Clamprite Drawbar Clamping Gage, Clamprite Drawbar Clamping Tester


We offer the Clamprite Drawbar Clamping Gage or Clamprite Drawbar Clamping Tester and all known spindle taper adapters for measuring the drawbar clamping force of CNC milling spindles.

Benefits of Testing Drawbar Clamping Force

The Clamprite Drawbar Clamping Gage is the best selling testing tool for spindle drawbar clamping force of CNC Machining Centers. The use of this economical spindle drawbar checking tool will maximize machine up time and minimize spindle down time costs associated with wear and damage of drawbars, clamping collets, pull studs, toolholders and tool tapers. By measuring the tool clamping force with this gage, spindle repair costs and repair time can be minimized.

Frequently Asked Questions

1) I have various machines with drawbar retention force specifications up to 4600 Lbs. Can I use the 5000 PSI pressure gage for all the smaller, lower force machines?

Yes. However, reading the numbers on the 3000 PSI pressure gage is easier than on a 5000 PSI pressure gage because there are fewer numbers on the dial. But a slight difference in reading accuracy is not that important because there is little concern about 10 or 20 Lbs — or even 100 Lbs. The real concern
is when a machine is 20% – 30% low on drawbar force. Spindle technicians usually accept 10% variation when rebuilding a drawbar.

2) What about the accuracy of the Clamprite Drawbar Clamping Gage versus the higher priced electronic and digital drawbar force gages? It is mentioned how the digital are “extremely accurate” in your sales brochure. Isn’t it better to have high accuracy when available?

Yes … and no! More accuracy is good when FREE. We believe the electronic gages with digital display are overkill for the shop floor in terms of accuracy and cost. The extremely accurate and expensive digital drawbar force check gages are similar to what we use to certify all Clamprite Gages. Our accurate and rugged tool clamping force gage is better suited to a shop environment where the delicate electronic spindle drawbar force gages can be easily damaged. If a Clamprite Drawbar Clamping Gage is dropped, a pressure gage could cost $40. A digital gage can cost over $1,000 to repair. So … our Drawbar Clamping Gage can be purchased new for less than the repair cost of an electronic drawbar checking gauge with digital display. The Clamprite Gage Kit includes a Certificate of Accuracy traceable to National Institute of Standards and Technology (NIST). These digital drawbar force check gages are sometimes called a spindle drawbar dynamometer. Unfortunately, they neither measure power nor the two components of power   …   rotational speed and torque. In fact, for safety reasons, there can be no spindle rotation while doing the axial force check. These spindle check gages simply measure the drawbar axial pull force   …   there is no power check and a drawbar clamping gage is not a spindle dynamometer.

3) My CNC Mill has a high speed HSK type spindle. Can I use the same gage head to test this machine?

Yes, the gage head used for testing the common long taper spindles is the same for testing the HSK spindle system. An HSK adapter is required for HSK spindle testing. These HSK adapters are available for all HSK spindle sizes and are accurate, durable and low in cost.

HSK spindles must be tested frequently to assure that they are within 75% of spec at all times. The tool holders will not seat correctly to the face if the drawbar is pulling less than 75% of spec.

We highly recommend to anyone who sells, uses, or services HSK machinery or tooling that you read the “HSK Handbook”. It is well written (by Dr. E. Kocherovsky) and not only tells the history of HSK and it’s full story, it also has about 150 excellent color charts, graphs, and pictures explaining how it works. There is even a complete set of blueprints for each size of spindles and arbors. The contents of the book will amaze you, and make you glad that your shop is not making HSK tooling!! It is well worth our price of only $149 when you purchase a gage kit.

4) I can’t find the specified drawbar force for my machine anywhere in the manuals. Can you tell me what the drawbar force my machine should provide?

There are so many variations, even among similar machines. Only the manufacturer can say for sure. Some general figures are: 1800 to 2500 Lbs for most 40-taper spindles, 3800 to 4800 lb for most 50-taper spindles except heavy-duty versions which may be considerably higher. This is only a reference, and not a guide for any specific machine or spindle. To obtain the specific force for any particular machine, get the model number, the serial number, and date it was built from the nameplate and consult the factory representative.

5) What is the difference between using a standard tool holder as the spindle adapter vs. using the specially designed Clamprite Spindle Adapters?

