What's new
What's new

Creating a rough estimate of machining time without CNC-preparation

hanermo, even though your illustration in a nutshell tells me that as a buying company the 24fold difference in MRR will still only come out as a 10% price difference (;)), I get your point and it makes me look at different ways that one would be able to estimate a machining time without using MRR.
What do you think of an approach that uses questions to assign points? I.E. amount of material removed (x points), number of refittings required (x points), highest precision required (x points), does the part include double curved areas (x points)... And then use a sum of points to give a production time-range?


StoopidEngineer, thank you for your kind words on the project and my english. Yes, its a second language :)
Well we have a prototype shop inhouse that they use to make when they only need 1 for prototypes, and sometimes they just need a part quick and it doesnt really matter what the cost is, but what we are working on is mainly to be used in serial production when the shops that gets the job can plan it in properly meaning that they will probably do it in the way that gives them the best margins.

Another fact that I have forgotten to mention is that the main area of research for the project we are doing is additive manufacturing, so we wont be looking at anything that is sizewise beyond a buildchamber of a printer (250x250x325mm). This ofcourse limits machining time severly.
 
StoopidEngineer, thank you for your kind words on the project and my english. Yes, its a second language :)

Just an observation, your grasp of the English language is far better than many who only speak English. I was surprised to see your updated profile location as Sweden vs somewhere in the USA because you were easy to understand. :cheers:
 
Just an observation, your grasp of the English language is far better than many who only speak English. I was surprised to see your updated profile location as Sweden vs somewhere in the USA because you were easy to understand. :cheers:

Alas, if only my grasp of machining was greater then my grasp of english then this wretched task would soon be done :nopity:
 
It seems like we have to forgo the search for an average MRR and take another route to the target. (This is what i meant with not having a map or a compass!) Now I would greatly appreciate help from the knowledgable people of this forum. If you could take a minut to quote the following factors and give them a number from 1 to 7 (with 1 being the most important) for how much they affect machining time and cost then i would be very thankful:

Finest tolerances
Amount of material removed
Curved/doublecurved areas
Number of operations
Amount of manual labor
Tool wear and cost
Part complexity (more smaller areas vs fewer larger ones etc)

And, if you would be so kind, do it once for aluminium and once for titanium. This would be a huge help.
 
Last edited:
Amount of material removed3
Finest tolerances 1
Curved areas4
Doublecurved areas5
Number of re-fixations 2

First, not 1-10, but 1-5 in order of how much they affect cost, which is what you're really trying to figure, right?

As a rough rule, every number to the right of the decimal (or the comma, if you're like the rest of the Europeans I know :D) increases your price. So always work with the loosest tolerances you can to still have a functional part. So I made that number 1.

Material removal is farther down the list than you might think because, honestly, if we're talking a few more cubic inches, for most shops with decent machines, that's only a few extra minutes of work. The time/labor is more involved in making the initial program and fixture, which requires human interaction and thought. Once the piece is in the machine and running, a minute or two here and there doesn't mean much more to us than the normal machine rate.


Number of re-fixations, or as we generally call them, operations is a big factor. The less a shop has to handle your parts, the less they'll charge you for time and effort.

4 and 5 are about the same, really. Once the program is made, which is the hard part, a machine doesn't care if it's cutting curves or angles or holes or what, it just runs.
 
Thanks TPM! I edited the post according to your suggestion of ordering and added some more factors that we think might be important.
 
The report we create is supposed to be in Swedish but with this forum being as helpful as it is, if there is an interest for it, i promise to translate as much as possible, maybe even write the whole damn thing in english, and post a link to it here once it is done.
 
It seems like we have to forgo the search for an average MRR and take another route to the target. (This is what i meant with not having a map or a compass!) Now I would greatly appreciate help from the knowledgable people of this forum. If you could take a minut to quote the following factors and give them a number from 1 to 7 (with 1 being the most important) for how much they affect machining time and cost then i would be very thankful:

Finest tolerances
Amount of material removed
Curved/doublecurved areas
Number of operations
Amount of manual labor
Tool wear and cost
Part complexity (more smaller areas vs fewer larger ones etc)

And, if you would be so kind, do it once for aluminium and once for titanium. This would be a huge help.

Watch out for that "Finest Tolerances" one - it's not going to be a linear relationship. A couple of tenths (5 microns or so) is a completely different animal than a tolerance twice that size, and a lot of guys here would tell you they can hit +/- .001" on a clapped out machine all day long, where some shops might struggle with that (depending on part size, geometry, etc.)

This is where the machines/shops available becomes important. A guy running a Bridgeport in his garage can kick butt on price on simpler stuff in smaller quantities, but loses his advantage to CNCs when doing more complex geometry, or higher quantity parts (just one very basic example.)

Everything else on your list are reasonable questions to ask, and not terribly difficult to capture - most shops and shop managers have a pretty good "feel" for these types of things when it comes to quoting.
 
