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Rtldan
04-01-2014, 06:18 PM
Hi guys!
Complete newbie here to home machining. I've cut a few test parts but nothing much.

Anyhow, I just tried to mill a test part yesterday, but encountered a question I can't find an answer to through google (at least with the terminology I'm using!).

The stock I have purchased from my local Metal Supermarkets is not cut completely straight in it's length. It varies by maybe a 1/32" (or more?).
Of course, it meets the minimum length that I requested, but some areas are longer.

So my question is really this: When I'm modeling my stock, do I take the longest length and use that for my stock dimensions, or do I use the actual requested length/the shortest length of my stock?

To make it even more complicated...
If I'm setting a top left origin point, my x coordinates would be different if taken at the longer end vs the shorter end...which would affect my origin point and ultimately, the entire part.

So, how are others dealing with imperfect stock in their home shops?

Thanks!

hareng
04-01-2014, 06:27 PM
Rule number 1 you have to have at least one working edge, the opposite dimension comes from that, easy.

Arthur.Marks
04-01-2014, 06:46 PM
Rtidan, it is called "squaring" -- i.e. "squaring up stock" in common parlance. http://www.machinistblog.com/how-to-square-up-stock-on-a-mill/

Toolguy
04-01-2014, 06:49 PM
Square it to size first. If you do this accurately enough, you can measure off any side or end. If not right on the money, designate two adjacent sides and one end as datum planes. All measurements then originate from those surfaces. Mark the surfaces with a magic marker or some other marking so you can keep track of which ones they are as you work through the part.

CCWKen
04-01-2014, 07:07 PM
You can never rely on mill cuts or forms to be accurate or straight. You must allow extra material for squaring and finishing unless you purchase precision ground material. Even then, the cut ends still need squaring.

Just to eliminate confusion, "mill cuts" refers to the manufacturer or distributor of the material not the process of milling with a milling machine. :)

Bob Fisher
04-01-2014, 08:31 PM
What Toolguy said. I like to mark mine with small prick punch marks that don't rub off during handling. Bob.

Rtldan
04-02-2014, 04:01 AM
Thanks to all for your quick and helpful replies!
I watched the video and it was very insightful.
Two questions though...

1) What is a general depth of cut for squaring surfaces of the stock that are close to square - such as the z and y axis of an extruded piece of bar stock that have not been cut by the supplier.

2) In the video, he talks about getting the oversized stock to a closer dimension to the part.
Say we have a part that is 1x1x1 inch square. How much stock should we leave after squaring for the part milling operation?

I hope my questions make sense! :confused:
Thanks again for the quick help!
-Daniel

Lu47Dan
04-02-2014, 10:23 AM
Daniel, the amount left on the part varies with the type of final finish operation.
If the part with be finished to size in the mill once squaring operations are done. Than take the final squaring cut to the final size.
If surface ground you need to leave enough material to remove the tool marks left by the end mill.
Dan.

RussZHC
04-02-2014, 10:24 AM
1] I think I know what you are asking but don't know that there is a "general rule", referring back to the video, the first side you want to machine fully but with minimal stock removal. A bit of an assumption being you are in the ball park in terms of rough material size compared to needed finished size. It's not vital, clearly you need enough stock size rough to "contain" the whole part but larger is OK but can be wasteful. Just saying, if I need to turn down to a 1/2" round, I will try not to start with something say 4" round.

2] At about 4:45 in the video, Tom Griffin has three machined sides of that block to use as reference and so can then machine one of those sides much closer to his finished size. His preference is for 2 more cuts, a roughing and a finishing. The finishing cut very often means a better surface finish so you machinery and tooling plays a role here, and depending on those and other details, there will be a "range" that will work but with experience there will be one "optimal" (for lack of a better description). If you know what finishing cut is needed, you sort of backtrack (in terms of roughing) between what that will be and where you are "at" currently. Related, you need to be aware of what you machines "do", by that I mean when I saw a part to rough length, I know that end will be maybe .100" away from truly square to the sides (round stock) so in the overall length, I need to make allowances of that amount to finish turn that end so it is square. If I want round stock to end up say 9" finished, and I am cutting individual pieces, I may saw cut both ends, leaving say 1/4" extra to allow for finishing. Much more and its a waste, much less and I am closer to not having enough length if those saw cuts are off more than expected. That is part of the reason, in a lathe, if possible, you tend to leave a rough part long...if you mess up you still have that extra length and maybe able to salvage a part.