A standard end mill holder can be used as the interface for drawbar testing. Our Adapters cost less than most tool holders, and since there are no tool changer flanges, the 50 taper (one unit) adapter fits all CAT, BT, and ISO spindles. Finding the right holder can be time consuming because it is often back in use on a machine when it is needed.

But the best reason to use a Clamprite Spindle Adapter is because we have found tool holders to be out of tolerance to the gage line dimension of the spindle. In fact, some tool holders (from reputable manufacturers) were .0125 over the high tolerance limit. Some were nearly that amount below the low tolerance limits. This amount of variation can cause great differences in spindle drawbar force readings.

All Clamprite Spindle Adapters are inspected with spindle taper gages to verify gage-length accuracy. A spindle that has .001 wear on the surface will allow a tool to seat .0069 deeper into the spindle pocket — well past the gage-line tolerance limits, and enough to cause reduced overall drawbar holding force!

6) I have a 50-taper spindle on a geared head machine designed for cutting steel. Can I use the 5000 PSI pressure gauge to test it?

Most 50-taper machines are below 5000 PSI, usually from 3800 to 4800 PSI. But some heavy duty 50-taper machines can go to 7800, or more. If you over pressure the gauge, it may become inaccurate.
See precautions in the instruction manual. As a courtesy to our customers, Clamprite will replace a pressure gauge with another gauge at no labor cost, if shipping is prepaid both ways. The complete re-certifying procedure is done for a nominal fee of just $59.95 (includes certificate traceable to NIST).

7) One of our machines is very large and was designed for heavy duty use. Someone said it may provide over 10,000 lb of drawbar force. Can the CG be used to test its drawbar?

The Clamprite Gage II is a heavy duty version of the standard Clamprite Drawbar Clamping Gage. The standard Clamprite Drawbar Clamping Gage will test up to 10,000 lb when fitted with the proper pressure gauge. The Clamprite Gage II will test up to 15,000 lb. Contact us for testing of forces over 15,000 lb. If you attempt to test the spindle and it is over 10,000 lb, the 10,000 PSI pressure gauge may be ruined and hydraulic fluid may squirt out causing injury. See precautions in the Instruction Manual.
Your gage head can be re-fitted with a higher reading pressure gauge at no charge if courier charges are paid both ways. There is a nominal charge for re-certification, recording and issuing a new Certificate of Accuracy traceable to NIST ($59.95).

8) Can a Clamprite Drawbar Clamping Gage fitted with a 10,000 PSI pressure gauge be used to test spindles with 2000 lb of force?

Yes, but reading a 3000 PSI gauge is easier than reading a 10,000 PSI gauge because there are fewer numbers on the face of the pressure gauge but the 10,000 PSI gauge will show you the drawbar force that your machine is producing. Changing pressure gauges should only be done by qualified hydraulic repair personnel. As a courtesy to our customers, Clamprite will replace a pressure gauge with another one of different pressure at no charge, except for the gauge and re-certification. Labor is free. Use caution if you attempt replacement yourself, as the fluid can squirt out and cause skin irritation, and improper bleeding of the gauge and body may cause false readings. Also, commercial gauges can vary greatly in accuracy. Clamprite uses only high accuracy stainless steel pressure gauges, and we also test and certify each assembly for accuracy on a certified electronic test bench.

9) We are a small shop with no customer requirements to verify our drawbar force. How long can we go without checking or re-certifying the Clamprite Spindle Drawbar Clamping Gage?

A lot of attention is being given to machine condition today. Some companies, especially aerospace, are even condition-coding machines and requiring accuracy tests such as “ball-bar testing” to be performed by vendors before parts can pass QC requirements. It is just a matter of time before they realize that a drawbar is just as critical for consistent machining as the axis drives and ball screw condition. But under your circumstances, you might go a few years without worrying about the gauge accuracy. If the Clamprite Drawbar Clamping Gage is not over-pressured, it should remain accurate for many years. However, Clamprite will check the accuracy of your gauge for free as long as you own it, if shipping is paid both ways. We do not record and issue a new Certificate of Accuracy when doing a free courtesy check. But it will verify that your gage is still in the original condition. There is a nominal fee of $59.95 for a complete re-certification.