Watch out for that "Finest Tolerances" one - it's not going to be a linear relationship. A couple of tenths (5 microns or so) is a completely different animal than a tolerance twice that size, and a lot of guys here would tell you they can hit +/- .001" on a clapped out machine all day long, where some shops might struggle with that (depending on part size, geometry, etc.)

This is where the machines/shops available becomes important. A guy running a Bridgeport in his garage can kick butt on price on simpler stuff in smaller quantities, but loses his advantage to CNCs when doing more complex geometry, or higher quantity parts (just one very basic example.)

Everything else on your list are reasonable questions to ask, and not terribly difficult to capture - most shops and shop managers have a pretty good "feel" for these types of things when it comes to quoting.

Whats your feel for it then? (aka would you like order the numbers?)

Yeah, the finest tolerances will probably have some kind of exponential growth in its effect on price in the calculations, still brainstorming about how to implement the info we collect.

Manufacturers we use have some decent stuff, like DMC 80H Linear and even if our parts dont end up in that type of machines all the time they still affect the overhead, so thats some of the assumptions we are working from.
 
I have made this type of spreadsheet twice at previous companies to allow our purchasing team to set price targets for quotes. It works pretty good for typically "production style" CNC were you face a part and drill and tap some holes. As many people have mentioned once you are doing slotting, pockets, ect the accuracy goes WAY downhill.

As both an engineer and cnc guy I understand the value of this type of spreadsheet, I recently needed to order around 150 CNC parts for a project and needed to come up with a budget to make my manufacturing quote. There was no way I could wait to get 200 quotes back. If you are going to be using the system for the long term, setup a database that tracks the estimated cost vs the real cost so you can fine tune your model.
 
If you are doing small parts, MRR flies entirely out the window and it becomes a game of fixtures, machine acceleration, spindle speed and tool change time.
 
It seems like we have to forgo the search for an average MRR and take another route to the target. (This is what i meant with not having a map or a compass!) Now I would greatly appreciate help from the knowledgable people of this forum. If you could take a minut to quote the following factors and give them a number from 1 to 7 (with 1 being the most important) for how much they affect machining time and cost then i would be very thankful:

Finest tolerances
Amount of material removed
Curved/doublecurved areas
Number of operations
Amount of manual labor
Tool wear and cost
Part complexity (more smaller areas vs fewer larger ones etc)

And, if you would be so kind, do it once for aluminium and once for titanium. This would be a huge help.

Look into the Pugh Decision Matrix ( http://www.decision-making-confidence.com/pugh-matrix.html )

I would use a charting method such as that to get a singular output. There's nothing special about that matrix - it is just the first one that came to mind when thinking about a simple X/Y chart with "value modifiers" to give you outputs for various "options". This may come in handy when you are in your "value engineering" phase of design. This is what I call the design phase that focuses on increasing ease of manufacturing and reducing manufacturing costs. You could compare various options quite quickly this way.

This is a clear, efficient, and effective means of defining your design decisions to justify revisions or 'value engineering' results if you are questioned or required to justify your engineering change-order / revision.

//edit - better URL supplied for decision matrix //
 
Hi Practical Machinist!

Me and a student friend are trying to create an excel spreadsheet that will be used to evaluate different manufacturing methods when ordering for a company that has no in-house production. We are working on a method using average MRR for the large-volume removal and then using a different MRR for the finishing. Do you think this is a viable method of getting a good estimate of machining time? The whole idea is to not have to do a proper CNC-preparation since it might end up being manufactured in a different way and then the preparation would be a waste. Any ideas on how to improve this? Do you think this is a viable method?
.
each tool in tool holder has a limit on metal removal rate assuming what material you are machining and part shape.
....... also machine horsepower and rigidity has a effect. i use a cnc mill that has a 40 hp limit with the vertical head attached and 25 hp with the horizontal head attached. and i have often seen a 6" facemill at 0.2" depth of cut at 35" inch per minute feed with a cast iron casting even if horsepower is available the part can vibrate so much from the cutting force as to destroy the carbide cutting inserts quickly.
....... there is often a limit on how fast you can remove material that is a solid square cube shape compared to thinner sections vibrating from cutting forces. often bigger tools can put tons of force on a part. even taking finish cuts to meet less than .0004" flatness and straightness tolerances often extra finishing cuts of very small amounts are required as the cutting forces can push or distort the part when cutting. or a 20" long tool can vibrate taking even a relatively light cut and needs to be used with only very light cuts to limit tool vibration
so you also need to factor in what machine you are using, what tool and tool holder you are using, and part shape. part shape can effect machining times 1000% easily. many shapes are difficult to machine
 
Whats your feel for it then? (aka would you like order the numbers?)

Yeah, the finest tolerances will probably have some kind of exponential growth in its effect on price in the calculations, still brainstorming about how to implement the info we collect.

Manufacturers we use have some decent stuff, like DMC 80H Linear and even if our parts dont end up in that type of machines all the time they still affect the overhead, so thats some of the assumptions we are working from.