3] related to both, one needs to be very much aware of (a) how far out metal as supplied can be and (b) how much metal moves (so, as example, you may think a surface is already pretty good and decide to leave it and remove all material to get to your dimension from one side, you are going to end up with a banana of some degree...the amount varying by material etc. etc.). And this [3] is not just regarding machining, one can often run into some pretty serious problems welding as well, stock that is called square can be pretty far from actual 90 degree corners and or with a twist, as example.

Rtldan
04-02-2014, 03:57 PM
Thank you again for all your replies!
Just to help my understanding...

Suppose I have a part that the tightest possible stock dimensions could be is: X 1.54", Y 0.48", Z 1.18".
I believe the Metal Supermarket's sales guy said that their cuts are within 1/16.
I will not be grinding any surfaces. The cut in the mill will be the final dimensions.

How would a machinist determine the amount of material to add to his stock dimensions for roughing?
Would it make sense in my example to add, say, 1/32" to all sides (assuming it's in keeping with the Metal Suppliers cuts) PLUS 1/16" for tolerance?
Or, since the Metal Supplier is cutting at specific dimensions, would I just ask them to go to their closest size that is at least 1/16" larger than the required stock size?

Sorry for the confusing question. I wish I knew how to ask it more simply!
Thanks again for trying to help decipher this for me!
-Daniel

RussZHC
04-02-2014, 04:13 PM
I'd be looking for material 1/2" x 1.25" making "X" the other and cut that myself...bit chancy on the 1/2" (as final is .48").
2nd choice would be material 1.625 x 1.25 and I cut the "Y" since that is the one a bit chancy the other way and the 1 5/8" x 1 1/4" is likely a stock size and so may end up being a better choice.
I've got rounds from a scrap dealer w rust and based on measurements the manufacturer likely called them 1" even with the surface rust. Its not 1.25" and, locally, getting 1.125 is not the easiest as over an inch local seem to make 1/4" incremental jumps.
I can get 1" finished from them but need to be pretty careful...first cut is already very near that 1" when one considers what a roughing cut can be...I am estimating a 32nd per radius "extra" (not sure what the standard from a mill would be in terms of "larger than").

Your last (to closest w 1/16" large) will, generally, be a safe bet IMO.

May sound a bit frivolous but sometimes, being a "home shop", and when the part dimensions are not vital, I modify part sizes based on what I know is commonly stocked at local suppliers...can end up a bit wasteful but knowing they have it allows for planning on the bit longer term.

Edit: been pondering the dimensions you gave in your last example (1.54 x .48 x 1.18) and now think the reality would be the first suggestion as to size I made would be too tight, skills and machines that I know I don't have and the second suggestion would be pushing the limits (that is more about having to actually machine all sides of the "as is" purchased stock), a chunk of 1 3/4" X 1 1/2" would be a bit more wasteful but allow more wiggle room (and can almost guarantee that size would be in stock).
This question also got me checking.
One online supplier house list Bronze(s) rounds as being 1/16" larger than stated size and is intended to finish to the list size,
Stressproof cold finished round bars go by size up to 1 1/2" being - .002, 1 1/2" to 2 1/2" being - .003 and 2 1/2" to 4" being -.004 (all those are minus tolerances)
Brass changing by shape, round +- .002 to .005, square +- .004 to .005, flats +- .004 to .012 (those are all plus or minus tolerances).
So just from that I am now more aware materials play a role too. I can only imagine the allowances someone would need to make if you were using hollow/cored bronze.

Jaakko Fagerlund
04-02-2014, 04:51 PM
Find out what the stock sizes are and find an item that is a little bit larger than needed. When I saw things to size, I tend to shoot for 2-5 mm oversize in overall dimensions, depending on the material thickness as the saw tends to cut crooked. For small stuff, like under 100 mm in any dimension, 2 mm extra in dimensions is just fine.

For example, if I had a ready dimensions of 48 mm diameter and 100 mm in length, I would either get 50 mm cold drawn with 103 mm length (I know that the tolerance for cold drawn diameter is h9, thus leaving me 2 mm to play with) OR I would get the next size up which is 60 mm in diameter and order that. Of course one has to always think of how the part is to be machined, as some work has to be done in one fixturing and thus needs to be really oversize in one dimension so that it can be clamped, be it a vise or a 3-jaw or the like.

Usually my suppliers will list the dimensions they cut to as -0/+2, meaning if I order something 100 mm long, it will be 100-102 mm long. Though that doesn't mean I can machine it perfectly to be 100 mm, so I would order 103 or 105 mm long piece so that I know I will be able to produce the part as per drawing.