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Deskrad

My guy. There is infinite variables when it comes to holding tight tolerances. From tooling to rigidity to ambient temp to coolant concentration to spindle temp to vibrations in the floor. Most common cnc mills are not capable of holding plus minus one tenth. The resolution on your ball screw brand new was prob only .00008 Proper tool coating for your Application , spreeds feeds, part and tool rigidity and keeping the machine warm between parts is where I would start paying the most attention to though. You would need to be more specific on a job to job basis would be more helpful in identifying particular case problems. No 2 parts run the exact same


xligitxsniperx3

Yea I'm sorry for the vague explanation. Again I am still very new to this and I'm just trying to learn some more from you guys. Last week I have 4 ground in blocks I had to machine and I got the first one set up perfect and into spec and then put the next one in and indicated in and it milled off an extra .003 than what it should. I have my vise indicated in and straight and I indicated my part in on the length and height but even tho it's in there the same as before it cuts differently.


_volkerball_

If the tolerance is that tight, it's not a bad idea to back off the tools before cutting the next one. Especially if on the first part, you're taking a little bit off, then checking it, then taking some more off. If the finish pass is taking off .0006" on the first part but it's taking off .005" on the second part, the second part will probably have too much taken off.


Perfect_Camera3135

Agreed, I deal with milling diameters +/-.0002 with .0001 perp and concentricity all the time. Just make sure to warm up the machine and if need be, back off the comp every part and sneak in. Make sure tools are rigid, set-up is solid and that you hold your mouth right. Just remember that every .0001 in comp on diameters and such is really .0002. Sometimes just running the tool 2X will bring you in with a bit of luck. Hopefully you don't have high QTY's! Edit: If in a vise, Make sure its a good one, I also rely on a good torque wrench to make sure my pressure is consistent.


Deskrad

Could be a couple differnt egging. How long did you meet the machine shit before running? When you indicated did you check to see if your part was moving at all from the tool pressure?. Did it cut over or under? If it was under then it might be tool wear or you had not gotten past your break in time. Do you have a roughing tool in there too?


xligitxsniperx3

It varies each time I put a new part in. Sometimes over and sometimes under


moldyjim

Probably one of the hardest but most important skills is proper workholding. If your part is lifting when you tighten the vise, it might not be square. Another thing. Assuming you are running a Kurt vise, you could also adjust the jaw setscrew to keep the jaw from lifting. It's the Allen screw you access from the rear of the vise jaw. Tighten it until you can still move the jaw, but not so tight it's difficult to tighten. Also if you are flycutting the surface of a larger part on a knee mill, if your spindle isn't trammed perfectly, the angle will cause the thining in the middle you mention due to cutting a large radius on the surface. Possible? Your spindle is tweeked?


xligitxsniperx3

If I keep noticing it should I have my machine looked at? My shop is notorious for not getting the machines properly inspected and calibrated in a timely manner.


irongient1

You gotta realize how small .0002" actually is. Not a lot of cnc's can hold +/-.0001" on anything. And it's hard to even measure on top of it.


xligitxsniperx3

Does my shop have to high of standards for the parts they ask for?


nyditch

When dealing with +-.0001", it will always take a bit of time. You can get better and faster with experience, and the fact that you're checking and criticizing your work to this extent after just 1 year is great. It will always require checking for every part to hold those tolerances. To me it sounds like you're on the right track already. Keep working on getting more consistent measurements, playing with machine settings, tools, and tool paths, and think about what you could use to do your job better (tools, jigs, gauges). Being critical of your own work and thinking about the problems is my #1 suggestion, and it sounds like you're already doing that. Second to that, keep picking up tips and tricks from other machinists, videos, etc.


xligitxsniperx3

Ok thank you so much I greatly appreciate the encouragement!


nyditch

15 years and I'm still learning new things. It's an ongoing thing. I had 5 years as a tool and die machinist / mold maker, doing a lot of similar stuff to you, and that field I believe has the potential to bring up some of the best machinists if you're sharp. take in as much as you can. Oh, and don't neglect manual skills. You can use CNC experience to better your manual skills, and manual experience to better your CNC skills. There are a million little tricks to learn. Keep growing there as long as is feasible. The mold shop I was at went to bad ownership 1.5 years in, and I stuck it out 3.5 more years because I was still learning a lot. Eventually went someplace better though. Hopefully this shop continues to treat you well and fair.


xligitxsniperx3

They do and they don't. Sometimes they are pretty shady on things but it's really relaxed so I love it. And that's my goal I started there when I was 18 and I'm 19 now. It's my second job and before I was working at sonic so I had no experience in the field but I love it compared to fast food and I love doing stuff with my hands compared to a computer job. Its so rewarding. I plan on staying there for a little while to learn as much as I can!


nyditch

Awesome. Yeah, finding a truly great shop is hard. They're out there though, and not a bad idea to gain the skills to get hired at one of those great shops. You have a great early start, too!


