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Join Date: Feb 2016
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UHMWPE Surface Issue

02/25/2016 5:12 AM

Dear Sir,

We are Orthopedic industry and manufacturer of tibial insert of UHMWPE material. We want to achieve the Ra value <1 micrometers in CNC milling machine. But for last month we are not able to acheive it. Following are the Trials we took:

Trial 1 :

Cutter 10 Ball Nose Cutter

Speed 3000 rpm

feed rate 1600 mm/min

depth of cut 0.3 mm

Run out of cutter 5-8 micron deviation

Trial 2 :

Millining machine balancing

But still we are not able to get Ra value less than 1

If any suggestion You have please Give it. I will be Highly obliged

with regards

Pralhad

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#1

Re: UHMWPE Surface issue

02/25/2016 8:48 AM

You may be asking the impossible .

Try buffing, flame polishing, or as a last resort injection molding in a high polished mold.

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#4
In reply to #1

Re: UHMWPE Surface issue

02/25/2016 10:42 PM

Dear sir,

Thank you for suggestion . I would like to know more about Flame Polishing in your Point of view.

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#6
In reply to #4

Re: UHMWPE Surface issue

02/25/2016 10:57 PM

Flame polishing is not magic. Google it.

Also ask your material supplier about lot to lot variation of their product. Have you begun using a new lot of material?

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#2

Re: UHMWPE Surface issue

02/25/2016 10:20 AM

Such Ra values correspond to superfinish and this is NOT possible on only a milling machine with cutting tools.

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#3

Re: UHMWPE Surface issue

02/25/2016 2:11 PM

Micrometer finish and micron deviation? Are you sure you have your units right (just checking)?

How close are you getting at the moment? As others have said, the repeatable accuracy you are asking for doesn't appear possible using a single CNC process on this type of material.

However, if you were somehow getting it before (which you seem to indicate) then you need to check what has changed in your CNC setup or CNC machine as even slight variations in machine tool, holder and machine mechanical tolerances or wear could cause deviations at such a fine accuracy level. Perhaps even external vibration transferring itself to the milling head.

Even variations between two supposedly identical brand new milling tools from the same manufacturer and batch could account for this.

Perhaps it is time to contact the CNC manufacturer and ask them there advice. They will probably advise you the same that it is not possible to ensure finish repeatability at this accuracy level.

Let us know what you find.

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#5
In reply to #3

Re: UHMWPE Surface issue

02/25/2016 10:49 PM

Dear sir,

Thank You For your input. The Ra value is in the range of 1.3 -1.9 (which is not acceptable). We also have done the trails with old parameters, old tooling by resharping. Even then the Problem is not solved.

2 months the same machine VMC used to give us Ra upto 0.8-0.9 . Also concerned the manufacturer of VMC and checked the balancing , coolant Ph value and vibrations even then we have the same issue.

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#7

Re: UHMWPE Surface issue

02/26/2016 12:12 AM

You may want to explore another direction on this. Is the Ra value < 1 micro actually needed or is this spec being generated by an overly aggressive design engineer who may not know much about the actual product use or manufacturing.

In my (limited) experience with medical devices the accuracy you are looking for is basicly being wasted. I know nothing about your product, but I am assuming it is either a part that is being surgically implanted into a patient or a sub assembly of a part going into a patient.

In either case there is a doctor that is preparing the implant location with surgical tools. Their accuracy will not be anywhere close to the precision you are looking for in your manufacturing. All your effort is lost when the part gets placed in the patient. There is no sense wasting the effort and money being precise when the destination is too variable to take advantage of the precision.

If your issue is a smooth bio-compatable surface then you should really be looking at surfacing processes and not the machining. Your real goal would be surface quality and not sub micro precision.

My suggestion would be to stop what you are doing and have a frank discussion with the customer and/or design engineers. Talk about the precision/cost tradeoff and the achievability of their design spec.

With a little creativity and an honest discussion you have a good chance of figuring out a way to make the product more effectively. This could mean lower costs for them, more profit for you and still hitting the true design goal.

I have seen so many times when the designer put completely unreasonable tolerances in Solidworks without ever having a clue what it really meant. That problem wont get fixed without you working with the customer/designers.

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#8
In reply to #7

Re: UHMWPE Surface issue

02/26/2016 1:43 AM

Actually , This tibial tray is in continuous contact with Shine metal part . So if the Ra value is not matched then it will result in wear and tear . and will damage the whole component. This is the Input which is given by my Design team. And Previous month the same VMC machine used to give 0.9 Ra . If any More input required let me know

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#9
In reply to #7

Re: UHMWPE Surface issue

02/26/2016 5:07 AM

There are two separate issues here, of accuracy of shape and roughness. It is the roughness parameter which is of interest, and yes, an Ra value of 1 mu or less is appropriate for surfaces which need to slide (a higher Ra is needed for surfaces which need to bond to bone).

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#10

Re: UHMWPE Surface issue

02/26/2016 6:07 AM

Ball end cutter is not the tool I would chose for a flat wide surface the contact area is so very small and can deviate due to several factors at 1micron You do not mention the path pattern or the fixture, has that changed become worn or contaminated is the temperature stable in the cutting environ. The feed speed does it need to be so high? did you try a flat end triple flute with the corners backed off.

We had a very sensitive CMM that required at least 1 hour of stabilisation before it was on point if someone left the door open for any time.

Flame polishing will get you a smooth surface but not always an accurate one and you might have to try several fluid mixes to get your results..

SO Go back to basics try a different material one harder one softer , cutter try one bigger one smaller, vary the feed speed, check the lubrication on the slides, try a Collete and spindle balance vibration test and alignment all these things will add to the mix. Do you have a safe spec-ed unit for comparison.. and.....

Exactly how are you measuring this piece are you sure that is accurate are you digging in the wrong place

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#11
In reply to #10

Re: UHMWPE Surface issue

02/26/2016 9:48 AM

The surface is wide but not flat, so a ball-end cutter has its justification (assuming that milling is the best way to produce the surface).

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#12

Re: UHMWPE Surface Issue

02/27/2016 10:09 PM

You may a spot of swarf on a collet or holder surface. The resharpened tool may have been done in a machine that has a small spot of swarf etc. Try resharpeneing the tool while clamped in it's holder. Having said that my money would be on the material. Do you have some that you used when you got the good finish? Is it made with recycled plastic?

I don't know the shape but i am guessing it is for a knee replacement. A good finish can be obtained by scraping but this requires a high degree of hand skills and the compound curves in a knee joint could be destroyed by aggressive scraping.

Jim

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#14
In reply to #12

Re: UHMWPE Surface Issue

02/29/2016 11:17 PM

you are very correct about Knee replacement. It is a Knee ReplaCement product. Right now We are Focusing on the tool Geometry. Any Suggestion regarding tool suggestion will be marvelous.

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#13

Re: UHMWPE Surface Issue

02/27/2016 10:15 PM

I'm due a tibia head replacement in 2017. It's bad enough feeling the joint grinding together.

I don't want to feel your bodged engineering crunching about.

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#15

Re: UHMWPE Surface Issue

03/03/2016 12:11 AM

Finally Ra value achieved. Tool Geometry was the Problem . We were using resharpening tools from last month. Now with Silmax 8 Ball nose cutter. we were able to achieve below 1 micrometers.

Thanks to all for your Interest. It was a third eye to me

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#16
In reply to #15

Re: UHMWPE Surface Issue

03/04/2016 5:18 AM

Thanks for posting the solution.

Jim

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