[How do I?] Estimate Life Of A Roughing End Mill

JimDawson

JimDawson

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I am bidding a job of 750 pieces. Material 2 x 2 x 0.250 wall, square, structural steel tubing (A36?). The customer is specifying the material, I have no say in that.

Machining: Cut a notch about 3 inches along the length and about an inch deep into the width, though both sides of the tube.

Because of the radius in the corner, I will be using a 5/8 TiN coated, cobalt rougher. The feed & speed will be optimized for the job, and I'll be using a heavy mist coolant. The total length of cut is about 5 inches per piece, and I'll do it in one pass.

The question is: How many pieces can I cut before the end mill is toast?

I need to order enough end mills to do the job, and of course estimate my tooling cost. At $36 each the tooling cost will be substantial. I'm guessing I can get about 30 pieces (about 150 cut inches total, or about 150 minutes), but some of you production guys have a lot more experience at this kind of thing than I do. Any ideas?

Drawing1.jpg
 
So, you're getting end mills with flute length sufficient to cut both walls at the same time? Aware of the poor finish that will be left by the rougher?
 
Yes, 2.125 flute length. The notch is where another part fits, and will be welded together, so the surface finish is not important.
 
Well, I'll tell you how I would bid the job. I never itemize tooling unless it is exotic, special, or dedicated to that job, such as a jig or fixture. Even then, if the quantity is high enough, even that I ignore. My shop rate is intended to cover expendable tooling like inserts, drills, end mills etc.. Never has a customer asked for a breakdown of the costs. In your case, the life length would be more of a need to know how many to buy type information.

One factor which will be a heavy influence is exactly what end mill you buy. Cobalt? Coated? Coarse or Fine tooth? You are going to run them on your CNC vertical, I presume. So "chip time" is a figure that you can use as part of the calculation. As long as the end mill you use will work under your programmed parameters, the chip time can be very accurate. Handling the pieces, in and out of the mill is hand time, and you should be able to estimate that. I believe that I would rig up to hold two pieces on the mill. They would be roughly end to end and have the program mirrored. While one is cutting, the other would be getting swapped out. All you need is a stop in your program if needed to get the timing right. I always used the chip time I got from programmers and, depending on the complexity of the part, used a x1.5 or x2 factor to cover the tooling changes, and part changes. Are you planning to do any major deburring or any other operations after milling?
 
Tony Wells said:
Well, I'll tell you how I would bid the job. I never itemize tooling unless it is exotic, special, or dedicated to that job, such as a jig or fixture. Even then, if the quantity is high enough, even that I ignore. My shop rate is intended to cover expendable tooling like inserts, drills, end mills etc.. Never has a customer asked for a breakdown of the costs. In your case, the life length would be more of a need to know how many to buy type information.

One factor which will be a heavy influence is exactly what end mill you buy. Cobalt? Coated? Coarse or Fine tooth? You are going to run them on your CNC vertical, I presume. So "chip time" is a figure that you can use as part of the calculation. As long as the end mill you use will work under your programmed parameters, the chip time can be very accurate. Handling the pieces, in and out of the mill is hand time, and you should be able to estimate that. I believe that I would rig up to hold two pieces on the mill. They would be roughly end to end and have the program mirrored. While one is cutting, the other would be getting swapped out. All you need is a stop in your program if needed to get the timing right. I always used the chip time I got from programmers and, depending on the complexity of the part, used a x1.5 or x2 factor to cover the tooling changes, and part changes. Are you planning to do any major deburring or any other operations after milling?
Click to expand...

The end mills will be Cobalt, TiN coated, Course tooth. I may run the parts on Alloy's Shizouka that's sitting in my shop. It's a heaver machine than mine and has a tool changer, what isn't shown is a 11/16 hole that needs to be drilled also. I was thinking about a second opp to do that, but with a tool changer it would save a second opp.

I calculate about 5 minutes of chip time per part, and do plan on loading 2 parts at a time, I may even be able to put 4 parts on the fixture, not sure yet, I haven't gotten that far. I got the RFQ at about 17:00 this evening. Should be less than a minute to load & unload. Deburring will be a quick pass with the angle grinder.

The customer is not asking for a break down, but I need to estimate my tooling cost to calculate the bid. I normally use $85/hr + tooling. Normally I can make a pretty good guess at how long the tools will last and what I'll need for the job, but I have not machined much of this material. Structural steel is what they make cutting torches for.;) I'm guessing I'll need about 25 end mills, at $36 each that's $900, so about $1.20 per part in tooling cost.

Maybe I'm bidding stuff wrong. Maybe I should just bump up the machine time rate a bit. I also charge a one time setup and fixture charge for runs like this. Maybe I should roll those charges into the per piece price.

It's good to have someone to bounce this stuff off of.
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This is fascinating, totally new to this so call me a "greenhorn". From your data your maximum estimated production rate will be 10 units per hour at minimum cost of $8.70 each plus set up and fixture expenses. No doubt you have calculated a production rate somewhat less than this and have a good idea about that. Going back to some Industrial Engineering training I guesstimated a time involved for each function from your data.
Function Time Min Sec
1. Place units on machine 0.25 15
2. Cut groove reposition & cut 2nd groove 4.15 249
3. Change tool to drill hole & position 0.10 6
4. Drill Hole 0.10 6
5. Reposition & drill hole 0.15 9
6. Remove units 0.25 15
7. Total 5.00 300
I notice you are estimating cut time at 1 inch per minute when talking about tooling so that would change the cut time above to 6.15 or 7.00 minutes. Tool changing time would needs to be added to this estimate as well.
Will keeping stock available to the machine, having coffee, answering telephone, storing finished product, etc. be done in the time the machine does the work? Obviously this is a newby estimate that overlooks many things but is something I would record as the work is done. Can the machine record the time of each function? I have been asked to do random jobs by neighbors that I totally don't know what to charge so most of it goes "gratis".
If I am not being to snoopy I would be interested to know how this turns out.
 
Setup time I calculate at 1 hour per tool, using shop rate, then amortize over the run.
 
Rock_breaker, that is a great break down, and is in line with my guesses. I think 10 parts per hour is right on the money, and I did figure out how to get 4 parts on the table at a time and figured out a fixture that will allow quick load & unload. Most of my work is product development and machine design/building and tool & die work. Normally those are time and materials jobs. 5 of the same part is a lot for me. I normally don't do production work but occasionally I get asked to run a production job, and if there is enough profit in it, or it's for a really good customer I'll do it. Like you, I do odd jobs for the neighbors too, usually for a Sunday dinner or something. ''Will work for food'' :grin: The neighbors barbecue up some great steaks and hamburgers.! We just kinda do stuff for each other. If I get this job, I'll post the actual run times.


Tony, I like that formula. In this case I'm going to have about 3 hours and a couple hundred bucks in materials for the fixture. But this will be a recurring job, so I only have to build the fixture once.
 
On a worst day, may get 10 pieces per endmill. On a good day, maybe 30 pieces. Also depends on how dull you want to make them before changing out. Or how often you break off one. Shame you couldn't run a 3/4 rougher! Be much stiffer less likely to break off....
 
He is going to run a corncobb, but he has to contend with a 2"+ flute length to do the part in one pass. I might point out that the feedrate will need to change as you cut through the full 2" wall at the terminus of the cut. Might make a performance difference starting on the side vs starting on the end. Have to think about that.

I think I'd order 10 end mills, or wherever I could get a price break and do some metrics on tool life. I assume you have ready access to end mills?
 
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