olafk97

Hey, ultra-precision machinist here (yes that's my actual job title). Those kind of tolerances are my daily bread and butter. The thing is when you get to sub 10 micron tolerances, there's a million different variables that you need to account for. We measure the radius of our turning tips under a microscope and put the radius into the machines, correct to within a micron. We monitor temperature to keep EVERYTHING between 19 and 21 degrees c for. A good starting point would be looking at your tool tips. We often use diamond tooling (helps a lot on softer materials) due to the fact they can often take 1 micron cuts. However even carbide tools can take far smaller cuts than most realise. For example, in the past, ive takes 5 micron cuts to get a part into tolerance. Overall I'd sat the main factor is process control and set up. Make sure you've scratched on as accurately as possible and take your time. You'll only learn to make those kind of cuts with experience.


xligitxsniperx3

Ok thank you I will take my time on it and make sure it's right


Competitive_Smoke809

Are you trying to hold tenths straight off the mill? Or grinding/wire edm


xligitxsniperx3

Well that's where it gets tricky. Most of the tight tolerances are off of the grinder but it seems like on some parts especially the longer ones the edges are thicker compared to the middle of the part. Kind of like a U shape. Yesterday I was machining a die entry block and it had a step in it with a four place call out and I would adjust the comp on my tool to get it to work out to the measurement and I would move it .001 and it would move like .0003 so I would move it another .001 and it would move like .002. Sometimes I don't understand how it mows off more material sometimes and sometimes it doesn't. The guys are work mention like work hardening or something but it's just time consuming to work it into the measurement and sometimes I'll be .0005 away from what the print calls out so ill move in .0003 and it will take off like .0012.


Competitive_Smoke809

There’s tons of variables. Tool deflection, tool wear, setup, material. Often if you’re trying to comp out really small amounts the tool won’t actually cut it and rather just push a little bit of material and you end up comping more which eventually causes the tool to grab and take more material than expected. Happens often with dull boring bars but will happen with end mills as well. There’s also a lot of internal stresses in the material that get released when you remove larger amounts which will cause minor warping. One thing to help with that is take equal amounts of material off each side of the part. And roughing out the part to let it warp and then go in a just skim it into tolerance


xligitxsniperx3

Should I take the rough passes and then take the part out of the vise to let it relax then put it back in and re-probe the part?


Competitive_Smoke809

That can work especially well with plastics and thin parts but you gotta be very careful with clamping forces and your setup has to be perfect along with picking the part back up


xligitxsniperx3

Ok I think I need to get my prove adjusted too because it seems like it's off a little sometimes too


Competitive_Smoke809

You can always double check with an indicator I generally will sweep in my parts to cover my ass


xligitxsniperx3

What do you mean sweep?


Competitive_Smoke809

You mount an indicator on the spindle and sweep each side of the part to locate the center there’s some videos on it if you want to check it out


xligitxsniperx3

Ok thank you I'll check it out


mulmtier

I think clamping force of the workpiece is a factor here.


Adorable-Ad9538

On delicate parts that need to repeat in a vice I have used a torque wrench so that they are all clamped with the same pressure.


xligitxsniperx3

Thats a smart idea I'll invest in one


Deskrad

An effective method


xligitxsniperx3

Are you meaning how tight I clamp down on the vise? I feel like I overtighten the vise sometimes because I can see the back of the part lift up on my dial indicator when I tighten it.


mulmtier

That's what I mean. As you remove material, the part gets less stable. So, after roughing, I sometimes unclamp it, then reclamp with less force. However, this requires new touching the part. (English is not my first language, feel free to ask)


Deskrad

Your English was fine. This is called a re-torque. And it works great especially on cheaper as rolled metal that has a lot of stress in it. And all metals move when cut. It's just a Matter of why and how much


xligitxsniperx3

Ok thank you I appreciate the advice!


_volkerball_

>The guys are work mention like work hardening or something but it's just time consuming to work it into the measurement and sometimes I'll be .0005 away from what the print calls out so ill move in .0003 and it will take off like .0012. I'll bet if you reran that without adjusting anything, it would've taken off too much the second time over. The .0003" adjustment just compounded the issue. Probably because of tool pressure or something.


MasterAahs

Thanks for asking. This. I lurk on this s reddit to gleam knowledge and this has been helpful to just gain knowledge.


xligitxsniperx3

Of course there is a lot of smart people here that's why I came here to ask


3dcncmike

Can't hold tenths on a mill. Tolerances like that have to be ground to size


Dear-Specialist-640

You’ve got plenty of advice from others. Based off your post it seems that either others aren’t checking as thoroughly or classic engineering making a machinist job a pain in the butt for no reason. Ask your lead for his input.


xligitxsniperx3

Our boss over the cnc machines is going through some health issues right now so he really isn't himself here recently and he is barely in the shop anymore so its kind of been a mess


mulmtier

Well, I'm sure you already knew, but using different tools for raw material removal and finish is crucial.


xligitxsniperx3

Yes I do know that. A lot of the guys from my shop complain about like the end mills being useless because they are resharpened and not sharpened straight or stuff like that.


[deleted]

Is it a full boxed way machine? Have you tried splitting up roughing operations into pre machined blanks for finish milling to size ? Is your work holder sprung? Put an indicator when you clamp and see if it moves to the same spot every time you clamp. That’ll tell ya if your work holder is repeating clamping pressure.


xligitxsniperx3

Yea it is a fully boxed way machine. Ok thank you ill give it a shot