch my imagination, this could be due to a large amount of Y-axis backlash. The easiest way to find out what your backlash means to you in real terms is to machine a circular boss and measure the diameter, preferably with a good old analog micrometer (not one of those new-fangled digital thingies ). The points of interest are just before and after (when rotating clockwise) 12:00/06:00 o'clock for Y-Axis backlash, and before and after 03:00/09:00 for X-Axis backlash. It's therefore better to start the finishing pass at about 45° so as not to affect these areas. When climb milling, the values measured before these points will be slightly smaller than the values measured afterwards. The difference will be twice the backlash. You may even be able to just measure the diameter of the protrusion with a Vernier if the backlash really is in the region of 3 thou.
And no, you're not too picky
Step

[mbi]

This will not resolve your backlash problem however it may help. Tighten up your spine tolerance. Caution it will make your program larger.

[kstrauss]

Are all of the parts machine in the same direction identical? You appear to be securing the blanks to a fixture with SHCS. Is slippage during machining a possibility? You could verify by cutting a group of pieces from a larger billet and then checking for identical shapes.

[mike sr1]

Are all of the parts machine in the same direction identical? You appear to be securing the blanks to a fixture with SHCS. Is slippage during machining a possibility? You could verify by cutting a group of pieces from a larger billet and then checking for identical shapes.

No, the profile on the second part is machined in the opposite direction as the part is flipped over to mate with the previous one. The second part could slip possibly but it isnt likely as all 5 have the same discrepancy, and are machined all at the same time.

[mike sr1]

My first thought was that you may be using the protrusion on the inside to locate the two halves. If the protrusion and matching hole were not machined in the same ops as the respective outside profiles then a mismatch is easily possible. On the other hand, if I stretch my imagination, this could be due to a large amount of Y-axis backlash. The easiest way to find out what your backlash means to you in real terms is to machine a circular boss and measure the diameter, preferably with a good old analog micrometer (not one of those new-fangled digital thingies ). The points of interest are just before and after (when rotating clockwise) 12:00/06:00 o'clock for Y-Axis backlash, and before and after 03:00/09:00 for X-Axis backlash. It's therefore better to start the finishing pass at about 45° so as not to affect these areas. When climb milling, the values measured before these points will be slightly smaller than the values measured afterwards. The difference will be twice the backlash. You may even be able to just measure the diameter of the protrusion with a Vernier if the backlash really is in the region of 3 thou.
And no, you're not too picky
Step

I think you are on to the problem Step, and the simple solution is to machine the profiles so that the parts when mated, the profile ops were machined in the same direction. The rest of the tolerances are not super critical, this one is because its visible if it doesnt match perfectly.

I just need to change the holes in the fixture and profile from the other side of the part so that when its flipped and bolted together the profiles will match.

I had just never run into this problem before, kind of had me scratching my head for about a week wondering what was causing it!

[Beezle]

I make vise-held fixtures like that.

Couldn't you finish profile them as a set?

[mike sr1]

I make vise-held fixtures like that.

Couldn't you finish profile them as a set?

Those fixtures work well, but I am running out of material for them, need to go check the scrap yards etc.

I am thinking about finishing as a set, or using the bottom side of that fixture to profile them from the other side, that will work too. These arent needed right away so its no rush on them.

[SCzEngrgGroup]

The only way to get a really perfect match is to mate the parts, and profile both at the same time. The basic accuracy/repeatability of all of the machines in this size/price class is +/-0.001" at best, so the small mis-match you're seeing on those parts is almost certainly just an artifact of the limit of accuracy of the machine itself. Getting better accuracy also requires extreme care in fixturing and setup. There's a reason $100K machines cost $100K, and not $10K - extreme accuracy costs a LOT of money, and still requires a highly skilled operator and very precise fixturing and setup.

Regards,
Ray L.

[mike sr1]

right

[Steve Seebold]

Attachment 232478Attachment 232480Attachment 232482

These are pics of the parts in question,

I make 5 at a time on this fixture of two different groups of parts, I mounted two different singles ones to show the direction of profile for each part, which when mated they are not profiled in the same direction and I am wondering if that could be the reason they are off a bit?

the last pic, where the left bolt is, is where its off a bit, pics tend to make things look better than they actually are, maybe I am just too picky?

Hey Mike, let's get real here. That part is for a model boat. .002 or .003 mismatch is no big thing, and if you ad a .005 or .010 chamfer around the part, no one would could feel the mismatch anyway.

[mike sr1]

Right on Steve, and thats probably whats going to happen.... Junior wanted a part that would work with the thin flange stainless headers, kept me busy thru some of the cold months though.

It sure had me scratching my head as to why it was doing that though ha!

I am new to CNC, I always wanted to learn the trade, now I have the time, this kind of problem really makes things interesting in the learning process ha!

[Steve Seebold]

Another way to make your parts would be to use material that has 1/4 inch of stock on the back side, then you could run your part complete and then flip it over and fly cut the back side off.

[mike sr1]

That would work as well, I think I can fix the problem either with a chamfer or machine the two profiles in the same direction.

I have some scrap parts that have no profile cut on them yet, so I am going to setup and profile the other direction and see what happens then I will have a better idea of whats going on.

[mike sr1]

Hey Mike, let's get real here. That part is for a model boat. .002 or .003 mismatch is no big thing, and if you ad a .005 or .010 chamfer around the part, no one would could feel the mismatch anyway.

Steve,
I wrote a small program and profiled the parts from the other side of the fixture, now both parts are profiled in the same direction and it fixed the problem! No perceptible differences that I can feel, even when rotating it 180 degrees its right on as well.

All I really wasted was about 4 feet of 2" x 3/8" flat 6061 if I scrap the parts.

I learned something new today in the process and I know it works (seeing is believing) !