I agree with Teach's order of importance, but (there are always "buts" with this type of thing, hence we all said it is difficult) I would think (and may be wrong) that a part with no tolerance tighter that +/-.010" that requires 4 setups would wind up costing more than a part with a +/-.0005" tolerance that can be done in one setup. Balancing this is the tough thing, even for guys with a lot of experience.

The other thing to consider on this topic is available equipment: it's a lot more difficult (time consuming and expensive) to try to hold something dead flat on a mill. You might be better off surface grinding it afterwards if you have a surface grinder. If a shop has a grinder sitting there, ready to go, it's not a big deal. If they don't have one, or theirs is busy and it has to go down the street to somebody else, it will cost more and/or take longer.
 
Pugh Decision matrix seems to be an awesome solution, thank you! We are checking it out now by building a modified version where we have options A, B and C be the production methods, criterias be the different aspects of the part (like complexity, tolerances etc) and letting the user input the weighting for that specific part. Seems to work out nicely sofar and gives a good amount of freedom since we can program the different cells with IF statements that makes it easy to have non-linear changes
 
What are the major cost drivers when it comes to machining?
.
a part that is not a square cube can vibrate if machined at too high a metal removal rate.
.
tight tolerance can require taking multiple light finish passes and operator checking part
.
drilling deep holes often hss drills are used and long carbide drills can break easy. since hss drills feed at slow rates just drilling holes can take time.
.
multiple setups. everytime a part has to be setup in a different orientation and possibly different parallels or shims and or different zero location it requires time.
.
long reach using long length tools and i often use tools over 18" long that cannot handle a high metal removal rate. again many parts are not simple square cubes.
 
About 15 years ago a fellow I worked with developed a fairly sophisticated Excel spreadsheet for our company to do just what you are looking at. I worked with it a lot myself and felt it was a pretty powerful way to estimate part fabrication costs. The purchasing department quickly lost enthusiasm for our work because they lacked the most basic understanding of it and managed to accomplish little other than infuriating the supplier base of fabrication shops. Another division of the company bought proprietary software to do the same thing, paid well into the 5 figures for it and used it with some success. At the time (2001) our company was pretty close to Autodesk so we shared the method with them. At the time of my retirement late 2002 they hadn't done anything with it although the method seems a natural for 3d CAD systems to give designers a real time cost estimate of their design as it develops. Maybe that's actually happening now. But when I was working the design engineers still threw the designs "over the wall" and the purchasing people in the manufacturing department looked like heroes when they compared their production volume contract costs with the prototype costs even if the company's competitors were buying similar parts for half the price.
The foundation of our spreadsheet approach was the estimating method outlined by Ostwald in the 1984 edition of the AM Cost Estimator. A rare book even in the USA. The basic idea was to use the retail cost of hours on a specific machine or work center and estimate minutes of time the job is being processed there. Add the material cost and a scrap and rework factor. The idea is that all the overhead and profit stuff the company accountants deal with to satisfy the tax collectors are combined into one hourly rate on each process. That's the way job shops in the USA quote their jobs. The additional effort on the part of the company planning to buy a job is to maintain credible local cost information on various machining processes as well as materials. Ostwald's book had that info for 30 years ago. A lot has changed in fabrication technology. But a lot hasn't. You still have to set up machines, load and unload parts, change tools, cut metal. Technology changes the time constants; but the operations are still there. And note here we are counting work center time here. Labor hours are just part of that single work center price whether it's a CNC machine in a lights out line or a simple assembly bench.
Ed Weldon

Hi Practical Machinist!
Me and a student friend are trying to create an excel spreadsheet that will be used to evaluate different manufacturing methods when ordering for a company that has no in-house production. .....Any ideas on how to improve this? Do you think this is a viable method?
 
You've picked a tough one. As a person who did the very thing you asking for a defense contractor and was paid well for it, I suggest picking one type of machine...Mill...lathe... CMM (yes CMM)...shearing ect ...ect... and do a spreadsheet for that.

We spent weeks doing just what you're trying to do and believe me, it is complicated to cover all on one spreadsheet. There are soooooo many variables it almost impossible to cover everything. Understand, it's not a "just plug in" the numbers and you will have an answer. If you could, you would be millionaires with that program.

Example, I just was paid 80 hrs at $100/hr to do a five part famiy production estimate last year, and that was just that, a non-firm estimate.
 
I'm starting to think I should quote someone 200 bucks... just to quote them. Because I use my CAM to estimate machining time, I have to CAD/CAM it first, then put it into my spreadsheet for the final quote which includes the cost of the CAD CAM I've just done. Maybe its a really bad way of doing business or a really good way of loosing money if they don't go ahead with it. Would be interested to see other ways. At least if I say "It will be 200 to quote you". Pressures on us both to go ahead with work. Might work in my favour, may blow up in my face like a Peter North film. Could be good or bad, depending on how you look at it. LOL
 








 
Back
Top