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Milling Question

B

BradH

Member
Anyone know what is a reasonable expectation for parallelism on a milled surface?

Try as I might, I can't get down as close as I would hope. Typically it is in the range of 0.005". over about 6".

I know nothing is perfect... but that seems a bit big.

I have trammed the head of my Hartford 8 ways to Sunday, I have measured the vise - it is within 0.001" over the 6" width. The vise squared in. I have used my flycutter, and recently a large face mill, and still in the same ballpark.

Thoughts?

Thanks in advance,

Brad.
 
Hey @BradH

As I was reading your note, you covered the obvious stuff well.

Here are some thoughts:

Is your mill levelled? (with gravity, not the floor)

Could it be an effect of chips gumming, or tool "wear" during use?
(Cuts less well at one end than the other?)
(Sharp HSS dulls if heated too
Much at the edge?)

Practical machinist would be grilling you looking for feeds and speeds, haha

What about cutting fluid somehow playing a part?
 
Morning JW:

Level, no. But as one fellow on Practical Machinist pointed out, many machines on Navy ships are never "level" but still produce good results as long as the cutter relationship with the table is correct... so I am going to agree with that and say I don't think that is the issue.

Have done single pointing (fly cutter) and multi-point cutting with a 5 insert carbide face mill. Similar results.

The issue is a lack of parallelism, so yes, the machine basically cuts a taper / wedge shape from one end to the other. Thinking recently it may be stress relief in the material, but that doesn't precisely answer the issue.

I took a milling course once upon a time in BC, and I wish I could recall how well a "known good" machine would perform. I know, for example, a 3 jaw chuck is usually accurate within a couple of thou., so moving the work will always give you a bit of error. But 0.005" seems excessive. Half a thou I would be very pleased and think that exceptional.

Don't think that the parts are heating up enough to affect the depth of cut.

Typically use cutting fluid for finish and prevent chip welding, but again, the depth of cut and feed are not sufficient to generate enough heat. I generate way more heat in parts turned on the lathe and get closer tolerances.

Anyway, that is why I wondered if anyone else had experience / reasonable expectations for what a good mill should cut.

My mill is a Chinese Bridgeport clone, which had a hard life from all evidence. It was likely beat like a rented mule and probably poorly maintained. There is a fair bit of slop in the lead screw. I set the gibs to give a fairly consistent feel through the entire travel, but if there were excessive wear on the ways (to give a 5 thou difference of cut) I would expect to see that when dialing against the floor of the vise. Ditto if dents in the table were causing the vise to be jacked up when mounted.

I am starting to think it is operator error in workholding... going to play some more.
 
I know with my craftex 601 locking or loosening the x,y,z changes the numbers do you snug up the locks?
 
You probably checked this stuff already, but just in case...

- any chance you are using parallels/riser blocks that are introducing error?

- does it cut the wedge profile on exact same side every time? (example left side X-axis always thinner regardless of setup or material orientation).

- after you make your finishing cut, leave the part in vise, & z-axis locked. Apply blue. Now run cutter in opposite axis (traverse across Y-axis if you were going X). Any difference? Does it remove material somewhere?

- when you checked the vise, does the DTI read the same across the vise bed? (ie if part or parallels are resting on this surface it will cock the setup regardless if the jaws are square)

- with a nicely finished/flat part, no edge burrs etc. resting gently in the vise just snugged but pressed firmly down, take DTI readings on all 4 corners. Doesn't really matter the numbers fro now. Now tighten vise like you normally do. Does one end consistently float up this same 0.005" amount? I used to have a POS Chinese vise that was a bugger this way. Never really figured out what the issue was. There is a whole procedure about always clamping against the back jaw + using a float on back jaw like soft wire + rotating part in specific orientation order... but assume you know all this & still getting this discrepancy.

- just to eliminate any possibility of post stress relief (CRS is notorious), do you get exact same result from say chunk of aluminum or other material?

- also wondering if a light skim cut yields the same 0.005" wedge vs. heavier depth cut? You wont be able to test this too hard with fly cutter but wondering out loud if the quill is floating a bit, thrust bearing play related.... although I cant quite visualize what is 'should be doing'.

Hope this helps, keep us posted.
 
Hey:
@ Tom O - yes, the locks often move the table a bit, but that shouldn't change the orientation of the part, just the positioning.

@ Peter T - I have dialed the "floor" of the vise and it is within a thou across the plate. I agree - RE: the setup being bad. I have tried soft wire between the part and the rear jaw. I haven't tried bluing and looking for a change in cut. I planned to determine if it is consistent to one side, but rather suspect it is. I file burrs off so the material is clean. Aluminum and steel both act similar.

Will try some of your suggestions and see what I can see... again, I suspect operator error!

Brad.
 
what kind of vise do you have? I originaly had the busy bee centering vise and I had problems keeping the part square.
 
I have been thinking about this post for a long time and I am finally ready to chime in. Took me about a month to find this sheet of paper lol. This is the system I use and it helps to compensate for a few variables. Credit goes to the guy on the top right. Try it!
image.jpg
 
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