10) My machine uses hydraulic pressure to hold the tools in the spindle. Will the Clamprite Drawbar Clamping Gage work for this application?

Yes. Whatever the means of creating tool retention force, that force can be measured with the Clamprite Gage.

However, there is some confusion about this. Nearly all machines use the spring stack method of tool retention. But the method of releasing the tool from the spring pressure comes from a hydraulic piston which is usually energized by an air cylinder. Compressed air is supplied to the air-over-hydraulic piston (usually located on top of the spindle) which acts like a small jack-type press for compressing the spring stack which in turn releases the clamping force acting on the tool holder’s pull-stud. Sounds a little complicated, but is actually fairly simple once you have seen all the parts laid out. Sometimes people are confused about how the hydraulics work. The air-over-hydraulic piston can be heard during a tool change, but it is usually used to release the tools, not to hold them.

11) In advertising and in your Instruction Sheet you spell the word gauge as simply “gage” at times. Why the difference?

Gage (spelled this way) is somewhat traditional in the metal working trades, except when specifically discussing a pressure gauge. The bourdon-tube type of pressure gauge has come to be known as simply “a gauge“, whereas many other gages (like radius gages, depth gages, etc.) are more like a method of measuring. Most tool making companies like the L.S. Starrett Co. for example, still use the older spelling of gage when describing their products. Most dictionaries list both spellings as acceptable.

12) You used to offer an adapter to test drawbar force on a Bridgeport mill. Why test drawbar force on a manual machine?

The Bridgeport Adapter was designed to give people a way to see and feel the kind of force that can be generated simply by turning a screw with a wrench. Most people are surprised when they use the Clamprite Gage on a Bridgeport type mill. It showed that most people applied twice the tool holding force than most 40-taper CNC Machining Centers when they are new! It was a real eye opener and a lot of fun, and it might have saved some Bridgeports from damage.

Why Test Drawbar Clamping Force, and Why Check for Drawbar Clamping Force Loss?

Why Check – Why Check Drawbar Clamping Force

Drawbar clamping force is often the first thing to weaken or fail on a CNC Machining Center!

A stack of 100 to 150 Belleville springs provide the clamping force that pulls the toolholder into the spindle taper. Belleville springs wear grooves into the drawbar and the friction between these springs and the drawbar can result in drawbar clamping force loss, which can permit chatter, bad finish, reduced cutter life, and scrapped parts. Belleville springs can and do fatigue and break, resulting in extreme loss of drawbar clamping force. Programmers will often compensate for this, unaware that one spring after another is failing.

The toolholder can eventually pull out of the spindle taper during a cut, causing damage to the spindle taper, bearings, toolholder and pullstud, cutter, and fixture. A routine force check with a spindle drawbar clamping gage would have revealed drawbar force loss, which would be the cue to initiate corrective action.

How to Use the Clamprite Gage

By placing the Clamprite Gage (with an appropriate taper adapter) into the spindle taper, and causing the drawbar to pull on the pullstud (or retention knob) that you have threaded into the internal transfer screw coupling, it takes less than one minute to check the drawbar clamping force against the machine manufacturer’s recommended level. The clamping force gauge is simply hydraulic, with no batteries, no cables and no delicate electronics to go wrong. Since the piston surface area is 1 square inch, a pressure gauge converts directly to pounds of force. Many shops use the Clamprite Gage and most say it paid for itself the first time they used it … because they caught a drawbar exerting low clamping force and they corrected it, before that caused a crash and cost a spindle taper regrind or a full spindle repair. Keep a running record of each spindle’s measured drawbar clamping pull, and be alerted to drawbar force loss and trends over time. The more frequently you check spindle drawbar pull, the sooner a spindle drawbar clamping force loss is detected.

How to order a Drawbar Force Gage

Select a gagehead with sufficient capacity to measure your highest drawbar clamping pull. Select a spindle taper adapter for each size and type of taper in your plant.   Or, select a kit consisting of a gagehead and spindle taper adapters. Order additional adapters as needed. For pricing, see: Pricing – Spindle Drawbar Force Gages.

For a Quotation to order your spindle drawbar force gage – Click or Contact Us in the footer below, or the header above!   or Call 519-671-3911.